Enable external metrics server for Puma

Production Change

Change Summary

As per gitlab-org/gitlab#350548 (closed)

We are extracting the in-process metrics server from Puma into its own process. This has already happened on staging-ref.

The change is currently behind an environment variable: PUMA_EXTERNAL_METRICS_SERVER.

Change Details

1. Services Impacted - webservice
2. Change Technician - DRI for the execution of this change
3. Change Reviewer - DRI for the review of this change
4. Time tracking - Time, in minutes, needed to execute all change steps, including rollback
5. Downtime Component - If there is a need for downtime, include downtime estimate here

Detailed steps for the change

Pre-Change Steps - steps to be completed before execution of the change

Change Steps - steps to take to execute the change

Set the PUMA_EXTERNAL_METRICS_SERVER env var to something truthy in all environments:

• Staging: gitlab-com/gl-infra/k8s-workloads/gitlab-com!1578 (merged)
• Prod cny: gitlab-com/gl-infra/k8s-workloads/gitlab-com!1609 (merged)
• All envs: gitlab-com/gl-infra/k8s-workloads/gitlab-com!1610 (merged)

Post-Change Steps - steps to take to verify the change

• Verify metrics are still served for Puma.
• Verify metrics are served from external process (web_exporter_ruby_* metrics should exist; these come from the new Ruby process)

Rollback

Rollback steps - steps to be taken in the event of a need to rollback this change

Set the PUMA_EXTERNAL_METRICS_SERVER env var to something falsey.

Monitoring

Key metrics to observe

• Metric: web_exporter_ruby_process_start_time_seconds
  • Location: https://thanos-query.ops.gitlab.net/graph?g0.expr=avg%20by%20(env)%20(web_exporter_ruby_process_start_time_seconds%7Bapp%3D%22webservice%22%7D)&g0.tab=0&g0.stacked=0&g0.range_input=1h&g0.max_source_resolution=0s&g0.deduplicate=1&g0.partial_response=0&g0.store_matches=%5B%5D
  • What changes to this metric should prompt a rollback: it does not exist. If other metrics are still served, it means that we merely failed to start the process but are serving from the in-app server. This might not necessitate a rollback, just investigation and a follow-up fix.

Summary of infrastructure changes

We are starting 1 extra Ruby process per Puma pod. In past measurements I found the extra memory use to be acceptable (around 50M), but it may vary over time and depend on metrics volume.

Change Reviewer checklist

C4 C3 C2 C1:

• The scheduled day and time of execution of the change is appropriate.
• The change plan is technically accurate.
• The change plan includes estimated timing values based on previous testing.
• The change plan includes a viable rollback plan.
• The specified metrics/monitoring dashboards provide sufficient visibility for the change.

C2 C1:

• The complexity of the plan is appropriate for the corresponding risk of the change. (i.e. the plan contains clear details).
• The change plan includes success measures for all steps/milestones during the execution.
• The change adequately minimizes risk within the environment/service.
• The performance implications of executing the change are well-understood and documented.
• The specified metrics/monitoring dashboards provide sufficient visibility for the change. - If not, is it possible (or necessary) to make changes to observability platforms for added visibility?
• The change has a primary and secondary SRE with knowledge of the details available during the change window.

Change Technician checklist

• This issue has a criticality label (e.g. C1, C2, C3, C4) and a change-type label (e.g. changeunscheduled, changescheduled) based on the Change Management Criticalities.
• This issue has the change technician as the assignee.
• Pre-Change, Change, Post-Change, and Rollback steps and have been filled out and reviewed.
• This Change Issue is linked to the appropriate Issue and/or Epic
• Necessary approvals have been completed based on the Change Management Workflow.
• Change has been tested in staging and results noted in a comment on this issue.
• A dry-run has been conducted and results noted in a comment on this issue.
• SRE on-call has been informed prior to change being rolled out. (In #production channel, mention @sre-oncall and this issue and await their acknowledgement.)
• Release managers have been informed (If needed! Cases include DB change) prior to change being rolled out. (In #production channel, mention @release-managers and this issue and await their acknowledgment.)
• There are currently no active incidents.