Background

We intend for Tvix tooling to be more decoupled than the existing, monolithic Nix implementation. In practice, we expect to gain several benefits from this, such as:

• Ability to use different builders
• Ability to use different store implementations
• No monopolisation of the implementation, allowing users to replace components that they are unhappy with (up to and including the language evaluator)
• Less hidden intra-dependencies between tools due to explicit RPC/IPC boundaries

Communication between different components of the system will use gRPC. The rest of this document outlines the components.

Components

Coordinator

Purpose: The coordinator (in the simplest case, the Tvix CLI tool) oversees the flow of a build process and delegates tasks to the right subcomponents. For example, if a user runs the equivalent of nix-build in a folder containing a default.nix file, the coordinator will invoke the evaluator, pass the resulting derivations to the builder and coordinate any necessary store interactions (for substitution and other purposes).

While many users are likely to use the CLI tool as their primary method of interacting with Tvix, it is not unlikely that alternative coordinators (e.g. for a distributed, "Nix-native" CI system) would be implemented. To facilitate this, we are considering implementing the coordinator on top of a state-machine model that would make it possible to reuse the FSM logic without tying it to any particular kind of application.

Evaluator

Purpose: Eval takes care of evaluating Nix code. In a typical build flow it would be responsible for producing derivations. It can also be used as a standalone tool, for example, in use-cases where Nix is used to generate configuration without any build or store involvement.

Requirements: For now, it will run on the machine invoking the build command itself. We give it filesystem access to handle things like imports or builtins.readFile.

To support IFD, the Evaluator also needs access to store paths. This could be implemented by having the coordinator provide an interface to retrieve files from a store path, or by ensuring a "realized version of the store" is accessible by the evaluator (this could be a FUSE filesystem, or the "real" /nix/store on disk.

We might be okay with running the evaluator with filesystem access for now and can extend the interface if the need arises.

Builder

Purpose: A builder receives derivations from the coordinator and builds them.

By making builder a standardised interface it's possible to make the sandboxing mechanism used by the build process pluggable.

Nix is currently using a hard-coded libseccomp based sandboxing mechanism and another one based on sandboxd on macOS. These are only separated by compiler preprocessor macros within the same source files despite having very little in common with each other.

This makes experimentation with alternative backends difficult and porting Nix to other platforms harder than it has to be. We want to write a new Linux builder which uses OCI, the current dominant Linux containerisation technology, by default.

With a well-defined builder abstraction, it's also easy to imagine other backends such as a Kubernetes-based one in the future.

The environment in which builds happen is currently very Nix-specific. We might want to avoid having to maintain all the intricacies of a Nix-specific sandboxing environment in every builder, and instead only provide a more generic interface, receiving build requests (and have the coordinator translate derivations to that format). ¹

To build, the builder needs to be able to mount all build inputs into the build environment. For this, it needs the store to expose a filesystem interface.

Store

Purpose: Store takes care of storing build results. It provides a unified interface to get store paths and upload new ones, as well as querying for the existence of a store path and its metadata (references, signatures, …).

Tvix natively uses an improved store protocol. Instead of transferring around NAR files, which don't provide an index and don't allow seekable access, a concept similar to git tree hashing is used.

This allows more granular substitution, chunk reusage and parallel download of individual files, reducing bandwidth usage. As these chunks are content-addressed, it opens up the potential for peer-to-peer trustless substitution of most of the data, as long as we sign the root of the index.

Tvix still keeps the old-style signatures, NAR hashes and NAR size around. In the case of NAR hash / NAR size, this data is strictly required in some cases. The old-style signatures are valuable for communication with existing implementations.

Old-style binary caches (like cache.nixos.org) can still be exposed via the new interface, by doing on-the-fly (re)chunking/ingestion.

Most likely, there will be multiple implementations of store, some storing things locally, some exposing a "remote view".

A few possible ones that come to mind are:

• Local store
• SFTP/ GCP / S3 / HTTP
• NAR/NARInfo protocol: HTTP, S3

A remote Tvix store can be connected by simply connecting to its gRPC interface, possibly using SSH tunneling, but there doesn't need to be an additional "wire format" like the Nix ssh(+ng):// protocol.

Settling on one interface allows composition of stores, meaning it becomes possible to express substitution from remote caches as a proxy layer.

It'd also be possible to write a FUSE implementation on top of the RPC interface, exposing a lazily-substituting /nix/store mountpoint. Using this in remote build context dramatically reduces the amount of data transferred to a builder, as only the files really accessed during the build are substituted.

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