refactor(tvix): move castore into tvix-castore crate r/6629

This splits the pure content-addressed layers from tvix-store into a
`castore` crate, and only leaves PathInfo related things, as well as the
CLI entrypoint in the tvix-store crate.

Notable changes:
 - `fixtures` and `utils` had to be moved out of the `test` cfg, so they
   can be imported from tvix-store.
 - Some ad-hoc fixtures in the test were moved to proper fixtures in the
   same step.
 - The protos are now created by a (more static) recipe in the protos/
   directory.

The (now two) golang targets are commented out, as it's not possible to
update them properly in the same CL. This will be done by a followup CL
once this is merged (and whitby deployed)

Bug: https://b.tvl.fyi/issues/301

Change-Id: I8d675d4bf1fb697eb7d479747c1b1e3635718107
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9370
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

46 files changed, 7392 insertions, 0 deletions

diff --git a/tvix/castore/Cargo.toml b/tvix/castore/Cargo.toml
new file mode 100644
index 0000000000..8bef5bb147
--- /dev/null
+++ b/tvix/castore/Cargo.toml
@@ -0,0 +1,42 @@
+[package]
+name = "tvix-castore"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+anyhow = "1.0.68"
+async-stream = "0.3.5"
+blake3 = { version = "1.3.1", features = ["rayon", "std"] }
+bytes = "1.4.0"
+data-encoding = "2.3.3"
+futures = "0.3.28"
+lazy_static = "1.4.0"
+pin-project-lite = "0.2.13"
+prost = "0.11.2"
+sled = { version = "0.34.7", features = ["compression"] }
+thiserror = "1.0.38"
+tokio-stream = { version = "0.1.14", features = ["fs", "net"] }
+tokio-util = { version = "0.7.8", features = ["io", "io-util"] }
+tokio = { version = "1.28.0", features = ["fs", "net", "rt-multi-thread", "signal"] }
+tonic = "0.8.2"
+tower = "0.4.13"
+tracing = "0.1.37"
+url = "2.4.0"
+walkdir = "2.4.0"
+
+[dependencies.tonic-reflection]
+optional = true
+version = "0.5.0"
+
+[build-dependencies]
+prost-build = "0.11.2"
+tonic-build = "0.8.2"
+
+[dev-dependencies]
+test-case = "2.2.2"
+tempfile = "3.3.0"
+tonic-mock = { git = "https://github.com/brainrake/tonic-mock", branch = "bump-dependencies" }
+
+[features]
+default = ["reflection"]
+reflection = ["tonic-reflection"]
\ No newline at end of file
diff --git a/tvix/castore/build.rs b/tvix/castore/build.rs
new file mode 100644
index 0000000000..9f06c88333
--- /dev/null
+++ b/tvix/castore/build.rs
@@ -0,0 +1,38 @@
+use std::io::Result;
+
+fn main() -> Result<()> {
+    #[allow(unused_mut)]
+    let mut builder = tonic_build::configure();
+
+    #[cfg(feature = "reflection")]
+    {
+        let out_dir = std::path::PathBuf::from(std::env::var("OUT_DIR").unwrap());
+        let descriptor_path = out_dir.join("tvix.castore.v1.bin");
+
+        builder = builder.file_descriptor_set_path(descriptor_path);
+    };
+
+    // https://github.com/hyperium/tonic/issues/908
+    let mut config = prost_build::Config::new();
+    config.bytes(["."]);
+
+    builder
+        .build_server(true)
+        .build_client(true)
+        .compile_with_config(
+            config,
+            &[
+                "tvix/castore/protos/castore.proto",
+                "tvix/castore/protos/rpc_blobstore.proto",
+                "tvix/castore/protos/rpc_directory.proto",
+            ],
+            // If we are in running `cargo build` manually, using `../..` works fine,
+            // but in case we run inside a nix build, we need to instead point PROTO_ROOT
+            // to a sparseTree containing that structure.
+            &[match std::env::var_os("PROTO_ROOT") {
+                Some(proto_root) => proto_root.to_str().unwrap().to_owned(),
+                None => "../..".to_string(),
+            }],
+        )?;
+    Ok(())
+}
diff --git a/tvix/castore/default.nix b/tvix/castore/default.nix
new file mode 100644
index 0000000000..09f6c0bea3
--- /dev/null
+++ b/tvix/castore/default.nix
@@ -0,0 +1,5 @@
+{ depot, pkgs, ... }:
+
+depot.tvix.crates.workspaceMembers.tvix-castore.build.override {
+  runTests = true;
+}
diff --git a/tvix/castore/protos/LICENSE b/tvix/castore/protos/LICENSE
new file mode 100644
index 0000000000..2034ada6fd
--- /dev/null
+++ b/tvix/castore/protos/LICENSE
@@ -0,0 +1,21 @@
+Copyright © The Tvix Authors
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+“Software”), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
diff --git a/tvix/castore/protos/castore.go b/tvix/castore/protos/castore.go
new file mode 100644
index 0000000000..102ba4bff7
--- /dev/null
+++ b/tvix/castore/protos/castore.go
@@ -0,0 +1,164 @@
+package castorev1
+
+import (
+	"bytes"
+	"encoding/base64"
+	"fmt"
+	"google.golang.org/protobuf/proto"
+	"lukechampine.com/blake3"
+)
+
+// The size of a directory is calculated by summing up the numbers of
+// `directories`, `files` and `symlinks`, and for each directory, its size
+// field.
+func (d *Directory) Size() uint32 {
+	var size uint32
+	size = uint32(len(d.Files) + len(d.Symlinks))
+	for _, d := range d.Directories {
+		size += 1 + d.Size
+	}
+	return size
+}
+
+func (d *Directory) Digest() ([]byte, error) {
+	b, err := proto.MarshalOptions{
+		Deterministic: true,
+	}.Marshal(d)
+
+	if err != nil {
+		return nil, fmt.Errorf("error while marshalling directory: %w", err)
+	}
+
+	h := blake3.New(32, nil)
+
+	_, err = h.Write(b)
+	if err != nil {
+		return nil, fmt.Errorf("error writing to hasher: %w", err)
+	}
+
+	return h.Sum(nil), nil
+}
+
+// isValidName checks a name for validity.
+// We disallow slashes, null bytes, '.', '..' and the empty string.
+// Depending on the context, a *Node message with an empty string as name is
+// allowed, but they don't occur inside a Directory message.
+func isValidName(n []byte) bool {
+	if len(n) == 0 || bytes.Equal(n, []byte("..")) || bytes.Equal(n, []byte{'.'}) || bytes.Contains(n, []byte{'\x00'}) || bytes.Contains(n, []byte{'/'}) {
+		return false
+	}
+	return true
+}
+
+// Validate thecks the Directory message for invalid data, such as:
+// - violations of name restrictions
+// - invalid digest lengths
+// - not properly sorted lists
+// - duplicate names in the three lists
+func (d *Directory) Validate() error {
+	// seenNames contains all seen names so far.
+	// We populate this to ensure node names are unique across all three lists.
+	seenNames := make(map[string]interface{})
+
+	// We also track the last seen name in each of the three lists,
+	// to ensure nodes are sorted by their names.
+	var lastDirectoryName, lastFileName, lastSymlinkName []byte
+
+	// helper function to only insert in sorted order.
+	// used with the three lists above.
+	// Note this consumes a *pointer to* a string,  as it mutates it.
+	insertIfGt := func(lastName *[]byte, name []byte) error {
+		// update if it's greater than the previous name
+		if bytes.Compare(name, *lastName) == 1 {
+			*lastName = name
+			return nil
+		} else {
+			return fmt.Errorf("%v is not in sorted order", name)
+		}
+	}
+
+	// insertOnce inserts into seenNames if the key doesn't exist yet.
+	insertOnce := func(name []byte) error {
+		encoded := base64.StdEncoding.EncodeToString(name)
+		if _, found := seenNames[encoded]; found {
+			return fmt.Errorf("duplicate name: %v", string(name))
+		}
+		seenNames[encoded] = nil
+		return nil
+	}
+
+	// Loop over all Directories, Files and Symlinks individually.
+	// Check the name for validity, check a potential digest for length,
+	// then check for sorting in the current list, and uniqueness across all three lists.
+	for _, directoryNode := range d.Directories {
+		directoryName := directoryNode.GetName()
+
+		// check name for validity
+		if !isValidName(directoryName) {
+			return fmt.Errorf("invalid name for DirectoryNode: %v", directoryName)
+		}
+
+		// check digest to be 32 bytes
+		digestLen := len(directoryNode.GetDigest())
+		if digestLen != 32 {
+			return fmt.Errorf("invalid digest length for DirectoryNode: %d", digestLen)
+		}
+
+		// ensure names are sorted
+		if err := insertIfGt(&lastDirectoryName, directoryName); err != nil {
+			return err
+		}
+
+		// add to seenNames
+		if err := insertOnce(directoryName); err != nil {
+			return err
+		}
+
+	}
+
+	for _, fileNode := range d.Files {
+		fileName := fileNode.GetName()
+
+		// check name for validity
+		if !isValidName(fileName) {
+			return fmt.Errorf("invalid name for FileNode: %v", fileName)
+		}
+
+		// check digest to be 32 bytes
+		digestLen := len(fileNode.GetDigest())
+		if digestLen != 32 {
+			return fmt.Errorf("invalid digest length for FileNode: %d", digestLen)
+		}
+
+		// ensure names are sorted
+		if err := insertIfGt(&lastFileName, fileName); err != nil {
+			return err
+		}
+
+		// add to seenNames
+		if err := insertOnce(fileName); err != nil {
+			return err
+		}
+	}
+
+	for _, symlinkNode := range d.Symlinks {
+		symlinkName := symlinkNode.GetName()
+
+		// check name for validity
+		if !isValidName(symlinkName) {
+			return fmt.Errorf("invalid name for SymlinkNode: %v", symlinkName)
+		}
+
+		// ensure names are sorted
+		if err := insertIfGt(&lastSymlinkName, symlinkName); err != nil {
+			return err
+		}
+
+		// add to seenNames
+		if err := insertOnce(symlinkName); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
diff --git a/tvix/castore/protos/castore.pb.go b/tvix/castore/protos/castore.pb.go
new file mode 100644
index 0000000000..5323d6c923
--- /dev/null
+++ b/tvix/castore/protos/castore.pb.go
@@ -0,0 +1,580 @@
+// SPDX-FileCopyrightText: edef <edef@unfathomable.blue>
+// SPDX-License-Identifier: OSL-3.0 OR MIT OR Apache-2.0
+
+// Code generated by protoc-gen-go. DO NOT EDIT.
+// versions:
+// 	protoc-gen-go v1.31.0
+// 	protoc        (unknown)
+// source: tvix/castore/protos/castore.proto
+
+package castorev1
+
+import (
+	protoreflect "google.golang.org/protobuf/reflect/protoreflect"
+	protoimpl "google.golang.org/protobuf/runtime/protoimpl"
+	reflect "reflect"
+	sync "sync"
+)
+
+const (
+	// Verify that this generated code is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(20 - protoimpl.MinVersion)
+	// Verify that runtime/protoimpl is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(protoimpl.MaxVersion - 20)
+)
+
+// A Directory can contain Directory, File or Symlink nodes.
+// Each of these nodes have a name attribute, which is the basename in that directory
+// and node type specific attributes.
+// The name attribute:
+//   - MUST not contain slashes or null bytes
+//   - MUST not be '.' or '..'
+//   - MUST be unique across all three lists
+//
+// Elements in each list need to be lexicographically ordered by the name
+// attribute.
+type Directory struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	Directories []*DirectoryNode `protobuf:"bytes,1,rep,name=directories,proto3" json:"directories,omitempty"`
+	Files       []*FileNode      `protobuf:"bytes,2,rep,name=files,proto3" json:"files,omitempty"`
+	Symlinks    []*SymlinkNode   `protobuf:"bytes,3,rep,name=symlinks,proto3" json:"symlinks,omitempty"`
+}
+
+func (x *Directory) Reset() {
+	*x = Directory{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_castore_proto_msgTypes[0]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *Directory) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*Directory) ProtoMessage() {}
+
+func (x *Directory) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_castore_proto_msgTypes[0]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use Directory.ProtoReflect.Descriptor instead.
+func (*Directory) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_castore_proto_rawDescGZIP(), []int{0}
+}
+
+func (x *Directory) GetDirectories() []*DirectoryNode {
+	if x != nil {
+		return x.Directories
+	}
+	return nil
+}
+
+func (x *Directory) GetFiles() []*FileNode {
+	if x != nil {
+		return x.Files
+	}
+	return nil
+}
+
+func (x *Directory) GetSymlinks() []*SymlinkNode {
+	if x != nil {
+		return x.Symlinks
+	}
+	return nil
+}
+
+// A DirectoryNode represents a directory in a Directory.
+type DirectoryNode struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// The (base)name of the directory
+	Name []byte `protobuf:"bytes,1,opt,name=name,proto3" json:"name,omitempty"`
+	// The blake3 hash of a Directory message, serialized in protobuf canonical form.
+	Digest []byte `protobuf:"bytes,2,opt,name=digest,proto3" json:"digest,omitempty"`
+	// Number of child elements in the Directory referred to by `digest`.
+	// Calculated by summing up the numbers of `directories`, `files` and
+	// `symlinks`, and for each directory, its size field. Used for inode
+	// number calculation.
+	// This field is precisely as verifiable as any other Merkle tree edge.
+	// Resolve `digest`, and you can compute it incrementally. Resolve the
+	// entire tree, and you can fully compute it from scratch.
+	// A credulous implementation won't reject an excessive size, but this is
+	// harmless: you'll have some ordinals without nodes. Undersizing is
+	// obvious and easy to reject: you won't have an ordinal for some nodes.
+	Size uint32 `protobuf:"varint,3,opt,name=size,proto3" json:"size,omitempty"`
+}
+
+func (x *DirectoryNode) Reset() {
+	*x = DirectoryNode{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_castore_proto_msgTypes[1]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *DirectoryNode) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*DirectoryNode) ProtoMessage() {}
+
+func (x *DirectoryNode) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_castore_proto_msgTypes[1]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use DirectoryNode.ProtoReflect.Descriptor instead.
+func (*DirectoryNode) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_castore_proto_rawDescGZIP(), []int{1}
+}
+
+func (x *DirectoryNode) GetName() []byte {
+	if x != nil {
+		return x.Name
+	}
+	return nil
+}
+
+func (x *DirectoryNode) GetDigest() []byte {
+	if x != nil {
+		return x.Digest
+	}
+	return nil
+}
+
+func (x *DirectoryNode) GetSize() uint32 {
+	if x != nil {
+		return x.Size
+	}
+	return 0
+}
+
+// A FileNode represents a regular or executable file in a Directory.
+type FileNode struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// The (base)name of the file
+	Name []byte `protobuf:"bytes,1,opt,name=name,proto3" json:"name,omitempty"`
+	// The blake3 digest of the file contents
+	Digest []byte `protobuf:"bytes,2,opt,name=digest,proto3" json:"digest,omitempty"`
+	// The file content size
+	Size uint32 `protobuf:"varint,3,opt,name=size,proto3" json:"size,omitempty"`
+	// Whether the file is executable
+	Executable bool `protobuf:"varint,4,opt,name=executable,proto3" json:"executable,omitempty"`
+}
+
+func (x *FileNode) Reset() {
+	*x = FileNode{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_castore_proto_msgTypes[2]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *FileNode) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*FileNode) ProtoMessage() {}
+
+func (x *FileNode) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_castore_proto_msgTypes[2]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use FileNode.ProtoReflect.Descriptor instead.
+func (*FileNode) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_castore_proto_rawDescGZIP(), []int{2}
+}
+
+func (x *FileNode) GetName() []byte {
+	if x != nil {
+		return x.Name
+	}
+	return nil
+}
+
+func (x *FileNode) GetDigest() []byte {
+	if x != nil {
+		return x.Digest
+	}
+	return nil
+}
+
+func (x *FileNode) GetSize() uint32 {
+	if x != nil {
+		return x.Size
+	}
+	return 0
+}
+
+func (x *FileNode) GetExecutable() bool {
+	if x != nil {
+		return x.Executable
+	}
+	return false
+}
+
+// A SymlinkNode represents a symbolic link in a Directory.
+type SymlinkNode struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// The (base)name of the symlink
+	Name []byte `protobuf:"bytes,1,opt,name=name,proto3" json:"name,omitempty"`
+	// The target of the symlink.
+	Target []byte `protobuf:"bytes,2,opt,name=target,proto3" json:"target,omitempty"`
+}
+
+func (x *SymlinkNode) Reset() {
+	*x = SymlinkNode{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_castore_proto_msgTypes[3]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *SymlinkNode) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*SymlinkNode) ProtoMessage() {}
+
+func (x *SymlinkNode) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_castore_proto_msgTypes[3]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use SymlinkNode.ProtoReflect.Descriptor instead.
+func (*SymlinkNode) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_castore_proto_rawDescGZIP(), []int{3}
+}
+
+func (x *SymlinkNode) GetName() []byte {
+	if x != nil {
+		return x.Name
+	}
+	return nil
+}
+
+func (x *SymlinkNode) GetTarget() []byte {
+	if x != nil {
+		return x.Target
+	}
+	return nil
+}
+
+// A Node is either a DirectoryNode, FileNode or SymlinkNode.
+type Node struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// Types that are assignable to Node:
+	//
+	//	*Node_Directory
+	//	*Node_File
+	//	*Node_Symlink
+	Node isNode_Node `protobuf_oneof:"node"`
+}
+
+func (x *Node) Reset() {
+	*x = Node{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_castore_proto_msgTypes[4]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *Node) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*Node) ProtoMessage() {}
+
+func (x *Node) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_castore_proto_msgTypes[4]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use Node.ProtoReflect.Descriptor instead.
+func (*Node) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_castore_proto_rawDescGZIP(), []int{4}
+}
+
+func (m *Node) GetNode() isNode_Node {
+	if m != nil {
+		return m.Node
+	}
+	return nil
+}
+
+func (x *Node) GetDirectory() *DirectoryNode {
+	if x, ok := x.GetNode().(*Node_Directory); ok {
+		return x.Directory
+	}
+	return nil
+}
+
+func (x *Node) GetFile() *FileNode {
+	if x, ok := x.GetNode().(*Node_File); ok {
+		return x.File
+	}
+	return nil
+}
+
+func (x *Node) GetSymlink() *SymlinkNode {
+	if x, ok := x.GetNode().(*Node_Symlink); ok {
+		return x.Symlink
+	}
+	return nil
+}
+
+type isNode_Node interface {
+	isNode_Node()
+}
+
+type Node_Directory struct {
+	Directory *DirectoryNode `protobuf:"bytes,1,opt,name=directory,proto3,oneof"`
+}
+
+type Node_File struct {
+	File *FileNode `protobuf:"bytes,2,opt,name=file,proto3,oneof"`
+}
+
+type Node_Symlink struct {
+	Symlink *SymlinkNode `protobuf:"bytes,3,opt,name=symlink,proto3,oneof"`
+}
+
+func (*Node_Directory) isNode_Node() {}
+
+func (*Node_File) isNode_Node() {}
+
+func (*Node_Symlink) isNode_Node() {}
+
+var File_tvix_castore_protos_castore_proto protoreflect.FileDescriptor
+
+var file_tvix_castore_protos_castore_proto_rawDesc = []byte{
+	0x0a, 0x21, 0x74, 0x76, 0x69, 0x78, 0x2f, 0x63, 0x61, 0x73, 0x74, 0x6f, 0x72, 0x65, 0x2f, 0x70,
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+	0x31, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
+}
+
+var (
+	file_tvix_castore_protos_castore_proto_rawDescOnce sync.Once
+	file_tvix_castore_protos_castore_proto_rawDescData = file_tvix_castore_protos_castore_proto_rawDesc
+)
+
+func file_tvix_castore_protos_castore_proto_rawDescGZIP() []byte {
+	file_tvix_castore_protos_castore_proto_rawDescOnce.Do(func() {
+		file_tvix_castore_protos_castore_proto_rawDescData = protoimpl.X.CompressGZIP(file_tvix_castore_protos_castore_proto_rawDescData)
+	})
+	return file_tvix_castore_protos_castore_proto_rawDescData
+}
+
+var file_tvix_castore_protos_castore_proto_msgTypes = make([]protoimpl.MessageInfo, 5)
+var file_tvix_castore_protos_castore_proto_goTypes = []interface{}{
+	(*Directory)(nil),     // 0: tvix.castore.v1.Directory
+	(*DirectoryNode)(nil), // 1: tvix.castore.v1.DirectoryNode
+	(*FileNode)(nil),      // 2: tvix.castore.v1.FileNode
+	(*SymlinkNode)(nil),   // 3: tvix.castore.v1.SymlinkNode
+	(*Node)(nil),          // 4: tvix.castore.v1.Node
+}
+var file_tvix_castore_protos_castore_proto_depIdxs = []int32{
+	1, // 0: tvix.castore.v1.Directory.directories:type_name -> tvix.castore.v1.DirectoryNode
+	2, // 1: tvix.castore.v1.Directory.files:type_name -> tvix.castore.v1.FileNode
+	3, // 2: tvix.castore.v1.Directory.symlinks:type_name -> tvix.castore.v1.SymlinkNode
+	1, // 3: tvix.castore.v1.Node.directory:type_name -> tvix.castore.v1.DirectoryNode
+	2, // 4: tvix.castore.v1.Node.file:type_name -> tvix.castore.v1.FileNode
+	3, // 5: tvix.castore.v1.Node.symlink:type_name -> tvix.castore.v1.SymlinkNode
+	6, // [6:6] is the sub-list for method output_type
+	6, // [6:6] is the sub-list for method input_type
+	6, // [6:6] is the sub-list for extension type_name
+	6, // [6:6] is the sub-list for extension extendee
+	0, // [0:6] is the sub-list for field type_name
+}
+
+func init() { file_tvix_castore_protos_castore_proto_init() }
+func file_tvix_castore_protos_castore_proto_init() {
+	if File_tvix_castore_protos_castore_proto != nil {
+		return
+	}
+	if !protoimpl.UnsafeEnabled {
+		file_tvix_castore_protos_castore_proto_msgTypes[0].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*Directory); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_tvix_castore_protos_castore_proto_msgTypes[1].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*DirectoryNode); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_tvix_castore_protos_castore_proto_msgTypes[2].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*FileNode); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_tvix_castore_protos_castore_proto_msgTypes[3].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*SymlinkNode); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_tvix_castore_protos_castore_proto_msgTypes[4].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*Node); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+	}
+	file_tvix_castore_protos_castore_proto_msgTypes[4].OneofWrappers = []interface{}{
+		(*Node_Directory)(nil),
+		(*Node_File)(nil),
+		(*Node_Symlink)(nil),
+	}
+	type x struct{}
+	out := protoimpl.TypeBuilder{
+		File: protoimpl.DescBuilder{
+			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
+			RawDescriptor: file_tvix_castore_protos_castore_proto_rawDesc,
+			NumEnums:      0,
+			NumMessages:   5,
+			NumExtensions: 0,
+			NumServices:   0,
+		},
+		GoTypes:           file_tvix_castore_protos_castore_proto_goTypes,
+		DependencyIndexes: file_tvix_castore_protos_castore_proto_depIdxs,
+		MessageInfos:      file_tvix_castore_protos_castore_proto_msgTypes,
+	}.Build()
+	File_tvix_castore_protos_castore_proto = out.File
+	file_tvix_castore_protos_castore_proto_rawDesc = nil
+	file_tvix_castore_protos_castore_proto_goTypes = nil
+	file_tvix_castore_protos_castore_proto_depIdxs = nil
+}
diff --git a/tvix/castore/protos/castore.proto b/tvix/castore/protos/castore.proto
new file mode 100644
index 0000000000..d99df43857
--- /dev/null
+++ b/tvix/castore/protos/castore.proto
@@ -0,0 +1,72 @@
+// SPDX-FileCopyrightText: edef <edef@unfathomable.blue>
+// SPDX-License-Identifier: OSL-3.0 OR MIT OR Apache-2.0
+
+syntax = "proto3";
+
+package tvix.castore.v1;
+
+option go_package = "code.tvl.fyi/tvix/castore/protos;castorev1";
+
+// A Directory can contain Directory, File or Symlink nodes.
+// Each of these nodes have a name attribute, which is the basename in that directory
+// and node type specific attributes.
+// The name attribute:
+//  - MUST not contain slashes or null bytes
+//  - MUST not be '.' or '..'
+//  - MUST be unique across all three lists
+// Elements in each list need to be lexicographically ordered by the name
+// attribute.
+message Directory {
+    repeated DirectoryNode directories = 1;
+    repeated FileNode files = 2;
+    repeated SymlinkNode symlinks = 3;
+}
+
+// A DirectoryNode represents a directory in a Directory.
+message DirectoryNode {
+    // The (base)name of the directory
+    bytes name = 1;
+    // The blake3 hash of a Directory message, serialized in protobuf canonical form.
+    bytes digest = 2;
+    // Number of child elements in the Directory referred to by `digest`.
+    // Calculated by summing up the numbers of `directories`, `files` and
+    // `symlinks`, and for each directory, its size field. Used for inode
+    // number calculation.
+    // This field is precisely as verifiable as any other Merkle tree edge.
+    // Resolve `digest`, and you can compute it incrementally. Resolve the
+    // entire tree, and you can fully compute it from scratch.
+    // A credulous implementation won't reject an excessive size, but this is
+    // harmless: you'll have some ordinals without nodes. Undersizing is
+    // obvious and easy to reject: you won't have an ordinal for some nodes.
+    uint32 size = 3;
+}
+
+// A FileNode represents a regular or executable file in a Directory.
+message FileNode {
+    // The (base)name of the file
+    bytes name = 1;
+    // The blake3 digest of the file contents
+    bytes digest = 2;
+    // The file content size
+    uint32 size = 3;
+    // Whether the file is executable
+    bool executable = 4;
+}
+
+// A SymlinkNode represents a symbolic link in a Directory.
+message SymlinkNode {
+    // The (base)name of the symlink
+    bytes name = 1;
+    // The target of the symlink.
+    bytes target = 2;
+}
+
+// A Node is either a DirectoryNode, FileNode or SymlinkNode.
+message Node {
+    oneof node {
+        DirectoryNode directory = 1;
+        FileNode file = 2;
+        SymlinkNode symlink = 3;
+    }
+}
+
diff --git a/tvix/castore/protos/castore_test.go b/tvix/castore/protos/castore_test.go
new file mode 100644
index 0000000000..958d399d76
--- /dev/null
+++ b/tvix/castore/protos/castore_test.go
@@ -0,0 +1,271 @@
+package castorev1_test
+
+import (
+	"testing"
+
+	castorev1pb "code.tvl.fyi/tvix/castore/protos"
+	"github.com/stretchr/testify/assert"
+)
+
+var (
+	dummyDigest = []byte{
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00,
+	}
+)
+
+func TestDirectorySize(t *testing.T) {
+	t.Run("empty", func(t *testing.T) {
+		d := castorev1pb.Directory{
+			Directories: []*castorev1pb.DirectoryNode{},
+			Files:       []*castorev1pb.FileNode{},
+			Symlinks:    []*castorev1pb.SymlinkNode{},
+		}
+
+		assert.Equal(t, uint32(0), d.Size())
+	})
+
+	t.Run("containing single empty directory", func(t *testing.T) {
+		d := castorev1pb.Directory{
+			Directories: []*castorev1pb.DirectoryNode{{
+				Name:   []byte([]byte("foo")),
+				Digest: dummyDigest,
+				Size:   0,
+			}},
+			Files:    []*castorev1pb.FileNode{},
+			Symlinks: []*castorev1pb.SymlinkNode{},
+		}
+
+		assert.Equal(t, uint32(1), d.Size())
+	})
+
+	t.Run("containing single non-empty directory", func(t *testing.T) {
+		d := castorev1pb.Directory{
+			Directories: []*castorev1pb.DirectoryNode{{
+				Name:   []byte("foo"),
+				Digest: dummyDigest,
+				Size:   4,
+			}},
+			Files:    []*castorev1pb.FileNode{},
+			Symlinks: []*castorev1pb.SymlinkNode{},
+		}
+
+		assert.Equal(t, uint32(5), d.Size())
+	})
+
+	t.Run("containing single file", func(t *testing.T) {
+		d := castorev1pb.Directory{
+			Directories: []*castorev1pb.DirectoryNode{},
+			Files: []*castorev1pb.FileNode{{
+				Name:       []byte("foo"),
+				Digest:     dummyDigest,
+				Size:       42,
+				Executable: false,
+			}},
+			Symlinks: []*castorev1pb.SymlinkNode{},
+		}
+
+		assert.Equal(t, uint32(1), d.Size())
+	})
+
+	t.Run("containing single symlink", func(t *testing.T) {
+		d := castorev1pb.Directory{
+			Directories: []*castorev1pb.DirectoryNode{},
+			Files:       []*castorev1pb.FileNode{},
+			Symlinks: []*castorev1pb.SymlinkNode{{
+				Name:   []byte("foo"),
+				Target: []byte("bar"),
+			}},
+		}
+
+		assert.Equal(t, uint32(1), d.Size())
+	})
+
+}
+func TestDirectoryDigest(t *testing.T) {
+	d := castorev1pb.Directory{
+		Directories: []*castorev1pb.DirectoryNode{},
+		Files:       []*castorev1pb.FileNode{},
+		Symlinks:    []*castorev1pb.SymlinkNode{},
+	}
+
+	dgst, err := d.Digest()
+	assert.NoError(t, err, "calling Digest() on a directory shouldn't error")
+	assert.Equal(t, []byte{
+		0xaf, 0x13, 0x49, 0xb9, 0xf5, 0xf9, 0xa1, 0xa6, 0xa0, 0x40, 0x4d, 0xea, 0x36, 0xdc,
+		0xc9, 0x49, 0x9b, 0xcb, 0x25, 0xc9, 0xad, 0xc1, 0x12, 0xb7, 0xcc, 0x9a, 0x93, 0xca,
+		0xe4, 0x1f, 0x32, 0x62,
+	}, dgst)
+}
+
+func TestDirectoryValidate(t *testing.T) {
+	t.Run("empty", func(t *testing.T) {
+		d := castorev1pb.Directory{
+			Directories: []*castorev1pb.DirectoryNode{},
+			Files:       []*castorev1pb.FileNode{},
+			Symlinks:    []*castorev1pb.SymlinkNode{},
+		}
+
+		assert.NoError(t, d.Validate())
+	})
+
+	t.Run("invalid names", func(t *testing.T) {
+		{
+			d := castorev1pb.Directory{
+				Directories: []*castorev1pb.DirectoryNode{{
+					Name:   []byte{},
+					Digest: dummyDigest,
+					Size:   42,
+				}},
+				Files:    []*castorev1pb.FileNode{},
+				Symlinks: []*castorev1pb.SymlinkNode{},
+			}
+
+			assert.ErrorContains(t, d.Validate(), "invalid name")
+		}
+		{
+			d := castorev1pb.Directory{
+				Directories: []*castorev1pb.DirectoryNode{{
+					Name:   []byte("."),
+					Digest: dummyDigest,
+					Size:   42,
+				}},
+				Files:    []*castorev1pb.FileNode{},
+				Symlinks: []*castorev1pb.SymlinkNode{},
+			}
+
+			assert.ErrorContains(t, d.Validate(), "invalid name")
+		}
+		{
+			d := castorev1pb.Directory{
+				Directories: []*castorev1pb.DirectoryNode{},
+				Files: []*castorev1pb.FileNode{{
+					Name:       []byte(".."),
+					Digest:     dummyDigest,
+					Size:       42,
+					Executable: false,
+				}},
+				Symlinks: []*castorev1pb.SymlinkNode{},
+			}
+
+			assert.ErrorContains(t, d.Validate(), "invalid name")
+		}
+		{
+			d := castorev1pb.Directory{
+				Directories: []*castorev1pb.DirectoryNode{},
+				Files:       []*castorev1pb.FileNode{},
+				Symlinks: []*castorev1pb.SymlinkNode{{
+					Name:   []byte("\x00"),
+					Target: []byte("foo"),
+				}},
+			}
+
+			assert.ErrorContains(t, d.Validate(), "invalid name")
+		}
+		{
+			d := castorev1pb.Directory{
+				Directories: []*castorev1pb.DirectoryNode{},
+				Files:       []*castorev1pb.FileNode{},
+				Symlinks: []*castorev1pb.SymlinkNode{{
+					Name:   []byte("foo/bar"),
+					Target: []byte("foo"),
+				}},
+			}
+
+			assert.ErrorContains(t, d.Validate(), "invalid name")
+		}
+	})
+
+	t.Run("invalid digest", func(t *testing.T) {
+		d := castorev1pb.Directory{
+			Directories: []*castorev1pb.DirectoryNode{{
+				Name:   []byte("foo"),
+				Digest: nil,
+				Size:   42,
+			}},
+			Files:    []*castorev1pb.FileNode{},
+			Symlinks: []*castorev1pb.SymlinkNode{},
+		}
+
+		assert.ErrorContains(t, d.Validate(), "invalid digest length")
+	})
+
+	t.Run("sorting", func(t *testing.T) {
+		// "b" comes before "a", bad.
