diff options
Diffstat (limited to 'tvix/store/docs')
| -rw-r--r-- | tvix/store/docs/api.md | 31 | ||||
| -rw-r--r-- | tvix/store/docs/castore.md | 50 | ||||
| -rw-r--r-- | tvix/store/docs/why-not-git-trees.md | 57 |
3 files changed, 18 insertions, 120 deletions
diff --git a/tvix/store/docs/api.md b/tvix/store/docs/api.md index 6a4b98911c2c..c1dacc89a598 100644 --- a/tvix/store/docs/api.md +++ b/tvix/store/docs/api.md @@ -1,10 +1,12 @@ -tvix-store API +tvix-[ca]store API ============== -This document outlines the design of the API exposed by tvix-store, as -well as other implementations of this store protocol. +This document outlines the design of the API exposed by tvix-castore and tvix- +store, as well as other implementations of this store protocol. -This document is meant to be read side-by-side with [castore.md](./castore.md) which describes the data model in more detail. +This document is meant to be read side-by-side with +[castore.md](../../tvix-castore/docs/castore.md) which describes the data model +in more detail. The store API has four main consumers: @@ -115,8 +117,9 @@ content-addressed world to a physical path. ### PathInfo As most paths in the Nix store currently are input-addressed [^input-addressed], -we need something mapping from an input-addressed "output path hash" to the -contents in the content- addressed world. +and the `tvix-castore` data model is also not intrinsically using NAR hashes, +we need something mapping from an input-addressed "output path hash" (or a Nix- +specific content-addressed path) to the contents in the `tvix-castore` world. That's what `PathInfo` provides. It embeds the root node (Directory, File or Symlink) at a given store path. @@ -215,13 +218,15 @@ This is useful for people running a Tvix-only system, or running builds on a In a system with Nix installed, we can't simply manually "extract" things to `/nix/store`, as Nix assumes to own all writes to this location. In these use cases, we're probably better off exposing a tvix-store as a local -binary cache (that's what nar-bridge does). +binary cache (that's what `//tvix/nar-bridge` does). Assuming we are in an environment where we control `/nix/store` exclusively, a -"realize to disk" would either "extract" things from the tvix-store to a -filesystem, or expose a FUSE filesystem. The latter would be particularly -interesting for remote build workloads, as build inputs can be realized on- -demand, which saves copying around a lot of never-accessed files. +"realize to disk" would either "extract" things from the `tvix-store` to a +filesystem, or expose a `FUSE`/`virtio-fs` filesystem. + +The latter is already implemented, and particularly interesting for (remote) +build workloads, as build inputs can be realized on-demand, which saves copying +around a lot of never- accessed files. In both cases, the API interactions are similar. * The *PathInfoService* is asked for the `PathInfo` of the requested store path. @@ -253,7 +258,7 @@ As already described above, the only non-content-addressed service is the This means, all other messages (such as `Blob` and `Directory` messages) can be substituted from many different, untrusted sources/mirrors, which will make plugging in additional substitution strategies like IPFS, local network -neighbors super simple. +neighbors super simple. That's also why it's living in the `tvix-castore` crate. As for `PathInfo`, we don't specify an additional signature mechanism yet, but carry the NAR-based signatures from Nix along. @@ -268,7 +273,7 @@ rather than a whole NAR file. A future signature mechanism, that is only signing (parts of) the `PathInfo` message, which only points to content-addressed data will enable verified partial access into a store path, opening up opportunities for lazy filesystem -access, which is very useful in remote builder scenarios. +access etc. diff --git a/tvix/store/docs/castore.md b/tvix/store/docs/castore.md deleted file mode 100644 index f555ba5a861b..000000000000 --- a/tvix/store/docs/castore.md +++ /dev/null @@ -1,50 +0,0 @@ -# //tvix/store/docs/castore.md - -This provides some more notes on the fields used in castore.proto. - -It's meant to supplement `//tvix/store/docs/api.md`. - -## Directory message -`Directory` messages use the blake3 hash of their canonical protobuf -serialization as its identifier. - -A `Directory` message contains three lists, `directories`, `files` and -`symlinks`, holding `DirectoryNode`, `FileNode` and `SymlinkNode` messages -respectively. They describe all the direct child elements