Call for testing: Cargo sparse-registry

June 22, 2022 · Arlo Siemsen on behalf of The Cargo Team

Note: Sparse registry support has been stabilized for the 1.68 release. See Help test Cargo's new index protocol for updated information.

The Cargo nightly sparse-registry feature is ready for testing. The feature causes Cargo to access the index over HTTP, rather than git. It can provide a significant performance improvement, especially if the local copy of the git index is out-of-date or not yet cloned.


To try it out, add the -Z sparse-registry flag on a recent nightly build of Cargo. For example, to update dependencies:

rustup update nightly
cargo +nightly -Z sparse-registry update

The feature can also be enabled by setting the environment variable CARGO_UNSTABLE_SPARSE_REGISTRY=true. Setting this variable will have no effect on stable Cargo, making it easy to opt-in for CI jobs.

The minimum Cargo version is cargo 2022-06-17, which is bundled with rustc 2022-06-20.

You can leave feedback on the internals thread.

If you see any issues please report them on the Cargo repo. The output of Cargo with the environment variable CARGO_LOG=cargo::sources::registry::http_remote=trace set will be helpful in debugging.


Accessing the index over HTTP allows to continue growing without hampering performance. The current git index continues to grow as new crates are published, and clients must download the entire index. The HTTP index only requires downloading metadata for crates in your dependency tree.

The performance improvement for clients should be especially noticeable in CI environments, particularly if no local cache of the index exists.

On the server side, the HTTP protocol is much simpler to cache on a CDN, which improves scalability and reduces server load. Due to this caching, crate updates may take an extra minute to appear in the index.

The Cargo team plans to eventually make this the default way to access (though the git index will remain for compatibility with older versions of Cargo and external tools). Cargo.lock files will continue to reference the existing index on GitHub to avoid churn.

The -Z sparse-registry flag also enables alternative registries to be accessed over HTTP. For more details, see the tracking issue.

Thank you

This project has been in the works for over 2.5 years with collaboration from the, infra, and Cargo teams.

@kornelski wrote the sparse-index RFC and initial performance proof of concept. @jonhoo created the initial implementation in Cargo and gathered performance data. @arlosi completed the implementation in Cargo and implemented the changes to to serve the index. @eh2406 provided numerous reviews and feedback to get all the changes landed. Many others from the community helped by providing suggestions, feedback, and testing.

Thank you to everyone involved!