+		{
+			d := castorev1pb.Directory{
+				Directories: []*castorev1pb.DirectoryNode{{
+					Name:   []byte("b"),
+					Digest: dummyDigest,
+					Size:   42,
+				}, {
+					Name:   []byte("a"),
+					Digest: dummyDigest,
+					Size:   42,
+				}},
+				Files:    []*castorev1pb.FileNode{},
+				Symlinks: []*castorev1pb.SymlinkNode{},
+			}
+			assert.ErrorContains(t, d.Validate(), "is not in sorted order")
+		}
+
+		// "a" exists twice, bad.
+		{
+			d := castorev1pb.Directory{
+				Directories: []*castorev1pb.DirectoryNode{{
+					Name:   []byte("a"),
+					Digest: dummyDigest,
+					Size:   42,
+				}},
+				Files: []*castorev1pb.FileNode{{
+					Name:       []byte("a"),
+					Digest:     dummyDigest,
+					Size:       42,
+					Executable: false,
+				}},
+				Symlinks: []*castorev1pb.SymlinkNode{},
+			}
+			assert.ErrorContains(t, d.Validate(), "duplicate name")
+		}
+
+		// "a" comes before "b", all good.
+		{
+			d := castorev1pb.Directory{
+				Directories: []*castorev1pb.DirectoryNode{{
+					Name:   []byte("a"),
+					Digest: dummyDigest,
+					Size:   42,
+				}, {
+					Name:   []byte("b"),
+					Digest: dummyDigest,
+					Size:   42,
+				}},
+				Files:    []*castorev1pb.FileNode{},
+				Symlinks: []*castorev1pb.SymlinkNode{},
+			}
+			assert.NoError(t, d.Validate(), "shouldn't error")
+		}
+
+		// [b, c] and [a] are both properly sorted.
+		{
+			d := castorev1pb.Directory{
+				Directories: []*castorev1pb.DirectoryNode{{
+					Name:   []byte("b"),
+					Digest: dummyDigest,
+					Size:   42,
+				}, {
+					Name:   []byte("c"),
+					Digest: dummyDigest,
+					Size:   42,
+				}},
+				Files: []*castorev1pb.FileNode{},
+				Symlinks: []*castorev1pb.SymlinkNode{{
+					Name:   []byte("a"),
+					Target: []byte("foo"),
+				}},
+			}
+			assert.NoError(t, d.Validate(), "shouldn't error")
+		}
+	})
+}
diff --git a/tvix/castore/protos/go.mod b/tvix/castore/protos/go.mod
new file mode 100644
index 0000000000..35219aba5c
--- /dev/null
+++ b/tvix/castore/protos/go.mod
@@ -0,0 +1,19 @@
+module code.tvl.fyi/tvix/castore/protos
+
+go 1.19
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/golang/protobuf v1.5.2 // indirect
+	github.com/klauspost/cpuid/v2 v2.0.9 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/stretchr/testify v1.8.1 // indirect
+	golang.org/x/net v0.0.0-20220722155237-a158d28d115b // indirect
+	golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f // indirect
+	golang.org/x/text v0.4.0 // indirect
+	google.golang.org/genproto v0.0.0-20200526211855-cb27e3aa2013 // indirect
+	google.golang.org/grpc v1.51.0 // indirect
+	google.golang.org/protobuf v1.28.1 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+	lukechampine.com/blake3 v1.1.7 // indirect
+)
diff --git a/tvix/castore/protos/go.sum b/tvix/castore/protos/go.sum
new file mode 100644
index 0000000000..7a603cdb12
--- /dev/null
+++ b/tvix/castore/protos/go.sum
@@ -0,0 +1,96 @@
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diff --git a/tvix/castore/protos/rpc_blobstore.pb.go b/tvix/castore/protos/rpc_blobstore.pb.go
new file mode 100644
index 0000000000..1afc826744
--- /dev/null
+++ b/tvix/castore/protos/rpc_blobstore.pb.go
@@ -0,0 +1,414 @@
+// SPDX-License-Identifier: MIT
+// Copyright © 2022 The Tvix Authors
+
+// Code generated by protoc-gen-go. DO NOT EDIT.
+// versions:
+// 	protoc-gen-go v1.31.0
+// 	protoc        (unknown)
+// source: tvix/castore/protos/rpc_blobstore.proto
+
+package castorev1
+
+import (
+	protoreflect "google.golang.org/protobuf/reflect/protoreflect"
+	protoimpl "google.golang.org/protobuf/runtime/protoimpl"
+	reflect "reflect"
+	sync "sync"
+)
+
+const (
+	// Verify that this generated code is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(20 - protoimpl.MinVersion)
+	// Verify that runtime/protoimpl is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(protoimpl.MaxVersion - 20)
+)
+
+type StatBlobRequest struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// The blake3 digest of the blob requested
+	Digest []byte `protobuf:"bytes,1,opt,name=digest,proto3" json:"digest,omitempty"`
+}
+
+func (x *StatBlobRequest) Reset() {
+	*x = StatBlobRequest{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[0]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *StatBlobRequest) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*StatBlobRequest) ProtoMessage() {}
+
+func (x *StatBlobRequest) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[0]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use StatBlobRequest.ProtoReflect.Descriptor instead.
+func (*StatBlobRequest) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_rpc_blobstore_proto_rawDescGZIP(), []int{0}
+}
+
+func (x *StatBlobRequest) GetDigest() []byte {
+	if x != nil {
+		return x.Digest
+	}
+	return nil
+}
+
+type BlobMeta struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+}
+
+func (x *BlobMeta) Reset() {
+	*x = BlobMeta{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[1]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *BlobMeta) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*BlobMeta) ProtoMessage() {}
+
+func (x *BlobMeta) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[1]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use BlobMeta.ProtoReflect.Descriptor instead.
+func (*BlobMeta) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_rpc_blobstore_proto_rawDescGZIP(), []int{1}
+}
+
+type ReadBlobRequest struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// The blake3 digest of the blob or chunk requested
+	Digest []byte `protobuf:"bytes,1,opt,name=digest,proto3" json:"digest,omitempty"`
+}
+
+func (x *ReadBlobRequest) Reset() {
+	*x = ReadBlobRequest{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[2]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *ReadBlobRequest) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*ReadBlobRequest) ProtoMessage() {}
+
+func (x *ReadBlobRequest) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[2]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use ReadBlobRequest.ProtoReflect.Descriptor instead.
+func (*ReadBlobRequest) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_rpc_blobstore_proto_rawDescGZIP(), []int{2}
+}
+
+func (x *ReadBlobRequest) GetDigest() []byte {
+	if x != nil {
+		return x.Digest
+	}
+	return nil
+}
+
+// This represents some bytes of a blob.
+// Blobs are sent in smaller chunks to keep message sizes manageable.
+type BlobChunk struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	Data []byte `protobuf:"bytes,1,opt,name=data,proto3" json:"data,omitempty"`
+}
+
+func (x *BlobChunk) Reset() {
+	*x = BlobChunk{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[3]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *BlobChunk) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*BlobChunk) ProtoMessage() {}
+
+func (x *BlobChunk) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[3]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use BlobChunk.ProtoReflect.Descriptor instead.
+func (*BlobChunk) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_rpc_blobstore_proto_rawDescGZIP(), []int{3}
+}
+
+func (x *BlobChunk) GetData() []byte {
+	if x != nil {
+		return x.Data
+	}
+	return nil
+}
+
+type PutBlobResponse struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// The blake3 digest of the data that was sent.
+	Digest []byte `protobuf:"bytes,1,opt,name=digest,proto3" json:"digest,omitempty"`
+}
+
+func (x *PutBlobResponse) Reset() {
+	*x = PutBlobResponse{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[4]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *PutBlobResponse) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*PutBlobResponse) ProtoMessage() {}
+
+func (x *PutBlobResponse) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[4]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use PutBlobResponse.ProtoReflect.Descriptor instead.
+func (*PutBlobResponse) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_rpc_blobstore_proto_rawDescGZIP(), []int{4}
+}
+
+func (x *PutBlobResponse) GetDigest() []byte {
+	if x != nil {
+		return x.Digest
+	}
+	return nil
+}
+
+var File_tvix_castore_protos_rpc_blobstore_proto protoreflect.FileDescriptor
+
+var file_tvix_castore_protos_rpc_blobstore_proto_rawDesc = []byte{
+	0x0a, 0x27, 0x74, 0x76, 0x69, 0x78, 0x2f, 0x63, 0x61, 0x73, 0x74, 0x6f, 0x72, 0x65, 0x2f, 0x70,
+	0x72, 0x6f, 0x74, 0x6f, 0x73, 0x2f, 0x72, 0x70, 0x63, 0x5f, 0x62, 0x6c, 0x6f, 0x62, 0x73, 0x74,
+	0x6f, 0x72, 0x65, 0x2e, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x12, 0x0f, 0x74, 0x76, 0x69, 0x78, 0x2e,
+	0x63, 0x61, 0x73, 0x74, 0x6f, 0x72, 0x65, 0x2e, 0x76, 0x31, 0x22, 0x29, 0x0a, 0x0f, 0x53, 0x74,
+	0x61, 0x74, 0x42, 0x6c, 0x6f, 0x62, 0x52, 0x65, 0x71, 0x75, 0x65, 0x73, 0x74, 0x12, 0x16, 0x0a,
+	0x06, 0x64, 0x69, 0x67, 0x65, 0x73, 0x74, 0x18, 0x01, 0x20, 0x01, 0x28, 0x0c, 0x52, 0x06, 0x64,
+	0x69, 0x67, 0x65, 0x73, 0x74, 0x22, 0x0a, 0x0a, 0x08, 0x42, 0x6c, 0x6f, 0x62, 0x4d, 0x65, 0x74,
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+	0x75, 0x65, 0x73, 0x74, 0x12, 0x16, 0x0a, 0x06, 0x64, 0x69, 0x67, 0x65, 0x73, 0x74, 0x18, 0x01,
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+	0x42, 0x6c, 0x6f, 0x62, 0x43, 0x68, 0x75, 0x6e, 0x6b, 0x12, 0x12, 0x0a, 0x04, 0x64, 0x61, 0x74,
+	0x61, 0x18, 0x01, 0x20, 0x01, 0x28, 0x0c, 0x52, 0x04, 0x64, 0x61, 0x74, 0x61, 0x22, 0x29, 0x0a,
+	0x0f, 0x50, 0x75, 0x74, 0x42, 0x6c, 0x6f, 0x62, 0x52, 0x65, 0x73, 0x70, 0x6f, 0x6e, 0x73, 0x65,
+	0x12, 0x16, 0x0a, 0x06, 0x64, 0x69, 0x67, 0x65, 0x73, 0x74, 0x18, 0x01, 0x20, 0x01, 0x28, 0x0c,
+	0x52, 0x06, 0x64, 0x69, 0x67, 0x65, 0x73, 0x74, 0x32, 0xe1, 0x01, 0x0a, 0x0b, 0x42, 0x6c, 0x6f,
+	0x62, 0x53, 0x65, 0x72, 0x76, 0x69, 0x63, 0x65, 0x12, 0x43, 0x0a, 0x04, 0x53, 0x74, 0x61, 0x74,
+	0x12, 0x20, 0x2e, 0x74, 0x76, 0x69, 0x78, 0x2e, 0x63, 0x61, 0x73, 0x74, 0x6f, 0x72, 0x65, 0x2e,
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+	0x65, 0x2e, 0x76, 0x31, 0x2e, 0x42, 0x6c, 0x6f, 0x62, 0x4d, 0x65, 0x74, 0x61, 0x12, 0x46, 0x0a,
+	0x04, 0x52, 0x65, 0x61, 0x64, 0x12, 0x20, 0x2e, 0x74, 0x76, 0x69, 0x78, 0x2e, 0x63, 0x61, 0x73,
+	0x74, 0x6f, 0x72, 0x65, 0x2e, 0x76, 0x31, 0x2e, 0x52, 0x65, 0x61, 0x64, 0x42, 0x6c, 0x6f, 0x62,
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+	0x61, 0x73, 0x74, 0x6f, 0x72, 0x65, 0x2e, 0x76, 0x31, 0x2e, 0x42, 0x6c, 0x6f, 0x62, 0x43, 0x68,
+	0x75, 0x6e, 0x6b, 0x30, 0x01, 0x12, 0x45, 0x0a, 0x03, 0x50, 0x75, 0x74, 0x12, 0x1a, 0x2e, 0x74,
+	0x76, 0x69, 0x78, 0x2e, 0x63, 0x61, 0x73, 0x74, 0x6f, 0x72, 0x65, 0x2e, 0x76, 0x31, 0x2e, 0x42,
+	0x6c, 0x6f, 0x62, 0x43, 0x68, 0x75, 0x6e, 0x6b, 0x1a, 0x20, 0x2e, 0x74, 0x76, 0x69, 0x78, 0x2e,
+	0x63, 0x61, 0x73, 0x74, 0x6f, 0x72, 0x65, 0x2e, 0x76, 0x31, 0x2e, 0x50, 0x75, 0x74, 0x42, 0x6c,
+	0x6f, 0x62, 0x52, 0x65, 0x73, 0x70, 0x6f, 0x6e, 0x73, 0x65, 0x28, 0x01, 0x42, 0x2c, 0x5a, 0x2a,
+	0x63, 0x6f, 0x64, 0x65, 0x2e, 0x74, 0x76, 0x6c, 0x2e, 0x66, 0x79, 0x69, 0x2f, 0x74, 0x76, 0x69,
+	0x78, 0x2f, 0x63, 0x61, 0x73, 0x74, 0x6f, 0x72, 0x65, 0x2f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x73,
+	0x3b, 0x63, 0x61, 0x73, 0x74, 0x6f, 0x72, 0x65, 0x76, 0x31, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74,
+	0x6f, 0x33,
+}
+
+var (
+	file_tvix_castore_protos_rpc_blobstore_proto_rawDescOnce sync.Once
+	file_tvix_castore_protos_rpc_blobstore_proto_rawDescData = file_tvix_castore_protos_rpc_blobstore_proto_rawDesc
+)
+
+func file_tvix_castore_protos_rpc_blobstore_proto_rawDescGZIP() []byte {
+	file_tvix_castore_protos_rpc_blobstore_proto_rawDescOnce.Do(func() {
+		file_tvix_castore_protos_rpc_blobstore_proto_rawDescData = protoimpl.X.CompressGZIP(file_tvix_castore_protos_rpc_blobstore_proto_rawDescData)
+	})
+	return file_tvix_castore_protos_rpc_blobstore_proto_rawDescData
+}
+
+var file_tvix_castore_protos_rpc_blobstore_proto_msgTypes = make([]protoimpl.MessageInfo, 5)
+var file_tvix_castore_protos_rpc_blobstore_proto_goTypes = []interface{}{
+	(*StatBlobRequest)(nil), // 0: tvix.castore.v1.StatBlobRequest
+	(*BlobMeta)(nil),        // 1: tvix.castore.v1.BlobMeta
+	(*ReadBlobRequest)(nil), // 2: tvix.castore.v1.ReadBlobRequest
+	(*BlobChunk)(nil),       // 3: tvix.castore.v1.BlobChunk
+	(*PutBlobResponse)(nil), // 4: tvix.castore.v1.PutBlobResponse
+}
+var file_tvix_castore_protos_rpc_blobstore_proto_depIdxs = []int32{
+	0, // 0: tvix.castore.v1.BlobService.Stat:input_type -> tvix.castore.v1.StatBlobRequest
+	2, // 1: tvix.castore.v1.BlobService.Read:input_type -> tvix.castore.v1.ReadBlobRequest
+	3, // 2: tvix.castore.v1.BlobService.Put:input_type -> tvix.castore.v1.BlobChunk
+	1, // 3: tvix.castore.v1.BlobService.Stat:output_type -> tvix.castore.v1.BlobMeta
+	3, // 4: tvix.castore.v1.BlobService.Read:output_type -> tvix.castore.v1.BlobChunk
+	4, // 5: tvix.castore.v1.BlobService.Put:output_type -> tvix.castore.v1.PutBlobResponse
+	3, // [3:6] is the sub-list for method output_type
+	0, // [0:3] is the sub-list for method input_type
+	0, // [0:0] is the sub-list for extension type_name
+	0, // [0:0] is the sub-list for extension extendee
+	0, // [0:0] is the sub-list for field type_name
+}
+
+func init() { file_tvix_castore_protos_rpc_blobstore_proto_init() }
+func file_tvix_castore_protos_rpc_blobstore_proto_init() {
+	if File_tvix_castore_protos_rpc_blobstore_proto != nil {
+		return
+	}
+	if !protoimpl.UnsafeEnabled {
+		file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[0].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*StatBlobRequest); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[1].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*BlobMeta); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[2].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*ReadBlobRequest); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[3].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*BlobChunk); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_tvix_castore_protos_rpc_blobstore_proto_msgTypes[4].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*PutBlobResponse); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+	}
+	type x struct{}
+	out := protoimpl.TypeBuilder{
+		File: protoimpl.DescBuilder{
+			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
+			RawDescriptor: file_tvix_castore_protos_rpc_blobstore_proto_rawDesc,
+			NumEnums:      0,
+			NumMessages:   5,
+			NumExtensions: 0,
+			NumServices:   1,
+		},
+		GoTypes:           file_tvix_castore_protos_rpc_blobstore_proto_goTypes,
+		DependencyIndexes: file_tvix_castore_protos_rpc_blobstore_proto_depIdxs,
+		MessageInfos:      file_tvix_castore_protos_rpc_blobstore_proto_msgTypes,
+	}.Build()
+	File_tvix_castore_protos_rpc_blobstore_proto = out.File
+	file_tvix_castore_protos_rpc_blobstore_proto_rawDesc = nil
+	file_tvix_castore_protos_rpc_blobstore_proto_goTypes = nil
+	file_tvix_castore_protos_rpc_blobstore_proto_depIdxs = nil
+}
diff --git a/tvix/castore/protos/rpc_blobstore.proto b/tvix/castore/protos/rpc_blobstore.proto
new file mode 100644
index 0000000000..6ee9a80f0a
--- /dev/null
+++ b/tvix/castore/protos/rpc_blobstore.proto
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: MIT
+// Copyright © 2022 The Tvix Authors
+syntax = "proto3";
+
+package tvix.castore.v1;
+
+option go_package = "code.tvl.fyi/tvix/castore/protos;castorev1";
+
+service BlobService {
+    // In the future, Stat will expose more metadata about a given blob,
+    // such as more granular chunking, baos.
+    // For now, it's only used to check for the existence of a blob, as asking
+    // this for a non-existing Blob will return a Status::not_found gRPC error.
+    rpc Stat(StatBlobRequest) returns (BlobMeta);
+
+    // Read returns a stream of BlobChunk, which is just a stream of bytes with
+    // the digest specified in ReadBlobRequest.
+    //
+    // The server may decide on whatever chunking it may seem fit as a size for
+    // the individual BlobChunk sent in the response stream.
+    rpc Read(ReadBlobRequest) returns (stream BlobChunk);
+
+    // Put uploads a Blob, by reading a stream of bytes.
+    //
+    // The way the data is chunked up in individual BlobChunk messages sent in
+    // the stream has no effect on how the server ends up chunking blobs up.
+    rpc Put(stream BlobChunk) returns (PutBlobResponse);
+}
+
+message StatBlobRequest {
+    // The blake3 digest of the blob requested
+    bytes digest = 1;
+}
+
+message BlobMeta {
+}
+
+message ReadBlobRequest {
+    // The blake3 digest of the blob or chunk requested
+    bytes digest = 1;
+}
+
+// This represents some bytes of a blob.
+// Blobs are sent in smaller chunks to keep message sizes manageable.
+message BlobChunk {
+    bytes data = 1;
+}
+
+message PutBlobResponse {
+    // The blake3 digest of the data that was sent.
+    bytes digest = 1;
+}
diff --git a/tvix/castore/protos/rpc_blobstore_grpc.pb.go b/tvix/castore/protos/rpc_blobstore_grpc.pb.go
new file mode 100644
index 0000000000..0876bcc4e9
--- /dev/null
+++ b/tvix/castore/protos/rpc_blobstore_grpc.pb.go
@@ -0,0 +1,274 @@
+// SPDX-License-Identifier: MIT
+// Copyright © 2022 The Tvix Authors
+
+// Code generated by protoc-gen-go-grpc. DO NOT EDIT.
+// versions:
+// - protoc-gen-go-grpc v1.3.0
+// - protoc             (unknown)
+// source: tvix/castore/protos/rpc_blobstore.proto
+
+package castorev1
+
+import (
+	context "context"
+	grpc "google.golang.org/grpc"
+	codes "google.golang.org/grpc/codes"
+	status "google.golang.org/grpc/status"
+)
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the grpc package it is being compiled against.
+// Requires gRPC-Go v1.32.0 or later.
+const _ = grpc.SupportPackageIsVersion7
+
+const (
+	BlobService_Stat_FullMethodName = "/tvix.castore.v1.BlobService/Stat"
+	BlobService_Read_FullMethodName = "/tvix.castore.v1.BlobService/Read"
+	BlobService_Put_FullMethodName  = "/tvix.castore.v1.BlobService/Put"
+)
+
+// BlobServiceClient is the client API for BlobService service.
+//
+// For semantics around ctx use and closing/ending streaming RPCs, please refer to https://pkg.go.dev/google.golang.org/grpc/?tab=doc#ClientConn.NewStream.
+type BlobServiceClient interface {
+	// In the future, Stat will expose more metadata about a given blob,
+	// such as more granular chunking, baos.
+	// For now, it's only used to check for the existence of a blob, as asking
+	// this for a non-existing Blob will return a Status::not_found gRPC error.
+	Stat(ctx context.Context, in *StatBlobRequest, opts ...grpc.CallOption) (*BlobMeta, error)
+	// Read returns a stream of BlobChunk, which is just a stream of bytes with
+	// the digest specified in ReadBlobRequest.
+	//
+	// The server may decide on whatever chunking it may seem fit as a size for
+	// the individual BlobChunk sent in the response stream.
+	Read(ctx context.Context, in *ReadBlobRequest, opts ...grpc.CallOption) (BlobService_ReadClient, error)
+	// Put uploads a Blob, by reading a stream of bytes.
+	//
+	// The way the data is chunked up in individual BlobChunk messages sent in
+	// the stream has no effect on how the server ends up chunking blobs up.
+	Put(ctx context.Context, opts ...grpc.CallOption) (BlobService_PutClient, error)
+}
+
+type blobServiceClient struct {
+	cc grpc.ClientConnInterface
+}
+
+func NewBlobServiceClient(cc grpc.ClientConnInterface) BlobServiceClient {
+	return &blobServiceClient{cc}
+}
+
+func (c *blobServiceClient) Stat(ctx context.Context, in *StatBlobRequest, opts ...grpc.CallOption) (*BlobMeta, error) {
+	out := new(BlobMeta)
+	err := c.cc.Invoke(ctx, BlobService_Stat_FullMethodName, in, out, opts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+func (c *blobServiceClient) Read(ctx context.Context, in *ReadBlobRequest, opts ...grpc.CallOption) (BlobService_ReadClient, error) {
+	stream, err := c.cc.NewStream(ctx, &BlobService_ServiceDesc.Streams[0], BlobService_Read_FullMethodName, opts...)
+	if err != nil {
+		return nil, err
+	}
+	x := &blobServiceReadClient{stream}
+	if err := x.ClientStream.SendMsg(in); err != nil {
+		return nil, err
+	}
+	if err := x.ClientStream.CloseSend(); err != nil {
+		return nil, err
+	}
+	return x, nil
+}
+
+type BlobService_ReadClient interface {
+	Recv() (*BlobChunk, error)
+	grpc.ClientStream
+}
+
+type blobServiceReadClient struct {
+	grpc.ClientStream
+}
+
+func (x *blobServiceReadClient) Recv() (*BlobChunk, error) {
+	m := new(BlobChunk)
+	if err := x.ClientStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+func (c *blobServiceClient) Put(ctx context.Context, opts ...grpc.CallOption) (BlobService_PutClient, error) {
+	stream, err := c.cc.NewStream(ctx, &BlobService_ServiceDesc.Streams[1], BlobService_Put_FullMethodName, opts...)
+	if err != nil {
+		return nil, err
+	}
+	x := &blobServicePutClient{stream}
+	return x, nil
+}
+
+type BlobService_PutClient interface {
+	Send(*BlobChunk) error
+	CloseAndRecv() (*PutBlobResponse, error)
+	grpc.ClientStream
+}
+
+type blobServicePutClient struct {
+	grpc.ClientStream
+}
+
+func (x *blobServicePutClient) Send(m *BlobChunk) error {
+	return x.ClientStream.SendMsg(m)
+}
+
+func (x *blobServicePutClient) CloseAndRecv() (*PutBlobResponse, error) {
+	if err := x.ClientStream.CloseSend(); err != nil {
+		return nil, err
+	}
+	m := new(PutBlobResponse)
+	if err := x.ClientStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+// BlobServiceServer is the server API for BlobService service.
+// All implementations must embed UnimplementedBlobServiceServer
+// for forward compatibility
+type BlobServiceServer interface {
+	// In the future, Stat will expose more metadata about a given blob,
+	// such as more granular chunking, baos.
+	// For now, it's only used to check for the existence of a blob, as asking
+	// this for a non-existing Blob will return a Status::not_found gRPC error.
+	Stat(context.Context, *StatBlobRequest) (*BlobMeta, error)
+	// Read returns a stream of BlobChunk, which is just a stream of bytes with
+	// the digest specified in ReadBlobRequest.
+	//
+	// The server may decide on whatever chunking it may seem fit as a size for
+	// the individual BlobChunk sent in the response stream.
+	Read(*ReadBlobRequest, BlobService_ReadServer) error
+	// Put uploads a Blob, by reading a stream of bytes.
+	//
+	// The way the data is chunked up in individual BlobChunk messages sent in
+	// the stream has no effect on how the server ends up chunking blobs up.
+	Put(BlobService_PutServer) error
+	mustEmbedUnimplementedBlobServiceServer()
+}
+
+// UnimplementedBlobServiceServer must be embedded to have forward compatible implementations.
+type UnimplementedBlobServiceServer struct {
+}
+
+func (UnimplementedBlobServiceServer) Stat(context.Context, *StatBlobRequest) (*BlobMeta, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method Stat not implemented")
+}
+func (UnimplementedBlobServiceServer) Read(*ReadBlobRequest, BlobService_ReadServer) error {
+	return status.Errorf(codes.Unimplemented, "method Read not implemented")
+}
+func (UnimplementedBlobServiceServer) Put(BlobService_PutServer) error {
+	return status.Errorf(codes.Unimplemented, "method Put not implemented")
+}
+func (UnimplementedBlobServiceServer) mustEmbedUnimplementedBlobServiceServer() {}
+
+// UnsafeBlobServiceServer may be embedded to opt out of forward compatibility for this service.
+// Use of this interface is not recommended, as added methods to BlobServiceServer will
+// result in compilation errors.
+type UnsafeBlobServiceServer interface {
+	mustEmbedUnimplementedBlobServiceServer()
+}
+
+func RegisterBlobServiceServer(s grpc.ServiceRegistrar, srv BlobServiceServer) {
+	s.RegisterService(&BlobService_ServiceDesc, srv)
+}
+
+func _BlobService_Stat_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(StatBlobRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(BlobServiceServer).Stat(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: BlobService_Stat_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(BlobServiceServer).Stat(ctx, req.(*StatBlobRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
+func _BlobService_Read_Handler(srv interface{}, stream grpc.ServerStream) error {
+	m := new(ReadBlobRequest)
+	if err := stream.RecvMsg(m); err != nil {
+		return err
+	}
+	return srv.(BlobServiceServer).Read(m, &blobServiceReadServer{stream})
+}
+
+type BlobService_ReadServer interface {
+	Send(*BlobChunk) error
+	grpc.ServerStream
+}
+
+type blobServiceReadServer struct {
+	grpc.ServerStream
+}
+
+func (x *blobServiceReadServer) Send(m *BlobChunk) error {
+	return x.ServerStream.SendMsg(m)
+}
+
+func _BlobService_Put_Handler(srv interface{}, stream grpc.ServerStream) error {
+	return srv.(BlobServiceServer).Put(&blobServicePutServer{stream})
+}
+
+type BlobService_PutServer interface {
+	SendAndClose(*PutBlobResponse) error
+	Recv() (*BlobChunk, error)
+	grpc.ServerStream
+}
+
+type blobServicePutServer struct {
+	grpc.ServerStream
+}
+
+func (x *blobServicePutServer) SendAndClose(m *PutBlobResponse) error {
+	return x.ServerStream.SendMsg(m)
+}
+
+func (x *blobServicePutServer) Recv() (*BlobChunk, error) {
+	m := new(BlobChunk)
+	if err := x.ServerStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+// BlobService_ServiceDesc is the grpc.ServiceDesc for BlobService service.