that are contained in -a directory. - -All three message types have a `name` field, specifying the (base)name of the -element (which MUST not contain slashes or null bytes, and MUST not be '.' or '..'). -For reproducibility reasons, the lists MUST be sorted by that name and also -MUST be unique across all three lists. - -In addition to the `name` field, the various *Node messages have the following -fields: - -## DirectoryNode -A `DirectoryNode` message represents a child directory. - -It has a `digest` field, which points to the identifier of another `Directory` -message, making a `Directory` a merkle tree (or strictly speaking, a graph, as -two elements pointing to a child directory with the same contents would point -to the same `Directory` message. - -There's also a `size` field, containing the (total) number of all child -elements in the referenced `Directory`, which helps for inode calculation. - -## FileNode -A `FileNode` message represents a child (regular) file. - -Its `digest` field contains the blake3 hash of the file contents. It can be -looked up in the `BlobService`. - -The `size` field contains the size of the blob the `digest` field refers to. - -The `executable` field specifies whether the file should be marked as -executable or not. - -## SymlinkNode -A `SymlinkNode` message represents a child symlink. - -In addition to the `name` field, the only additional field is the `target`, -which is a string containing the target of the symlink. diff --git a/tvix/store/docs/why-not-git-trees.md b/tvix/store/docs/why-not-git-trees.md deleted file mode 100644 index fd46252cf55c..000000000000 --- a/tvix/store/docs/why-not-git-trees.md +++ /dev/null @@ -1,57 +0,0 @@ -## Why not git tree objects? - -We've been experimenting with (some variations of) the git tree and object -format, and ultimately decided against using it as an internal format, and -instead adapted the one documented in the other documents here. - -While the tvix-store API protocol shares some similarities with the format used -in git for trees and objects, the git one has shown some significant -disadvantages: - -### The binary encoding itself - -#### trees -The git tree object format is a very binary, error-prone and -"made-to-be-read-and-written-from-C" format. - -Tree objects are a combination of null-terminated strings, and fields of known -length. References to other tree objects use the literal sha1 hash of another -tree object in this encoding. -Extensions of the format/changes are very hard to do right, because parsers are -not aware they might be parsing something different. - -The tvix-store protocol uses a canonical protobuf serialization, and uses -the [blake3][blake3] hash of that serialization to point to other `Directory` -messages. -It's both compact and with a wide range of libraries for encoders and decoders -in many programming languages. -The choice of protobuf makes it easy to add new fields, and make old clients -aware of some unknown fields being detected [^adding-fields]. - -#### blob -On disk, git blob objects start with a "blob" prefix, then the size of the -payload, and then the data itself. The hash of a blob is the literal sha1sum -over all of this - which makes it something very git specific to request for. - -tvix-store simply uses the [blake3][blake3] hash of the literal contents -when referring to a file/blob, which makes it very easy to ask other data -sources for the same data, as no git-specific payload is included in the hash. -This also plays very well together with things like [iroh][iroh-discussion], -which plans to provide a way to substitute (large)blobs by their blake3 hash -over the IPFS network. - -In addition to that, [blake3][blake3] makes it possible to do -[verified streaming][bao], as already described in other parts of the -documentation. - -The git tree object format uses sha1 both for references to other trees and -hashes of blobs, which isn't really a hash function to fundamentally base -everything on in 2023. -The [migration to sha256][git-sha256] also has been dead for some years now, -and it's unclear how a "blake3" version of this would even look like. - -[bao]: https://github.com/oconnor663/bao -[blake3]: https://github.com/BLAKE3-team/BLAKE3 -[git-sha256]: https://git-scm.com/docs/hash-function-transition/ -[iroh-discussion]: https://github.com/n0-computer/iroh/discussions/707#discussioncomment-5070197 -[^adding-fields]: Obviously, adding new fields will change hashes, but it's something that's easy to detect. \ No newline at end of file |