This page is for rule writers who are planning to make their rules available to others.
We recommend you start a new ruleset from the template repository: https://github.com/bazel-contrib/rules-template That template follows the recommendations below, and includes API documentation generation and sets up a CI/CD pipeline to make it trivial to distribute your ruleset.
Hosting and naming rules
New rules should go into their own GitHub repository under your organization. Start a thread on GitHub 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 should follow this same format when publishing your Bazel rules.
Make sure to use a descriptive GitHub repository description and README.md
title, example:
- Repository name:
bazelbuild/rules_go - Repository description: Go rules for Bazel
- Repository tags:
golang,bazel README.mdheader: 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 (such as Scala), runtime platform (such as Android), or framework (such as Spring).
Repository content
Every rule repository should have a certain layout so that users can quickly understand new rules.
For example, when writing new rules for the (make-believe)
mockascript language, the rule repository would have the following structure:
/
LICENSE
README
WORKSPACE
mockascript/
constraints/
BUILD
runfiles/
BUILD
runfiles.mocs
BUILD
defs.bzl
tests/
BUILD
some_test.sh
another_test.py
examples/
BUILD
bin.mocs
lib.mocs
test.mocs
WORKSPACE
In the project's WORKSPACE, you should define the name that users will use
to reference your rules. If your rules belong to the
bazelbuild organization, you must use
rules_<lang> (such as rules_mockascript). Otherwise, you should name your
repository <org>_rules_<lang> (such as build_stack_rules_proto). Please
start a thread on GitHub
if you feel like your rules should follow the convention for rules in the
bazelbuild organization.
In the following sections, assume the repository belongs to the bazelbuild organization.
workspace(name = "rules_mockascript")
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.
Runfiles library
If your rule provides a standard library for accessing runfiles, it should be
in the form of a library target located at //<LANG>/runfiles (an abbreviation
of //<LANG>/runfiles:runfiles). User targets that need to access their data
dependencies will typically add this target to their deps attribute.
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.
Examples (optional)
It is useful to users to have an examples/ directory that shows users a couple
of basic ways that the rules can be used.
CI/CD
Many rulesets use GitHub Actions. See the configuration used in the rules-template repo, which are simplified using a "reusable workflow" hosted in the bazel-contrib
org. ci.yaml runs tests on each PR and main comit, and release.yaml runs anytime you push a tag to the repository.
See comments in the rules-template repo for more information.
If your repository is under the bazelbuild organization, you can ask to add it to ci.bazel.build.
Documentation
See the Stardoc documentation for instructions on how to comment your rules so that documentation can be generated automatically.
The rules-template docs/ folder
shows a simple way to ensure the Markdown content in the docs/ folder is always up-to-date
as Starlark files are updated.
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.md 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.