+// It's only intended for direct use with grpc.RegisterService,
+// and not to be introspected or modified (even as a copy)
+var BlobService_ServiceDesc = grpc.ServiceDesc{
+	ServiceName: "tvix.castore.v1.BlobService",
+	HandlerType: (*BlobServiceServer)(nil),
+	Methods: []grpc.MethodDesc{
+		{
+			MethodName: "Stat",
+			Handler:    _BlobService_Stat_Handler,
+		},
+	},
+	Streams: []grpc.StreamDesc{
+		{
+			StreamName:    "Read",
+			Handler:       _BlobService_Read_Handler,
+			ServerStreams: true,
+		},
+		{
+			StreamName:    "Put",
+			Handler:       _BlobService_Put_Handler,
+			ClientStreams: true,
+		},
+	},
+	Metadata: "tvix/castore/protos/rpc_blobstore.proto",
+}
diff --git a/tvix/castore/protos/rpc_directory.pb.go b/tvix/castore/protos/rpc_directory.pb.go
new file mode 100644
index 0000000000..f658c6b60c
--- /dev/null
+++ b/tvix/castore/protos/rpc_directory.pb.go
@@ -0,0 +1,273 @@
+// SPDX-License-Identifier: MIT
+// Copyright © 2022 The Tvix Authors
+
+// Code generated by protoc-gen-go. DO NOT EDIT.
+// versions:
+// 	protoc-gen-go v1.31.0
+// 	protoc        (unknown)
+// source: tvix/castore/protos/rpc_directory.proto
+
+package castorev1
+
+import (
+	protoreflect "google.golang.org/protobuf/reflect/protoreflect"
+	protoimpl "google.golang.org/protobuf/runtime/protoimpl"
+	reflect "reflect"
+	sync "sync"
+)
+
+const (
+	// Verify that this generated code is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(20 - protoimpl.MinVersion)
+	// Verify that runtime/protoimpl is sufficiently up-to-date.
+	_ = protoimpl.EnforceVersion(protoimpl.MaxVersion - 20)
+)
+
+type GetDirectoryRequest struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	// Types that are assignable to ByWhat:
+	//
+	//	*GetDirectoryRequest_Digest
+	ByWhat isGetDirectoryRequest_ByWhat `protobuf_oneof:"by_what"`
+	// If set to true, recursively resolve all child Directory messages.
+	// Directory messages SHOULD be streamed in a recursive breadth-first walk,
+	// but other orders are also fine, as long as Directory messages are only
+	// sent after they are referred to from previously sent Directory messages.
+	Recursive bool `protobuf:"varint,2,opt,name=recursive,proto3" json:"recursive,omitempty"`
+}
+
+func (x *GetDirectoryRequest) Reset() {
+	*x = GetDirectoryRequest{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_rpc_directory_proto_msgTypes[0]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *GetDirectoryRequest) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*GetDirectoryRequest) ProtoMessage() {}
+
+func (x *GetDirectoryRequest) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_rpc_directory_proto_msgTypes[0]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use GetDirectoryRequest.ProtoReflect.Descriptor instead.
+func (*GetDirectoryRequest) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_rpc_directory_proto_rawDescGZIP(), []int{0}
+}
+
+func (m *GetDirectoryRequest) GetByWhat() isGetDirectoryRequest_ByWhat {
+	if m != nil {
+		return m.ByWhat
+	}
+	return nil
+}
+
+func (x *GetDirectoryRequest) GetDigest() []byte {
+	if x, ok := x.GetByWhat().(*GetDirectoryRequest_Digest); ok {
+		return x.Digest
+	}
+	return nil
+}
+
+func (x *GetDirectoryRequest) GetRecursive() bool {
+	if x != nil {
+		return x.Recursive
+	}
+	return false
+}
+
+type isGetDirectoryRequest_ByWhat interface {
+	isGetDirectoryRequest_ByWhat()
+}
+
+type GetDirectoryRequest_Digest struct {
+	// The blake3 hash of the (root) Directory message, serialized in
+	// protobuf canonical form.
+	// Keep in mind this can be a subtree of another root.
+	Digest []byte `protobuf:"bytes,1,opt,name=digest,proto3,oneof"`
+}
+
+func (*GetDirectoryRequest_Digest) isGetDirectoryRequest_ByWhat() {}
+
+type PutDirectoryResponse struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	RootDigest []byte `protobuf:"bytes,1,opt,name=root_digest,json=rootDigest,proto3" json:"root_digest,omitempty"`
+}
+
+func (x *PutDirectoryResponse) Reset() {
+	*x = PutDirectoryResponse{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_tvix_castore_protos_rpc_directory_proto_msgTypes[1]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *PutDirectoryResponse) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*PutDirectoryResponse) ProtoMessage() {}
+
+func (x *PutDirectoryResponse) ProtoReflect() protoreflect.Message {
+	mi := &file_tvix_castore_protos_rpc_directory_proto_msgTypes[1]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use PutDirectoryResponse.ProtoReflect.Descriptor instead.
+func (*PutDirectoryResponse) Descriptor() ([]byte, []int) {
+	return file_tvix_castore_protos_rpc_directory_proto_rawDescGZIP(), []int{1}
+}
+
+func (x *PutDirectoryResponse) GetRootDigest() []byte {
+	if x != nil {
+		return x.RootDigest
+	}
+	return nil
+}
+
+var File_tvix_castore_protos_rpc_directory_proto protoreflect.FileDescriptor
+
+var file_tvix_castore_protos_rpc_directory_proto_rawDesc = []byte{
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+	0x63, 0x6f, 0x64, 0x65, 0x2e, 0x74, 0x76, 0x6c, 0x2e, 0x66, 0x79, 0x69, 0x2f, 0x74, 0x76, 0x69,
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+	0x6f, 0x33,
+}
+
+var (
+	file_tvix_castore_protos_rpc_directory_proto_rawDescOnce sync.Once
+	file_tvix_castore_protos_rpc_directory_proto_rawDescData = file_tvix_castore_protos_rpc_directory_proto_rawDesc
+)
+
+func file_tvix_castore_protos_rpc_directory_proto_rawDescGZIP() []byte {
+	file_tvix_castore_protos_rpc_directory_proto_rawDescOnce.Do(func() {
+		file_tvix_castore_protos_rpc_directory_proto_rawDescData = protoimpl.X.CompressGZIP(file_tvix_castore_protos_rpc_directory_proto_rawDescData)
+	})
+	return file_tvix_castore_protos_rpc_directory_proto_rawDescData
+}
+
+var file_tvix_castore_protos_rpc_directory_proto_msgTypes = make([]protoimpl.MessageInfo, 2)
+var file_tvix_castore_protos_rpc_directory_proto_goTypes = []interface{}{
+	(*GetDirectoryRequest)(nil),  // 0: tvix.castore.v1.GetDirectoryRequest
+	(*PutDirectoryResponse)(nil), // 1: tvix.castore.v1.PutDirectoryResponse
+	(*Directory)(nil),            // 2: tvix.castore.v1.Directory
+}
+var file_tvix_castore_protos_rpc_directory_proto_depIdxs = []int32{
+	0, // 0: tvix.castore.v1.DirectoryService.Get:input_type -> tvix.castore.v1.GetDirectoryRequest
+	2, // 1: tvix.castore.v1.DirectoryService.Put:input_type -> tvix.castore.v1.Directory
+	2, // 2: tvix.castore.v1.DirectoryService.Get:output_type -> tvix.castore.v1.Directory
+	1, // 3: tvix.castore.v1.DirectoryService.Put:output_type -> tvix.castore.v1.PutDirectoryResponse
+	2, // [2:4] is the sub-list for method output_type
+	0, // [0:2] is the sub-list for method input_type
+	0, // [0:0] is the sub-list for extension type_name
+	0, // [0:0] is the sub-list for extension extendee
+	0, // [0:0] is the sub-list for field type_name
+}
+
+func init() { file_tvix_castore_protos_rpc_directory_proto_init() }
+func file_tvix_castore_protos_rpc_directory_proto_init() {
+	if File_tvix_castore_protos_rpc_directory_proto != nil {
+		return
+	}
+	file_tvix_castore_protos_castore_proto_init()
+	if !protoimpl.UnsafeEnabled {
+		file_tvix_castore_protos_rpc_directory_proto_msgTypes[0].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*GetDirectoryRequest); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+		file_tvix_castore_protos_rpc_directory_proto_msgTypes[1].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*PutDirectoryResponse); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
+	}
+	file_tvix_castore_protos_rpc_directory_proto_msgTypes[0].OneofWrappers = []interface{}{
+		(*GetDirectoryRequest_Digest)(nil),
+	}
+	type x struct{}
+	out := protoimpl.TypeBuilder{
+		File: protoimpl.DescBuilder{
+			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
+			RawDescriptor: file_tvix_castore_protos_rpc_directory_proto_rawDesc,
+			NumEnums:      0,
+			NumMessages:   2,
+			NumExtensions: 0,
+			NumServices:   1,
+		},
+		GoTypes:           file_tvix_castore_protos_rpc_directory_proto_goTypes,
+		DependencyIndexes: file_tvix_castore_protos_rpc_directory_proto_depIdxs,
+		MessageInfos:      file_tvix_castore_protos_rpc_directory_proto_msgTypes,
+	}.Build()
+	File_tvix_castore_protos_rpc_directory_proto = out.File
+	file_tvix_castore_protos_rpc_directory_proto_rawDesc = nil
+	file_tvix_castore_protos_rpc_directory_proto_goTypes = nil
+	file_tvix_castore_protos_rpc_directory_proto_depIdxs = nil
+}
diff --git a/tvix/castore/protos/rpc_directory.proto b/tvix/castore/protos/rpc_directory.proto
new file mode 100644
index 0000000000..8d4c228285
--- /dev/null
+++ b/tvix/castore/protos/rpc_directory.proto
@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: MIT
+// Copyright © 2022 The Tvix Authors
+syntax = "proto3";
+
+package tvix.castore.v1;
+
+import "tvix/castore/protos/castore.proto";
+
+option go_package = "code.tvl.fyi/tvix/castore/protos;castorev1";
+
+service DirectoryService {
+  // Get retrieves a stream of Directory messages, by using the lookup
+  // parameters in GetDirectoryRequest.
+  // Keep in mind multiple DirectoryNodes in different parts of the graph might
+  // have the same digest if they have the same underlying contents,
+  // so sending subsequent ones can be omitted.
+  rpc Get(GetDirectoryRequest) returns (stream Directory);
+
+  // Put uploads a graph of Directory messages.
+  // Individual Directory messages need to be send in an order walking up
+  // from the leaves to the root - a Directory message can only refer to
+  // Directory messages previously sent in the same stream.
+  // Keep in mind multiple DirectoryNodes in different parts of the graph might
+  // have the same digest if they have the same underlying contents,
+  // so sending subsequent ones can be omitted.
+  // We might add a separate method, allowing to send partial graphs at a later
+  // time, if requiring to send the full graph turns out to be a problem.
+  rpc Put(stream Directory) returns (PutDirectoryResponse);
+}
+
+message GetDirectoryRequest {
+  oneof by_what {
+      // The blake3 hash of the (root) Directory message, serialized in
+      // protobuf canonical form.
+      // Keep in mind this can be a subtree of another root.
+      bytes digest = 1;
+  }
+
+  // If set to true, recursively resolve all child Directory messages.
+  // Directory messages SHOULD be streamed in a recursive breadth-first walk,
+  // but other orders are also fine, as long as Directory messages are only
+  // sent after they are referred to from previously sent Directory messages.
+  bool recursive = 2;
+}
+
+message PutDirectoryResponse {
+  bytes root_digest = 1;
+}
diff --git a/tvix/castore/protos/rpc_directory_grpc.pb.go b/tvix/castore/protos/rpc_directory_grpc.pb.go
new file mode 100644
index 0000000000..f19e457d86
--- /dev/null
+++ b/tvix/castore/protos/rpc_directory_grpc.pb.go
@@ -0,0 +1,238 @@
+// SPDX-License-Identifier: MIT
+// Copyright © 2022 The Tvix Authors
+
+// Code generated by protoc-gen-go-grpc. DO NOT EDIT.
+// versions:
+// - protoc-gen-go-grpc v1.3.0
+// - protoc             (unknown)
+// source: tvix/castore/protos/rpc_directory.proto
+
+package castorev1
+
+import (
+	context "context"
+	grpc "google.golang.org/grpc"
+	codes "google.golang.org/grpc/codes"
+	status "google.golang.org/grpc/status"
+)
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the grpc package it is being compiled against.
+// Requires gRPC-Go v1.32.0 or later.
+const _ = grpc.SupportPackageIsVersion7
+
+const (
+	DirectoryService_Get_FullMethodName = "/tvix.castore.v1.DirectoryService/Get"
+	DirectoryService_Put_FullMethodName = "/tvix.castore.v1.DirectoryService/Put"
+)
+
+// DirectoryServiceClient is the client API for DirectoryService service.
+//
+// For semantics around ctx use and closing/ending streaming RPCs, please refer to https://pkg.go.dev/google.golang.org/grpc/?tab=doc#ClientConn.NewStream.
+type DirectoryServiceClient interface {
+	// Get retrieves a stream of Directory messages, by using the lookup
+	// parameters in GetDirectoryRequest.
+	// Keep in mind multiple DirectoryNodes in different parts of the graph might
+	// have the same digest if they have the same underlying contents,
+	// so sending subsequent ones can be omitted.
+	Get(ctx context.Context, in *GetDirectoryRequest, opts ...grpc.CallOption) (DirectoryService_GetClient, error)
+	// Put uploads a graph of Directory messages.
+	// Individual Directory messages need to be send in an order walking up
+	// from the leaves to the root - a Directory message can only refer to
+	// Directory messages previously sent in the same stream.
+	// Keep in mind multiple DirectoryNodes in different parts of the graph might
+	// have the same digest if they have the same underlying contents,
+	// so sending subsequent ones can be omitted.
+	// We might add a separate method, allowing to send partial graphs at a later
+	// time, if requiring to send the full graph turns out to be a problem.
+	Put(ctx context.Context, opts ...grpc.CallOption) (DirectoryService_PutClient, error)
+}
+
+type directoryServiceClient struct {
+	cc grpc.ClientConnInterface
+}
+
+func NewDirectoryServiceClient(cc grpc.ClientConnInterface) DirectoryServiceClient {
+	return &directoryServiceClient{cc}
+}
+
+func (c *directoryServiceClient) Get(ctx context.Context, in *GetDirectoryRequest, opts ...grpc.CallOption) (DirectoryService_GetClient, error) {
+	stream, err := c.cc.NewStream(ctx, &DirectoryService_ServiceDesc.Streams[0], DirectoryService_Get_FullMethodName, opts...)
+	if err != nil {
+		return nil, err
+	}
+	x := &directoryServiceGetClient{stream}
+	if err := x.ClientStream.SendMsg(in); err != nil {
+		return nil, err
+	}
+	if err := x.ClientStream.CloseSend(); err != nil {
+		return nil, err
+	}
+	return x, nil
+}
+
+type DirectoryService_GetClient interface {
+	Recv() (*Directory, error)
+	grpc.ClientStream
+}
+
+type directoryServiceGetClient struct {
+	grpc.ClientStream
+}
+
+func (x *directoryServiceGetClient) Recv() (*Directory, error) {
+	m := new(Directory)
+	if err := x.ClientStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+func (c *directoryServiceClient) Put(ctx context.Context, opts ...grpc.CallOption) (DirectoryService_PutClient, error) {
+	stream, err := c.cc.NewStream(ctx, &DirectoryService_ServiceDesc.Streams[1], DirectoryService_Put_FullMethodName, opts...)
+	if err != nil {
+		return nil, err
+	}
+	x := &directoryServicePutClient{stream}
+	return x, nil
+}
+
+type DirectoryService_PutClient interface {
+	Send(*Directory) error
+	CloseAndRecv() (*PutDirectoryResponse, error)
+	grpc.ClientStream
+}
+
+type directoryServicePutClient struct {
+	grpc.ClientStream
+}
+
+func (x *directoryServicePutClient) Send(m *Directory) error {
+	return x.ClientStream.SendMsg(m)
+}
+
+func (x *directoryServicePutClient) CloseAndRecv() (*PutDirectoryResponse, error) {
+	if err := x.ClientStream.CloseSend(); err != nil {
+		return nil, err
+	}
+	m := new(PutDirectoryResponse)
+	if err := x.ClientStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+// DirectoryServiceServer is the server API for DirectoryService service.
+// All implementations must embed UnimplementedDirectoryServiceServer
+// for forward compatibility
+type DirectoryServiceServer interface {
+	// Get retrieves a stream of Directory messages, by using the lookup
+	// parameters in GetDirectoryRequest.
+	// Keep in mind multiple DirectoryNodes in different parts of the graph might
+	// have the same digest if they have the same underlying contents,
+	// so sending subsequent ones can be omitted.
+	Get(*GetDirectoryRequest, DirectoryService_GetServer) error
+	// Put uploads a graph of Directory messages.
+	// Individual Directory messages need to be send in an order walking up
+	// from the leaves to the root - a Directory message can only refer to
+	// Directory messages previously sent in the same stream.
+	// Keep in mind multiple DirectoryNodes in different parts of the graph might
+	// have the same digest if they have the same underlying contents,
+	// so sending subsequent ones can be omitted.
+	// We might add a separate method, allowing to send partial graphs at a later
+	// time, if requiring to send the full graph turns out to be a problem.
+	Put(DirectoryService_PutServer) error
+	mustEmbedUnimplementedDirectoryServiceServer()
+}
+
+// UnimplementedDirectoryServiceServer must be embedded to have forward compatible implementations.
+type UnimplementedDirectoryServiceServer struct {
+}
+
+func (UnimplementedDirectoryServiceServer) Get(*GetDirectoryRequest, DirectoryService_GetServer) error {
+	return status.Errorf(codes.Unimplemented, "method Get not implemented")
+}
+func (UnimplementedDirectoryServiceServer) Put(DirectoryService_PutServer) error {
+	return status.Errorf(codes.Unimplemented, "method Put not implemented")
+}
+func (UnimplementedDirectoryServiceServer) mustEmbedUnimplementedDirectoryServiceServer() {}
+
+// UnsafeDirectoryServiceServer may be embedded to opt out of forward compatibility for this service.
+// Use of this interface is not recommended, as added methods to DirectoryServiceServer will
+// result in compilation errors.
+type UnsafeDirectoryServiceServer interface {
+	mustEmbedUnimplementedDirectoryServiceServer()
+}
+
+func RegisterDirectoryServiceServer(s grpc.ServiceRegistrar, srv DirectoryServiceServer) {
+	s.RegisterService(&DirectoryService_ServiceDesc, srv)
+}
+
+func _DirectoryService_Get_Handler(srv interface{}, stream grpc.ServerStream) error {
+	m := new(GetDirectoryRequest)
+	if err := stream.RecvMsg(m); err != nil {
+		return err
+	}
+	return srv.(DirectoryServiceServer).Get(m, &directoryServiceGetServer{stream})
+}
+
+type DirectoryService_GetServer interface {
+	Send(*Directory) error
+	grpc.ServerStream
+}
+
+type directoryServiceGetServer struct {
+	grpc.ServerStream
+}
+
+func (x *directoryServiceGetServer) Send(m *Directory) error {
+	return x.ServerStream.SendMsg(m)
+}
+
+func _DirectoryService_Put_Handler(srv interface{}, stream grpc.ServerStream) error {
+	return srv.(DirectoryServiceServer).Put(&directoryServicePutServer{stream})
+}
+
+type DirectoryService_PutServer interface {
+	SendAndClose(*PutDirectoryResponse) error
+	Recv() (*Directory, error)
+	grpc.ServerStream
+}
+
+type directoryServicePutServer struct {
+	grpc.ServerStream
+}
+
+func (x *directoryServicePutServer) SendAndClose(m *PutDirectoryResponse) error {
+	return x.ServerStream.SendMsg(m)
+}
+
+func (x *directoryServicePutServer) Recv() (*Directory, error) {
+	m := new(Directory)
+	if err := x.ServerStream.RecvMsg(m); err != nil {
+		return nil, err
+	}
+	return m, nil
+}
+
+// DirectoryService_ServiceDesc is the grpc.ServiceDesc for DirectoryService service.
+// It's only intended for direct use with grpc.RegisterService,
+// and not to be introspected or modified (even as a copy)
+var DirectoryService_ServiceDesc = grpc.ServiceDesc{
+	ServiceName: "tvix.castore.v1.DirectoryService",
+	HandlerType: (*DirectoryServiceServer)(nil),
+	Methods:     []grpc.MethodDesc{},
+	Streams: []grpc.StreamDesc{
+		{
+			StreamName:    "Get",
+			Handler:       _DirectoryService_Get_Handler,
+			ServerStreams: true,
+		},
+		{
+			StreamName:    "Put",
+			Handler:       _DirectoryService_Put_Handler,
+			ClientStreams: true,
+		},
+	},
+	Metadata: "tvix/castore/protos/rpc_directory.proto",
+}
diff --git a/tvix/castore/src/blobservice/from_addr.rs b/tvix/castore/src/blobservice/from_addr.rs
new file mode 100644
index 0000000000..2e0a30697d
--- /dev/null
+++ b/tvix/castore/src/blobservice/from_addr.rs
@@ -0,0 +1,30 @@
+use std::sync::Arc;
+use url::Url;
+
+use super::{BlobService, GRPCBlobService, MemoryBlobService, SledBlobService};
+
+/// Constructs a new instance of a [BlobService] from an URI.
+///
+/// The following schemes are supported by the following services:
+/// - `memory://` ([MemoryBlobService])
+/// - `sled://` ([SledBlobService])
+/// - `grpc+*://` ([GRPCBlobService])
+///
+/// See their `from_url` methods for more details about their syntax.
+pub fn from_addr(uri: &str) -> Result<Arc<dyn BlobService>, crate::Error> {
+    let url = Url::parse(uri)
+        .map_err(|e| crate::Error::StorageError(format!("unable to parse url: {}", e)))?;
+
+    Ok(if url.scheme() == "memory" {
+        Arc::new(MemoryBlobService::from_url(&url)?)
+    } else if url.scheme() == "sled" {
+        Arc::new(SledBlobService::from_url(&url)?)
+    } else if url.scheme().starts_with("grpc+") {
+        Arc::new(GRPCBlobService::from_url(&url)?)
+    } else {
+        Err(crate::Error::StorageError(format!(
+            "unknown scheme: {}",
+            url.scheme()
+        )))?
+    })
+}
diff --git a/tvix/castore/src/blobservice/grpc.rs b/tvix/castore/src/blobservice/grpc.rs
new file mode 100644
index 0000000000..db9d4a9c00
--- /dev/null
+++ b/tvix/castore/src/blobservice/grpc.rs
@@ -0,0 +1,426 @@
+use super::{naive_seeker::NaiveSeeker, BlobReader, BlobService, BlobWriter};
+use crate::{proto, B3Digest};
+use futures::sink::SinkExt;
+use futures::TryFutureExt;
+use std::{
+    collections::VecDeque,
+    io::{self},
+    pin::pin,
+    task::Poll,
+};
+use tokio::io::AsyncWriteExt;
+use tokio::{net::UnixStream, task::JoinHandle};
+use tokio_stream::{wrappers::ReceiverStream, StreamExt};
+use tokio_util::{
+    io::{CopyToBytes, SinkWriter},
+    sync::{PollSendError, PollSender},
+};
+use tonic::{async_trait, transport::Channel, Code, Status};
+use tracing::instrument;
+
+/// Connects to a (remote) tvix-store BlobService over gRPC.
+#[derive(Clone)]
+pub struct GRPCBlobService {
+    /// The internal reference to a gRPC client.
+    /// Cloning it is cheap, and it internally handles concurrent requests.
+    grpc_client: proto::blob_service_client::BlobServiceClient<Channel>,
+}
+
+impl GRPCBlobService {
+    /// construct a [GRPCBlobService] from a [proto::blob_service_client::BlobServiceClient].
+    /// panics if called outside the context of a tokio runtime.
+    pub fn from_client(
+        grpc_client: proto::blob_service_client::BlobServiceClient<Channel>,
+    ) -> Self {
+        Self { grpc_client }
+    }
+}
+
+#[async_trait]
+impl BlobService for GRPCBlobService {
+    /// Constructs a [GRPCBlobService] from the passed [url::Url]:
+    /// - scheme has to match `grpc+*://`.
+    ///   That's normally grpc+unix for unix sockets, and grpc+http(s) for the HTTP counterparts.
+    /// - In the case of unix sockets, there must be a path, but may not be a host.
+    /// - In the case of non-unix sockets, there must be a host, but no path.
+    fn from_url(url: &url::Url) -> Result<Self, crate::Error> {
+        // Start checking for the scheme to start with grpc+.
+        match url.scheme().strip_prefix("grpc+") {
+            None => Err(crate::Error::StorageError("invalid scheme".to_string())),
+            Some(rest) => {
+                if rest == "unix" {
+                    if url.host_str().is_some() {
+                        return Err(crate::Error::StorageError(
+                            "host may not be set".to_string(),
+                        ));
+                    }
+                    let path = url.path().to_string();
+                    let channel = tonic::transport::Endpoint::try_from("http://[::]:50051") // doesn't matter
+                        .unwrap()
+                        .connect_with_connector_lazy(tower::service_fn(
+                            move |_: tonic::transport::Uri| UnixStream::connect(path.clone()),
+                        ));
+                    let grpc_client = proto::blob_service_client::BlobServiceClient::new(channel);
+                    Ok(Self::from_client(grpc_client))
+                } else {
+                    // ensure path is empty, not supported with gRPC.
+                    if !url.path().is_empty() {
+                        return Err(crate::Error::StorageError(
+                            "path may not be set".to_string(),
+                        ));
+                    }
+
+                    // clone the uri, and drop the grpc+ from the scheme.
+                    // Recreate a new uri with the `grpc+` prefix dropped from the scheme.
+                    // We can't use `url.set_scheme(rest)`, as it disallows
+                    // setting something http(s) that previously wasn't.
+                    let url = {
+                        let url_str = url.to_string();
+                        let s_stripped = url_str.strip_prefix("grpc+").unwrap();
+                        url::Url::parse(s_stripped).unwrap()
+                    };
+                    let channel = tonic::transport::Endpoint::try_from(url.to_string())
+                        .unwrap()
+                        .connect_lazy();
+
+                    let grpc_client = proto::blob_service_client::BlobServiceClient::new(channel);
+                    Ok(Self::from_client(grpc_client))
+                }
+            }
+        }
+    }
+
+    #[instrument(skip(self, digest), fields(blob.digest=%digest))]
+    async fn has(&self, digest: &B3Digest) -> Result<bool, crate::Error> {
+        let mut grpc_client = self.grpc_client.clone();
+        let resp = grpc_client
+            .stat(proto::StatBlobRequest {
+                digest: digest.clone().into(),
+            })
+            .await;
+
+        match resp {
+            Ok(_blob_meta) => Ok(true),
+            Err(e) if e.code() == Code::NotFound => Ok(false),
+            Err(e) => Err(crate::Error::StorageError(e.to_string())),
+        }
+    }
+
+    // On success, this returns a Ok(Some(io::Read)), which can be used to read
+    // the contents of the Blob, identified by the digest.
+    async fn open_read(
+        &self,
+        digest: &B3Digest,
+    ) -> Result<Option<Box<dyn BlobReader>>, crate::Error> {
+        // Get a new handle to the gRPC client, and copy the digest.
+        let mut grpc_client = self.grpc_client.clone();
+
+        // Get a stream of [proto::BlobChunk], or return an error if the blob
+        // doesn't exist.
+        let resp = grpc_client
+            .read(proto::ReadBlobRequest {
+                digest: digest.clone().into(),
+            })
+            .await;
+
+        // This runs the task to completion, which on success will return a stream.
+        // On reading from it, we receive individual [proto::BlobChunk], so we
+        // massage this to a stream of bytes,
+        // then create an [AsyncRead], which we'll turn into a [io::Read],
+        // that's returned from the function.
+        match resp {
+            Ok(stream) => {
+                // map the stream of proto::BlobChunk to bytes.
+                let data_stream = stream.into_inner().map(|x| {
+                    x.map(|x| VecDeque::from(x.data.to_vec()))
+                        .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidInput, e))
+                });
+
+                // Use StreamReader::new to convert to an AsyncRead.
+                let data_reader = tokio_util::io::StreamReader::new(data_stream);
+
+                Ok(Some(Box::new(NaiveSeeker::new(data_reader))))
+            }
+            Err(e) if e.code() == Code::NotFound => Ok(None),
+            Err(e) => Err(crate::Error::StorageError(e.to_string())),
+        }
+    }
+
+    /// Returns a BlobWriter, that'll internally wrap each write in a
+    // [proto::BlobChunk], which is send to the gRPC server.
+    async fn open_write(&self) -> Box<dyn BlobWriter> {
+        let mut grpc_client = self.grpc_client.clone();
+
+        // set up an mpsc channel passing around Bytes.
+        let (tx, rx) = tokio::sync::mpsc::channel::<bytes::Bytes>(10);
+
+        // bytes arriving on the RX side are wrapped inside a
+        // [proto::BlobChunk], and a [ReceiverStream] is constructed.
+        let blobchunk_stream = ReceiverStream::new(rx).map(|x| proto::BlobChunk { data: x });
+
+        // That receiver stream is used as a stream in the gRPC BlobService.put rpc call.
+        let task: JoinHandle<Result<_, Status>> =
+            tokio::spawn(async move { Ok(grpc_client.put(blobchunk_stream).await?.into_inner()) });
+
+        // The tx part of the channel is converted to a sink of byte chunks.
+
+        // We need to make this a function pointer, not a closure.
+        fn convert_error(_: PollSendError<bytes::Bytes>) -> io::Error {
+            io::Error::from(io::ErrorKind::BrokenPipe)
+        }
+
+        let sink = PollSender::new(tx)
+            .sink_map_err(convert_error as fn(PollSendError<bytes::Bytes>) -> io::Error);
+        // We need to explicitly cast here, otherwise rustc does error with "expected fn pointer, found fn item"
+
+        // … which is turned into an [tokio::io::AsyncWrite].
+        let writer = SinkWriter::new(CopyToBytes::new(sink));
+
+        Box::new(GRPCBlobWriter {
+            task_and_writer: Some((task, writer)),
+            digest: None,
+        })
+    }
+}
+
+pub struct GRPCBlobWriter<W: tokio::io::AsyncWrite> {
+    /// The task containing the put request, and the inner writer, if we're still writing.
+    task_and_writer: Option<(JoinHandle<Result<proto::PutBlobResponse, Status>>, W)>,
+
+    /// The digest that has been returned, if we successfully closed.
+    digest: Option<B3Digest>,
+}
+
+#[async_trait]
+impl<W: tokio::io::AsyncWrite + Send + Sync + Unpin + 'static> BlobWriter for GRPCBlobWriter<W> {
+    async fn close(&mut self) -> Result<B3Digest, crate::Error> {
+        if self.task_and_writer.is_none() {
+            // if we're already closed, return the b3 digest, which must exist.
+            // If it doesn't, we already closed and failed once, and didn't handle the error.
+            match &self.digest {
+                Some(digest) => Ok(digest.clone()),
+                None => Err(crate::Error::StorageError(
+                    "previously closed with error".to_string(),
+                )),
+            }
+        } else {
+            let (task, mut writer) = self.task_and_writer.take().unwrap();
+
+            // invoke shutdown, so the inner writer closes its internal tx side of
+            // the channel.
+            writer
+                .shutdown()
+                .map_err(|e| crate::Error::StorageError(e.to_string()))
+                .await?;
+
+            // block on the RPC call to return.
