Central cache for external repositories

Part 1: Caching HttpDownloader values

Author: jingwen@google.com

Status: Implemented

Reviewers: kchodorow@, jcater@, dmarting@


With bazel fetch //repo:..., external repositories and artifacts are downloaded from remote sources with repository functions, such as http_archive() or maven_jar(). Sometimes, network access is not available, or redownloading of already-downloaded repositories is wasteful.

With a central cache, Bazel can check it for the requested repositories before hitting the internet. This allows multiple workspaces to share the same repository cache. The cache should also be portable, that is, tar’ing up the cache and reusing it on another system.

Make maven_jar and friends smarter by re-using previously fetched artifacts across different projects #1752.


Implement an additional step prior to fetching that checks a directory $repository_cache for the requested external repository.


  • This does not change the way output_base/external is organized. It is delegating the actual location of the downloaded repositories to somewhere outside of output_base/external (which means outside of individual Bazel invocation folders), and symlinking to those locations instead.
  • This does not require a change in CI configuration until it’s switched on by default. It can be used with the flag --experimental_repository_cache during the development process.

Not in scope

  • (Skylark) Reconciling caching strategies between Bazel and other tools like Maven, which has their own strategies. It’s possible to cache the output of repository_ctx.execute, while still respecting the external tool’s caching strategy, but that is not in the scope of this design.
  • Human-readable cache layouts. Simplifies inspection of cache content.
  • Garbage collection / cache cleanup of non-referenced cache content.
  • Using a remote filesystem as cache (NFS, ssh-fuse, and so on).
  • Cache invalidation

Implementation plan

  1. Implement a new command line option: --experimental_repository_cache that takes in a string argument, which is the path of the directory. This path will be used directly in BazelRepositoryModule#handleOptions. There is no default value for this option; the user will need to provide an absolute path. a. Acts as a feature gate until the feature is ready to become a default behavior, after which it’ll will be used to specify custom locations or turn caching off.
  2. Option will be implemented in the new RepositoryOptions class, which is added to FetchCommand and BuildCommand.
  3. Create a class, RepositoryCache, that defines the caching strategy when used in HttpDownloader. Skylark’s download/download_and_extract methods will also get this caching for free.
  4. Prior to downloading, if the --repository_cache option is specified && the SHA checksum is specified in the download parameters in HttpDownloader, set the cache key as the SHA checksum. If cache hit in $repository_cache, skip the downloading step and copy the cache value from $repository_cache to $output_base/external.
  5. If not specified, download into $output_base/external as usual. After downloading, if the --repository_cache option is specified and there’s a cache miss, cache load into $repository_cache and create a copy back to $output_base/external.

Cache structure

  • repository_cache: $HOME/.cache/bazel/repository_cache
  • content_addressable_cache: ${repository_cache}/content_addressable
  • cache_entry: ${content_addressable_cache}/sha{1, 256}/file
  • Cache keys: SHA checksum of ${cache_entry}, for example the tar.gz of a http_archive.