Rule Compatibility

Bazel Starlark rules can break compatibility with Bazel LTS releases in the following two scenarios:

1. The rule breaks compatibility with future LTS releases because a feature it depends on is removed from Bazel at HEAD.
2. The rule breaks compatibility with the current or older LTS releases because a feature it depends on is only available in newer Bazel LTS releases.

Meanwhile, the rule itself can ship incompatible changes for their users as well. When combined with breaking changes in Bazel, upgrading the rule version and Bazel version can often be a source of frustration for Bazel users. This page covers how rules authors should maintain rule compatibility with Bazel to make it easier for users to upgrade Bazel and rules.

Manageable migration process

While it’s obviously not feasible to guarantee compatibility between every version of Bazel and every version of the rule, our aim is to ensure that the migration process remains manageable for Bazel users. A manageable migration process is defined as a process where users are not forced to upgrade the rule’s major version and Bazel’s major version simultaneously, thereby allowing users to handle incompatible changes from one source at a time.

For example, with the following compatibility matrix:

• Migrating from rules_foo 1.x + Bazel 4.x to rules_foo 2.x + Bazel 5.x is not considered manageable, as the users need to upgrade the major version of rules_foo and Bazel at the same time.
• Migrating from rules_foo 2.x + Bazel 5.x to rules_foo 3.x + Bazel 6.x is considered manageable, as the users can first upgrade rules_foo from 2.x to 3.x without changing the major Bazel version, then upgrade Bazel from 5.x to 6.x.

	rules_foo 1.x	rules_foo 2.x	rules_foo 3.x	HEAD
Bazel 4.x	✅	❌	❌	❌
Bazel 5.x	❌	✅	✅	❌
Bazel 6.x	❌	❌	✅	✅
HEAD	❌	❌	❌	✅

❌: No version of the major rule version is compatible with the Bazel LTS release.

✅: At least one version of the rule is compatible with the latest version of the Bazel LTS release.

Best practices

As Bazel rules authors, you can ensure a manageable migration process for users by following these best practices:

1. The rule should follow Semantic Versioning: minor versions of the same major version are backward compatible.
2. The rule at HEAD should be compatible with the latest Bazel LTS release.
3. The rule at HEAD should be compatible with Bazel at HEAD. To achieve this, you can
   • Set up your own CI testing with Bazel at HEAD
   • Add your project to Bazel downstream testing; the Bazel team files issues to your project if breaking changes in Bazel affect your project, and you must follow our downstream project policies to address issues timely.
4. The latest major version of the rule must be compatible with the latest Bazel LTS release.
5. A new major version of the rule should be compatible with the last Bazel LTS release supported by the previous major version of the rule.

Achieving 2. and 3. is the most important task since it allows achieving 4. and 5. naturally.

To make it easier to keep compatibility with both Bazel at HEAD and the latest Bazel LTS release, rules authors can:

• Request backward-compatible features to be back-ported to the latest LTS release, check out release process for more details.
• Use bazel_features to do Bazel feature detection.

In general, with the recommended approaches, rules should be able to migrate for Bazel incompatible changes and make use of new Bazel features at HEAD without dropping compatibility with the latest Bazel LTS release.