+            // This ensures all chunks are sent out, and have been received by the
+            // backend.
+
+            match task.await? {
+                Ok(resp) => {
+                    // return the digest from the response, and store it in self.digest for subsequent closes.
+                    let digest: B3Digest = resp.digest.try_into().map_err(|_| {
+                        crate::Error::StorageError(
+                            "invalid root digest length in response".to_string(),
+                        )
+                    })?;
+                    self.digest = Some(digest.clone());
+                    Ok(digest)
+                }
+                Err(e) => Err(crate::Error::StorageError(e.to_string())),
+            }
+        }
+    }
+}
+
+impl<W: tokio::io::AsyncWrite + Unpin> tokio::io::AsyncWrite for GRPCBlobWriter<W> {
+    fn poll_write(
+        mut self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+        buf: &[u8],
+    ) -> std::task::Poll<Result<usize, io::Error>> {
+        match &mut self.task_and_writer {
+            None => Poll::Ready(Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            ))),
+            Some((_, ref mut writer)) => {
+                let pinned_writer = pin!(writer);
+                pinned_writer.poll_write(cx, buf)
+            }
+        }
+    }
+
+    fn poll_flush(
+        mut self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        match &mut self.task_and_writer {
+            None => Poll::Ready(Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            ))),
+            Some((_, ref mut writer)) => {
+                let pinned_writer = pin!(writer);
+                pinned_writer.poll_flush(cx)
+            }
+        }
+    }
+
+    fn poll_shutdown(
+        self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        // TODO(raitobezarius): this might not be a graceful shutdown of the
+        // channel inside the gRPC connection.
+        Poll::Ready(Ok(()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+    use std::thread;
+
+    use tempfile::TempDir;
+    use tokio::net::UnixListener;
+    use tokio::time;
+    use tokio_stream::wrappers::UnixListenerStream;
+
+    use crate::blobservice::MemoryBlobService;
+    use crate::fixtures;
+    use crate::proto::GRPCBlobServiceWrapper;
+
+    use super::BlobService;
+    use super::GRPCBlobService;
+
+    /// This uses the wrong scheme
+    #[test]
+    fn test_invalid_scheme() {
+        let url = url::Url::parse("http://foo.example/test").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme for a unix socket.
+    /// The fact that /path/to/somewhere doesn't exist yet is no problem, because we connect lazily.
+    #[tokio::test]
+    async fn test_valid_unix_path() {
+        let url = url::Url::parse("grpc+unix:///path/to/somewhere").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_ok());
+    }
+
+    /// This uses the correct scheme for a unix socket,
+    /// but sets a host, which is unsupported.
+    #[tokio::test]
+    async fn test_invalid_unix_path_with_domain() {
+        let url =
+            url::Url::parse("grpc+unix://host.example/path/to/somewhere").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme for a HTTP server.
+    /// The fact that nothing is listening there is no problem, because we connect lazily.
+    #[tokio::test]
+    async fn test_valid_http() {
+        let url = url::Url::parse("grpc+http://localhost").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_ok());
+    }
+
+    /// This uses the correct scheme for a HTTPS server.
+    /// The fact that nothing is listening there is no problem, because we connect lazily.
+    #[tokio::test]
+    async fn test_valid_https() {
+        let url = url::Url::parse("grpc+https://localhost").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_ok());
+    }
+
+    /// This uses the correct scheme, but also specifies
+    /// an additional path, which is not supported for gRPC.
+    /// The fact that nothing is listening there is no problem, because we connect lazily.
+    #[tokio::test]
+    async fn test_invalid_http_with_path() {
+        let url = url::Url::parse("grpc+https://localhost/some-path").expect("must parse");
+
+        assert!(GRPCBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme for a unix socket, and provides a server on the other side.
+    /// This is not a tokio::test, because spawn two separate tokio runtimes and
+    // want to have explicit control.
+    #[test]
+    fn test_valid_unix_path_ping_pong() {
+        let tmpdir = TempDir::new().unwrap();
+        let path = tmpdir.path().join("daemon");
+
+        let path_clone = path.clone();
+
+        // Spin up a server, in a thread far away, which spawns its own tokio runtime,
+        // and blocks on the task.
+        thread::spawn(move || {
+            // Create the runtime
+            let rt = tokio::runtime::Runtime::new().unwrap();
+
+            let task = rt.spawn(async {
+                let uds = UnixListener::bind(path_clone).unwrap();
+                let uds_stream = UnixListenerStream::new(uds);
+
+                // spin up a new server
+                let mut server = tonic::transport::Server::builder();
+                let router =
+                    server.add_service(crate::proto::blob_service_server::BlobServiceServer::new(
+                        GRPCBlobServiceWrapper::from(
+                            Arc::new(MemoryBlobService::default()) as Arc<dyn BlobService>
+                        ),
+                    ));
+                router.serve_with_incoming(uds_stream).await
+            });
+
+            rt.block_on(task).unwrap().unwrap();
+        });
+
+        // Now create another tokio runtime which we'll use in the main test code.
+        let rt = tokio::runtime::Runtime::new().unwrap();
+
+        let task = rt.spawn(async move {
+            // wait for the socket to be created
+            {
+                let mut socket_created = false;
+                // TODO: exponential backoff urgently
+                for _try in 1..20 {
+                    if path.exists() {
+                        socket_created = true;
+                        break;
+                    }
+                    tokio::time::sleep(time::Duration::from_millis(20)).await;
+                }
+
+                assert!(
+                    socket_created,
+                    "expected socket path to eventually get created, but never happened"
+                );
+            }
+
+            // prepare a client
+            let client = {
+                let mut url =
+                    url::Url::parse("grpc+unix:///path/to/somewhere").expect("must parse");
+                url.set_path(path.to_str().unwrap());
+                GRPCBlobService::from_url(&url).expect("must succeed")
+            };
+
+            let has = client
+                .has(&fixtures::BLOB_A_DIGEST)
+                .await
+                .expect("must not be err");
+
+            assert!(!has);
+        });
+        rt.block_on(task).unwrap()
+    }
+}
diff --git a/tvix/castore/src/blobservice/memory.rs b/tvix/castore/src/blobservice/memory.rs
new file mode 100644
index 0000000000..383127344a
--- /dev/null
+++ b/tvix/castore/src/blobservice/memory.rs
@@ -0,0 +1,196 @@
+use std::io::{self, Cursor, Write};
+use std::task::Poll;
+use std::{
+    collections::HashMap,
+    sync::{Arc, RwLock},
+};
+use tonic::async_trait;
+use tracing::instrument;
+
+use super::{BlobReader, BlobService, BlobWriter};
+use crate::{B3Digest, Error};
+
+#[derive(Clone, Default)]
+pub struct MemoryBlobService {
+    db: Arc<RwLock<HashMap<B3Digest, Vec<u8>>>>,
+}
+
+#[async_trait]
+impl BlobService for MemoryBlobService {
+    /// Constructs a [MemoryBlobService] from the passed [url::Url]:
+    /// - scheme has to be `memory://`
+    /// - there may not be a host.
+    /// - there may not be a path.
+    fn from_url(url: &url::Url) -> Result<Self, Error> {
+        if url.scheme() != "memory" {
+            return Err(crate::Error::StorageError("invalid scheme".to_string()));
+        }
+
+        if url.has_host() || !url.path().is_empty() {
+            return Err(crate::Error::StorageError("invalid url".to_string()));
+        }
+
+        Ok(Self::default())
+    }
+
+    #[instrument(skip(self, digest), fields(blob.digest=%digest))]
+    async fn has(&self, digest: &B3Digest) -> Result<bool, Error> {
+        let db = self.db.read().unwrap();
+        Ok(db.contains_key(digest))
+    }
+
+    async fn open_read(&self, digest: &B3Digest) -> Result<Option<Box<dyn BlobReader>>, Error> {
+        let db = self.db.read().unwrap();
+
+        match db.get(digest).map(|x| Cursor::new(x.clone())) {
+            Some(result) => Ok(Some(Box::new(result))),
+            None => Ok(None),
+        }
+    }
+
+    #[instrument(skip(self))]
+    async fn open_write(&self) -> Box<dyn BlobWriter> {
+        Box::new(MemoryBlobWriter::new(self.db.clone()))
+    }
+}
+
+pub struct MemoryBlobWriter {
+    db: Arc<RwLock<HashMap<B3Digest, Vec<u8>>>>,
+
+    /// Contains the buffer Vec and hasher, or None if already closed
+    writers: Option<(Vec<u8>, blake3::Hasher)>,
+
+    /// The digest that has been returned, if we successfully closed.
+    digest: Option<B3Digest>,
+}
+
+impl MemoryBlobWriter {
+    fn new(db: Arc<RwLock<HashMap<B3Digest, Vec<u8>>>>) -> Self {
+        Self {
+            db,
+            writers: Some((Vec::new(), blake3::Hasher::new())),
+            digest: None,
+        }
+    }
+}
+impl tokio::io::AsyncWrite for MemoryBlobWriter {
+    fn poll_write(
+        mut self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+        b: &[u8],
+    ) -> std::task::Poll<Result<usize, io::Error>> {
+        Poll::Ready(match &mut self.writers {
+            None => Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            )),
+            Some((ref mut buf, ref mut hasher)) => {
+                let bytes_written = buf.write(b)?;
+                hasher.write(&b[..bytes_written])
+            }
+        })
+    }
+
+    fn poll_flush(
+        self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        Poll::Ready(match self.writers {
+            None => Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            )),
+            Some(_) => Ok(()),
+        })
+    }
+
+    fn poll_shutdown(
+        self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        // shutdown is "instantaneous", we only write to memory.
+        Poll::Ready(Ok(()))
+    }
+}
+
+#[async_trait]
+impl BlobWriter for MemoryBlobWriter {
+    async fn close(&mut self) -> Result<B3Digest, Error> {
+        if self.writers.is_none() {
+            match &self.digest {
+                Some(digest) => Ok(digest.clone()),
+                None => Err(crate::Error::StorageError(
+                    "previously closed with error".to_string(),
+                )),
+            }
+        } else {
+            let (buf, hasher) = self.writers.take().unwrap();
+
+            // We know self.hasher is doing blake3 hashing, so this won't fail.
+            let digest: B3Digest = hasher.finalize().as_bytes().into();
+
+            // Only insert if the blob doesn't already exist.
+            let db = self.db.read()?;
+            if !db.contains_key(&digest) {
+                // drop the read lock, so we can open for writing.
+                drop(db);
+
+                // open the database for writing.
+                let mut db = self.db.write()?;
+
+                // and put buf in there. This will move buf out.
+                db.insert(digest.clone(), buf);
+            }
+
+            self.digest = Some(digest.clone());
+
+            Ok(digest)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::BlobService;
+    use super::MemoryBlobService;
+
+    /// This uses a wrong scheme.
+    #[test]
+    fn test_invalid_scheme() {
+        let url = url::Url::parse("http://foo.example/test").expect("must parse");
+
+        assert!(MemoryBlobService::from_url(&url).is_err());
+    }
+
+    /// This correctly sets the scheme, and doesn't set a path.
+    #[test]
+    fn test_valid_scheme() {
+        let url = url::Url::parse("memory://").expect("must parse");
+
+        assert!(MemoryBlobService::from_url(&url).is_ok());
+    }
+
+    /// This sets the host to `foo`
+    #[test]
+    fn test_invalid_host() {
+        let url = url::Url::parse("memory://foo").expect("must parse");
+
+        assert!(MemoryBlobService::from_url(&url).is_err());
+    }
+
+    /// This has the path "/", which is invalid.
+    #[test]
+    fn test_invalid_has_path() {
+        let url = url::Url::parse("memory:///").expect("must parse");
+
+        assert!(MemoryBlobService::from_url(&url).is_err());
+    }
+
+    /// This has the path "/foo", which is invalid.
+    #[test]
+    fn test_invalid_path2() {
+        let url = url::Url::parse("memory:///foo").expect("must parse");
+
+        assert!(MemoryBlobService::from_url(&url).is_err());
+    }
+}
diff --git a/tvix/castore/src/blobservice/mod.rs b/tvix/castore/src/blobservice/mod.rs
new file mode 100644
index 0000000000..5ecf25ac13
--- /dev/null
+++ b/tvix/castore/src/blobservice/mod.rs
@@ -0,0 +1,62 @@
+use std::io;
+use tonic::async_trait;
+
+use crate::{B3Digest, Error};
+
+mod from_addr;
+mod grpc;
+mod memory;
+mod naive_seeker;
+mod sled;
+
+#[cfg(test)]
+mod tests;
+
+pub use self::from_addr::from_addr;
+pub use self::grpc::GRPCBlobService;
+pub use self::memory::MemoryBlobService;
+pub use self::sled::SledBlobService;
+
+/// The base trait all BlobService services need to implement.
+/// It provides functions to check whether a given blob exists,
+/// a way to get a [io::Read] to a blob, and a method to initiate writing a new
+/// Blob, which will return something implmenting io::Write, and providing a
+/// close funtion, to finalize a blob and get its digest.
+#[async_trait]
+pub trait BlobService: Send + Sync {
+    /// Create a new instance by passing in a connection URL.
+    /// TODO: check if we want to make this async, instead of lazily connecting
+    fn from_url(url: &url::Url) -> Result<Self, Error>
+    where
+        Self: Sized;
+
+    /// Check if the service has the blob, by its content hash.
+    async fn has(&self, digest: &B3Digest) -> Result<bool, Error>;
+
+    /// Request a blob from the store, by its content hash.
+    async fn open_read(&self, digest: &B3Digest) -> Result<Option<Box<dyn BlobReader>>, Error>;
+
+    /// Insert a new blob into the store. Returns a [BlobWriter], which
+    /// implements [io::Write] and a [BlobWriter::close].
+    async fn open_write(&self) -> Box<dyn BlobWriter>;
+}
+
+/// A [tokio::io::AsyncWrite] that you need to close() afterwards, and get back
+/// the digest of the written blob.
+#[async_trait]
+pub trait BlobWriter: tokio::io::AsyncWrite + Send + Sync + Unpin + 'static {
+    /// Signal there's no more data to be written, and return the digest of the
+    /// contents written.
+    ///
+    /// Closing a already-closed BlobWriter is a no-op.
+    async fn close(&mut self) -> Result<B3Digest, Error>;
+}
+
+/// A [tokio::io::AsyncRead] that also allows seeking.
+pub trait BlobReader:
+    tokio::io::AsyncRead + tokio::io::AsyncSeek + tokio::io::AsyncBufRead + Send + Unpin + 'static
+{
+}
+
+/// A [`io::Cursor<Vec<u8>>`] can be used as a BlobReader.
+impl BlobReader for io::Cursor<Vec<u8>> {}
diff --git a/tvix/castore/src/blobservice/naive_seeker.rs b/tvix/castore/src/blobservice/naive_seeker.rs
new file mode 100644
index 0000000000..e65a82c7f4
--- /dev/null
+++ b/tvix/castore/src/blobservice/naive_seeker.rs
@@ -0,0 +1,269 @@
+use super::BlobReader;
+use pin_project_lite::pin_project;
+use std::io;
+use std::task::Poll;
+use tokio::io::AsyncRead;
+use tracing::{debug, instrument};
+
+pin_project! {
+    /// This implements [tokio::io::AsyncSeek] for and [tokio::io::AsyncRead] by
+    /// simply skipping over some bytes, keeping track of the position.
+    /// It fails whenever you try to seek backwards.
+    ///
+    /// ## Pinning concerns:
+    ///
+    /// [NaiveSeeker] is itself pinned by callers, and we do not need to concern
+    /// ourselves regarding that.
+    ///
+    /// Though, its fields as per
+    /// <https://doc.rust-lang.org/std/pin/#pinning-is-not-structural-for-field>
+    /// can be pinned or unpinned.
+    ///
+    /// So we need to go over each field and choose our policy carefully.
+    ///
+    /// The obvious cases are the bookkeeping integers we keep in the structure,
+    /// those are private and not shared to anyone, we never build a
+    /// `Pin<&mut X>` out of them at any point, therefore, we can safely never
+    /// mark them as pinned. Of course, it is expected that no developer here
+    /// attempt to `pin!(self.pos)` to pin them because it makes no sense. If
+    /// they have to become pinned, they should be marked `#[pin]` and we need
+    /// to discuss it.
+    ///
+    /// So the bookkeeping integers are in the right state with respect to their
+    /// pinning status. The projection should offer direct access.
+    ///
+    /// On the `r` field, i.e. a `BufReader<R>`, given that
+    /// <https://docs.rs/tokio/latest/tokio/io/struct.BufReader.html#impl-Unpin-for-BufReader%3CR%3E>
+    /// is available, even a `Pin<&mut BufReader<R>>` can be safely moved.
+    ///
+    /// The only care we should have regards the internal reader itself, i.e.
+    /// the `R` instance, see that Tokio decided to `#[pin]` it too:
+    /// <https://docs.rs/tokio/latest/src/tokio/io/util/buf_reader.rs.html#29>
+    ///
+    /// In general, there's no `Unpin` instance for `R: tokio::io::AsyncRead`
+    /// (see <https://docs.rs/tokio/latest/tokio/io/trait.AsyncRead.html>).
+    ///
+    /// Therefore, we could keep it unpinned and pin it in every call site
+    /// whenever we need to call `poll_*` which can be confusing to the non-
+    /// expert developer and we have a fair share amount of situations where the
+    /// [BufReader] instance is naked, i.e. in its `&mut BufReader<R>`
+    /// form, this is annoying because it could lead to expose the naked `R`
+    /// internal instance somehow and would produce a risk of making it move
+    /// unexpectedly.
+    ///
+    /// We choose the path of the least resistance as we have no reason to have
+    /// access to the raw `BufReader<R>` instance, we just `#[pin]` it too and
+    /// enjoy its `poll_*` safe APIs and push the unpinning concerns to the
+    /// internal implementations themselves, which studied the question longer
+    /// than us.
+    pub struct NaiveSeeker<R: tokio::io::AsyncRead> {
+        #[pin]
+        r: tokio::io::BufReader<R>,
+        pos: u64,
+        bytes_to_skip: u64,
+    }
+}
+
+impl<R: tokio::io::AsyncRead> NaiveSeeker<R> {
+    pub fn new(r: R) -> Self {
+        NaiveSeeker {
+            r: tokio::io::BufReader::new(r),
+            pos: 0,
+            bytes_to_skip: 0,
+        }
+    }
+}
+
+impl<R: tokio::io::AsyncRead> tokio::io::AsyncRead for NaiveSeeker<R> {
+    fn poll_read(
+        self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+        buf: &mut tokio::io::ReadBuf<'_>,
+    ) -> Poll<std::io::Result<()>> {
+        // The amount of data read can be determined by the increase
+        // in the length of the slice returned by `ReadBuf::filled`.
+        let filled_before = buf.filled().len();
+        let this = self.project();
+        let pos: &mut u64 = this.pos;
+
+        match this.r.poll_read(cx, buf) {
+            Poll::Ready(a) => {
+                let bytes_read = buf.filled().len() - filled_before;
+                *pos += bytes_read as u64;
+
+                Poll::Ready(a)
+            }
+            Poll::Pending => Poll::Pending,
+        }
+    }
+}
+
+impl<R: tokio::io::AsyncRead> tokio::io::AsyncBufRead for NaiveSeeker<R> {
+    fn poll_fill_buf(
+        self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+    ) -> Poll<io::Result<&[u8]>> {
+        self.project().r.poll_fill_buf(cx)
+    }
+
+    fn consume(self: std::pin::Pin<&mut Self>, amt: usize) {
+        let this = self.project();
+        this.r.consume(amt);
+        let pos: &mut u64 = this.pos;
+        *pos += amt as u64;
+    }
+}
+
+impl<R: tokio::io::AsyncRead> tokio::io::AsyncSeek for NaiveSeeker<R> {
+    #[instrument(skip(self))]
+    fn start_seek(
+        self: std::pin::Pin<&mut Self>,
+        position: std::io::SeekFrom,
+    ) -> std::io::Result<()> {
+        let absolute_offset: u64 = match position {
+            io::SeekFrom::Start(start_offset) => {
+                if start_offset < self.pos {
+                    return Err(io::Error::new(
+                        io::ErrorKind::Unsupported,
+                        format!("can't seek backwards ({} -> {})", self.pos, start_offset),
+                    ));
+                } else {
+                    start_offset
+                }
+            }
+            // we don't know the total size, can't support this.
+            io::SeekFrom::End(_end_offset) => {
+                return Err(io::Error::new(
+                    io::ErrorKind::Unsupported,
+                    "can't seek from end",
+                ));
+            }
+            io::SeekFrom::Current(relative_offset) => {
+                if relative_offset < 0 {
+                    return Err(io::Error::new(
+                        io::ErrorKind::Unsupported,
+                        "can't seek backwards relative to current position",
+                    ));
+                } else {
+                    self.pos + relative_offset as u64
+                }
+            }
+        };
+
+        debug!(absolute_offset=?absolute_offset, "seek");
+
+        // we already know absolute_offset is larger than self.pos
+        debug_assert!(
+            absolute_offset >= self.pos,
+            "absolute_offset {} is larger than self.pos {}",
+            absolute_offset,
+            self.pos
+        );
+
+        // calculate bytes to skip
+        *self.project().bytes_to_skip = absolute_offset - self.pos;
+
+        Ok(())
+    }
+
+    #[instrument(skip(self))]
+    fn poll_complete(
+        mut self: std::pin::Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+    ) -> Poll<std::io::Result<u64>> {
+        if self.bytes_to_skip == 0 {
+            // return the new position (from the start of the stream)
+            return Poll::Ready(Ok(self.pos));
+        }
+
+        // discard some bytes, until pos is where we want it to be.
+        // We create a buffer that we'll discard later on.
+        let mut buf = [0; 1024];
+
+        // Loop until we've reached the desired seek position. This is done by issuing repeated
+        // `poll_read` calls. If the data is not available yet, we will yield back to the executor
+        // and wait to be polled again.
+        loop {
+            // calculate the length we want to skip at most, which is either a max
+            // buffer size, or the number of remaining bytes to read, whatever is
+            // smaller.
+            let bytes_to_skip = std::cmp::min(self.bytes_to_skip as usize, buf.len());
+
+            let mut read_buf = tokio::io::ReadBuf::new(&mut buf[..bytes_to_skip]);
+
+            match self.as_mut().poll_read(cx, &mut read_buf) {
+                Poll::Ready(_a) => {
+                    let bytes_read = read_buf.filled().len() as u64;
+
+                    if bytes_read == 0 {
+                        return Poll::Ready(Err(io::Error::new(
+                            io::ErrorKind::UnexpectedEof,
+                            format!(
+                                "tried to skip {} bytes, but only was able to skip {} until reaching EOF",
+                                bytes_to_skip, bytes_read
+                            ),
+                        )));
+                    }
+
+                    // calculate bytes to skip
+                    let bytes_to_skip = self.bytes_to_skip - bytes_read;
+
+                    *self.as_mut().project().bytes_to_skip = bytes_to_skip;
+
+                    if bytes_to_skip == 0 {
+                        return Poll::Ready(Ok(self.pos));
+                    }
+                }
+                Poll::Pending => return Poll::Pending,
+            };
+        }
+    }
+}
+
+impl<R: tokio::io::AsyncRead + Send + Unpin + 'static> BlobReader for NaiveSeeker<R> {}
+
+#[cfg(test)]
+mod tests {
+    use super::NaiveSeeker;
+    use std::io::{Cursor, SeekFrom};
+    use tokio::io::{AsyncReadExt, AsyncSeekExt};
+
+    /// This seek requires multiple `poll_read` as we use a 1024 bytes internal
+    /// buffer when doing the seek.
+    /// This ensures we don't hang indefinitely.
+    #[tokio::test]
+    async fn seek() {
+        let buf = vec![0u8; 4096];
+        let reader = Cursor::new(&buf);
+        let mut seeker = NaiveSeeker::new(reader);
+        seeker.seek(SeekFrom::Start(4000)).await.unwrap();
+    }
+
+    #[tokio::test]
+    async fn seek_read() {
+        let mut buf = vec![0u8; 2048];
+        buf.extend_from_slice(&[1u8; 2048]);
+        buf.extend_from_slice(&[2u8; 2048]);
+
+        let reader = Cursor::new(&buf);
+        let mut seeker = NaiveSeeker::new(reader);
+
+        let mut read_buf = vec![0u8; 1024];
+        seeker.read_exact(&mut read_buf).await.expect("must read");
+        assert_eq!(read_buf.as_slice(), &[0u8; 1024]);
+
+        seeker
+            .seek(SeekFrom::Current(1024))
+            .await
+            .expect("must seek");
+        seeker.read_exact(&mut read_buf).await.expect("must read");
+        assert_eq!(read_buf.as_slice(), &[1u8; 1024]);
+
+        seeker
+            .seek(SeekFrom::Start(2 * 2048))
+            .await
+            .expect("must seek");
+        seeker.read_exact(&mut read_buf).await.expect("must read");
+        assert_eq!(read_buf.as_slice(), &[2u8; 1024]);
+    }
+}
diff --git a/tvix/castore/src/blobservice/sled.rs b/tvix/castore/src/blobservice/sled.rs
new file mode 100644
index 0000000000..209f0b76fc
--- /dev/null
+++ b/tvix/castore/src/blobservice/sled.rs
@@ -0,0 +1,249 @@
+use super::{BlobReader, BlobService, BlobWriter};
+use crate::{B3Digest, Error};
+use std::{
+    io::{self, Cursor, Write},
+    path::PathBuf,
+    task::Poll,
+};
+use tonic::async_trait;
+use tracing::instrument;
+
+#[derive(Clone)]
+pub struct SledBlobService {
+    db: sled::Db,
+}
+
+impl SledBlobService {
+    pub fn new(p: PathBuf) -> Result<Self, sled::Error> {
+        let config = sled::Config::default().use_compression(true).path(p);
+        let db = config.open()?;
+
+        Ok(Self { db })
+    }
+
+    pub fn new_temporary() -> Result<Self, sled::Error> {
+        let config = sled::Config::default().temporary(true);
+        let db = config.open()?;
+
+        Ok(Self { db })
+    }
+}
+
+#[async_trait]
+impl BlobService for SledBlobService {
+    /// Constructs a [SledBlobService] from the passed [url::Url]:
+    /// - scheme has to be `sled://`
+    /// - there may not be a host.
+    /// - a path to the sled needs to be provided (which may not be `/`).
+    fn from_url(url: &url::Url) -> Result<Self, Error> {
+        if url.scheme() != "sled" {
+            return Err(crate::Error::StorageError("invalid scheme".to_string()));
+        }
+
+        if url.has_host() {
+            return Err(crate::Error::StorageError(format!(
+                "invalid host: {}",
+                url.host().unwrap()
+            )));
+        }
+
+        // TODO: expose compression and other parameters as URL parameters, drop new and new_temporary?
+        if url.path().is_empty() {
+            Self::new_temporary().map_err(|e| Error::StorageError(e.to_string()))
+        } else if url.path() == "/" {
+            Err(crate::Error::StorageError(
+                "cowardly refusing to open / with sled".to_string(),
+            ))
+        } else {
+            Self::new(url.path().into()).map_err(|e| Error::StorageError(e.to_string()))
+        }
+    }
+
+    #[instrument(skip(self), fields(blob.digest=%digest))]
+    async fn has(&self, digest: &B3Digest) -> Result<bool, Error> {
+        match self.db.contains_key(digest.to_vec()) {
+            Ok(has) => Ok(has),
+            Err(e) => Err(Error::StorageError(e.to_string())),
+        }
+    }
+
+    #[instrument(skip(self), fields(blob.digest=%digest))]
+    async fn open_read(&self, digest: &B3Digest) -> Result<Option<Box<dyn BlobReader>>, Error> {
+        match self.db.get(digest.to_vec()) {
+            Ok(None) => Ok(None),
+            Ok(Some(data)) => Ok(Some(Box::new(Cursor::new(data[..].to_vec())))),
+            Err(e) => Err(Error::StorageError(e.to_string())),
+        }
+    }
+
+    #[instrument(skip(self))]
+    async fn open_write(&self) -> Box<dyn BlobWriter> {
+        Box::new(SledBlobWriter::new(self.db.clone()))
+    }
+}
+
+pub struct SledBlobWriter {
+    db: sled::Db,
+
+    /// Contains the buffer Vec and hasher, or None if already closed
+    writers: Option<(Vec<u8>, blake3::Hasher)>,
+
+    /// The digest that has been returned, if we successfully closed.
+    digest: Option<B3Digest>,
+}
+
+impl SledBlobWriter {
+    pub fn new(db: sled::Db) -> Self {
+        Self {
+            db,
+            writers: Some((Vec::new(), blake3::Hasher::new())),
+            digest: None,
+        }
+    }
+}
+
+impl tokio::io::AsyncWrite for SledBlobWriter {
+    fn poll_write(
+        mut self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+        b: &[u8],
+    ) -> std::task::Poll<Result<usize, io::Error>> {
+        Poll::Ready(match &mut self.writers {
+            None => Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            )),
+            Some((ref mut buf, ref mut hasher)) => {
+                let bytes_written = buf.write(b)?;
+                hasher.write(&b[..bytes_written])
+            }
+        })
+    }
+
+    fn poll_flush(
+        mut self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        Poll::Ready(match &mut self.writers {
+            None => Err(io::Error::new(
+                io::ErrorKind::NotConnected,
+                "already closed",
+            )),
+            Some(_) => Ok(()),
+        })
+    }
+
+    fn poll_shutdown(
+        self: std::pin::Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Result<(), io::Error>> {
+        // shutdown is "instantaneous", we only write to a Vec<u8> as buffer.
+        Poll::Ready(Ok(()))
+    }
+}
+
+#[async_trait]
+impl BlobWriter for SledBlobWriter {
+    async fn close(&mut self) -> Result<B3Digest, Error> {
+        if self.writers.is_none() {
+            match &self.digest {
+                Some(digest) => Ok(digest.clone()),
+                None => Err(crate::Error::StorageError(
+                    "previously closed with error".to_string(),
+                )),
+            }
+        } else {
+            let (buf, hasher) = self.writers.take().unwrap();
+
+            let digest: B3Digest = hasher.finalize().as_bytes().into();
+
+            // Only insert if the blob doesn't already exist.
+            if !self.db.contains_key(digest.to_vec()).map_err(|e| {
+                Error::StorageError(format!("Unable to check if we have blob {}: {}", digest, e))
+            })? {
+                // put buf in there. This will move buf out.
+                self.db
+                    .insert(digest.to_vec(), buf)
+                    .map_err(|e| Error::StorageError(format!("unable to insert blob: {}", e)))?;
+            }
+
+            self.digest = Some(digest.clone());
+
+            Ok(digest)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use tempfile::TempDir;
+
+    use super::BlobService;
+    use super::SledBlobService;
+
+    /// This uses a wrong scheme.
+    #[test]
+    fn test_invalid_scheme() {
+        let url = url::Url::parse("http://foo.example/test").expect("must parse");
+
+        assert!(SledBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme, and doesn't specify a path (temporary sled).
