[Feature flag] Rollout of `load_balancer_low_statement_timeout`

Summary

This issue is to roll out a lower statement timeout for load balancer health check queries (#472161 (closed)) that is currently behind the load_balancer_low_statement_timeout feature flag.

Owners

Most appropriate Slack channel to reach out to: #g_database
Best individual to reach out to: @stomlinson

Expectations

What are we expecting to happen?

No changes to the application should happen unless a database replica disk becomes unresponsive. In that case, we expect to stop using the replica instead of getting stuck querying it and locking up the application

What can go wrong and how would we detect it?

The feature uses a new connection from the replica's connection pool to check replica status with a different statement timeout. Connection counts to replica pgbouncer connection pools may increase slightly, but it should be so slight that it's unnoticable.

If there's a logic bug or other problem, it's possible that we could incorrectly remove replicas from load balancing. In that case we'd want to disable the flag.

We can monitor load balancing logs with https://log.gprd.gitlab.net/app/r/s/QlAV5 (excluding normal host_list_update and service_discovery_failure messages which happen every few minutes and when new pods boot but consul isn't ready yet, respectively).

We should also look for any abrupt shift in traffic from replicas to the primary, either for the main or ci databases. We can monitor that from patroni overview dashboards: main, ci

Any abrupt shift means we should turn off the flag.

Rollout Steps

Note: Please make sure to run the chatops commands in the Slack channel that gets impacted by the command.

Rollout on non-production environments

Verify the MR with the feature flag is merged to master and have been deployed to non-production environments with /chatops run auto_deploy status <merge-commit-of-your-feature>
Enable the feature globally on non-production environments with /chatops run feature set load_balancer_low_statement_timeout true --dev --pre --staging --staging-ref
Verify that the feature works as expected. The best environment to validate the feature in is staging-canary as this is the first environment deployed to. Make sure you are configured to use canary.
If the feature flag causes end-to-end tests to fail, disable the feature flag on staging to avoid blocking deployments.
- See #qa-staging Slack channel and look for the following messages:
  - test kicked off: Feature flag load_balancer_low_statement_timeout has been set to true on **gstg**
  - test result: This pipeline was triggered due to toggling of load_balancer_low_statement_timeout feature flag

For assistance with end-to-end test failures, please reach out via the #test-platform Slack channel. Note that end-to-end test failures on staging-ref don't block deployments.

Preparation before global rollout

Set a milestone to this rollout issue to signal for enabling and removing the feature flag when it is stable.
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 exists for the feature, and the version history text has been updated.
Leave a comment on the feature issue announcing estimated time when this feature flag 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 the #support_gitlab-com Slack channel and your team channel (more guidance when this is necessary in the dev docs).
Ensure that the feature flag rollout plan is reviewed by another developer familiar with the domain.

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).

(Optional) Incrementally roll out the feature on production environment.
- Between every step wait for at least 15 minutes and monitor the appropriate graphs on https://dashboards.gitlab.net.
- Perform actor-based rollout: /chatops run feature set load_balancer_low_statement_timeout <rollout-percentage> --actors
Enable the feature globally on production environment: /chatops run feature set load_balancer_low_statement_timeout true
Observe appropriate graphs on https://dashboards.gitlab.net and verify that services are not affected.
Leave a comment on [the feature issue][main-issue] announcing 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

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.

See instructions if you're sure about enabling the feature globally through the feature flag definition

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:

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 the load_balancer_low_statement_timeout feature flag. Ask for review/approval/merge as usual. The MR should include the following changes:
- 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 https://gitlab.com/gitlab-org/gitlab/-/merge_requests/159633 17.2
Close [the feature issue][main-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 load_balancer_low_statement_timeout --dev --pre --staging --staging-ref --production
Close this rollout issue.

Rollback Steps

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

/chatops run feature set load_balancer_low_statement_timeout false

Disable the feature flag on non-production environments:

/chatops run feature set load_balancer_low_statement_timeout false --dev --pre --staging --staging-ref

Delete feature flag from all environments:

/chatops run feature delete load_balancer_low_statement_timeout --dev --pre --staging --staging-ref --production

Edited Oct 22, 2024 by Simon Tomlinson