cargo-chef

Cache the dependencies of your Rust project and speed up your Docker builds.

cargo-chef was initially developed for the deployment chapter of Zero to Production In Rust, a hands-on introduction to backend development using the Rust programming language.

Table of Contents

0. How to install
1. How to use
2. Benefits vs Limitations
3. License

How To Install

You can install cargo-chef from crates.io with

cargo install cargo-chef --locked

How to use

⚠️ cargo-chef is not meant to be run locally
Its primary use-case is to speed up container builds by running BEFORE the actual source code is copied over. Don't run it on existing codebases to avoid having files being overwritten.

cargo-chef exposes two commands: prepare and cook:

cargo chef --help

cargo-chef

USAGE:
    cargo chef <SUBCOMMAND>

SUBCOMMANDS:
    cook       Re-hydrate the minimum project skeleton identified by `cargo chef prepare` and
               build it to cache dependencies
    prepare    Analyze the current project to determine the minimum subset of files (Cargo.lock
               and Cargo.toml manifests) required to build it and cache dependencies

prepare examines your project and builds a recipe that captures the set of information required to build your dependencies.

cargo chef prepare --recipe-path recipe.json

Nothing too mysterious going on here, you can examine the recipe.json file: it contains the skeleton of your project (e.g. all the Cargo.toml files with their relative path, the Cargo.lock file is available) plus a few additional pieces of information.
In particular it makes sure that all libraries and binaries are explicitly declared in their respective Cargo.toml files even if they can be found at the canonical default location (src/main.rs for a binary, src/lib.rs for a library).

The recipe.json is the equivalent of the Python requirements.txt file - it is the only input required for cargo chef cook, the command that will build out our dependencies:

cargo chef cook --recipe-path recipe.json

If you want to build in --release mode:

cargo chef cook --release --recipe-path recipe.json

You can also choose to override which Rust toolchain should be used. E.g., to force the nightly toolchain:

cargo +nightly chef cook --recipe-path recipe.json

cargo-chef is designed to be leveraged in Dockerfiles:

FROM lukemathwalker/cargo-chef:latest-rust-1 AS chef
WORKDIR /app

FROM chef AS planner
COPY . .
RUN cargo chef prepare --recipe-path recipe.json

FROM chef AS builder 
COPY --from=planner /app/recipe.json recipe.json
# Build dependencies - this is the caching Docker layer!
RUN cargo chef cook --release --recipe-path recipe.json
# Build application
COPY . .
RUN cargo build --release --bin app

# We do not need the Rust toolchain to run the binary!
FROM debian:buster-slim AS runtime
WORKDIR /app
COPY --from=builder /app/target/release/app /usr/local/bin
ENTRYPOINT ["/usr/local/bin/app"]

We are using three stages: the first computes the recipe file, the second caches our dependencies and builds the binary, the third is our runtime environment.
As long as your dependencies do not change the recipe.json file will stay the same, therefore the outcome of cargo chef cook --release --recipe-path recipe.json will be cached, massively speeding up your builds (up to 5x measured on some commercial projects).

Pre-built images

We offer lukemathwalker/cargo-chef as a pre-built Docker image equipped with both Rust and cargo-chef.

The tagging scheme is <cargo-chef version>-rust-<rust version>.
For example, 0.1.22-rust-1.56.0.
You can choose to get the latest version of either cargo-chef or rust by using:

• latest-rust-1.56.0 (use latest cargo-chef with specific Rust version);
• 0.1.22-rust-latest (use latest Rust with specific cargo-chef version). You can find all the available tags on Dockerhub.

⚠️ You must use the same Rust version in all stages
If you use a different Rust version in one of the stages caching will not work as expected.

Without the pre-built image

If you do not want to use the lukemathwalker/cargo-chef image, you can simply install the CLI within the Dockerfile:

FROM rust:1 AS chef 
# We only pay the installation cost once, 
# it will be cached from the second build onwards
RUN cargo install cargo-chef 
WORKDIR app

FROM chef AS planner
COPY . .
RUN cargo chef prepare  --recipe-path recipe.json

FROM chef AS builder
COPY --from=planner /app/recipe.json recipe.json
# Build dependencies - this is the caching Docker layer!
RUN cargo chef cook --release --recipe-path recipe.json
# Build application
COPY . .
RUN cargo build --release --bin app

# We do not need the Rust toolchain to run the binary!
FROM debian:buster-slim AS runtime
WORKDIR app
COPY --from=builder /app/target/release/app /usr/local/bin
ENTRYPOINT ["/usr/local/bin/app"]

Running the binary in Alpine

If you want to run your application using the alpine distribution you need to create a fully static binary.
The recommended approach is to build for the x86_64-unknown-linux-musl target using muslrust.
cargo-chef works for x86_64-unknown-linux-musl, but we are cross-compiling - the target toolchain must be explicitly specified.

A sample Dockerfile looks like this:

# Using the `rust-musl-builder` as base image, instead of 
# the official Rust toolchain
FROM clux/muslrust:stable AS chef
USER root
RUN cargo install cargo-chef
WORKDIR /app

FROM chef AS planner
COPY . .
RUN cargo chef prepare --recipe-path recipe.json

FROM chef AS builder
COPY --from=planner /app/recipe.json recipe.json
# Notice that we are specifying the --target flag!
RUN cargo chef cook --release --target x86_64-unknown-linux-musl --recipe-path recipe.json
COPY . .
RUN cargo build --release --target x86_64-unknown-linux-musl --bin app

FROM alpine AS runtime
RUN addgroup -S myuser && adduser -S myuser -G myuser
COPY --from=builder /app/target/x86_64-unknown-linux-musl/release/app /usr/local/bin/
USER myuser
CMD ["/usr/local/bin/app"]

Benefits vs Limitations

cargo-chef has been tested on a few OpenSource projects and some of commercial projects, but our testing has definitely not exhausted the range of possibilities when it comes to cargo build customisations and we are sure that there are a few rough edges that will have to be smoothed out - please file issues on GitHub.

Benefits of cargo-chef:

A common alternative is to load a minimal main.rs into a container with Cargo.toml and Cargo.lock to build a Docker layer that consists of only your dependencies (more info here). This is fragile compared to cargo-chef which will instead:

• automatically pick up all crates in a workspace (and new ones as they are added)
• keep working when files or crates are moved around, which would instead require manual edits to the Dockerfile using the "manual" approach
• generate fewer intermediate Docker layers (for workspaces)

Limitations and caveats:

• cargo cook and cargo build must be executed from the same working directory. If you examine the *.d files under target/debug/deps for one of your projects using cat you will notice that they contain absolute paths referring to the project target directory. If moved around, cargo will not leverage them as cached dependencies;
• cargo build will build local dependencies (outside of the current project) from scratch, even if they are unchanged, due to the reliance of its fingerprinting logic on timestamps (see this long issue on cargo's repository);

License

Licensed under either of Apache License, Version 2.0 or MIT license at your option. Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.