+    #[test]
+    fn test_valid_scheme_temporary() {
+        let url = url::Url::parse("sled://").expect("must parse");
+
+        assert!(SledBlobService::from_url(&url).is_ok());
+    }
+
+    /// This sets the path to a location that doesn't exist, which should fail (as sled doesn't mkdir -p)
+    #[test]
+    fn test_nonexistent_path() {
+        let tmpdir = TempDir::new().unwrap();
+
+        let mut url = url::Url::parse("sled://foo.example").expect("must parse");
+        url.set_path(tmpdir.path().join("foo").join("bar").to_str().unwrap());
+
+        assert!(SledBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme, and specifies / as path (which should fail
+    // for obvious reasons)
+    #[test]
+    fn test_invalid_path_root() {
+        let url = url::Url::parse("sled:///").expect("must parse");
+
+        assert!(SledBlobService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme, and sets a tempdir as location.
+    #[test]
+    fn test_valid_scheme_path() {
+        let tmpdir = TempDir::new().unwrap();
+
+        let mut url = url::Url::parse("sled://").expect("must parse");
+        url.set_path(tmpdir.path().to_str().unwrap());
+
+        assert!(SledBlobService::from_url(&url).is_ok());
+    }
+
+    /// This sets a host, rather than a path, which should fail.
+    #[test]
+    fn test_invalid_host() {
+        let url = url::Url::parse("sled://foo.example").expect("must parse");
+
+        assert!(SledBlobService::from_url(&url).is_err());
+    }
+
+    /// This sets a host AND a valid path, which should fail
+    #[test]
+    fn test_invalid_host_and_path() {
+        let tmpdir = TempDir::new().unwrap();
+
+        let mut url = url::Url::parse("sled://foo.example").expect("must parse");
+        url.set_path(tmpdir.path().to_str().unwrap());
+
+        assert!(SledBlobService::from_url(&url).is_err());
+    }
+}
diff --git a/tvix/castore/src/blobservice/tests.rs b/tvix/castore/src/blobservice/tests.rs
new file mode 100644
index 0000000000..fe390b537e
--- /dev/null
+++ b/tvix/castore/src/blobservice/tests.rs
@@ -0,0 +1,246 @@
+use std::io;
+use std::pin::pin;
+
+use test_case::test_case;
+use tokio::io::AsyncReadExt;
+use tokio::io::AsyncSeekExt;
+
+use super::B3Digest;
+use super::BlobService;
+use super::MemoryBlobService;
+use super::SledBlobService;
+use crate::fixtures;
+
+// TODO: avoid having to define all different services we test against for all functions.
+// maybe something like rstest can be used?
+
+fn gen_memory_blob_service() -> impl BlobService {
+    MemoryBlobService::default()
+}
+fn gen_sled_blob_service() -> impl BlobService {
+    SledBlobService::new_temporary().unwrap()
+}
+
+// TODO: add GRPC blob service here.
+
+/// Using [BlobService::has] on a non-existing blob should return false
+#[test_case(gen_memory_blob_service(); "memory")]
+#[test_case(gen_sled_blob_service(); "sled")]
+fn has_nonexistent_false(blob_service: impl BlobService) {
+    tokio::runtime::Runtime::new().unwrap().block_on(async {
+        assert!(!blob_service
+            .has(&fixtures::BLOB_A_DIGEST)
+            .await
+            .expect("must not fail"));
+    })
+}
+
+/// Trying to read a non-existing blob should return a None instead of a reader.
+#[test_case(gen_memory_blob_service(); "memory")]
+#[test_case(gen_sled_blob_service(); "sled")]
+fn not_found_read(blob_service: impl BlobService) {
+    tokio::runtime::Runtime::new().unwrap().block_on(async {
+        assert!(blob_service
+            .open_read(&fixtures::BLOB_A_DIGEST)
+            .await
+            .expect("must not fail")
+            .is_none())
+    })
+}
+
+/// Put a blob in the store, check has, get it back.
+/// We test both with small and big blobs.
+#[test_case(gen_memory_blob_service(), &fixtures::BLOB_A, &fixtures::BLOB_A_DIGEST; "memory-small")]
+#[test_case(gen_sled_blob_service(), &fixtures::BLOB_A, &fixtures::BLOB_A_DIGEST; "sled-small")]
+#[test_case(gen_memory_blob_service(), &fixtures::BLOB_B, &fixtures::BLOB_B_DIGEST; "memory-big")]
+#[test_case(gen_sled_blob_service(), &fixtures::BLOB_B, &fixtures::BLOB_B_DIGEST; "sled-big")]
+fn put_has_get(blob_service: impl BlobService, blob_contents: &[u8], blob_digest: &B3Digest) {
+    tokio::runtime::Runtime::new().unwrap().block_on(async {
+        let mut w = blob_service.open_write().await;
+
+        let l = tokio::io::copy(&mut io::Cursor::new(blob_contents), &mut w)
+            .await
+            .expect("copy must succeed");
+        assert_eq!(
+            blob_contents.len(),
+            l as usize,
+            "written bytes must match blob length"
+        );
+
+        let digest = w.close().await.expect("close must succeed");
+
+        assert_eq!(*blob_digest, digest, "returned digest must be correct");
+
+        assert!(
+            blob_service.has(blob_digest).await.expect("must not fail"),
+            "blob service should now have the blob"
+        );
+
+        let mut r = blob_service
+            .open_read(blob_digest)
+            .await
+            .expect("open_read must succeed")
+            .expect("must be some");
+
+        let mut buf: Vec<u8> = Vec::new();
+        let mut pinned_reader = pin!(r);
+        let l = tokio::io::copy(&mut pinned_reader, &mut buf)
+            .await
+            .expect("copy must succeed");
+        // let l = io::copy(&mut r, &mut buf).expect("copy must succeed");
+
+        assert_eq!(
+            blob_contents.len(),
+            l as usize,
+            "read bytes must match blob length"
+        );
+
+        assert_eq!(blob_contents, buf, "read blob contents must match");
+    })
+}
+
+/// Put a blob in the store, and seek inside it a bit.
+#[test_case(gen_memory_blob_service(); "memory")]
+#[test_case(gen_sled_blob_service(); "sled")]
+fn put_seek(blob_service: impl BlobService) {
+    tokio::runtime::Runtime::new().unwrap().block_on(async {
+        let mut w = blob_service.open_write().await;
+
+        tokio::io::copy(&mut io::Cursor::new(&fixtures::BLOB_B.to_vec()), &mut w)
+            .await
+            .expect("copy must succeed");
+        w.close().await.expect("close must succeed");
+
+        // open a blob for reading
+        let mut r = blob_service
+            .open_read(&fixtures::BLOB_B_DIGEST)
+            .await
+            .expect("open_read must succeed")
+            .expect("must be some");
+
+        let mut pos: u64 = 0;
+
+        // read the first 10 bytes, they must match the data in the fixture.
+        {
+            let mut buf = [0; 10];
+            r.read_exact(&mut buf).await.expect("must succeed");
+
+            assert_eq!(
+                &fixtures::BLOB_B[pos as usize..pos as usize + buf.len()],
+                buf,
+                "expected first 10 bytes to match"
+            );
+
+            pos += buf.len() as u64;
+        }
+        // seek by 0 bytes, using SeekFrom::Start.
+        let p = r
+            .seek(io::SeekFrom::Start(pos))
+            .await
+            .expect("must not fail");
+        assert_eq!(pos, p);
+
+        // read the next 10 bytes, they must match the data in the fixture.
+        {
+            let mut buf = [0; 10];
+            r.read_exact(&mut buf).await.expect("must succeed");
+
+            assert_eq!(
+                &fixtures::BLOB_B[pos as usize..pos as usize + buf.len()],
+                buf,
+                "expected data to match"
+            );
+
+            pos += buf.len() as u64;
+        }
+
+        // seek by 5 bytes, using SeekFrom::Start.
+        let p = r
+            .seek(io::SeekFrom::Start(pos + 5))
+            .await
+            .expect("must not fail");
+        pos += 5;
+        assert_eq!(pos, p);
+
+        // read the next 10 bytes, they must match the data in the fixture.
+        {
+            let mut buf = [0; 10];
+            r.read_exact(&mut buf).await.expect("must succeed");
+
+            assert_eq!(
+                &fixtures::BLOB_B[pos as usize..pos as usize + buf.len()],
+                buf,
+                "expected data to match"
+            );
+
+            pos += buf.len() as u64;
+        }
+
+        // seek by 12345 bytes, using SeekFrom::
+        let p = r
+            .seek(io::SeekFrom::Current(12345))
+            .await
+            .expect("must not fail");
+        pos += 12345;
+        assert_eq!(pos, p);
+
+        // read the next 10 bytes, they must match the data in the fixture.
+        {
+            let mut buf = [0; 10];
+            r.read_exact(&mut buf).await.expect("must succeed");
+
+            assert_eq!(
+                &fixtures::BLOB_B[pos as usize..pos as usize + buf.len()],
+                buf,
+                "expected data to match"
+            );
+
+            #[allow(unused_assignments)]
+            {
+                pos += buf.len() as u64;
+            }
+        }
+
+        // seeking to the end is okay…
+        let p = r
+            .seek(io::SeekFrom::Start(fixtures::BLOB_B.len() as u64))
+            .await
+            .expect("must not fail");
+        pos = fixtures::BLOB_B.len() as u64;
+        assert_eq!(pos, p);
+
+        {
+            // but it returns no more data.
+            let mut buf: Vec<u8> = Vec::new();
+            r.read_to_end(&mut buf).await.expect("must not fail");
+            assert!(buf.is_empty(), "expected no more data to be read");
+        }
+
+        // seeking past the end…
+        match r
+            .seek(io::SeekFrom::Start(fixtures::BLOB_B.len() as u64 + 1))
+            .await
+        {
+            // should either be ok, but then return 0 bytes.
+            // this matches the behaviour or a Cursor<Vec<u8>>.
+            Ok(_pos) => {
+                let mut buf: Vec<u8> = Vec::new();
+                r.read_to_end(&mut buf).await.expect("must not fail");
+                assert!(buf.is_empty(), "expected no more data to be read");
+            }
+            // or not be okay.
+            Err(_) => {}
+        }
+
+        // TODO: this is only broken for the gRPC version
+        // We expect seeking backwards or relative to the end to fail.
+        // r.seek(io::SeekFrom::Current(-1))
+        //     .expect_err("SeekFrom::Current(-1) expected to fail");
+
+        // r.seek(io::SeekFrom::Start(pos - 1))
+        //     .expect_err("SeekFrom::Start(pos-1) expected to fail");
+
+        // r.seek(io::SeekFrom::End(0))
+        //     .expect_err("SeekFrom::End(_) expected to fail");
+    })
+}
diff --git a/tvix/castore/src/digests.rs b/tvix/castore/src/digests.rs
new file mode 100644
index 0000000000..4df11b389e
--- /dev/null
+++ b/tvix/castore/src/digests.rs
@@ -0,0 +1,72 @@
+use bytes::Bytes;
+use data_encoding::BASE64;
+use thiserror::Error;
+
+#[derive(PartialEq, Eq, Hash, Debug)]
+pub struct B3Digest(Bytes);
+
+// TODO: allow converting these errors to crate::Error
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("invalid digest length: {0}")]
+    InvalidDigestLen(usize),
+}
+
+impl B3Digest {
+    // returns a copy of the inner [Vec<u8>].
+    pub fn to_vec(&self) -> Vec<u8> {
+        self.0.to_vec()
+    }
+}
+
+impl From<B3Digest> for bytes::Bytes {
+    fn from(val: B3Digest) -> Self {
+        val.0
+    }
+}
+
+impl TryFrom<Vec<u8>> for B3Digest {
+    type Error = Error;
+
+    // constructs a [B3Digest] from a [Vec<u8>].
+    // Returns an error if the digest has the wrong length.
+    fn try_from(value: Vec<u8>) -> Result<Self, Self::Error> {
+        if value.len() != 32 {
+            Err(Error::InvalidDigestLen(value.len()))
+        } else {
+            Ok(Self(value.into()))
+        }
+    }
+}
+
+impl TryFrom<bytes::Bytes> for B3Digest {
+    type Error = Error;
+
+    // constructs a [B3Digest] from a [bytes::Bytes].
+    // Returns an error if the digest has the wrong length.
+    fn try_from(value: bytes::Bytes) -> Result<Self, Self::Error> {
+        if value.len() != 32 {
+            Err(Error::InvalidDigestLen(value.len()))
+        } else {
+            Ok(Self(value))
+        }
+    }
+}
+
+impl From<&[u8; 32]> for B3Digest {
+    fn from(value: &[u8; 32]) -> Self {
+        Self(value.to_vec().into())
+    }
+}
+
+impl Clone for B3Digest {
+    fn clone(&self) -> Self {
+        Self(self.0.to_owned())
+    }
+}
+
+impl std::fmt::Display for B3Digest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "b3:{}", BASE64.encode(&self.0))
+    }
+}
diff --git a/tvix/castore/src/directoryservice/from_addr.rs b/tvix/castore/src/directoryservice/from_addr.rs
new file mode 100644
index 0000000000..776cf06109
--- /dev/null
+++ b/tvix/castore/src/directoryservice/from_addr.rs
@@ -0,0 +1,36 @@
+use std::sync::Arc;
+use url::Url;
+
+use super::{DirectoryService, GRPCDirectoryService, MemoryDirectoryService, SledDirectoryService};
+
+/// Constructs a new instance of a [DirectoryService] from an URI.
+///
+/// The following URIs are supported:
+/// - `memory:`
+///   Uses a in-memory implementation.
+/// - `sled:`
+///   Uses a in-memory sled implementation.
+/// - `sled:///absolute/path/to/somewhere`
+///   Uses sled, using a path on the disk for persistency. Can be only opened
+///   from one process at the same time.
+/// - `grpc+unix:///absolute/path/to/somewhere`
+///   Connects to a local tvix-store gRPC service via Unix socket.
+/// - `grpc+http://host:port`, `grpc+https://host:port`
+///    Connects to a (remote) tvix-store gRPC service.
+pub fn from_addr(uri: &str) -> Result<Arc<dyn DirectoryService>, crate::Error> {
+    let url = Url::parse(uri)
+        .map_err(|e| crate::Error::StorageError(format!("unable to parse url: {}", e)))?;
+
+    Ok(if url.scheme() == "memory" {
+        Arc::new(MemoryDirectoryService::from_url(&url)?)
+    } else if url.scheme() == "sled" {
+        Arc::new(SledDirectoryService::from_url(&url)?)
+    } else if url.scheme().starts_with("grpc+") {
+        Arc::new(GRPCDirectoryService::from_url(&url)?)
+    } else {
+        Err(crate::Error::StorageError(format!(
+            "unknown scheme: {}",
+            url.scheme()
+        )))?
+    })
+}
diff --git a/tvix/castore/src/directoryservice/grpc.rs b/tvix/castore/src/directoryservice/grpc.rs
new file mode 100644
index 0000000000..0f03053412
--- /dev/null
+++ b/tvix/castore/src/directoryservice/grpc.rs
@@ -0,0 +1,541 @@
+use std::collections::HashSet;
+use std::pin::Pin;
+
+use super::{DirectoryPutter, DirectoryService};
+use crate::proto::{self, get_directory_request::ByWhat};
+use crate::{B3Digest, Error};
+use async_stream::try_stream;
+use futures::Stream;
+use tokio::net::UnixStream;
+use tokio::spawn;
+use tokio::sync::mpsc::UnboundedSender;
+use tokio::task::JoinHandle;
+use tokio_stream::wrappers::UnboundedReceiverStream;
+use tonic::async_trait;
+use tonic::Code;
+use tonic::{transport::Channel, Status};
+use tracing::{instrument, warn};
+
+/// Connects to a (remote) tvix-store DirectoryService over gRPC.
+#[derive(Clone)]
+pub struct GRPCDirectoryService {
+    /// The internal reference to a gRPC client.
+    /// Cloning it is cheap, and it internally handles concurrent requests.
+    grpc_client: proto::directory_service_client::DirectoryServiceClient<Channel>,
+}
+
+impl GRPCDirectoryService {
+    /// construct a [GRPCDirectoryService] from a [proto::directory_service_client::DirectoryServiceClient].
+    /// panics if called outside the context of a tokio runtime.
+    pub fn from_client(
+        grpc_client: proto::directory_service_client::DirectoryServiceClient<Channel>,
+    ) -> Self {
+        Self { grpc_client }
+    }
+}
+
+#[async_trait]
+impl DirectoryService for GRPCDirectoryService {
+    /// Constructs a [GRPCDirectoryService] from the passed [url::Url]:
+    /// - scheme has to match `grpc+*://`.
+    ///   That's normally grpc+unix for unix sockets, and grpc+http(s) for the HTTP counterparts.
+    /// - In the case of unix sockets, there must be a path, but may not be a host.
+    /// - In the case of non-unix sockets, there must be a host, but no path.
+    fn from_url(url: &url::Url) -> Result<Self, crate::Error> {
+        // Start checking for the scheme to start with grpc+.
+        match url.scheme().strip_prefix("grpc+") {
+            None => Err(crate::Error::StorageError("invalid scheme".to_string())),
+            Some(rest) => {
+                if rest == "unix" {
+                    if url.host_str().is_some() {
+                        return Err(crate::Error::StorageError(
+                            "host may not be set".to_string(),
+                        ));
+                    }
+                    let path = url.path().to_string();
+                    let channel = tonic::transport::Endpoint::try_from("http://[::]:50051") // doesn't matter
+                        .unwrap()
+                        .connect_with_connector_lazy(tower::service_fn(
+                            move |_: tonic::transport::Uri| UnixStream::connect(path.clone()),
+                        ));
+                    let grpc_client =
+                        proto::directory_service_client::DirectoryServiceClient::new(channel);
+                    Ok(Self::from_client(grpc_client))
+                } else {
+                    // ensure path is empty, not supported with gRPC.
+                    if !url.path().is_empty() {
+                        return Err(crate::Error::StorageError(
+                            "path may not be set".to_string(),
+                        ));
+                    }
+
+                    // clone the uri, and drop the grpc+ from the scheme.
+                    // Recreate a new uri with the `grpc+` prefix dropped from the scheme.
+                    // We can't use `url.set_scheme(rest)`, as it disallows
+                    // setting something http(s) that previously wasn't.
+                    let url = {
+                        let url_str = url.to_string();
+                        let s_stripped = url_str.strip_prefix("grpc+").unwrap();
+                        url::Url::parse(s_stripped).unwrap()
+                    };
+                    let channel = tonic::transport::Endpoint::try_from(url.to_string())
+                        .unwrap()
+                        .connect_lazy();
+
+                    let grpc_client =
+                        proto::directory_service_client::DirectoryServiceClient::new(channel);
+                    Ok(Self::from_client(grpc_client))
+                }
+            }
+        }
+    }
+
+    async fn get(
+        &self,
+        digest: &B3Digest,
+    ) -> Result<Option<crate::proto::Directory>, crate::Error> {
+        // Get a new handle to the gRPC client, and copy the digest.
+        let mut grpc_client = self.grpc_client.clone();
+        let digest_cpy = digest.clone();
+        let message = async move {
+            let mut s = grpc_client
+                .get(proto::GetDirectoryRequest {
+                    recursive: false,
+                    by_what: Some(ByWhat::Digest(digest_cpy.into())),
+                })
+                .await?
+                .into_inner();
+
+            // Retrieve the first message only, then close the stream (we set recursive to false)
+            s.message().await
+        };
+
+        let digest = digest.clone();
+        match message.await {
+            Ok(Some(directory)) => {
+                // Validate the retrieved Directory indeed has the
+                // digest we expect it to have, to detect corruptions.
+                let actual_digest = directory.digest();
+                if actual_digest != digest {
+                    Err(crate::Error::StorageError(format!(
+                        "requested directory with digest {}, but got {}",
+                        digest, actual_digest
+                    )))
+                } else if let Err(e) = directory.validate() {
+                    // Validate the Directory itself is valid.
+                    warn!("directory failed validation: {}", e.to_string());
+                    Err(crate::Error::StorageError(format!(
+                        "directory {} failed validation: {}",
+                        digest, e,
+                    )))
+                } else {
+                    Ok(Some(directory))
+                }
+            }
+            Ok(None) => Ok(None),
+            Err(e) if e.code() == Code::NotFound => Ok(None),
+            Err(e) => Err(crate::Error::StorageError(e.to_string())),
+        }
+    }
+
+    async fn put(&self, directory: crate::proto::Directory) -> Result<B3Digest, crate::Error> {
+        let mut grpc_client = self.grpc_client.clone();
+
+        let resp = grpc_client.put(tokio_stream::iter(vec![directory])).await;
+
+        match resp {
+            Ok(put_directory_resp) => Ok(put_directory_resp
+                .into_inner()
+                .root_digest
+                .try_into()
+                .map_err(|_| {
+                    Error::StorageError("invalid root digest length in response".to_string())
+                })?),
+            Err(e) => Err(crate::Error::StorageError(e.to_string())),
+        }
+    }
+
+    #[instrument(skip_all, fields(directory.digest = %root_directory_digest))]
+    fn get_recursive(
+        &self,
+        root_directory_digest: &B3Digest,
+    ) -> Pin<Box<dyn Stream<Item = Result<proto::Directory, Error>> + Send>> {
+        let mut grpc_client = self.grpc_client.clone();
+        let root_directory_digest = root_directory_digest.clone();
+
+        let stream = try_stream! {
+            let mut stream = grpc_client
+                .get(proto::GetDirectoryRequest {
+                    recursive: true,
+                    by_what: Some(ByWhat::Digest(root_directory_digest.clone().into())),
+                })
+                .await
+                .map_err(|e| crate::Error::StorageError(e.to_string()))?
+                .into_inner();
+
+            // The Directory digests we received so far
+            let mut received_directory_digests: HashSet<B3Digest> = HashSet::new();
+            // The Directory digests we're still expecting to get sent.
+            let mut expected_directory_digests: HashSet<B3Digest> = HashSet::from([root_directory_digest]);
+
+            loop {
+                match stream.message().await {
+                    Ok(Some(directory)) => {
+                        // validate the directory itself.
+                        if let Err(e) = directory.validate() {
+                            Err(crate::Error::StorageError(format!(
+                                "directory {} failed validation: {}",
+                                directory.digest(),
+                                e,
+                            )))?;
+                        }
+                        // validate we actually expected that directory, and move it from expected to received.
+                        let directory_digest = directory.digest();
+                        let was_expected = expected_directory_digests.remove(&directory_digest);
+                        if !was_expected {
+                            // FUTUREWORK: dumb clients might send the same stuff twice.
+                            // as a fallback, we might want to tolerate receiving
+                            // it if it's in received_directory_digests (as that
+                            // means it once was in expected_directory_digests)
+                            Err(crate::Error::StorageError(format!(
+                                "received unexpected directory {}",
+                                directory_digest
+                            )))?;
+                        }
+                        received_directory_digests.insert(directory_digest);
+
+                        // register all children in expected_directory_digests.
+                        for child_directory in &directory.directories {
+                            // We ran validate() above, so we know these digests must be correct.
+                            let child_directory_digest =
+                                child_directory.digest.clone().try_into().unwrap();
+
+                            expected_directory_digests
+                                .insert(child_directory_digest);
+                        }
+
+                        yield directory;
+                    },
+                    Ok(None) => {
+                        // If we were still expecting something, that's an error.
+                        if !expected_directory_digests.is_empty() {
+                            Err(crate::Error::StorageError(format!(
+                                "still expected {} directories, but got premature end of stream",
+                                expected_directory_digests.len(),
+                            )))?
+                        } else {
+                            return
+                        }
+                    },
+                    Err(e) => {
+                        Err(crate::Error::StorageError(e.to_string()))?;
+                    },
+                }
+            }
+        };
+
+        Box::pin(stream)
+    }
+
+    #[instrument(skip_all)]
+    fn put_multiple_start(&self) -> Box<(dyn DirectoryPutter + 'static)>
+    where
+        Self: Clone,
+    {
+        let mut grpc_client = self.grpc_client.clone();
+
+        let (tx, rx) = tokio::sync::mpsc::unbounded_channel();
+
+        let task: JoinHandle<Result<proto::PutDirectoryResponse, Status>> = spawn(async move {
+            let s = grpc_client
+                .put(UnboundedReceiverStream::new(rx))
+                .await?
+                .into_inner();
+
+            Ok(s)
+        });
+
+        Box::new(GRPCPutter::new(tx, task))
+    }
+}
+
+/// Allows uploading multiple Directory messages in the same gRPC stream.
+pub struct GRPCPutter {
+    /// Data about the current request - a handle to the task, and the tx part
+    /// of the channel.
+    /// The tx part of the pipe is used to send [proto::Directory] to the ongoing request.
+    /// The task will yield a [proto::PutDirectoryResponse] once the stream is closed.
+    #[allow(clippy::type_complexity)] // lol
+    rq: Option<(
+        JoinHandle<Result<proto::PutDirectoryResponse, Status>>,
+        UnboundedSender<proto::Directory>,
+    )>,
+}
+
+impl GRPCPutter {
+    pub fn new(
+        directory_sender: UnboundedSender<proto::Directory>,
+        task: JoinHandle<Result<proto::PutDirectoryResponse, Status>>,
+    ) -> Self {
+        Self {
+            rq: Some((task, directory_sender)),
+        }
+    }
+}
+
+#[async_trait]
+impl DirectoryPutter for GRPCPutter {
+    async fn put(&mut self, directory: proto::Directory) -> Result<(), crate::Error> {
+        match self.rq {
+            // If we're not already closed, send the directory to directory_sender.
+            Some((_, ref directory_sender)) => {
+                if directory_sender.send(directory).is_err() {
+                    // If the channel has been prematurely closed, invoke close (so we can peek at the error code)
+                    // That error code is much more helpful, because it
+                    // contains the error message from the server.
+                    self.close().await?;
+                }
+                Ok(())
+            }
+            // If self.close() was already called, we can't put again.
+            None => Err(Error::StorageError(
+                "DirectoryPutter already closed".to_string(),
+            )),
+        }
+    }
+
+    /// Closes the stream for sending, and returns the value
+    async fn close(&mut self) -> Result<B3Digest, crate::Error> {
+        // get self.rq, and replace it with None.
+        // This ensures we can only close it once.
+        match std::mem::take(&mut self.rq) {
+            None => Err(Error::StorageError("already closed".to_string())),
+            Some((task, directory_sender)) => {
+                // close directory_sender, so blocking on task will finish.
+                drop(directory_sender);
+
+                let root_digest = task
+                    .await?
+                    .map_err(|e| Error::StorageError(e.to_string()))?
+                    .root_digest;
+
+                root_digest.try_into().map_err(|_| {
+                    Error::StorageError("invalid root digest length in response".to_string())
+                })
+            }
+        }
+    }
+
+    // allows checking if the tx part of the channel is closed.
+    fn is_closed(&self) -> bool {
+        match self.rq {
+            None => true,
+            Some((_, ref directory_sender)) => directory_sender.is_closed(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use core::time;
+    use std::thread;
+
+    use futures::StreamExt;
+    use tempfile::TempDir;
+    use tokio::net::{UnixListener, UnixStream};
+    use tokio_stream::wrappers::UnixListenerStream;
+    use tonic::transport::{Endpoint, Server, Uri};
+
+    use crate::{
+        directoryservice::DirectoryService,
+        fixtures::{DIRECTORY_A, DIRECTORY_B},
+        proto,
+        proto::{directory_service_server::DirectoryServiceServer, GRPCDirectoryServiceWrapper},
+        utils::gen_directory_service,
+    };
+
+    #[test]
+    fn test() -> anyhow::Result<()> {
+        let tmpdir = TempDir::new().unwrap();
+        let socket_path = tmpdir.path().join("socket");
+
+        // Spin up a server, in a thread far away, which spawns its own tokio runtime,
+        // and blocks on the task.
+        let socket_path_clone = socket_path.clone();
+        thread::spawn(move || {
+            // Create the runtime
+            let rt = tokio::runtime::Runtime::new().unwrap();
+            // Get a handle from this runtime
+            let handle = rt.handle();
+
+            let task = handle.spawn(async {
+                let uds = UnixListener::bind(socket_path_clone).unwrap();
+                let uds_stream = UnixListenerStream::new(uds);
+
+                // spin up a new DirectoryService
+                let mut server = Server::builder();
+                let router = server.add_service(DirectoryServiceServer::new(
+                    GRPCDirectoryServiceWrapper::from(gen_directory_service()),
+                ));
+                router.serve_with_incoming(uds_stream).await
+            });
+
+            handle.block_on(task)
+        });
+
+        // set up the local client runtime. This is similar to what the [tokio:test] macro desugars to.
+        let tester_runtime = tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap();
+
+        // wait for the socket to be created
+        {
+            let mut socket_created = false;
+            for _try in 1..20 {
+                if socket_path.exists() {
+                    socket_created = true;
+                    break;
+                }
+                std::thread::sleep(time::Duration::from_millis(20))
+            }
+
+            assert!(
+                socket_created,
+                "expected socket path to eventually get created, but never happened"
+            );
+        }
+
+        tester_runtime.block_on(async move {
+            // Create a channel, connecting to the uds at socket_path.
+            // The URI is unused.
+            let channel = Endpoint::try_from("http://[::]:50051")
+                .unwrap()
+                .connect_with_connector_lazy(tower::service_fn(move |_: Uri| {
+                    UnixStream::connect(socket_path.clone())
+                }));
+
+            let grpc_client = proto::directory_service_client::DirectoryServiceClient::new(channel);
+
+            // create the GrpcDirectoryService, using the tester_runtime.
+            let directory_service = super::GRPCDirectoryService::from_client(grpc_client);
+
+            // try to get DIRECTORY_A should return Ok(None)
+            assert_eq!(
+                None,
+                directory_service
+                    .get(&DIRECTORY_A.digest())
+                    .await
+                    .expect("must not fail")
+            );
+
+            // Now upload it
+            assert_eq!(
+                DIRECTORY_A.digest(),
+                directory_service
+                    .put(DIRECTORY_A.clone())
+                    .await
+                    .expect("must succeed")
+            );
+
+            // And retrieve it, compare for equality.
+            assert_eq!(
+                DIRECTORY_A.clone(),
+                directory_service
+                    .get(&DIRECTORY_A.digest())
+                    .await
+                    .expect("must succeed")
+                    .expect("must be some")
+            );
+
+            // Putting DIRECTORY_B alone should fail, because it refers to DIRECTORY_A.
+            directory_service
+                .put(DIRECTORY_B.clone())
+                .await
+                .expect_err("must fail");
+
+            // Putting DIRECTORY_B in a put_multiple will succeed, but the close
+            // will always fail.
+            {
+                let mut handle = directory_service.put_multiple_start();
+                handle.put(DIRECTORY_B.clone()).await.expect("must succeed");
+                handle.close().await.expect_err("must fail");
+            }
+
+            // Uploading A and then B should succeed, and closing should return the digest of B.
+            let mut handle = directory_service.put_multiple_start();
+            handle.put(DIRECTORY_A.clone()).await.expect("must succeed");
+            handle.put(DIRECTORY_B.clone()).await.expect("must succeed");
+            let digest = handle.close().await.expect("must succeed");
+            assert_eq!(DIRECTORY_B.digest(), digest);
+
+            // Now try to retrieve the closure of DIRECTORY_B, which should return B and then A.
