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This page provides information about how to handle backward compatibility, including migrating from one release to another and how to communicate incompatible changes. Bazel is evolving. Minor versions released as part of an LTS major version are fully backward-compatible. New major LTS releases may contain incompatible changes that require some migration effort. For more information about Bazel’s release model, please check out the Release Model page.

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Summary

  1. It is recommended to use --incompatible_* flags for breaking changes.
  2. For every --incompatible_* flag, a GitHub issue explains the change in behavior and aims to provide a migration recipe.
  3. Incompatible flags are recommended to be back-ported to the latest LTS release without enabling the flag by default.
  4. APIs and behavior guarded by an --experimental_* flag can change at any time.
  5. Never run production builds with --experimental_* or --incompatible_* flags.

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How to follow this policy

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What is stable functionality?

In general, APIs or behaviors without --experimental_... flags are considered stable, supported features in Bazel. This includes:
  • Starlark language and APIs
  • Rules bundled with Bazel
  • Bazel APIs such as Remote Execution APIs or Build Event Protocol
  • Flags and their semantics

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Incompatible changes and migration recipes

For every incompatible change in a new release, the Bazel team aims to provide a migration recipe that helps you update your code (BUILD and .bzl files, as well as any Bazel usage in scripts, usage of Bazel API, and so on). Incompatible changes should have an associated --incompatible_* flag and a corresponding GitHub issue. The incompatible flag and relevant changes are recommended to be back-ported to the latest LTS release without enabling the flag by default. This allows users to migrate for the incompatible changes before the next LTS release is available.

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Communicating incompatible changes

The primary source of information about incompatible changes are GitHub issues marked with an “incompatible-change” label. For every incompatible change, the issue specifies the following:
  • Name of the flag controlling the incompatible change
  • Description of the changed functionality
  • Migration recipe
When an incompatible change is ready for migration with Bazel at HEAD (therefore, also with the next Bazel rolling release), it should be marked with the migration-ready label. The incompatible change issue is closed when the incompatible flag is flipped at HEAD.