[Feature flag] Enable pausing background migration on high number of WAL segments pending archival

Summary

This issue is to rollout Pause migration when WAL queue pending archival... (#357249 - closed) on production, that is currently behind the batched_migrations_health_status_wal feature flag.

Owners

• Team: groupdatabase
• Most appropriate slack channel to reach out to: #g_database
• Best individual to reach out to: @krasio
• PM: @fzimmer

Stakeholders

Expectations

What are we expecting to happen?

When feature is enabled, batched background migration will be throttled if the number of WAL segments pending archival is above 42.

What might happen if this goes wrong?

In the worst case, batched background migrations are not executed (because of some bug in our code).

What can we monitor to detect problems with this?

Generic Kibana log search for batched migration jobs transitions - look for many jobs reported as failed.

What can we check for monitoring production after rollouts?

• Kibana log search for migrations put on hold
• Kibana log search for batched migration signals
• PostgreSQL Overview dashboard

Rollout Steps

Rollout on non-production environments

• Ensure that the feature MRs have been deployed to non-production environments.
  • /chatops run auto_deploy status d18535056bebb1fd3d561c3d83ddf69e44f82640
• Enable the feature globally on non-production environments.
  • /chatops run feature set batched_migrations_health_status_wal true --dev --staging --staging-ref
• Verify that the feature works as expected. Posting the QA result in this issue is preferable. The best environment to validate the feature in is staging-canary as this is the first environment deployed to. Note you will need to make sure you are configured to use canary as outlined here when accessing the staging environment in order to make sure you are testing appropriately.

Specific rollout on production

• Ensure that the feature MRs have been deployed to both production and canary.
  • /chatops run auto_deploy status d18535056bebb1fd3d561c3d83ddf69e44f82640

Preparation before global rollout

• 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 Pause migration when WAL queue pending archival... (#357249 - closed) an estimated time this will be enabled on GitLab.com.
• Ensure that any breaking changes have been announced following the release post process to ensure GitLab customers are aware.
• Notify #support_gitlab-com and your team channel (more guidance when this is necessary in the dev docs).

Global rollout on production

For visibility, all /chatops commands that target production should be executed in the #production slack channel and cross-posted (with the command results) to the responsible team's slack channel (#g_database).

• Incrementally roll out the feature.
  • Enable the feature globally on production environment.
    • /chatops run feature set batched_migrations_health_status_wal true
• Announce on Pause migration when WAL queue pending archival... (#357249 - closed) that the feature has been globally enabled.
• Wait for at least one day for the verification term.

(Optional) Release the feature with the feature flag

If you're still unsure whether the feature is deemed stable but want to release it in the current milestone, you can change the default state of the feature flag to be enabled. To do so, follow these steps:

• Create a merge request with the following changes. Ask for review and merge it.
  • Set the default_enabled attribute in the feature flag definition to true.
  • Create a changelog entry.
• Ensure that the default-enabling MR has been included in the release package. If the merge request was deployed before the monthly release was tagged, the feature can be officially announced in a release blog post.
  • /chatops run release check <merge-request-url> <milestone>
• Consider cleaning up the feature flag from all environments by running these chatops command in #production channel. Otherwise these settings may override the default enabled.
  • /chatops run feature delete <feature-flag-name> --dev --staging --staging-ref --production
• Close the feature issue to indicate the feature will be released in the current milestone.
• Set the next milestone to this rollout issue for scheduling the flag removal.
• (Optional) You can create a separate issue for scheduling the steps below to Release the feature.
  • Set the title to "[Feature flag] Cleanup <feature-flag-name>".
  • Execute the /copy_metadata <this-rollout-issue-link> quick action to copy the labels from this rollout issue.
  • Link this rollout issue as a related issue.
  • Close this rollout issue.

WARNING: This approach has the downside that it makes it difficult for us to clean up the flag. For example, on-premise users could disable the feature on their GitLab instance. But when you remove the flag at some point, they suddenly see the feature as enabled and they can't roll it back to the previous behavior. To avoid this potential breaking change, use this approach only for urgent matters.

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.

You can either create a follow-up issue for Feature Flag Cleanup or use the checklist below in this same issue.

• 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 included in the release package. If the merge request was deployed before the monthly release was tagged, the feature can be officially announced in a release blog post.
  • /chatops run release check <merge-request-url> <milestone>
• Close the feature issue to indicate the feature will be released in the current milestone.
• If not already done, clean up the feature flag from all environments by running these chatops command in #production channel:
  • /chatops run feature delete <feature-flag-name> --dev --staging --staging-ref --production
• Close this rollout issue.

Rollback Steps

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

/chatops run feature set batched_migrations_health_status_wal false