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Committer Guide

This guide is for committers and covers Beam's guidelines for reviewing and merging code.

Pull request review objectives

The review process aims for:

  • Review iterations should be efficient, timely and of quality (avoid tiny or out-of-context changes or huge mega-changes)
  • Support efficiency of authoring (don't want to wait on a review for a tiny bit because GitHub makes it very hard to stack up reviews in sequence / don't want to have major changes blocked because of difficulty of review)
  • Ease of first-time contribution (encourage to follow contribution guildelines but committer may absorb some extra effort for new contributors)
  • Pull requests and commit messages establish a clear history with purpose and origin of changes
  • Ability to perform a granular rollback, if necessary (also see policies)

Granularity of changes:

  • We prefer small independent, incremental PRs with descriptive, isolated commits. Each commit is a single clear change
  • It is OK to keep separate commits for different logical pieces of the code, if they make reviewing and revisiting code easier
  • Making commits isolated is a good practice, authors should be able to relatively easily split the PR upon reviewer's request
  • Generally, every commit should compile and pass tests.
  • Avoid keeping in history formatting messages such as checkstyle or spotless fixes. Squash such commits with previous one.

Always get to LGTM ("Looks good to me!")

After a pull request goes through rounds of reviews and revisions, it will become ready for merge. A reviewer signals their approval either by GitHub "approval" or by a comment such as "Looks good to me!" (LGTM).

  • If the author of the pull request is not a committer, a committer must be the one to approve the change.
  • If the author of the pull request is a committer, approval from their chosen reviewer is enough. A committer is trusted to choose an appropriate reviewer, even if the reviewer is not a committer.

Once a pull request is approved, any committer can merge it.

Exceptions to this rule are rare and made on a case-by-case basis. A committer may use their discretion for situations such as when the build is broken on the main branch, blocking all code contributions. In this case, you should still seek a review on the pull request! A common acronym you may see is "TBR" -- "to be reviewed".

Always go through a pull request, even if you won’t wait for the code review. Committers should never commit anything without going through a pull request, even when it is an urgent fix or rollback due to build breakage. Skipping pull request bypasses test coverage and could potentially cause the build to fail, or fail to fix breakage. In addition, pull requests ensure that changes are communicated properly and potential flaws or improvements can be spotted, even after the merge happens.

Contributor License Agreement

If you are merging a larger contribution, please make sure that the contributor has an ICLA on file with the Apache Secretary. You can view the list of committers here, as well as ICLA-signers who aren’t yet committers.

For smaller contributions, however, this is not required. In this case, we rely on clause five of the Apache License, Version 2.0, describing licensing of intentionally submitted contributions.

Tests

Before merging, please make sure that Jenkins tests pass, as visible in the GitHub pull request. Do not merge the pull request if there are test failures.

If the pull request contains changes that call for extra test coverage, you can ask Jenkins to run an extended test suite. For example, if the pull request modifies a runner, you can run the full ValidatesRunner suite with a comment such as "Run Spark ValidatesRunner". You can run the examples and some IO integration tests with "Run Java PostCommit".

Finishing touches

At some point in the review process, the change to the codebase will be complete. However, the pull request may have a collection of review-related commits that are not meaningful to preserve in the history. The reviewer should give the LGTM and then request that the author of the pull request rebase, squash, split, etc, the commits, so that the history is most useful:

  • Favor commits that do just one thing. The commit is the smallest unit of easy rollback; it is easy to roll back many commits, or a whole pull request, but harder to roll back part of a commit.
  • Commit messages should be descriptive and should reference the issue number that they address. It should later not be necessary to find a merge commit or first PR commit to find out what caused a change.
  • Pull request descriptions should contain a link to the issue being addressed by the changes.
  • CHANGES.md file should be updated with noteworthy changes (e.g. new features, backward incompatible changes, dependency changes, etc.).
  • Squash the "Fixup!", "Address comments" type of commits that resulted from review iterations.

Merging it!

While it is preferred that authors squash commits after review is complete, there may be situations where it is more practical for the committer to handle this (such as when the action to be taken is obvious or the author isn't available). The committer may use the "Squash and merge" option in Github (or modify the PR commits in other ways). The committer is ultimately responsible and we "trust the committer's judgment"!

After all the tests pass, there should be a green merge button at the bottom of the pull request. There are multiple choices. Unless you want to squash commits as part of the merge (see above) you should choose "Merge pull request" and ensure "Create a merge commit" is selected from the drop down. This preserves the commit history and adds a merge commit, so be sure the commit history has been curated appropriately.

Do not use the default GitHub commit message, which looks like this:

Merge pull request #1234 from some_user/transient_branch_name

[BEAM-7873] Fix the foo bizzle bazzle

Instead, pull it all into the subject line:

Merge pull request #1234: [BEAM-7873] Fix the foo bizzle bazzle

If you have comments to add, put them in the body of the commit message.

Seed jobs

As a committer, you can now run seed jobs! These are used to update our Jenkins configuration and can be run to test PRs modifying Groovy files before they are merged.

To make sure you have these permissions, put up a PR adding yourself to https://github.com/apache/beam/blob/master/.test-infra/jenkins/Committers.groovy