[Feature flag] Rollout of `between_uses_list_commits`

Summary

This issue is to rollout the feature on production, that is currently behind the between_uses_list_commits feature flag.

Owners

• Team: groupsource code
• Most appropriate slack channel to reach out to: #g_create_source-code
• Best individual to reach out to: Nick Thomas @nick.thomas
• PM: Sarah Waldner @sarahwaldner

Stakeholders

The Rollout Plan

• 50% rollout
• Rollout on GitLab.com for a couple of days
• Rollout Feature for everyone as soon as it's ready

We don't have easy access to a repository container (project/group) to gate the FF on, but the FF is more out of an abundance of caution anyway.

Testing Groups/Projects/Users

N/a

Expectations

What are we expecting to happen?

This should be a no-op, except we switch from a deprecated Gitaly RPC (CommitsBetween) to a new one (ListCommits) for the "get commits between these two revisions" operation. This code is most often executed by Git::BranchHooksService as part of the PostReceive worker executing

What might happen if this goes wrong?

Git pushes could get all messed up. Particularly, processing commit messages and hook execution.

What can we monitor to detect problems with this?

Two Sidekiq workers in particular are affected: PostReceive and UpdateMergeRequestsWorker:

https://dashboards.gitlab.net/d/sidekiq-worker-detail/sidekiq-worker-detail?orgId=1&var-PROMETHEUS_DS=Global&var-environment=gprd&var-stage=main&var-worker=PostReceive

https://dashboards.gitlab.net/d/sidekiq-worker-detail/sidekiq-worker-detail?orgId=1&var-PROMETHEUS_DS=Global&var-environment=gprd&var-stage=main&var-worker=UpdateMergeRequestsWorker

These workers will switch much of their traffic from the CommitsBetween RPC to the ListCommits RPC, without any change to the number of commits they are requesting. If ListCommits is more expensive in some way, this may be noticeable.

We can also watch these two RPCs in Gitaly:

• https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&refresh=30s&var-environment=gprd&var-method=CommitsBetween&var-prometheus=prometheus-01-inf-gprd

• https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&refresh=30s&var-environment=gprd&var-method=ListCommits&var-prometheus=prometheus-01-inf-gprd

We expect to see request rate to the former to drop, and to increase by a similar amount to the latter. Again, if ListCommits is more expensive for the same work, we'll see this here.

Rollout Steps

Rollout on non-production environments

• Ensure that the feature MRs have been deployed to non-production environments.
  • /chatops run auto_deploy status <merge-commit-of-your-feature>
• Enable the feature globally on non-production environments.
  • /chatops run feature set <feature-flag-name> true --dev
  • /chatops run feature set <feature-flag-name> true --staging
• Verify that the feature works as expected. Posting the QA result in this issue is preferable.

Preparation before production rollout

• Ensure that the feature MRs have been deployed to both production and canary.
  • /chatops run auto_deploy status <merge-commit-of-your-feature>
• Check if the feature flag change needs to be accompanied with a change management issue. Cross link the issue here if it does.
• Ensure that you or a representative in development can be available for at least 2 hours after feature flag updates in production. If a different developer will be covering, or an exception is needed, please inform the oncall SRE by using the @sre-oncall Slack alias.
• Ensure that documentation has been updated (More info).
• Announce on the feature issue an estimated time this will be enabled on GitLab.com.
• If the feature might impact the user experience, notify #support_gitlab-com and your team channel (more guidance when this is necessary in the dev docs).
• Verify that the feature works as expected. Posting the QA result in this issue is preferable.

Global rollout on production

• Incrementally roll out the feature.
  • If the feature flag in code has an actor, perform actor-based rollout.
    • /chatops run feature set <feature-flag-name> <rollout-percentage> --actors
  • If the feature flag in code does NOT have an actor, perform time-based rollout (random rollout).
    • /chatops run feature set <feature-flag-name> <rollout-percentage>
  • Enable the feature globally on production environment.
    • /chatops run feature set <feature-flag-name> true
• Announce on the feature issue that the feature has been globally enabled.
• Wait for at least one day for the verification term.

Release the feature

After the feature has been deemed stable, the clean up should be done as soon as possible to permanently enable the feature and reduce complexity in the codebase.

• Create a merge request to remove <feature-flag-name> feature flag. Ask for review and merge it.
  • Remove all references to the feature flag from the codebase.
  • Remove the YAML definitions for the feature from the repository.
  • Create a changelog entry.
• Ensure that the cleanup MR has been deployed to both production and canary. If the merge request was deployed before the code cutoff, the feature can be officially announced in a release blog post.
  • /chatops run auto_deploy status <merge-commit-of-cleanup-mr>
• Close the feature issue to indicate the feature will be released in the current milestone.
• Clean up the feature flag from all environments by running these chatops command in #production channel:
  • /chatops run feature delete <feature-flag-name> --dev
  • /chatops run feature delete <feature-flag-name> --staging
  • /chatops run feature delete <feature-flag-name>
• Close this rollout issue.

Rollback Steps

• This feature can be disabled by running the following Chatops command:

/chatops run feature set <feature-flag-name> false