Add post to call for testing of Cargo's -Zhttp-registry

Author: arlosi

posts/2022-06-22-sparse-registry-testing.md

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+---
+layout: post
+title: "Call for testing: Cargo sparse-registry"
+author: Arlo Siemsen
+team: The Cargo Team <https://www.rust-lang.org/governance/teams/dev-tools#cargo>
+---
+
+The Cargo nightly [`sparse-registry`][sparse-registry] feature is ready for testing. The
+feature causes Cargo to access the crates.io 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+the git index is out-of-date or not yet cloned.
+
+## Overview
+To try it out, add the `-Z sparse-registry` flag on `nightly-2022-06-20` or newer 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.
+
+You can [leave feedback on the internals thread][internals].
+
+If you see any issues [please report them on the Cargo repo][cargo]. The output of Cargo
+with the environment variable `CARGO_LOG=cargo::sources::registry::http_remote=trace` set
+will be helpful in debugging.
+
+## Details
+
+Accessing the index over HTTP allows crates.io 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 improvies
+scalability and reduces server load.
+
+The Cargo team plans to eventually make this the default way to access crates.io
+(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 crates.io
+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][tracking-issue].
+
+## Thank you
+
+This project has been in the works for over 2.5 years with collaboration from the crates.io,
+infra, and Cargo teams.
+
+@kornelski wrote the [sparse-index RFC][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 crates.io
+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!
+
+[rfc]: https://rust-lang.github.io/rfcs/2789-sparse-index.html
+[sparse-registry]: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#sparse-registry
+[internals]: https://internals.rust-lang.org/t/call-for-testing-cargo-sparse-registry/16862
+[tracking-issue]: https://github.com/rust-lang/cargo/issues/9069
+[cargo]: https://github.com/rust-lang/cargo/issues