Documentation

Deploying new Skylark rules

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

Where to put new rules

In general, 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.

You can see lots of examples of what your repository should look like on GitHub: see all of the repositories named rules_whatever. In particular, rules_scala is a nice example of how to set up your repo.

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

What a rule repository should contain

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

For example, suppose we are writing new Skylark rules for the (make-believe) chaiscript language. We would have the following structure:

.travis.yml
README.html
WORKSPACE
chaiscript/
  BUILD
  chaiscript.bzl
tests/
  BUILD
  some_test.sh
  another_test.py
examples/
  BUILD
  bin.chai
  lib.chai
  test.chai

README.html

At the top level, there should be a README.html 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 git_repository pointing to your GitHub repo and a macro call that downloads/configures any tools your rule needs. For example, for the Go rules, this looks like:

git_repository(
    name = "io_bazel_rules_go",
    remote = "https://github.com/bazelbuild/rules_go.git",
    tag = "0.0.2",
)
load("@io_bazel_rules_go//go:def.bzl", "go_repositories")

go_repositories()

If your rules depend on another repository's rules, specify both in the README.html (see the Skydoc rules, which depend on the Sass rules, for an example of this).

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:

language:
  - java
jdk:
  - oraclejdk8  # Building Bazel requires JDK8.
before_install:
  - wget https://github.com/bazelbuild/bazel/archive/0.3.0.zip  # Replace with desired version
  - unzip 0.3.0.zip
  - cd bazel-0.3.0
  - ./compile.sh
  - sudo cp output/bazel /usr/bin/bazel
  - cd ..
  - rm -rf bazel-0.3.0
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 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 section above.

We used to have all of the Skylark 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.