+            let mut directories_it = directory_service.get_recursive(&DIRECTORY_B.digest());
+            assert_eq!(
+                DIRECTORY_B.clone(),
+                directories_it
+                    .next()
+                    .await
+                    .expect("must be some")
+                    .expect("must succeed")
+            );
+            assert_eq!(
+                DIRECTORY_A.clone(),
+                directories_it
+                    .next()
+                    .await
+                    .expect("must be some")
+                    .expect("must succeed")
+            );
+
+            // Uploading B and then A should fail, because B refers to A, which
+            // hasn't been uploaded yet.
+            // However, the client can burst, so we might not have received the
+            // error back from the server.
+            {
+                let mut handle = directory_service.put_multiple_start();
+                // sending out B will always be fine
+                handle.put(DIRECTORY_B.clone()).await.expect("must succeed");
+
+                // whether we will be able to put A as well depends on whether we
+                // already received the error about B.
+                if handle.put(DIRECTORY_A.clone()).await.is_ok() {
+                    // If we didn't, and this was Ok(_), …
+                    // a subsequent close MUST fail (because it waits for the
+                    // server)
+                    handle.close().await.expect_err("must fail");
+                }
+            }
+
+            // Now we do the same test as before, send B, then A, but wait
+            // sufficiently enough for the server to have s
+            // to close us the stream,
+            // and then assert that uploading anything else via the handle will fail.
+            {
+                let mut handle = directory_service.put_multiple_start();
+                handle.put(DIRECTORY_B.clone()).await.expect("must succeed");
+
+                let mut is_closed = false;
+                for _try in 1..1000 {
+                    if handle.is_closed() {
+                        is_closed = true;
+                        break;
+                    }
+                    tokio::time::sleep(time::Duration::from_millis(10)).await;
+                }
+
+                assert!(
+                    is_closed,
+                    "expected channel to eventually close, but never happened"
+                );
+
+                handle
+                    .put(DIRECTORY_A.clone())
+                    .await
+                    .expect_err("must fail");
+            }
+        });
+
+        Ok(())
+    }
+}
diff --git a/tvix/castore/src/directoryservice/memory.rs b/tvix/castore/src/directoryservice/memory.rs
new file mode 100644
index 0000000000..ac67c999d0
--- /dev/null
+++ b/tvix/castore/src/directoryservice/memory.rs
@@ -0,0 +1,149 @@
+use crate::{proto, B3Digest, Error};
+use futures::Stream;
+use std::collections::HashMap;
+use std::pin::Pin;
+use std::sync::{Arc, RwLock};
+use tonic::async_trait;
+use tracing::{instrument, warn};
+
+use super::utils::{traverse_directory, SimplePutter};
+use super::{DirectoryPutter, DirectoryService};
+
+#[derive(Clone, Default)]
+pub struct MemoryDirectoryService {
+    db: Arc<RwLock<HashMap<B3Digest, proto::Directory>>>,
+}
+
+#[async_trait]
+impl DirectoryService for MemoryDirectoryService {
+    /// Constructs a [MemoryDirectoryService] from the passed [url::Url]:
+    /// - scheme has to be `memory://`
+    /// - there may not be a host.
+    /// - there may not be a path.
+    fn from_url(url: &url::Url) -> Result<Self, Error> {
+        if url.scheme() != "memory" {
+            return Err(crate::Error::StorageError("invalid scheme".to_string()));
+        }
+
+        if url.has_host() || !url.path().is_empty() {
+            return Err(crate::Error::StorageError("invalid url".to_string()));
+        }
+
+        Ok(Self::default())
+    }
+
+    #[instrument(skip(self, digest), fields(directory.digest = %digest))]
+    async fn get(&self, digest: &B3Digest) -> Result<Option<proto::Directory>, Error> {
+        let db = self.db.read()?;
+
+        match db.get(digest) {
+            // The directory was not found, return
+            None => Ok(None),
+
+            // The directory was found, try to parse the data as Directory message
+            Some(directory) => {
+                // Validate the retrieved Directory indeed has the
+                // digest we expect it to have, to detect corruptions.
+                let actual_digest = directory.digest();
+                if actual_digest != *digest {
+                    return Err(Error::StorageError(format!(
+                        "requested directory with digest {}, but got {}",
+                        digest, actual_digest
+                    )));
+                }
+
+                // Validate the Directory itself is valid.
+                if let Err(e) = directory.validate() {
+                    warn!("directory failed validation: {}", e.to_string());
+                    return Err(Error::StorageError(format!(
+                        "directory {} failed validation: {}",
+                        actual_digest, e,
+                    )));
+                }
+
+                Ok(Some(directory.clone()))
+            }
+        }
+    }
+
+    #[instrument(skip(self, directory), fields(directory.digest = %directory.digest()))]
+    async fn put(&self, directory: proto::Directory) -> Result<B3Digest, Error> {
+        let digest = directory.digest();
+
+        // validate the directory itself.
+        if let Err(e) = directory.validate() {
+            return Err(Error::InvalidRequest(format!(
+                "directory {} failed validation: {}",
+                digest, e,
+            )));
+        }
+
+        // store it
+        let mut db = self.db.write()?;
+        db.insert(digest.clone(), directory);
+
+        Ok(digest)
+    }
+
+    #[instrument(skip_all, fields(directory.digest = %root_directory_digest))]
+    fn get_recursive(
+        &self,
+        root_directory_digest: &B3Digest,
+    ) -> Pin<Box<dyn Stream<Item = Result<proto::Directory, Error>> + Send>> {
+        traverse_directory(self.clone(), root_directory_digest)
+    }
+
+    #[instrument(skip_all)]
+    fn put_multiple_start(&self) -> Box<(dyn DirectoryPutter + 'static)>
+    where
+        Self: Clone,
+    {
+        Box::new(SimplePutter::new(self.clone()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::DirectoryService;
+    use super::MemoryDirectoryService;
+
+    /// This uses a wrong scheme.
+    #[test]
+    fn test_invalid_scheme() {
+        let url = url::Url::parse("http://foo.example/test").expect("must parse");
+
+        assert!(MemoryDirectoryService::from_url(&url).is_err());
+    }
+
+    /// This correctly sets the scheme, and doesn't set a path.
+    #[test]
+    fn test_valid_scheme() {
+        let url = url::Url::parse("memory://").expect("must parse");
+
+        assert!(MemoryDirectoryService::from_url(&url).is_ok());
+    }
+
+    /// This sets the host to `foo`
+    #[test]
+    fn test_invalid_host() {
+        let url = url::Url::parse("memory://foo").expect("must parse");
+
+        assert!(MemoryDirectoryService::from_url(&url).is_err());
+    }
+
+    /// This has the path "/", which is invalid.
+    #[test]
+    fn test_invalid_has_path() {
+        let url = url::Url::parse("memory:///").expect("must parse");
+
+        assert!(MemoryDirectoryService::from_url(&url).is_err());
+    }
+
+    /// This has the path "/foo", which is invalid.
+    #[test]
+    fn test_invalid_path2() {
+        let url = url::Url::parse("memory:///foo").expect("must parse");
+
+        assert!(MemoryDirectoryService::from_url(&url).is_err());
+    }
+}
diff --git a/tvix/castore/src/directoryservice/mod.rs b/tvix/castore/src/directoryservice/mod.rs
new file mode 100644
index 0000000000..3b26f4baf7
--- /dev/null
+++ b/tvix/castore/src/directoryservice/mod.rs
@@ -0,0 +1,76 @@
+use crate::{proto, B3Digest, Error};
+use futures::Stream;
+use std::pin::Pin;
+use tonic::async_trait;
+
+mod from_addr;
+mod grpc;
+mod memory;
+mod sled;
+mod traverse;
+mod utils;
+
+pub use self::from_addr::from_addr;
+pub use self::grpc::GRPCDirectoryService;
+pub use self::memory::MemoryDirectoryService;
+pub use self::sled::SledDirectoryService;
+pub use self::traverse::descend_to;
+
+/// The base trait all Directory services need to implement.
+/// This is a simple get and put of [crate::proto::Directory], returning their
+/// digest.
+#[async_trait]
+pub trait DirectoryService: Send + Sync {
+    /// Create a new instance by passing in a connection URL.
+    /// TODO: check if we want to make this async, instead of lazily connecting
+    fn from_url(url: &url::Url) -> Result<Self, Error>
+    where
+        Self: Sized;
+
+    /// Get looks up a single Directory message by its digest.
+    /// In case the directory is not found, Ok(None) is returned.
+    async fn get(&self, digest: &B3Digest) -> Result<Option<proto::Directory>, Error>;
+    /// Get uploads a single Directory message, and returns the calculated
+    /// digest, or an error.
+    async fn put(&self, directory: proto::Directory) -> Result<B3Digest, Error>;
+
+    /// Looks up a closure of [proto::Directory].
+    /// Ideally this would be a `impl Stream<Item = Result<proto::Directory, Error>>`,
+    /// and we'd be able to add a default implementation for it here, but
+    /// we can't have that yet.
+    ///
+    /// This returns a pinned, boxed stream. The pinning allows for it to be polled easily,
+    /// and the box allows different underlying stream implementations to be returned since
+    /// Rust doesn't support this as a generic in traits yet. This is the same thing that
+    /// [async_trait] generates, but for streams instead of futures.
+    fn get_recursive(
+        &self,
+        root_directory_digest: &B3Digest,
+    ) -> Pin<Box<dyn Stream<Item = Result<proto::Directory, Error>> + Send>>;
+
+    /// Allows persisting a closure of [proto::Directory], which is a graph of
+    /// connected Directory messages.
+    fn put_multiple_start(&self) -> Box<dyn DirectoryPutter>;
+}
+
+/// Provides a handle to put a closure of connected [proto::Directory] elements.
+///
+/// The consumer can periodically call [DirectoryPutter::put], starting from the
+/// leaves. Once the root is reached, [DirectoryPutter::close] can be called to
+/// retrieve the root digest (or an error).
+#[async_trait]
+pub trait DirectoryPutter: Send {
+    /// Put a individual [proto::Directory] into the store.
+    /// Error semantics and behaviour is up to the specific implementation of
+    /// this trait.
+    /// Due to bursting, the returned error might refer to an object previously
+    /// sent via `put`.
+    async fn put(&mut self, directory: proto::Directory) -> Result<(), Error>;
+
+    /// Close the stream, and wait for any errors.
+    async fn close(&mut self) -> Result<B3Digest, Error>;
+
+    /// Return whether the stream is closed or not.
+    /// Used from some [DirectoryService] implementations only.
+    fn is_closed(&self) -> bool;
+}
diff --git a/tvix/castore/src/directoryservice/sled.rs b/tvix/castore/src/directoryservice/sled.rs
new file mode 100644
index 0000000000..0dc5496803
--- /dev/null
+++ b/tvix/castore/src/directoryservice/sled.rs
@@ -0,0 +1,213 @@
+use crate::directoryservice::DirectoryPutter;
+use crate::proto::Directory;
+use crate::{proto, B3Digest, Error};
+use futures::Stream;
+use prost::Message;
+use std::path::PathBuf;
+use std::pin::Pin;
+use tonic::async_trait;
+use tracing::{instrument, warn};
+
+use super::utils::{traverse_directory, SimplePutter};
+use super::DirectoryService;
+
+#[derive(Clone)]
+pub struct SledDirectoryService {
+    db: sled::Db,
+}
+
+impl SledDirectoryService {
+    pub fn new(p: PathBuf) -> Result<Self, sled::Error> {
+        let config = sled::Config::default().use_compression(true).path(p);
+        let db = config.open()?;
+
+        Ok(Self { db })
+    }
+
+    pub fn new_temporary() -> Result<Self, sled::Error> {
+        let config = sled::Config::default().temporary(true);
+        let db = config.open()?;
+
+        Ok(Self { db })
+    }
+}
+
+#[async_trait]
+impl DirectoryService for SledDirectoryService {
+    /// Constructs a [SledDirectoryService] from the passed [url::Url]:
+    /// - scheme has to be `sled://`
+    /// - there may not be a host.
+    /// - a path to the sled needs to be provided (which may not be `/`).
+    fn from_url(url: &url::Url) -> Result<Self, Error> {
+        if url.scheme() != "sled" {
+            return Err(crate::Error::StorageError("invalid scheme".to_string()));
+        }
+
+        if url.has_host() {
+            return Err(crate::Error::StorageError(format!(
+                "invalid host: {}",
+                url.host().unwrap()
+            )));
+        }
+
+        // TODO: expose compression and other parameters as URL parameters, drop new and new_temporary?
+        if url.path().is_empty() {
+            Self::new_temporary().map_err(|e| Error::StorageError(e.to_string()))
+        } else if url.path() == "/" {
+            Err(crate::Error::StorageError(
+                "cowardly refusing to open / with sled".to_string(),
+            ))
+        } else {
+            Self::new(url.path().into()).map_err(|e| Error::StorageError(e.to_string()))
+        }
+    }
+
+    #[instrument(skip(self, digest), fields(directory.digest = %digest))]
+    async fn get(&self, digest: &B3Digest) -> Result<Option<proto::Directory>, Error> {
+        match self.db.get(digest.to_vec()) {
+            // The directory was not found, return
+            Ok(None) => Ok(None),
+
+            // The directory was found, try to parse the data as Directory message
+            Ok(Some(data)) => match Directory::decode(&*data) {
+                Ok(directory) => {
+                    // Validate the retrieved Directory indeed has the
+                    // digest we expect it to have, to detect corruptions.
+                    let actual_digest = directory.digest();
+                    if actual_digest != *digest {
+                        return Err(Error::StorageError(format!(
+                            "requested directory with digest {}, but got {}",
+                            digest, actual_digest
+                        )));
+                    }
+
+                    // Validate the Directory itself is valid.
+                    if let Err(e) = directory.validate() {
+                        warn!("directory failed validation: {}", e.to_string());
+                        return Err(Error::StorageError(format!(
+                            "directory {} failed validation: {}",
+                            actual_digest, e,
+                        )));
+                    }
+
+                    Ok(Some(directory))
+                }
+                Err(e) => {
+                    warn!("unable to parse directory {}: {}", digest, e);
+                    Err(Error::StorageError(e.to_string()))
+                }
+            },
+            // some storage error?
+            Err(e) => Err(Error::StorageError(e.to_string())),
+        }
+    }
+
+    #[instrument(skip(self, directory), fields(directory.digest = %directory.digest()))]
+    async fn put(&self, directory: proto::Directory) -> Result<B3Digest, Error> {
+        let digest = directory.digest();
+
+        // validate the directory itself.
+        if let Err(e) = directory.validate() {
+            return Err(Error::InvalidRequest(format!(
+                "directory {} failed validation: {}",
+                digest, e,
+            )));
+        }
+        // store it
+        let result = self.db.insert(digest.to_vec(), directory.encode_to_vec());
+        if let Err(e) = result {
+            return Err(Error::StorageError(e.to_string()));
+        }
+        Ok(digest)
+    }
+
+    #[instrument(skip_all, fields(directory.digest = %root_directory_digest))]
+    fn get_recursive(
+        &self,
+        root_directory_digest: &B3Digest,
+    ) -> Pin<Box<(dyn Stream<Item = Result<proto::Directory, Error>> + Send + 'static)>> {
+        traverse_directory(self.clone(), root_directory_digest)
+    }
+
+    #[instrument(skip_all)]
+    fn put_multiple_start(&self) -> Box<(dyn DirectoryPutter + 'static)>
+    where
+        Self: Clone,
+    {
+        Box::new(SimplePutter::new(self.clone()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use tempfile::TempDir;
+
+    use super::DirectoryService;
+    use super::SledDirectoryService;
+
+    /// This uses a wrong scheme.
+    #[test]
+    fn test_invalid_scheme() {
+        let url = url::Url::parse("http://foo.example/test").expect("must parse");
+
+        assert!(SledDirectoryService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme, and doesn't specify a path (temporary sled).
+    #[test]
+    fn test_valid_scheme_temporary() {
+        let url = url::Url::parse("sled://").expect("must parse");
+
+        assert!(SledDirectoryService::from_url(&url).is_ok());
+    }
+
+    /// This sets the path to a location that doesn't exist, which should fail (as sled doesn't mkdir -p)
+    #[test]
+    fn test_nonexistent_path() {
+        let tmpdir = TempDir::new().unwrap();
+
+        let mut url = url::Url::parse("sled://foo.example").expect("must parse");
+        url.set_path(tmpdir.path().join("foo").join("bar").to_str().unwrap());
+
+        assert!(SledDirectoryService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme, and specifies / as path (which should fail
+    // for obvious reasons)
+    #[test]
+    fn test_invalid_path_root() {
+        let url = url::Url::parse("sled:///").expect("must parse");
+
+        assert!(SledDirectoryService::from_url(&url).is_err());
+    }
+
+    /// This uses the correct scheme, and sets a tempdir as location.
+    #[test]
+    fn test_valid_scheme_path() {
+        let tmpdir = TempDir::new().unwrap();
+
+        let mut url = url::Url::parse("sled://").expect("must parse");
+        url.set_path(tmpdir.path().to_str().unwrap());
+
+        assert!(SledDirectoryService::from_url(&url).is_ok());
+    }
+
+    /// This sets a host, rather than a path, which should fail.
+    #[test]
+    fn test_invalid_host() {
+        let url = url::Url::parse("sled://foo.example").expect("must parse");
+
+        assert!(SledDirectoryService::from_url(&url).is_err());
+    }
+
+    /// This sets a host AND a valid path, which should fail
+    #[test]
+    fn test_invalid_host_and_path() {
+        let tmpdir = TempDir::new().unwrap();
+
+        let mut url = url::Url::parse("sled://foo.example").expect("must parse");
+        url.set_path(tmpdir.path().to_str().unwrap());
+
+        assert!(SledDirectoryService::from_url(&url).is_err());
+    }
+}
diff --git a/tvix/castore/src/directoryservice/traverse.rs b/tvix/castore/src/directoryservice/traverse.rs
new file mode 100644
index 0000000000..4f011c963c
--- /dev/null
+++ b/tvix/castore/src/directoryservice/traverse.rs
@@ -0,0 +1,228 @@
+use super::DirectoryService;
+use crate::{proto::NamedNode, B3Digest, Error};
+use std::{os::unix::ffi::OsStrExt, sync::Arc};
+use tracing::{instrument, warn};
+
+/// This descends from a (root) node to the given (sub)path, returning the Node
+/// at that path, or none, if there's nothing at that path.
+#[instrument(skip(directory_service))]
+pub async fn descend_to(
+    directory_service: Arc<dyn DirectoryService>,
+    root_node: crate::proto::node::Node,
+    path: &std::path::Path,
+) -> Result<Option<crate::proto::node::Node>, Error> {
+    // strip a possible `/` prefix from the path.
+    let path = {
+        if path.starts_with("/") {
+            path.strip_prefix("/").unwrap()
+        } else {
+            path
+        }
+    };
+
+    let mut cur_node = root_node;
+    let mut it = path.components();
+
+    loop {
+        match it.next() {
+            None => {
+                // the (remaining) path is empty, return the node we're current at.
+                return Ok(Some(cur_node));
+            }
+            Some(first_component) => {
+                match cur_node {
+                    crate::proto::node::Node::File(_) | crate::proto::node::Node::Symlink(_) => {
+                        // There's still some path left, but the current node is no directory.
+                        // This means the path doesn't exist, as we can't reach it.
+                        return Ok(None);
+                    }
+                    crate::proto::node::Node::Directory(directory_node) => {
+                        let digest: B3Digest = directory_node.digest.try_into().map_err(|_e| {
+                            Error::StorageError("invalid digest length".to_string())
+                        })?;
+
+                        // fetch the linked node from the directory_service
+                        match directory_service.get(&digest).await? {
+                            // If we didn't get the directory node that's linked, that's a store inconsistency, bail out!
+                            None => {
+                                warn!("directory {} does not exist", digest);
+
+                                return Err(Error::StorageError(format!(
+                                    "directory {} does not exist",
+                                    digest
+                                )));
+                            }
+                            Some(directory) => {
+                                // look for first_component in the [Directory].
+                                // FUTUREWORK: as the nodes() iterator returns in a sorted fashion, we
+                                // could stop as soon as e.name is larger than the search string.
+                                let child_node = directory.nodes().find(|n| {
+                                    n.get_name() == first_component.as_os_str().as_bytes()
+                                });
+
+                                match child_node {
+                                    // child node not found means there's no such element inside the directory.
+                                    None => {
+                                        return Ok(None);
+                                    }
+                                    // child node found, return to top-of loop to find the next
+                                    // node in the path.
+                                    Some(child_node) => {
+                                        cur_node = child_node;
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::path::PathBuf;
+
+    use crate::fixtures::{DIRECTORY_COMPLICATED, DIRECTORY_WITH_KEEP};
+    use crate::utils::gen_directory_service;
+
+    use super::descend_to;
+
+    #[tokio::test]
+    async fn test_descend_to() {
+        let directory_service = gen_directory_service();
+
+        let mut handle = directory_service.put_multiple_start();
+        handle
+            .put(DIRECTORY_WITH_KEEP.clone())
+            .await
+            .expect("must succeed");
+        handle
+            .put(DIRECTORY_COMPLICATED.clone())
+            .await
+            .expect("must succeed");
+
+        // construct the node for DIRECTORY_COMPLICATED
+        let node_directory_complicated =
+            crate::proto::node::Node::Directory(crate::proto::DirectoryNode {
+                name: "doesntmatter".into(),
+                digest: DIRECTORY_COMPLICATED.digest().into(),
+                size: DIRECTORY_COMPLICATED.size(),
+            });
+
+        // construct the node for DIRECTORY_COMPLICATED
+        let node_directory_with_keep = crate::proto::node::Node::Directory(
+            DIRECTORY_COMPLICATED.directories.first().unwrap().clone(),
+        );
+
+        // construct the node for the .keep file
+        let node_file_keep =
+            crate::proto::node::Node::File(DIRECTORY_WITH_KEEP.files.first().unwrap().clone());
+
+        // traversal to an empty subpath should return the root node.
+        {
+            let resp = descend_to(
+                directory_service.clone(),
+                node_directory_complicated.clone(),
+                &PathBuf::from(""),
+            )
+            .await
+            .expect("must succeed");
+
+            assert_eq!(Some(node_directory_complicated.clone()), resp);
+        }
+
+        // traversal to `keep` should return the node for DIRECTORY_WITH_KEEP
+        {
+            let resp = descend_to(
+                directory_service.clone(),
+                node_directory_complicated.clone(),
+                &PathBuf::from("keep"),
+            )
+            .await
+            .expect("must succeed");
+
+            assert_eq!(Some(node_directory_with_keep), resp);
+        }
+
+        // traversal to `keep/.keep` should return the node for the .keep file
+        {
+            let resp = descend_to(
+                directory_service.clone(),
+                node_directory_complicated.clone(),
+                &PathBuf::from("keep/.keep"),
+            )
+            .await
+            .expect("must succeed");
+
+            assert_eq!(Some(node_file_keep.clone()), resp);
+        }
+
+        // traversal to `keep/.keep` should return the node for the .keep file
+        {
+            let resp = descend_to(
+                directory_service.clone(),
+                node_directory_complicated.clone(),
+                &PathBuf::from("/keep/.keep"),
+            )
+            .await
+            .expect("must succeed");
+
+            assert_eq!(Some(node_file_keep), resp);
+        }
+
+        // traversal to `void` should return None (doesn't exist)
+        {
+            let resp = descend_to(
+                directory_service.clone(),
+                node_directory_complicated.clone(),
+                &PathBuf::from("void"),
+            )
+            .await
+            .expect("must succeed");
+
+            assert_eq!(None, resp);
+        }
+
+        // traversal to `void` should return None (doesn't exist)
+        {
+            let resp = descend_to(
+                directory_service.clone(),
+                node_directory_complicated.clone(),
+                &PathBuf::from("//v/oid"),
+            )
+            .await
+            .expect("must succeed");
+
+            assert_eq!(None, resp);
+        }
+
+        // traversal to `keep/.keep/404` should return None (the path can't be
+        // reached, as keep/.keep already is a file)
+        {
+            let resp = descend_to(
+                directory_service.clone(),
+                node_directory_complicated.clone(),
+                &PathBuf::from("keep/.keep/foo"),
+            )
+            .await
+            .expect("must succeed");
+
+            assert_eq!(None, resp);
+        }
+
+        // traversal to a subpath of '/' should return the root node.
+        {
+            let resp = descend_to(
+                directory_service.clone(),
+                node_directory_complicated.clone(),
+                &PathBuf::from("/"),
+            )
+            .await
+            .expect("must succeed");
+
+            assert_eq!(Some(node_directory_complicated), resp);
+        }
+    }
+}
diff --git a/tvix/castore/src/directoryservice/utils.rs b/tvix/castore/src/directoryservice/utils.rs
new file mode 100644
index 0000000000..4c5e7cfde3
--- /dev/null
+++ b/tvix/castore/src/directoryservice/utils.rs
@@ -0,0 +1,140 @@
+use super::DirectoryPutter;
+use super::DirectoryService;
+use crate::proto;
+use crate::B3Digest;
+use crate::Error;
+use async_stream::stream;
+use futures::Stream;
+use std::collections::{HashSet, VecDeque};
+use std::pin::Pin;
+use tonic::async_trait;
+use tracing::warn;
+
+/// Traverses a [proto::Directory] from the root to the children.
+///
+/// This is mostly BFS, but directories are only returned once.
+pub fn traverse_directory<DS: DirectoryService + 'static>(
+    directory_service: DS,
+    root_directory_digest: &B3Digest,
+) -> Pin<Box<dyn Stream<Item = Result<proto::Directory, Error>> + Send>> {
+    // The list of all directories that still need to be traversed. The next
+    // element is picked from the front, new elements are enqueued at the
+    // back.
+    let mut worklist_directory_digests: VecDeque<B3Digest> =
+        VecDeque::from([root_directory_digest.clone()]);
+    // The list of directory digests already sent to the consumer.
+    // We omit sending the same directories multiple times.
+    let mut sent_directory_digests: HashSet<B3Digest> = HashSet::new();
+
+    let stream = stream! {
+        while let Some(current_directory_digest) = worklist_directory_digests.pop_front() {
+            match directory_service.get(&current_directory_digest).await {
+                // if it's not there, we have an inconsistent store!
+                Ok(None) => {
+                    warn!("directory {} does not exist", current_directory_digest);
+                    yield Err(Error::StorageError(format!(
+                        "directory {} does not exist",
+                        current_directory_digest
+                    )));
+                }
+                Err(e) => {
+                    warn!("failed to look up directory");
+                    yield Err(Error::StorageError(format!(
+                        "unable to look up directory {}: {}",
+                        current_directory_digest, e
+                    )));
+                }
+
+                // if we got it
+                Ok(Some(current_directory)) => {
+                    // validate, we don't want to send invalid directories.
+                    if let Err(e) = current_directory.validate() {
+                        warn!("directory failed validation: {}", e.to_string());
+                        yield Err(Error::StorageError(format!(
+                            "invalid directory: {}",
+                            current_directory_digest
+                        )));
+                    }
+
+                    // We're about to send this directory, so let's avoid sending it again if a
+                    // descendant has it.
+                    sent_directory_digests.insert(current_directory_digest);
+
+                    // enqueue all child directory digests to the work queue, as
+                    // long as they're not part of the worklist or already sent.
+                    // This panics if the digest looks invalid, it's supposed to be checked first.
+                    for child_directory_node in &current_directory.directories {
+                        // TODO: propagate error
+                        let child_digest: B3Digest = child_directory_node.digest.clone().try_into().unwrap();
+
+                        if worklist_directory_digests.contains(&child_digest)
+                            || sent_directory_digests.contains(&child_digest)
+                        {
+                            continue;
+                        }
+                        worklist_directory_digests.push_back(child_digest);
+                    }
+
+                    yield Ok(current_directory);
+                }
+            };
+        }
+    };
+
+    Box::pin(stream)
+}
+
+/// This is a simple implementation of a Directory uploader.
+/// TODO: verify connectivity? Factor out these checks into generic helpers?
+pub struct SimplePutter<DS: DirectoryService> {
+    directory_service: DS,
+    last_directory_digest: Option<B3Digest>,
+    closed: bool,
+}
+
+impl<DS: DirectoryService> SimplePutter<DS> {
+    pub fn new(directory_service: DS) -> Self {
+        Self {
+            directory_service,
+            closed: false,
+            last_directory_digest: None,
+        }
+    }
+}
+
+#[async_trait]
+impl<DS: DirectoryService> DirectoryPutter for SimplePutter<DS> {
+    async fn put(&mut self, directory: proto::Directory) -> Result<(), Error> {
+        if self.closed {
+            return Err(Error::StorageError("already closed".to_string()));
+        }
+
+        let digest = self.directory_service.put(directory).await?;
+
+        // track the last directory digest
+        self.last_directory_digest = Some(digest);
+
+        Ok(())
+    }
+
+    /// We need to be mutable here, as that's the signature of the trait.
+    async fn close(&mut self) -> Result<B3Digest, Error> {
+        if self.closed {
+            return Err(Error::StorageError("already closed".to_string()));
+        }
+
+        match &self.last_directory_digest {
+            Some(last_digest) => {
+                self.closed = true;
+                Ok(last_digest.clone())
+            }
+            None => Err(Error::InvalidRequest(
+                "no directories sent, can't show root digest".to_string(),
+            )),
+        }
+    }
+
+    fn is_closed(&self) -> bool {
+        self.closed
+    }
+}
diff --git a/tvix/castore/src/errors.rs b/tvix/castore/src/errors.rs
new file mode 100644
index 0000000000..3b23f972b0
--- /dev/null
+++ b/tvix/castore/src/errors.rs
@@ -0,0 +1,45 @@
+use std::sync::PoisonError;
+use thiserror::Error;
+use tokio::task::JoinError;
+use tonic::Status;
+
+/// Errors related to communication with the store.
+#[derive(Debug, Error)]
+pub enum Error {
+    #[error("invalid request: {0}")]
+    InvalidRequest(String),
+
+    #[error("internal storage error: {0}")]
+    StorageError(String),
+}
+
+impl<T> From<PoisonError<T>> for Error {
+    fn from(value: PoisonError<T>) -> Self {
+        Error::StorageError(value.to_string())
+    }
+}
+
+impl From<JoinError> for Error {
+    fn from(value: JoinError) -> Self {
+        Error::StorageError(value.to_string())
+    }
+}
+
+impl From<Error> for Status {
+    fn from(value: Error) -> Self {
+        match value {
+            Error::InvalidRequest(msg) => Status::invalid_argument(msg),
+            Error::StorageError(msg) => Status::data_loss(format!("storage error: {}", msg)),
+        }
+    }
+}
+
+// TODO: this should probably go somewhere else?
