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Deploying Rules

This documentation is for rule writers who are planning to make their rules available to others.

Hosting and naming rules

New rules should go into their own GitHub repository under your organization. Contact the bazel-dev mailing list if you feel like your rules belong in the bazelbuild organization.

Repository names for Bazel rules are standardized on the following format: $ORGANIZATION/rules_$NAME. See examples on GitHub. For consistency, you must follow this same format when publishing your Bazel rules.

Make sure to use a descriptive GitHub repository description and README.html title, example:

  • Repository name: bazelbuild/rules_go
  • Repository description: Go rules for Bazel
  • Repository tags: golang, bazel
  • README.html header: Go rules for Bazel (note the link to https://bazel.build which will guide users who are unfamiliar with Bazel to the right place)

Rules can be grouped either by language (e.g., Scala) or some notion of platform (e.g., Android).

Repository content

Every rule repository should have a certain layout so that users can quickly understand new rules.

For example, suppose we are writing new rules for the (make-believe) mockascript language. The repository will be named rules_mockascript. We would have the following structure:

/
  LICENSE
  README
  WORKSPACE
  mockascript/
    constraints/
      BUILD
    BUILD
    defs.bzl
  tests/
    BUILD
    some_test.sh
    another_test.py
  examples/
    BUILD
    bin.mocs
    lib.mocs
    test.mocs

README

At the top level, there should be a README that contains (at least) what users will need to copy-paste into their WORKSPACE file to use your rule. In general, this will be a http_archive pointing to your GitHub release and a macro call that downloads/configures any tools your rule needs. For example, for the Go rules, this looks like:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
    name = "rules_go",
    urls = ["https://github.com/bazelbuild/rules_go/releases/download/0.18.5/rules_go-0.18.5.tar.gz"],
    sha256 = "a82a352bffae6bee4e95f68a8d80a70e87f42c4741e6a448bec11998fcc82329",
)
load("@rules_go//go:deps.bzl", "go_rules_dependencies", "go_register_toolchains")
go_rules_dependencies()
go_register_toolchains()

If your rules depend on another repository’s rules, specify that in the rules documentation (for example, see the Skydoc rules, which depend on the Sass rules), and provide a WORKSPACE macro that will download all dependencies (see rules_go above).

Rules

Often times there will be multiple rules provided by your repository. Create a directory named by the language and provide an entry point - defs.bzl file exporting all rules (also include a BUILD file so the directory is a package). For rules_mockascript that means there will be a directory named mockascript, and a BUILD file and a defs.bzl file inside:

/
  mockascript/
    BUILD
    defs.bzl

Constraints

If your rule defines toolchain rules, it’s possible that you’ll need to define custom constraint_settings and/or constraint_values. Put these into a //<LANG>/constraints package. Your directory structure will look like this:

/
  mockascript/
    constraints/
      BUILD
    BUILD
    defs.bzl

Please read github.com/bazelbuild/platforms for best practices, and to see what constraints are already present, and consider contributing your constraints there if they are language independent. Be mindful of introducing custom constraints, all users of your rules will use them to perform platform specific logic in their BUILD files (for example, using selects). With custom constraints, you define a language that the whole Bazel ecosystem will speak.

Repository Rules

Dependencies

Your rules might have external dependencies. To make depending on your rules simpler, please provide a WORKSPACE macro that will declare dependencies on those external dependencies. Do not declare dependencies of tests there, only dependencies that rules require to work. Put development dependencies into the WORKSPACE file.

Create a file named <LANG>/repositories.bzl and provide a single entry point macro named rules_<LANG>_dependencies. Our directory will look as follows:

/
  mockascript/
    constraints/
      BUILD
    BUILD
    defs.bzl
    repositories.bzl

Registering Toolchains

Your rules might also register toolchains. Please provide a separate WORKSPACE macro that registers these toolchains. This way users can decide to omit the previous macro and control dependencies manually, while still being allowed to register toolchains.

Therefore add a WORKSPACE macro named rules_<LANG>_toolchains into <LANG>/repositories.bzl file.

Note that in order to resolve toolchains in the analysis phase Bazel needs to analyze all toolchain targets that are registered. Bazel will not need to analyze all targets referenced by toolchain.toolchain attribute. If in order to register toolchains you need to perform complex computation in the repository, consider splitting the repository with toolchain targets from the repository with <LANG>_toolchain targets. Former will be always fetched, and the latter will only be fetched when user actually needs to build <LANG> code.

Release snippet

In your release announcement provide a snippet that your users can copy-paste into their WORKSPACE file. This snippet in general will look as follows:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
    name = "rules_<LANG>",
    urls = ["<url_to_the_release.zip"],
    sha256 = "4242424242",
)
load("@rules_<LANG>//<LANG>:repositories.bzl", "rules_<LANG>_dependencies", "rules_<LANG>_toolchains")
rules_<LANG>_dependencies()
rules_<LANG>_toolchains()

Tests

There should be tests that verify that the rules are working as expected. This can either be in the standard location for the language the rules are for or a tests/ directory at the top level.

Optional: Examples

It is useful to users to have an examples/ directory that shows users a couple of basic ways that the rules can be used.

Testing

Set up Travis as described in their getting started docs. Then add a .travis.yml file to your repository with the following content:

# On trusty images, the Bazel apt repository can be used.
addons:
  apt:
    sources:
    - sourceline: 'deb [arch=amd64] http://storage.googleapis.com/bazel-apt stable jdk1.8'
      key_url: 'https://bazel.build/bazel-release.pub.gpg'
    packages:
    - bazel

script:
  - bazel build //...
  - bazel test //...

Right now Bazel has to be compiled from source, as Travis does not support a version of GCC that works with the precompiled Bazel binaries. Thus, the before_install steps download the Bazel source, compile it, and “install” the Bazel binary in /usr/bin.

If your repository is under the bazelbuild organization, contact the bazel-dev list to have it added to ci.bazel.build.

Documentation

See the Skydoc documentation for instructions on how to comment your rules so that documentation can be generated automatically.

FAQs

Why can’t we add our rule to the main Bazel GitHub repository?

We want to decouple rules from Bazel releases as much as possible. It’s clearer who owns individual rules, reducing the load on Bazel developers. For our users, decoupling makes it easier to modify, upgrade, downgrade, and replace rules. Contributing to rules can be lighter weight than contributing to Bazel - depending on the rules -, including full submit access to the corresponding GitHub repository. Getting submit access to Bazel itself is a much more involved process.

The downside is a more complicated one-time installation process for our users: they have to copy-paste a rule into their WORKSPACE file, as shown in the README.html section above.

We used to have all of the rules in the Bazel repository (under //tools/build_rules or //tools/build_defs). We still have a couple rules there, but we are working on moving the remaining rules out.