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Incidents/2022-03-31 api errors

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document status: draft

Summary

Incident metadata (see Incident Scorecard)
Incident ID 2022-03-31 api errors Start 2022-03-31 05:18
Task T305119 End 2022-03-31 05:40
People paged 17 Responder count 2
Coordinators n/a Affected metrics/SLOs No relevant SLOs exist. The API Gateway SLO was unaffected. If the planned app server SLO existed, we would have consumed a small portion of its error budget.
Impact For 22 minutes, API server and app server availability were slightly decreased (~0.1% errors, all for s7-hosted wikis such as Spanish Wikipedia), and the latency of API servers was elevated as well.

After a code change [1] [2] rolled out in this week's train, the GlobalUsersPager class (part of CentralAuth) produced expensive DB queries that exhausted resources on s7 database replicas.

Backpressure from the databases tied up PHP-FPM workers on the API servers, triggering a paging alert for worker saturation. The slow queries were identified and manually killed on the database, which resolved the incident.

Because the alert fired and the queries were killed before available workers were fully exhausted, the impact was limited to s7. Full worker saturation would have resulted in a complete API outage.

Because only two engineers responded to the page and the response only took half an hour, we decided not to designate an incident coordinator, start a status doc, and so on. We didn't need those tools to organize the response, and they would have taken time away from solving the problem.

Documentation:

Actionables

  • Revert the patches generating the slow queries - done [3] [4]
  • Later (2022-04-06) it was discovered the query killer was using the old 'wikiuser' name, which prevented it from acting. Fixed in [5], deploying soon.

Scorecard

Incident Engagement™ ScoreCard
Question Score Notes
People Were the people responding to this incident sufficiently different than the previous five incidents? (score 1 for yes, 0 for no) 0 Info not logged
Were the people who responded prepared enough to respond effectively (score 1 for yes, 0 for no) 1
Were more than 5 people paged? (score 0 for yes, 1 for no) 0 paged via batphone
Were pages routed to the correct sub-team(s)? (score 1 for yes, 0 for no) 0 paged via batphone
Were pages routed to online (business hours) engineers? (score 1 for yes,  0 if people were paged after business hours) 0 paged via batphone
Process Was the incident status section actively updated during the incident? (score 1 for yes, 0 for no) 0
Was the public status page updated? (score 1 for yes, 0 for no) 0
Is there a phabricator task for the incident? (score 1 for yes, 0 for no) 1
Are the documented action items assigned?  (score 1 for yes, 0 for no) 1
Is this a repeat of an earlier incident (score 0 for yes, 1 for no) 1
Tooling Was there, before the incident occurred, open tasks that would prevent this incident / make mitigation easier if implemented? (score 0 for yes, 1 for no) 1
Were the people responding able to communicate effectively during the incident with the existing tooling? (score 1 for yes, 0 or no) 1
Did existing monitoring notify the initial responders? (score 1 for yes, 0 for no) 1
Were all engineering tools required available and in service? (score 1 for yes, 0 for no) 1
Was there a runbook for all known issues present? (score 1 for yes, 0 for no) 1
Total score 9