+impl From<Error> for std::io::Error {
+    fn from(value: Error) -> Self {
+        match value {
+            Error::InvalidRequest(msg) => Self::new(std::io::ErrorKind::InvalidInput, msg),
+            Error::StorageError(msg) => Self::new(std::io::ErrorKind::Other, msg),
+        }
+    }
+}
diff --git a/tvix/castore/src/fixtures.rs b/tvix/castore/src/fixtures.rs
new file mode 100644
index 0000000000..ed3d1ca6e8
--- /dev/null
+++ b/tvix/castore/src/fixtures.rs
@@ -0,0 +1,95 @@
+use crate::{
+    proto::{self, Directory, DirectoryNode, FileNode, SymlinkNode},
+    B3Digest,
+};
+use lazy_static::lazy_static;
+
+pub const HELLOWORLD_BLOB_CONTENTS: &[u8] = b"Hello World!";
+pub const EMPTY_BLOB_CONTENTS: &[u8] = b"";
+
+lazy_static! {
+    pub static ref DUMMY_DIGEST: B3Digest = {
+        let u: &[u8; 32] = &[
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00,
+        ];
+        u.into()
+    };
+    pub static ref DUMMY_DIGEST_2: B3Digest = {
+        let u: &[u8; 32] = &[
+            0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00,
+        ];
+        u.into()
+    };
+    pub static ref DUMMY_DATA_1: bytes::Bytes = vec![0x01, 0x02, 0x03].into();
+    pub static ref DUMMY_DATA_2: bytes::Bytes = vec![0x04, 0x05].into();
+
+    pub static ref HELLOWORLD_BLOB_DIGEST: B3Digest =
+        blake3::hash(HELLOWORLD_BLOB_CONTENTS).as_bytes().into();
+    pub static ref EMPTY_BLOB_DIGEST: B3Digest =
+        blake3::hash(EMPTY_BLOB_CONTENTS).as_bytes().into();
+
+    // 2 bytes
+    pub static ref BLOB_A: bytes::Bytes = vec![0x00, 0x01].into();
+    pub static ref BLOB_A_DIGEST: B3Digest = blake3::hash(&BLOB_A).as_bytes().into();
+
+    // 1MB
+    pub static ref BLOB_B: bytes::Bytes = (0..255).collect::<Vec<u8>>().repeat(4 * 1024).into();
+    pub static ref BLOB_B_DIGEST: B3Digest = blake3::hash(&BLOB_B).as_bytes().into();
+
+    // Directories
+    pub static ref DIRECTORY_WITH_KEEP: proto::Directory = proto::Directory {
+        directories: vec![],
+        files: vec![FileNode {
+            name: b".keep".to_vec().into(),
+            digest: EMPTY_BLOB_DIGEST.clone().into(),
+            size: 0,
+            executable: false,
+        }],
+        symlinks: vec![],
+    };
+    pub static ref DIRECTORY_COMPLICATED: proto::Directory = proto::Directory {
+        directories: vec![DirectoryNode {
+            name: b"keep".to_vec().into(),
+            digest: DIRECTORY_WITH_KEEP.digest().into(),
+            size: DIRECTORY_WITH_KEEP.size(),
+        }],
+        files: vec![FileNode {
+            name: b".keep".to_vec().into(),
+            digest: EMPTY_BLOB_DIGEST.clone().into(),
+            size: 0,
+            executable: false,
+        }],
+        symlinks: vec![SymlinkNode {
+            name: b"aa".to_vec().into(),
+            target: b"/nix/store/somewhereelse".to_vec().into(),
+        }],
+    };
+    pub static ref DIRECTORY_A: Directory = Directory::default();
+    pub static ref DIRECTORY_B: Directory = Directory {
+        directories: vec![DirectoryNode {
+            name: b"a".to_vec().into(),
+            digest: DIRECTORY_A.digest().into(),
+            size: DIRECTORY_A.size(),
+        }],
+        ..Default::default()
+    };
+    pub static ref DIRECTORY_C: Directory = Directory {
+        directories: vec![
+            DirectoryNode {
+                name: b"a".to_vec().into(),
+                digest: DIRECTORY_A.digest().into(),
+                size: DIRECTORY_A.size(),
+            },
+            DirectoryNode {
+                name: b"a'".to_vec().into(),
+                digest: DIRECTORY_A.digest().into(),
+                size: DIRECTORY_A.size(),
+            }
+        ],
+        ..Default::default()
+    };
+}
diff --git a/tvix/castore/src/import.rs b/tvix/castore/src/import.rs
new file mode 100644
index 0000000000..92f792bfe7
--- /dev/null
+++ b/tvix/castore/src/import.rs
@@ -0,0 +1,205 @@
+use crate::blobservice::BlobService;
+use crate::directoryservice::DirectoryPutter;
+use crate::directoryservice::DirectoryService;
+use crate::proto::node::Node;
+use crate::proto::Directory;
+use crate::proto::DirectoryNode;
+use crate::proto::FileNode;
+use crate::proto::SymlinkNode;
+use crate::Error as CastoreError;
+use std::os::unix::ffi::OsStrExt;
+use std::sync::Arc;
+use std::{
+    collections::HashMap,
+    fmt::Debug,
+    os::unix::prelude::PermissionsExt,
+    path::{Path, PathBuf},
+};
+use tracing::instrument;
+use walkdir::WalkDir;
+
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    #[error("failed to upload directory at {0}: {1}")]
+    UploadDirectoryError(PathBuf, CastoreError),
+
+    #[error("invalid encoding encountered for entry {0:?}")]
+    InvalidEncoding(PathBuf),
+
+    #[error("unable to stat {0}: {1}")]
+    UnableToStat(PathBuf, std::io::Error),
+
+    #[error("unable to open {0}: {1}")]
+    UnableToOpen(PathBuf, std::io::Error),
+
+    #[error("unable to read {0}: {1}")]
+    UnableToRead(PathBuf, std::io::Error),
+}
+
+impl From<CastoreError> for Error {
+    fn from(value: CastoreError) -> Self {
+        match value {
+            CastoreError::InvalidRequest(_) => panic!("tvix bug"),
+            CastoreError::StorageError(_) => panic!("error"),
+        }
+    }
+}
+
+// This processes a given [walkdir::DirEntry] and returns a
+// proto::node::Node, depending on the type of the entry.
+//
+// If the entry is a file, its contents are uploaded.
+// If the entry is a directory, the Directory is uploaded as well.
+// For this to work, it relies on the caller to provide the directory object
+// with the previously returned (child) nodes.
+//
+// It assumes entries to be returned in "contents first" order, means this
+// will only be called with a directory if all children of it have been
+// visited. If the entry is indeed a directory, it'll also upload that
+// directory to the store. For this, the so-far-assembled Directory object for
+// this path needs to be passed in.
+//
+// It assumes the caller adds returned nodes to the directories it assembles.
+#[instrument(skip_all, fields(entry.file_type=?&entry.file_type(),entry.path=?entry.path()))]
+async fn process_entry(
+    blob_service: Arc<dyn BlobService>,
+    directory_putter: &mut Box<dyn DirectoryPutter>,
+    entry: &walkdir::DirEntry,
+    maybe_directory: Option<Directory>,
+) -> Result<Node, Error> {
+    let file_type = entry.file_type();
+
+    if file_type.is_dir() {
+        let directory = maybe_directory
+            .expect("tvix bug: must be called with some directory in the case of directory");
+        let directory_digest = directory.digest();
+        let directory_size = directory.size();
+
+        // upload this directory
+        directory_putter
+            .put(directory)
+            .await
+            .map_err(|e| Error::UploadDirectoryError(entry.path().to_path_buf(), e))?;
+
+        return Ok(Node::Directory(DirectoryNode {
+            name: entry.file_name().as_bytes().to_owned().into(),
+            digest: directory_digest.into(),
+            size: directory_size,
+        }));
+    }
+
+    if file_type.is_symlink() {
+        let target: bytes::Bytes = std::fs::read_link(entry.path())
+            .map_err(|e| Error::UnableToStat(entry.path().to_path_buf(), e))?
+            .as_os_str()
+            .as_bytes()
+            .to_owned()
+            .into();
+
+        return Ok(Node::Symlink(SymlinkNode {
+            name: entry.file_name().as_bytes().to_owned().into(),
+            target,
+        }));
+    }
+
+    if file_type.is_file() {
+        let metadata = entry
+            .metadata()
+            .map_err(|e| Error::UnableToStat(entry.path().to_path_buf(), e.into()))?;
+
+        let mut file = tokio::fs::File::open(entry.path())
+            .await
+            .map_err(|e| Error::UnableToOpen(entry.path().to_path_buf(), e))?;
+
+        let mut writer = blob_service.open_write().await;
+
+        if let Err(e) = tokio::io::copy(&mut file, &mut writer).await {
+            return Err(Error::UnableToRead(entry.path().to_path_buf(), e));
+        };
+
+        let digest = writer.close().await?;
+
+        return Ok(Node::File(FileNode {
+            name: entry.file_name().as_bytes().to_vec().into(),
+            digest: digest.into(),
+            size: metadata.len() as u32,
+            // If it's executable by the user, it'll become executable.
+            // This matches nix's dump() function behaviour.
+            executable: metadata.permissions().mode() & 64 != 0,
+        }));
+    }
+    todo!("handle other types")
+}
+
+/// Ingests the contents at the given path into the tvix store,
+/// interacting with a [BlobService] and [DirectoryService].
+/// It returns the root node or an error.
+///
+/// It does not follow symlinks at the root, they will be ingested as actual
+/// symlinks.
+///
+/// It's not interacting with a PathInfoService (from tvix-store), or anything
+/// else giving it a "non-content-addressed name".
+/// It's up to the caller to possibly register it somewhere (and potentially
+/// rename it based on some naming scheme)
+#[instrument(skip(blob_service, directory_service), fields(path=?p))]
+pub async fn ingest_path<P: AsRef<Path> + Debug>(
+    blob_service: Arc<dyn BlobService>,
+    directory_service: Arc<dyn DirectoryService>,
+    p: P,
+) -> Result<Node, Error> {
+    let mut directories: HashMap<PathBuf, Directory> = HashMap::default();
+
+    // TODO: pass this one instead?
+    let mut directory_putter = directory_service.put_multiple_start();
+
+    for entry in WalkDir::new(p)
+        .follow_links(false)
+        .follow_root_links(false)
+        .contents_first(true)
+        .sort_by_file_name()
+    {
+        let entry = entry.unwrap();
+
+        // process_entry wants an Option<Directory> in case the entry points to a directory.
+        // make sure to provide it.
+        let maybe_directory: Option<Directory> = {
+            if entry.file_type().is_dir() {
+                Some(
+                    directories
+                        .entry(entry.path().to_path_buf())
+                        .or_default()
+                        .clone(),
+                )
+            } else {
+                None
+            }
+        };
+
+        let node = process_entry(
+            blob_service.clone(),
+            &mut directory_putter,
+            &entry,
+            maybe_directory,
+        )
+        .await?;
+
+        if entry.depth() == 0 {
+            return Ok(node);
+        } else {
+            // calculate the parent path, and make sure we register the node there.
+            // NOTE: entry.depth() > 0
+            let parent_path = entry.path().parent().unwrap().to_path_buf();
+
+            // record node in parent directory, creating a new [proto:Directory] if not there yet.
+            let parent_directory = directories.entry(parent_path).or_default();
+            match node {
+                Node::Directory(e) => parent_directory.directories.push(e),
+                Node::File(e) => parent_directory.files.push(e),
+                Node::Symlink(e) => parent_directory.symlinks.push(e),
+            }
+        }
+    }
+    // unreachable, we already bailed out before if root doesn't exist.
+    panic!("tvix bug")
+}
diff --git a/tvix/castore/src/lib.rs b/tvix/castore/src/lib.rs
new file mode 100644
index 0000000000..76490fb7e6
--- /dev/null
+++ b/tvix/castore/src/lib.rs
@@ -0,0 +1,15 @@
+mod digests;
+mod errors;
+
+pub mod blobservice;
+pub mod directoryservice;
+pub mod fixtures;
+pub mod import;
+pub mod proto;
+pub mod utils;
+
+pub use digests::B3Digest;
+pub use errors::Error;
+
+#[cfg(test)]
+mod tests;
diff --git a/tvix/castore/src/proto/grpc_blobservice_wrapper.rs b/tvix/castore/src/proto/grpc_blobservice_wrapper.rs
new file mode 100644
index 0000000000..93db1deef6
--- /dev/null
+++ b/tvix/castore/src/proto/grpc_blobservice_wrapper.rs
@@ -0,0 +1,177 @@
+use crate::blobservice::BlobService;
+use core::pin::pin;
+use futures::TryFutureExt;
+use std::{
+    collections::VecDeque,
+    io,
+    ops::{Deref, DerefMut},
+    pin::Pin,
+    sync::Arc,
+};
+use tokio_stream::StreamExt;
+use tokio_util::io::ReaderStream;
+use tonic::{async_trait, Request, Response, Status, Streaming};
+use tracing::{instrument, warn};
+
+pub struct GRPCBlobServiceWrapper {
+    blob_service: Arc<dyn BlobService>,
+}
+
+impl From<Arc<dyn BlobService>> for GRPCBlobServiceWrapper {
+    fn from(value: Arc<dyn BlobService>) -> Self {
+        Self {
+            blob_service: value,
+        }
+    }
+}
+
+// This is necessary because bytes::BytesMut comes up with
+// a default 64 bytes capacity that cannot be changed
+// easily if you assume a bytes::BufMut trait implementation
+// Therefore, we override the Default implementation here
+// TODO(raitobezarius?): upstream me properly
+struct BytesMutWithDefaultCapacity<const N: usize> {
+    inner: bytes::BytesMut,
+}
+
+impl<const N: usize> Deref for BytesMutWithDefaultCapacity<N> {
+    type Target = bytes::BytesMut;
+    fn deref(&self) -> &Self::Target {
+        &self.inner
+    }
+}
+
+impl<const N: usize> DerefMut for BytesMutWithDefaultCapacity<N> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.inner
+    }
+}
+
+impl<const N: usize> Default for BytesMutWithDefaultCapacity<N> {
+    fn default() -> Self {
+        BytesMutWithDefaultCapacity {
+            inner: bytes::BytesMut::with_capacity(N),
+        }
+    }
+}
+
+impl<const N: usize> bytes::Buf for BytesMutWithDefaultCapacity<N> {
+    fn remaining(&self) -> usize {
+        self.inner.remaining()
+    }
+
+    fn chunk(&self) -> &[u8] {
+        self.inner.chunk()
+    }
+
+    fn advance(&mut self, cnt: usize) {
+        self.inner.advance(cnt);
+    }
+}
+
+unsafe impl<const N: usize> bytes::BufMut for BytesMutWithDefaultCapacity<N> {
+    fn remaining_mut(&self) -> usize {
+        self.inner.remaining_mut()
+    }
+
+    unsafe fn advance_mut(&mut self, cnt: usize) {
+        self.inner.advance_mut(cnt);
+    }
+
+    fn chunk_mut(&mut self) -> &mut bytes::buf::UninitSlice {
+        self.inner.chunk_mut()
+    }
+}
+
+#[async_trait]
+impl super::blob_service_server::BlobService for GRPCBlobServiceWrapper {
+    // https://github.com/tokio-rs/tokio/issues/2723#issuecomment-1534723933
+    type ReadStream =
+        Pin<Box<dyn futures::Stream<Item = Result<super::BlobChunk, Status>> + Send + 'static>>;
+
+    #[instrument(skip(self))]
+    async fn stat(
+        &self,
+        request: Request<super::StatBlobRequest>,
+    ) -> Result<Response<super::BlobMeta>, Status> {
+        let rq = request.into_inner();
+        let req_digest = rq
+            .digest
+            .try_into()
+            .map_err(|_e| Status::invalid_argument("invalid digest length"))?;
+
+        match self.blob_service.has(&req_digest).await {
+            Ok(true) => Ok(Response::new(super::BlobMeta::default())),
+            Ok(false) => Err(Status::not_found(format!("blob {} not found", &req_digest))),
+            Err(e) => Err(e.into()),
+        }
+    }
+
+    #[instrument(skip(self))]
+    async fn read(
+        &self,
+        request: Request<super::ReadBlobRequest>,
+    ) -> Result<Response<Self::ReadStream>, Status> {
+        let rq = request.into_inner();
+
+        let req_digest = rq
+            .digest
+            .try_into()
+            .map_err(|_e| Status::invalid_argument("invalid digest length"))?;
+
+        match self.blob_service.open_read(&req_digest).await {
+            Ok(Some(reader)) => {
+                fn stream_mapper(
+                    x: Result<bytes::Bytes, io::Error>,
+                ) -> Result<super::BlobChunk, Status> {
+                    match x {
+                        Ok(bytes) => Ok(super::BlobChunk { data: bytes }),
+                        Err(e) => Err(Status::from(e)),
+                    }
+                }
+
+                let chunks_stream = ReaderStream::new(reader).map(stream_mapper);
+                Ok(Response::new(Box::pin(chunks_stream)))
+            }
+            Ok(None) => Err(Status::not_found(format!("blob {} not found", &req_digest))),
+            Err(e) => Err(e.into()),
+        }
+    }
+
+    #[instrument(skip(self))]
+    async fn put(
+        &self,
+        request: Request<Streaming<super::BlobChunk>>,
+    ) -> Result<Response<super::PutBlobResponse>, Status> {
+        let req_inner = request.into_inner();
+
+        let data_stream = req_inner.map(|x| {
+            x.map(|x| VecDeque::from(x.data.to_vec()))
+                .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidInput, e))
+        });
+
+        let mut data_reader = tokio_util::io::StreamReader::new(data_stream);
+
+        let mut blob_writer = pin!(self.blob_service.open_write().await);
+
+        tokio::io::copy(&mut data_reader, &mut blob_writer)
+            .await
+            .map_err(|e| {
+                warn!("error copying: {}", e);
+                Status::internal("error copying")
+            })?;
+
+        let digest = blob_writer
+            .close()
+            .map_err(|e| {
+                warn!("error closing stream: {}", e);
+                Status::internal("error closing stream")
+            })
+            .await?
+            .to_vec();
+
+        Ok(Response::new(super::PutBlobResponse {
+            digest: digest.into(),
+        }))
+    }
+}
diff --git a/tvix/castore/src/proto/grpc_directoryservice_wrapper.rs b/tvix/castore/src/proto/grpc_directoryservice_wrapper.rs
new file mode 100644
index 0000000000..5e143a7bd7
--- /dev/null
+++ b/tvix/castore/src/proto/grpc_directoryservice_wrapper.rs
@@ -0,0 +1,184 @@
+use crate::proto;
+use crate::{directoryservice::DirectoryService, B3Digest};
+use futures::StreamExt;
+use std::collections::HashMap;
+use std::sync::Arc;
+use tokio::{sync::mpsc::channel, task};
+use tokio_stream::wrappers::ReceiverStream;
+use tonic::{async_trait, Request, Response, Status, Streaming};
+use tracing::{debug, instrument, warn};
+
+pub struct GRPCDirectoryServiceWrapper {
+    directory_service: Arc<dyn DirectoryService>,
+}
+
+impl From<Arc<dyn DirectoryService>> for GRPCDirectoryServiceWrapper {
+    fn from(value: Arc<dyn DirectoryService>) -> Self {
+        Self {
+            directory_service: value,
+        }
+    }
+}
+
+#[async_trait]
+impl proto::directory_service_server::DirectoryService for GRPCDirectoryServiceWrapper {
+    type GetStream = ReceiverStream<tonic::Result<proto::Directory, Status>>;
+
+    #[instrument(skip(self))]
+    async fn get(
+        &self,
+        request: Request<proto::GetDirectoryRequest>,
+    ) -> Result<Response<Self::GetStream>, Status> {
+        let (tx, rx) = channel(5);
+
+        let req_inner = request.into_inner();
+
+        let directory_service = self.directory_service.clone();
+
+        let _task = {
+            // look at the digest in the request and put it in the top of the queue.
+            match &req_inner.by_what {
+                None => return Err(Status::invalid_argument("by_what needs to be specified")),
+                Some(proto::get_directory_request::ByWhat::Digest(ref digest)) => {
+                    let digest: B3Digest = digest
+                        .clone()
+                        .try_into()
+                        .map_err(|_e| Status::invalid_argument("invalid digest length"))?;
+
+                    task::spawn(async move {
+                        if !req_inner.recursive {
+                            let e: Result<proto::Directory, Status> =
+                                match directory_service.get(&digest).await {
+                                    Ok(Some(directory)) => Ok(directory),
+                                    Ok(None) => Err(Status::not_found(format!(
+                                        "directory {} not found",
+                                        digest
+                                    ))),
+                                    Err(e) => Err(e.into()),
+                                };
+
+                            if tx.send(e).await.is_err() {
+                                debug!("receiver dropped");
+                            }
+                        } else {
+                            // If recursive was requested, traverse via get_recursive.
+                            let mut directories_it = directory_service.get_recursive(&digest);
+
+                            while let Some(e) = directories_it.next().await {
+                                // map err in res from Error to Status
+                                let res = e.map_err(|e| Status::internal(e.to_string()));
+                                if tx.send(res).await.is_err() {
+                                    debug!("receiver dropped");
+                                    break;
+                                }
+                            }
+                        }
+                    });
+                }
+            }
+        };
+
+        let receiver_stream = ReceiverStream::new(rx);
+        Ok(Response::new(receiver_stream))
+    }
+
+    #[instrument(skip(self, request))]
+    async fn put(
+        &self,
+        request: Request<Streaming<proto::Directory>>,
+    ) -> Result<Response<proto::PutDirectoryResponse>, Status> {
+        let mut req_inner = request.into_inner();
+        // TODO: let this use DirectoryPutter to the store it's connected to,
+        // and move the validation logic into [SimplePutter].
+
+        // This keeps track of the seen directory keys, and their size.
+        // This is used to validate the size field of a reference to a previously sent directory.
+        // We don't need to keep the contents around, they're stored in the DB.
+        // https://github.com/rust-lang/rust-clippy/issues/5812
+        #[allow(clippy::mutable_key_type)]
+        let mut seen_directories_sizes: HashMap<B3Digest, u32> = HashMap::new();
+        let mut last_directory_dgst: Option<B3Digest> = None;
+
+        // Consume directories, and insert them into the store.
+        // Reject directory messages that refer to Directories not sent in the same stream.
+        while let Some(directory) = req_inner.message().await? {
+            // validate the directory itself.
+            if let Err(e) = directory.validate() {
+                return Err(Status::invalid_argument(format!(
+                    "directory {} failed validation: {}",
+                    directory.digest(),
+                    e,
+                )));
+            }
+
+            // for each child directory this directory refers to, we need
+            // to ensure it has been seen already in this stream, and that the size
+            // matches what we recorded.
+            for child_directory in &directory.directories {
+                let child_directory_digest: B3Digest = child_directory
+                    .digest
+                    .clone()
+                    .try_into()
+                    .map_err(|_e| Status::internal("invalid child directory digest len"))?;
+
+                match seen_directories_sizes.get(&child_directory_digest) {
+                    None => {
+                        return Err(Status::invalid_argument(format!(
+                            "child directory '{:?}' ({}) in directory '{}' not seen yet",
+                            child_directory.name,
+                            &child_directory_digest,
+                            &directory.digest(),
+                        )));
+                    }
+                    Some(seen_child_directory_size) => {
+                        if seen_child_directory_size != &child_directory.size {
+                            return Err(Status::invalid_argument(format!(
+                                    "child directory '{:?}' ({}) in directory '{}' referred with wrong size, expected {}, actual {}",
+                                    child_directory.name,
+                                    &child_directory_digest,
+                                    &directory.digest(),
+                                    seen_child_directory_size,
+                                    child_directory.size,
+                            )));
+                        }
+                    }
+                }
+            }
+
+            // NOTE: We can't know if a directory we're receiving actually is
+            // part of the closure, because we receive directories from the leaf nodes up to
+            // the root.
+            // The only thing we could to would be doing a final check when the
+            // last Directory was received, that all Directories received so far are
+            // reachable from that (root) node.
+
+            let dgst = directory.digest();
+            seen_directories_sizes.insert(dgst.clone(), directory.size());
+            last_directory_dgst = Some(dgst.clone());
+
+            // check if the directory already exists in the database. We can skip
+            // inserting if it's already there, as that'd be a no-op.
+            match self.directory_service.get(&dgst).await {
+                Err(e) => {
+                    warn!("error checking if directory already exists: {}", e);
+                    return Err(e.into());
+                }
+                // skip if already exists
+                Ok(Some(_)) => {}
+                // insert if it doesn't already exist
+                Ok(None) => {
+                    self.directory_service.put(directory).await?;
+                }
+            }
+        }
+
+        // We're done receiving. peek at last_directory_digest and either return the digest,
+        // or an error, if we received an empty stream.
+        match last_directory_dgst {
+            None => Err(Status::invalid_argument("no directories received")),
+            Some(last_directory_dgst) => Ok(Response::new(proto::PutDirectoryResponse {
+                root_digest: last_directory_dgst.into(),
+            })),
+        }
+    }
+}
diff --git a/tvix/castore/src/proto/mod.rs b/tvix/castore/src/proto/mod.rs
new file mode 100644
index 0000000000..2a44383fdd
--- /dev/null
+++ b/tvix/castore/src/proto/mod.rs
@@ -0,0 +1,279 @@
+#![allow(clippy::derive_partial_eq_without_eq, non_snake_case)]
+// https://github.com/hyperium/tonic/issues/1056
+use data_encoding::BASE64;
+use std::{collections::HashSet, iter::Peekable};
+use thiserror::Error;
+
+use prost::Message;
+
+mod grpc_blobservice_wrapper;
+mod grpc_directoryservice_wrapper;
+
+pub use grpc_blobservice_wrapper::GRPCBlobServiceWrapper;
+pub use grpc_directoryservice_wrapper::GRPCDirectoryServiceWrapper;
+
+use crate::B3Digest;
+
+tonic::include_proto!("tvix.castore.v1");
+
+#[cfg(feature = "reflection")]
+/// Compiled file descriptors for implementing [gRPC
+/// reflection](https://github.com/grpc/grpc/blob/master/doc/server-reflection.md) with e.g.
+/// [`tonic_reflection`](https://docs.rs/tonic-reflection).
+pub const FILE_DESCRIPTOR_SET: &[u8] = tonic::include_file_descriptor_set!("tvix.castore.v1");
+
+#[cfg(test)]
+mod tests;
+
+/// Errors that can occur during the validation of Directory messages.
+#[derive(Debug, PartialEq, Eq, Error)]
+pub enum ValidateDirectoryError {
+    /// Elements are not in sorted order
+    #[error("{} is not sorted", std::str::from_utf8(.0).unwrap_or(&BASE64.encode(.0)))]
+    WrongSorting(Vec<u8>),
+    /// Multiple elements with the same name encountered
+    #[error("{0:?} is a duplicate name")]
+    DuplicateName(Vec<u8>),
+    /// Invalid name encountered
+    #[error("Invalid name in {0:?}")]
+    InvalidName(Vec<u8>),
+    /// Invalid digest length encountered
+    #[error("Invalid Digest length: {0}")]
+    InvalidDigestLen(usize),
+}
+
+/// Checks a Node name for validity as an intermediate node, and returns an
+/// error that's generated from the supplied constructor.
+///
+/// We disallow slashes, null bytes, '.', '..' and the empty string.
+fn validate_node_name<E>(name: &[u8], err: fn(Vec<u8>) -> E) -> Result<(), E> {
+    if name.is_empty()
+        || name == b".."
+        || name == b"."
+        || name.contains(&0x00)
+        || name.contains(&b'/')
+    {
+        return Err(err(name.to_vec()));
+    }
+    Ok(())
+}
+
+/// NamedNode is implemented for [FileNode], [DirectoryNode] and [SymlinkNode]
+/// and [node::Node], so we can ask all of them for the name easily.
+pub trait NamedNode {
+    fn get_name(&self) -> &[u8];
+}
+
+impl NamedNode for &FileNode {
+    fn get_name(&self) -> &[u8] {
+        &self.name
+    }
+}
+
+impl NamedNode for &DirectoryNode {
+    fn get_name(&self) -> &[u8] {
+        &self.name
+    }
+}
+
+impl NamedNode for &SymlinkNode {
+    fn get_name(&self) -> &[u8] {
+        &self.name
+    }
+}
+
+impl NamedNode for node::Node {
+    fn get_name(&self) -> &[u8] {
+        match self {
+            node::Node::File(node_file) => &node_file.name,
+            node::Node::Directory(node_directory) => &node_directory.name,
+            node::Node::Symlink(node_symlink) => &node_symlink.name,
+        }
+    }
+}
+
+impl node::Node {
+    /// Returns the node with a new name.
+    pub fn rename(self, name: bytes::Bytes) -> Self {
+        match self {
+            node::Node::Directory(n) => node::Node::Directory(DirectoryNode { name, ..n }),
+            node::Node::File(n) => node::Node::File(FileNode { name, ..n }),
+            node::Node::Symlink(n) => node::Node::Symlink(SymlinkNode { name, ..n }),
+        }
+    }
+}
+
+/// Accepts a name, and a mutable reference to the previous name.
+/// If the passed name is larger than the previous one, the reference is updated.
+/// If it's not, an error is returned.
+fn update_if_lt_prev<'n>(
+    prev_name: &mut &'n [u8],
+    name: &'n [u8],
+) -> Result<(), ValidateDirectoryError> {
+    if *name < **prev_name {
+        return Err(ValidateDirectoryError::WrongSorting(name.to_vec()));
+    }
+    *prev_name = name;
+    Ok(())
+}
+
+/// Inserts the given name into a HashSet if it's not already in there.
+/// If it is, an error is returned.
+fn insert_once<'n>(
+    seen_names: &mut HashSet<&'n [u8]>,
+    name: &'n [u8],
+) -> Result<(), ValidateDirectoryError> {
+    if seen_names.get(name).is_some() {
+        return Err(ValidateDirectoryError::DuplicateName(name.to_vec()));
+    }
+    seen_names.insert(name);
+    Ok(())
+}
+
+impl Directory {
+    /// The size of a directory is the number of all regular and symlink elements,
+    /// the number of directory elements, and their size fields.
+    pub fn size(&self) -> u32 {
+        self.files.len() as u32
+            + self.symlinks.len() as u32
+            + self
+                .directories
+                .iter()
+                .fold(0, |acc: u32, e| (acc + 1 + e.size))
+    }
+
+    /// Calculates the digest of a Directory, which is the blake3 hash of a
+    /// Directory protobuf message, serialized in protobuf canonical form.
+    pub fn digest(&self) -> B3Digest {
+        let mut hasher = blake3::Hasher::new();
+
+        hasher
+            .update(&self.encode_to_vec())
+            .finalize()
+            .as_bytes()
+            .into()
+    }
+
+    /// validate checks the directory for invalid data, such as:
+    /// - violations of name restrictions
+    /// - invalid digest lengths
+    /// - not properly sorted lists
+    /// - duplicate names in the three lists
+    pub fn validate(&self) -> Result<(), ValidateDirectoryError> {
+        let mut seen_names: HashSet<&[u8]> = HashSet::new();
+
+        let mut last_directory_name: &[u8] = b"";
+        let mut last_file_name: &[u8] = b"";
+        let mut last_symlink_name: &[u8] = b"";
+
+        // check directories
+        for directory_node in &self.directories {
+            validate_node_name(&directory_node.name, ValidateDirectoryError::InvalidName)?;
+            // ensure the digest has the appropriate size.
