[Feature flag] Rollout of `drop_detached_partitions`

Summary

This issue is to rollout dropping partitions previously detached by partition pruning on production, that is currently behind the drop_detached_partitions feature flag.

Once partitions are detached by enabling the partition pruning feature flag, enabling this feature flag will drop the detached tables one week later.

Owners

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

Stakeholders

The Rollout Plan

• Partial Rollout on GitLab.com with testing groups
• Rollout on GitLab.com for a certain period (How long)
• Percentage Rollout on GitLab.com
• Rollout Feature for everyone as soon as it's ready

Testing Groups/Projects/Users

This feature flag can only be toggled globally

Expectations

What are we expecting to happen?

For each row in the detached_partitions table, the scheduled job behind the drop_detached_partitions feature flag will drop the table associated with that row and delete the row from the database.

What might happen if this goes wrong?

If bad table names end up in the detached_partitions table, we could drop the wrong table. This is mitigated by the fact that this feature will only drop tables if:

1. The table is NOT a partition of another table
2. The table is in the gitlab_partitions_dynamic schema

This makes this feature flag relatively safe, but if we do delete the wrong data by accident, we will need to recover it from a backup.

What can we monitor to detect problems with this?

This kibana link shows recent drop events, and should be monitored to make sure that we are dropping the correct tables.

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 drop_detached_partitions true --dev
  • /chatops run feature set drop_detached_partitions 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).
• If the feature flag in code has an actor, enable it on GitLab.com for testing groups/projects.
  • /chatops run feature set --<actor-type>=<actor> drop_detached_partitions true
• 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 drop_detached_partitions <rollout-percentage> --actors
  • If the feature flag in code does NOT have an actor, perform time-based rollout (random rollout).
    • /chatops run feature set drop_detached_partitions <rollout-percentage>
  • Enable the feature globally on production environment.
    • /chatops run feature set drop_detached_partitions true
• Announce on the feature issue 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 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-default-enabling-mr>
• 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 drop_detached_partitions".
  • 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.

• Create a merge request to remove drop_detached_partitions 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 drop_detached_partitions --dev
  • `/chatops run feature delete drop_detached_partitions --staging
  • /chatops run feature delete drop_detached_partitions
• Close this rollout issue.

Rollback Steps

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

/chatops run feature set drop_detached_partitions false