+            if TryInto::<B3Digest>::try_into(directory_node.digest.clone()).is_err() {
+                return Err(ValidateDirectoryError::InvalidDigestLen(
+                    directory_node.digest.len(),
+                ));
+            }
+
+            update_if_lt_prev(&mut last_directory_name, &directory_node.name)?;
+            insert_once(&mut seen_names, &directory_node.name)?;
+        }
+
+        // check files
+        for file_node in &self.files {
+            validate_node_name(&file_node.name, ValidateDirectoryError::InvalidName)?;
+            if TryInto::<B3Digest>::try_into(file_node.digest.clone()).is_err() {
+                return Err(ValidateDirectoryError::InvalidDigestLen(
+                    file_node.digest.len(),
+                ));
+            }
+
+            update_if_lt_prev(&mut last_file_name, &file_node.name)?;
+            insert_once(&mut seen_names, &file_node.name)?;
+        }
+
+        // check symlinks
+        for symlink_node in &self.symlinks {
+            validate_node_name(&symlink_node.name, ValidateDirectoryError::InvalidName)?;
+
+            update_if_lt_prev(&mut last_symlink_name, &symlink_node.name)?;
+            insert_once(&mut seen_names, &symlink_node.name)?;
+        }
+
+        Ok(())
+    }
+
+    /// Allows iterating over all three nodes ([DirectoryNode], [FileNode],
+    /// [SymlinkNode]) in an ordered fashion, as long as the individual lists
+    /// are sorted (which can be checked by the [Directory::validate]).
+    pub fn nodes(&self) -> DirectoryNodesIterator {
+        return DirectoryNodesIterator {
+            i_directories: self.directories.iter().peekable(),
+            i_files: self.files.iter().peekable(),
+            i_symlinks: self.symlinks.iter().peekable(),
+        };
+    }
+}
+
+/// Struct to hold the state of an iterator over all nodes of a Directory.
+///
+/// Internally, this keeps peekable Iterators over all three lists of a
+/// Directory message.
+pub struct DirectoryNodesIterator<'a> {
+    // directory: &Directory,
+    i_directories: Peekable<std::slice::Iter<'a, DirectoryNode>>,
+    i_files: Peekable<std::slice::Iter<'a, FileNode>>,
+    i_symlinks: Peekable<std::slice::Iter<'a, SymlinkNode>>,
+}
+
+/// looks at two elements implementing NamedNode, and returns true if "left
+/// is smaller / comes first".
+///
+/// Some(_) is preferred over None.
+fn left_name_lt_right<A: NamedNode, B: NamedNode>(left: Option<&A>, right: Option<&B>) -> bool {
+    match left {
+        // if left is None, right always wins
+        None => false,
+        Some(left_inner) => {
+            // left is Some.
+            match right {
+                // left is Some, right is None - left wins.
+                None => true,
+                Some(right_inner) => {
+                    // both are Some - compare the name.
+                    return left_inner.get_name() < right_inner.get_name();
+                }
+            }
+        }
+    }
+}
+
+impl Iterator for DirectoryNodesIterator<'_> {
+    type Item = node::Node;
+
+    // next returns the next node in the Directory.
+    // we peek at all three internal iterators, and pick the one with the
+    // smallest name, to ensure lexicographical ordering.
+    // The individual lists are already known to be sorted.
+    fn next(&mut self) -> Option<Self::Item> {
+        if left_name_lt_right(self.i_directories.peek(), self.i_files.peek()) {
+            // i_directories is still in the game, compare with symlinks
+            if left_name_lt_right(self.i_directories.peek(), self.i_symlinks.peek()) {
+                self.i_directories
+                    .next()
+                    .cloned()
+                    .map(node::Node::Directory)
+            } else {
+                self.i_symlinks.next().cloned().map(node::Node::Symlink)
+            }
+        } else {
+            // i_files is still in the game, compare with symlinks
+            if left_name_lt_right(self.i_files.peek(), self.i_symlinks.peek()) {
+                self.i_files.next().cloned().map(node::Node::File)
+            } else {
+                self.i_symlinks.next().cloned().map(node::Node::Symlink)
+            }
+        }
+    }
+}
diff --git a/tvix/castore/src/proto/tests/directory.rs b/tvix/castore/src/proto/tests/directory.rs
new file mode 100644
index 0000000000..eed49b2b59
--- /dev/null
+++ b/tvix/castore/src/proto/tests/directory.rs
@@ -0,0 +1,287 @@
+use crate::proto::{Directory, DirectoryNode, FileNode, SymlinkNode, ValidateDirectoryError};
+use lazy_static::lazy_static;
+
+lazy_static! {
+    static ref DUMMY_DIGEST: [u8; 32] = [
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00,
+    ];
+}
+#[test]
+fn size() {
+    {
+        let d = Directory::default();
+        assert_eq!(d.size(), 0);
+    }
+    {
+        let d = Directory {
+            directories: vec![DirectoryNode {
+                name: "foo".into(),
+                digest: DUMMY_DIGEST.to_vec().into(),
+                size: 0,
+            }],
+            ..Default::default()
+        };
+        assert_eq!(d.size(), 1);
+    }
+    {
+        let d = Directory {
+            directories: vec![DirectoryNode {
+                name: "foo".into(),
+                digest: DUMMY_DIGEST.to_vec().into(),
+                size: 4,
+            }],
+            ..Default::default()
+        };
+        assert_eq!(d.size(), 5);
+    }
+    {
+        let d = Directory {
+            files: vec![FileNode {
+                name: "foo".into(),
+                digest: DUMMY_DIGEST.to_vec().into(),
+                size: 42,
+                executable: false,
+            }],
+            ..Default::default()
+        };
+        assert_eq!(d.size(), 1);
+    }
+    {
+        let d = Directory {
+            symlinks: vec![SymlinkNode {
+                name: "foo".into(),
+                target: "bar".into(),
+            }],
+            ..Default::default()
+        };
+        assert_eq!(d.size(), 1);
+    }
+}
+
+#[test]
+fn digest() {
+    let d = Directory::default();
+
+    assert_eq!(
+        d.digest(),
+        vec![
+            0xaf, 0x13, 0x49, 0xb9, 0xf5, 0xf9, 0xa1, 0xa6, 0xa0, 0x40, 0x4d, 0xea, 0x36, 0xdc,
+            0xc9, 0x49, 0x9b, 0xcb, 0x25, 0xc9, 0xad, 0xc1, 0x12, 0xb7, 0xcc, 0x9a, 0x93, 0xca,
+            0xe4, 0x1f, 0x32, 0x62
+        ]
+        .try_into()
+        .unwrap()
+    )
+}
+
+#[test]
+fn validate_empty() {
+    let d = Directory::default();
+    assert_eq!(d.validate(), Ok(()));
+}
+
+#[test]
+fn validate_invalid_names() {
+    {
+        let d = Directory {
+            directories: vec![DirectoryNode {
+                name: "".into(),
+                digest: DUMMY_DIGEST.to_vec().into(),
+                size: 42,
+            }],
+            ..Default::default()
+        };
+        match d.validate().expect_err("must fail") {
+            ValidateDirectoryError::InvalidName(n) => {
+                assert_eq!(n, b"")
+            }
+            _ => panic!("unexpected error"),
+        };
+    }
+
+    {
+        let d = Directory {
+            directories: vec![DirectoryNode {
+                name: ".".into(),
+                digest: DUMMY_DIGEST.to_vec().into(),
+                size: 42,
+            }],
+            ..Default::default()
+        };
+        match d.validate().expect_err("must fail") {
+            ValidateDirectoryError::InvalidName(n) => {
+                assert_eq!(n, b".")
+            }
+            _ => panic!("unexpected error"),
+        };
+    }
+
+    {
+        let d = Directory {
+            files: vec![FileNode {
+                name: "..".into(),
+                digest: DUMMY_DIGEST.to_vec().into(),
+                size: 42,
+                executable: false,
+            }],
+            ..Default::default()
+        };
+        match d.validate().expect_err("must fail") {
+            ValidateDirectoryError::InvalidName(n) => {
+                assert_eq!(n, b"..")
+            }
+            _ => panic!("unexpected error"),
+        };
+    }
+
+    {
+        let d = Directory {
+            symlinks: vec![SymlinkNode {
+                name: "\x00".into(),
+                target: "foo".into(),
+            }],
+            ..Default::default()
+        };
+        match d.validate().expect_err("must fail") {
+            ValidateDirectoryError::InvalidName(n) => {
+                assert_eq!(n, b"\x00")
+            }
+            _ => panic!("unexpected error"),
+        };
+    }
+
+    {
+        let d = Directory {
+            symlinks: vec![SymlinkNode {
+                name: "foo/bar".into(),
+                target: "foo".into(),
+            }],
+            ..Default::default()
+        };
+        match d.validate().expect_err("must fail") {
+            ValidateDirectoryError::InvalidName(n) => {
+                assert_eq!(n, b"foo/bar")
+            }
+            _ => panic!("unexpected error"),
+        };
+    }
+}
+
+#[test]
+fn validate_invalid_digest() {
+    let d = Directory {
+        directories: vec![DirectoryNode {
+            name: "foo".into(),
+            digest: vec![0x00, 0x42].into(), // invalid length
+            size: 42,
+        }],
+        ..Default::default()
+    };
+    match d.validate().expect_err("must fail") {
+        ValidateDirectoryError::InvalidDigestLen(n) => {
+            assert_eq!(n, 2)
+        }
+        _ => panic!("unexpected error"),
+    }
+}
+
+#[test]
+fn validate_sorting() {
+    // "b" comes before "a", bad.
+    {
+        let d = Directory {
+            directories: vec![
+                DirectoryNode {
+                    name: "b".into(),
+                    digest: DUMMY_DIGEST.to_vec().into(),
+                    size: 42,
+                },
+                DirectoryNode {
+                    name: "a".into(),
+                    digest: DUMMY_DIGEST.to_vec().into(),
+                    size: 42,
+                },
+            ],
+            ..Default::default()
+        };
+        match d.validate().expect_err("must fail") {
+            ValidateDirectoryError::WrongSorting(s) => {
+                assert_eq!(s, b"a");
+            }
+            _ => panic!("unexpected error"),
+        }
+    }
+
+    // "a" exists twice, bad.
+    {
+        let d = Directory {
+            directories: vec![
+                DirectoryNode {
+                    name: "a".into(),
+                    digest: DUMMY_DIGEST.to_vec().into(),
+                    size: 42,
+                },
+                DirectoryNode {
+                    name: "a".into(),
+                    digest: DUMMY_DIGEST.to_vec().into(),
+                    size: 42,
+                },
+            ],
+            ..Default::default()
+        };
+        match d.validate().expect_err("must fail") {
+            ValidateDirectoryError::DuplicateName(s) => {
+                assert_eq!(s, b"a");
+            }
+            _ => panic!("unexpected error"),
+        }
+    }
+
+    // "a" comes before "b", all good.
+    {
+        let d = Directory {
+            directories: vec![
+                DirectoryNode {
+                    name: "a".into(),
+                    digest: DUMMY_DIGEST.to_vec().into(),
+                    size: 42,
+                },
+                DirectoryNode {
+                    name: "b".into(),
+                    digest: DUMMY_DIGEST.to_vec().into(),
+                    size: 42,
+                },
+            ],
+            ..Default::default()
+        };
+
+        d.validate().expect("validate shouldn't error");
+    }
+
+    // [b, c] and [a] are both properly sorted.
+    {
+        let d = Directory {
+            directories: vec![
+                DirectoryNode {
+                    name: "b".into(),
+                    digest: DUMMY_DIGEST.to_vec().into(),
+                    size: 42,
+                },
+                DirectoryNode {
+                    name: "c".into(),
+                    digest: DUMMY_DIGEST.to_vec().into(),
+                    size: 42,
+                },
+            ],
+            symlinks: vec![SymlinkNode {
+                name: "a".into(),
+                target: "foo".into(),
+            }],
+            ..Default::default()
+        };
+
+        d.validate().expect("validate shouldn't error");
+    }
+}
diff --git a/tvix/castore/src/proto/tests/directory_nodes_iterator.rs b/tvix/castore/src/proto/tests/directory_nodes_iterator.rs
new file mode 100644
index 0000000000..68f147a332
--- /dev/null
+++ b/tvix/castore/src/proto/tests/directory_nodes_iterator.rs
@@ -0,0 +1,78 @@
+use crate::proto::Directory;
+use crate::proto::DirectoryNode;
+use crate::proto::FileNode;
+use crate::proto::NamedNode;
+use crate::proto::SymlinkNode;
+
+#[test]
+fn iterator() {
+    let d = Directory {
+        directories: vec![
+            DirectoryNode {
+                name: "c".into(),
+                ..DirectoryNode::default()
+            },
+            DirectoryNode {
+                name: "d".into(),
+                ..DirectoryNode::default()
+            },
+            DirectoryNode {
+                name: "h".into(),
+                ..DirectoryNode::default()
+            },
+            DirectoryNode {
+                name: "l".into(),
+                ..DirectoryNode::default()
+            },
+        ],
+        files: vec![
+            FileNode {
+                name: "b".into(),
+                ..FileNode::default()
+            },
+            FileNode {
+                name: "e".into(),
+                ..FileNode::default()
+            },
+            FileNode {
+                name: "g".into(),
+                ..FileNode::default()
+            },
+            FileNode {
+                name: "j".into(),
+                ..FileNode::default()
+            },
+        ],
+        symlinks: vec![
+            SymlinkNode {
+                name: "a".into(),
+                ..SymlinkNode::default()
+            },
+            SymlinkNode {
+                name: "f".into(),
+                ..SymlinkNode::default()
+            },
+            SymlinkNode {
+                name: "i".into(),
+                ..SymlinkNode::default()
+            },
+            SymlinkNode {
+                name: "k".into(),
+                ..SymlinkNode::default()
+            },
+        ],
+    };
+
+    // We keep this strings here and convert to string to make the comparison
+    // less messy.
+    let mut node_names: Vec<String> = vec![];
+
+    for node in d.nodes() {
+        node_names.push(String::from_utf8(node.get_name().to_vec()).unwrap());
+    }
+
+    assert_eq!(
+        vec!["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l"],
+        node_names
+    );
+}
diff --git a/tvix/castore/src/proto/tests/grpc_blobservice.rs b/tvix/castore/src/proto/tests/grpc_blobservice.rs
new file mode 100644
index 0000000000..0d7b340b44
--- /dev/null
+++ b/tvix/castore/src/proto/tests/grpc_blobservice.rs
@@ -0,0 +1,100 @@
+use crate::fixtures::{BLOB_A, BLOB_A_DIGEST};
+use crate::proto::blob_service_server::BlobService as GRPCBlobService;
+use crate::proto::{BlobChunk, GRPCBlobServiceWrapper, ReadBlobRequest, StatBlobRequest};
+use crate::utils::gen_blob_service;
+use tokio_stream::StreamExt;
+
+fn gen_grpc_blob_service() -> GRPCBlobServiceWrapper {
+    let blob_service = gen_blob_service();
+    GRPCBlobServiceWrapper::from(blob_service)
+}
+
+/// Trying to read a non-existent blob should return a not found error.
+#[tokio::test]
+async fn not_found_read() {
+    let service = gen_grpc_blob_service();
+
+    let resp = service
+        .read(tonic::Request::new(ReadBlobRequest {
+            digest: BLOB_A_DIGEST.clone().into(),
+        }))
+        .await;
+
+    // We can't use unwrap_err here, because the Ok value doesn't implement
+    // debug.
+    if let Err(e) = resp {
+        assert_eq!(e.code(), tonic::Code::NotFound);
+    } else {
+        panic!("resp is not err")
+    }
+}
+
+/// Trying to stat a non-existent blob should return a not found error.
+#[tokio::test]
+async fn not_found_stat() {
+    let service = gen_grpc_blob_service();
+
+    let resp = service
+        .stat(tonic::Request::new(StatBlobRequest {
+            digest: BLOB_A_DIGEST.clone().into(),
+            ..Default::default()
+        }))
+        .await
+        .expect_err("must fail");
+
+    // The resp should be a status with Code::NotFound
+    assert_eq!(resp.code(), tonic::Code::NotFound);
+}
+
+/// Put a blob in the store, get it back.
+#[tokio::test]
+async fn put_read_stat() {
+    let service = gen_grpc_blob_service();
+
+    // Send blob A.
+    let put_resp = service
+        .put(tonic_mock::streaming_request(vec![BlobChunk {
+            data: BLOB_A.clone(),
+        }]))
+        .await
+        .expect("must succeed")
+        .into_inner();
+
+    assert_eq!(BLOB_A_DIGEST.to_vec(), put_resp.digest);
+
+    // Stat for the digest of A.
+    // We currently don't ask for more granular chunking data, as we don't
+    // expose it yet.
+    let _resp = service
+        .stat(tonic::Request::new(StatBlobRequest {
+            digest: BLOB_A_DIGEST.clone().into(),
+            ..Default::default()
+        }))
+        .await
+        .expect("must succeed")
+        .into_inner();
+
+    // Read the blob. It should return the same data.
+    let resp = service
+        .read(tonic::Request::new(ReadBlobRequest {
+            digest: BLOB_A_DIGEST.clone().into(),
+        }))
+        .await;
+
+    let mut rx = resp.ok().unwrap().into_inner();
+
+    // the stream should contain one element, a BlobChunk with the same contents as BLOB_A.
+    let item = rx
+        .next()
+        .await
+        .expect("must be some")
+        .expect("must succeed");
+
+    assert_eq!(BLOB_A.clone(), item.data);
+
+    // … and no more elements
+    assert!(rx.next().await.is_none());
+
+    // TODO: we rely here on the blob being small enough to not get broken up into multiple chunks.
+    // Test with some bigger blob too
+}
diff --git a/tvix/castore/src/proto/tests/grpc_directoryservice.rs b/tvix/castore/src/proto/tests/grpc_directoryservice.rs
new file mode 100644
index 0000000000..6e8cf1e4a7
--- /dev/null
+++ b/tvix/castore/src/proto/tests/grpc_directoryservice.rs
@@ -0,0 +1,239 @@
+use crate::fixtures::{DIRECTORY_A, DIRECTORY_B, DIRECTORY_C};
+use crate::proto::directory_service_server::DirectoryService as GRPCDirectoryService;
+use crate::proto::get_directory_request::ByWhat;
+use crate::proto::{Directory, DirectoryNode, SymlinkNode};
+use crate::proto::{GRPCDirectoryServiceWrapper, GetDirectoryRequest};
+use crate::utils::gen_directory_service;
+use tokio_stream::StreamExt;
+use tonic::Status;
+
+fn gen_grpc_service() -> GRPCDirectoryServiceWrapper {
+    let directory_service = gen_directory_service();
+    GRPCDirectoryServiceWrapper::from(directory_service)
+}
+
+/// Send the specified GetDirectoryRequest.
+/// Returns an error in the case of an error response, or an error in one of
+// the items in the stream, or a Vec<Directory> in the case of a successful
+/// request.
+async fn get_directories<S: GRPCDirectoryService>(
+    svc: &S,
+    get_directory_request: GetDirectoryRequest,
+) -> Result<Vec<Directory>, Status> {
+    let resp = svc.get(tonic::Request::new(get_directory_request)).await;
+
+    // if the response is an error itself, return the error, otherwise unpack
+    let stream = match resp {
+        Ok(resp) => resp,
+        Err(status) => return Err(status),
+    }
+    .into_inner();
+
+    let directory_results: Vec<Result<Directory, Status>> = stream.collect().await;
+
+    // turn Vec<Result<Directory, Status> into Result<Vec<Directory>,Status>
+    directory_results.into_iter().collect()
+}
+
+/// Trying to get a non-existent Directory should return a not found error.
+#[tokio::test]
+async fn not_found() {
+    let service = gen_grpc_service();
+
+    let resp = service
+        .get(tonic::Request::new(GetDirectoryRequest {
+            by_what: Some(ByWhat::Digest(DIRECTORY_A.digest().into())),
+            ..Default::default()
+        }))
+        .await;
+
+    let mut rx = resp.expect("must succeed").into_inner().into_inner();
+
+    // The stream should contain one element, an error with Code::NotFound.
+    let item = rx
+        .recv()
+        .await
+        .expect("must be some")
+        .expect_err("must be err");
+    assert_eq!(item.code(), tonic::Code::NotFound);
+
+    // … and nothing else
+    assert!(rx.recv().await.is_none());
+}
+
+/// Put a Directory into the store, get it back.
+#[tokio::test]
+async fn put_get() {
+    let service = gen_grpc_service();
+
+    let streaming_request = tonic_mock::streaming_request(vec![DIRECTORY_A.clone()]);
+    let put_resp = service
+        .put(streaming_request)
+        .await
+        .expect("must succeed")
+        .into_inner();
+
+    // the sent root_digest should match the calculated digest
+    assert_eq!(put_resp.root_digest, DIRECTORY_A.digest().to_vec());
+
+    // get it back
+    let items = get_directories(
+        &service,
+        GetDirectoryRequest {
+            by_what: Some(ByWhat::Digest(DIRECTORY_A.digest().into())),
+            ..Default::default()
+        },
+    )
+    .await
+    .expect("must not error");
+
+    assert_eq!(vec![DIRECTORY_A.clone()], items);
+}
+
+/// Put multiple Directories into the store, and get them back
+#[tokio::test]
+async fn put_get_multiple() {
+    let service = gen_grpc_service();
+
+    // sending "b" (which refers to "a") without sending "a" first should fail.
+    let put_resp = service
+        .put(tonic_mock::streaming_request(vec![DIRECTORY_B.clone()]))
+        .await
+        .expect_err("must fail");
+
+    assert_eq!(tonic::Code::InvalidArgument, put_resp.code());
+
+    // sending "a", then "b" should succeed, and the response should contain the digest of b.
+    let put_resp = service
+        .put(tonic_mock::streaming_request(vec![
+            DIRECTORY_A.clone(),
+            DIRECTORY_B.clone(),
+        ]))
+        .await
+        .expect("must succeed");
+
+    assert_eq!(
+        DIRECTORY_B.digest().to_vec(),
+        put_resp.into_inner().root_digest
+    );
+
+    // now, request b, first in non-recursive mode.
+    let items = get_directories(
+        &service,
+        GetDirectoryRequest {
+            recursive: false,
+            by_what: Some(ByWhat::Digest(DIRECTORY_B.digest().into())),
+        },
+    )
+    .await
+    .expect("must not error");
+
+    // We expect to only get b.
+    assert_eq!(vec![DIRECTORY_B.clone()], items);
+
+    // now, request b, but in recursive mode.
+    let items = get_directories(
+        &service,
+        GetDirectoryRequest {
+            recursive: true,
+            by_what: Some(ByWhat::Digest(DIRECTORY_B.digest().into())),
+        },
+    )
+    .await
+    .expect("must not error");
+
+    // We expect to get b, and then a, because that's how we traverse down.
+    assert_eq!(vec![DIRECTORY_B.clone(), DIRECTORY_A.clone()], items);
+}
+
+/// Put multiple Directories into the store, and omit duplicates.
+#[tokio::test]
+async fn put_get_dedup() {
+    let service = gen_grpc_service();
+
+    // Send "A", then "C", which refers to "A" two times
+    // Pretend we're a dumb client sending A twice.
+    let put_resp = service
+        .put(tonic_mock::streaming_request(vec![
+            DIRECTORY_A.clone(),
+            DIRECTORY_A.clone(),
+            DIRECTORY_C.clone(),
+        ]))
+        .await
+        .expect("must succeed");
+
+    assert_eq!(
+        DIRECTORY_C.digest().to_vec(),
+        put_resp.into_inner().root_digest
+    );
+
+    // Ask for "C" recursively. We expect to only get "A" once, as there's no point sending it twice.
+    let items = get_directories(
+        &service,
+        GetDirectoryRequest {
+            recursive: true,
+            by_what: Some(ByWhat::Digest(DIRECTORY_C.digest().into())),
+        },
+    )
+    .await
+    .expect("must not error");
+
+    // We expect to get C, and then A (once, as the second A has been deduplicated).
+    assert_eq!(vec![DIRECTORY_C.clone(), DIRECTORY_A.clone()], items);
+}
+
+/// Trying to upload a Directory failing validation should fail.
+#[tokio::test]
+async fn put_reject_failed_validation() {
+    let service = gen_grpc_service();
+
+    // construct a broken Directory message that fails validation
+    let broken_directory = Directory {
+        symlinks: vec![SymlinkNode {
+            name: "".into(),
+            target: "doesntmatter".into(),
+        }],
+        ..Default::default()
+    };
+    assert!(broken_directory.validate().is_err());
+
+    // send it over, it must fail
+    let put_resp = service
+        .put(tonic_mock::streaming_request(vec![broken_directory]))
+        .await
+        .expect_err("must fail");
+
+    assert_eq!(put_resp.code(), tonic::Code::InvalidArgument);
+}
+
+/// Trying to upload a Directory with wrong size should fail.
+#[tokio::test]
+async fn put_reject_wrong_size() {
+    let service = gen_grpc_service();
+
+    // Construct a directory referring to DIRECTORY_A, but with wrong size.
+    let broken_parent_directory = Directory {
+        directories: vec![DirectoryNode {
+            name: "foo".into(),
+            digest: DIRECTORY_A.digest().into(),
+            size: 42,
+        }],
+        ..Default::default()
+    };
+    // Make sure we got the size wrong.
+    assert_ne!(
+        broken_parent_directory.directories[0].size,
+        DIRECTORY_A.size()
+    );
+
+    // now upload both (first A, then the broken parent). This must fail.
+    let put_resp = service
+        .put(tonic_mock::streaming_request(vec![
+            DIRECTORY_A.clone(),
+            broken_parent_directory,
+        ]))
+        .await
+        .expect_err("must fail");
+
+    assert_eq!(put_resp.code(), tonic::Code::InvalidArgument);
+}
diff --git a/tvix/castore/src/proto/tests/mod.rs b/tvix/castore/src/proto/tests/mod.rs
new file mode 100644
index 0000000000..8b62fadeb5
--- /dev/null
+++ b/tvix/castore/src/proto/tests/mod.rs
@@ -0,0 +1,4 @@
+mod directory;
+mod directory_nodes_iterator;
+mod grpc_blobservice;
+mod grpc_directoryservice;
diff --git a/tvix/castore/src/tests/import.rs b/tvix/castore/src/tests/import.rs
new file mode 100644
index 0000000000..77ed6d21c0
--- /dev/null
+++ b/tvix/castore/src/tests/import.rs
@@ -0,0 +1,124 @@
+use crate::fixtures::*;
+use crate::import::ingest_path;
+use crate::proto;
+use crate::utils::{gen_blob_service, gen_directory_service};
+use tempfile::TempDir;
+
+#[cfg(target_family = "unix")]
+use std::os::unix::ffi::OsStrExt;
+
+#[cfg(target_family = "unix")]
+#[tokio::test]
+async fn symlink() {
+    let tmpdir = TempDir::new().unwrap();
+
+    std::fs::create_dir_all(&tmpdir).unwrap();
+    std::os::unix::fs::symlink(
+        "/nix/store/somewhereelse",
+        tmpdir.path().join("doesntmatter"),
+    )
+    .unwrap();
+
+    let root_node = ingest_path(
+        gen_blob_service(),
+        gen_directory_service(),
+        tmpdir.path().join("doesntmatter"),
+    )
+    .await
+    .expect("must succeed");
+
+    assert_eq!(
+        proto::node::Node::Symlink(proto::SymlinkNode {
+            name: "doesntmatter".into(),
+            target: "/nix/store/somewhereelse".into(),
+        }),
+        root_node,
+    )
+}
+
+#[tokio::test]
+async fn single_file() {
+    let tmpdir = TempDir::new().unwrap();
+
+    std::fs::write(tmpdir.path().join("root"), HELLOWORLD_BLOB_CONTENTS).unwrap();
+
+    let blob_service = gen_blob_service();
+
+    let root_node = ingest_path(
+        blob_service.clone(),
+        gen_directory_service(),
+        tmpdir.path().join("root"),
+    )
+    .await
+    .expect("must succeed");
+
+    assert_eq!(
+        proto::node::Node::File(proto::FileNode {
+            name: "root".into(),
+            digest: HELLOWORLD_BLOB_DIGEST.clone().into(),
+            size: HELLOWORLD_BLOB_CONTENTS.len() as u32,
+            executable: false,
+        }),
+        root_node,
+    );
+
+    // ensure the blob has been uploaded
+    assert!(blob_service.has(&HELLOWORLD_BLOB_DIGEST).await.unwrap());
+}
+
+#[cfg(target_family = "unix")]
+#[tokio::test]
+async fn complicated() {
+    let tmpdir = TempDir::new().unwrap();
+
+    // File ``.keep`
+    std::fs::write(tmpdir.path().join(".keep"), vec![]).unwrap();
+    // Symlink `aa`
+    std::os::unix::fs::symlink("/nix/store/somewhereelse", tmpdir.path().join("aa")).unwrap();
+    // Directory `keep`
+    std::fs::create_dir(tmpdir.path().join("keep")).unwrap();
+    // File ``keep/.keep`
+    std::fs::write(tmpdir.path().join("keep").join(".keep"), vec![]).unwrap();
+
+    let blob_service = gen_blob_service();
+    let directory_service = gen_directory_service();
+
+    let root_node = ingest_path(
+        blob_service.clone(),
+        directory_service.clone(),
+        tmpdir.path(),
+    )
+    .await
+    .expect("must succeed");
+
+    // ensure root_node matched expectations
+    assert_eq!(
+        proto::node::Node::Directory(proto::DirectoryNode {
+            name: tmpdir
+                .path()
+                .file_name()
+                .unwrap()
+                .as_bytes()
+                .to_owned()
+                .into(),
+            digest: DIRECTORY_COMPLICATED.digest().into(),
+            size: DIRECTORY_COMPLICATED.size(),
+        }),
+        root_node,
+    );
+
+    // ensure DIRECTORY_WITH_KEEP and DIRECTORY_COMPLICATED have been uploaded
+    assert!(directory_service
+        .get(&DIRECTORY_WITH_KEEP.digest())
+        .await
+        .unwrap()
+        .is_some());
+    assert!(directory_service
+        .get(&DIRECTORY_COMPLICATED.digest())
+        .await
+        .unwrap()
+        .is_some());
+
+    // ensure EMPTY_BLOB_CONTENTS has been uploaded
+    assert!(blob_service.has(&EMPTY_BLOB_DIGEST).await.unwrap());
+}
diff --git a/tvix/castore/src/tests/mod.rs b/tvix/castore/src/tests/mod.rs
new file mode 100644
index 0000000000..d016f3e0aa
--- /dev/null
+++ b/tvix/castore/src/tests/mod.rs
@@ -0,0 +1 @@
+mod import;
diff --git a/tvix/castore/src/utils.rs b/tvix/castore/src/utils.rs
new file mode 100644
index 0000000000..660cb2f4b0
--- /dev/null
+++ b/tvix/castore/src/utils.rs
@@ -0,0 +1,19 @@
+//! A crate containing constructors to provide instances of a BlobService and
+//! DirectoryService.
+//! Only used for testing purposes, but across crates.
+//! Should be removed once we have a better concept of a "Service registry".
+
+use std::sync::Arc;
+
+use crate::{
+    blobservice::{BlobService, MemoryBlobService},
+    directoryservice::{DirectoryService, MemoryDirectoryService},
+};
+
+pub fn gen_blob_service() -> Arc<dyn BlobService> {
+    Arc::new(MemoryBlobService::default())
+}
+
+pub fn gen_directory_service() -> Arc<dyn DirectoryService> {
+    Arc::new(MemoryDirectoryService::default())
+}