Incidents/2022-12-21 shellbox-syntaxhighlight

== Summary ==
…
A sudden spike in requests to shellbox-syntaxhighlight overloaded the service leading to slow response times and increased failures.
The majority of requests where originating from jobrunners and api_appservers.
==Timeline==
All times in UTC.
*09:00 OUTAGE BEGINS non paging alerts of failing http-api probes and api_appservers running out of idle workers where observed in #mediawiki-operations; Investigation started
*09:03 paging alert: (ProbeDown) firing: Service shellbox-syntaxhighlight:4014 has failed probes (http_shellbox-syntaxhighlight_ip4)
*09:05 shellbox-syntaxhighlight req/s increased from ~5 to ~70req/s
*09:05 shellbox-syntaxhighlight scaled up from 12 to 40 replicas
*09:09 Alerts recovering
*09:10 OUTAGE ENDS
*09:11 (CirrusSearchJobQueueBacklogTooBig) firing: CirrusSearch job topic eqiad.mediawiki.job.cirrusSearchLinksUpdate is heavily backlogged with 209k messages
*09:49 cirrusSearchLinksUpdate backlog was cleared
*09:54 shellbox-syntaxhighlight scaled back to 12 replicas
==Detection==
Initially detected by a human spotting alerts on IRC, closely followed by a page.
==Conclusions ==
===What went well?===
* Problem spotted early and countermeasures where taken quickly
* One of the SREs paged knew how to increase the number of syntaxhighlight runners (it's not mentioned on Shellbox)
===What went poorly?===
* shellbox-syntaxhighlight was probably overloaded by our own services (jobrunners), but currently this is an assumption.
===Where did we get lucky?===
* The burst in requests was rather short
==Links to relevant documentation==
* https://wikitech.wikimedia.org/wiki/Kubernetes/Deployments
==Actionables==
* Figure out what caused the burst (there's a suggestion a template was changed leading to a lot of pages needing re-rendering at once)
* Document how to scale up shellbox runners? (or link to it from Shellbox)
==Scorecard==
Incident Engagement ScoreCard
 	Question 	Answer
(yes/no)	Notes
People 	Were the people responding to this incident sufficiently different than the previous five incidents? 	Yes	
People 	Were the people who responded prepared enough to respond effectively 	Yes	
People 	Were fewer than five people paged? 	Yes	
People 	Were pages routed to the correct sub-team(s)? 	Yes	
People 	Were pages routed to online (business hours) engineers?  Answer “no” if engineers were paged after business hours.	Yes	
Process 	Was the "Incident status" section atop the Google Doc kept up-to-date during the incident? 	No 	no google doc was created, only 3 responders
Process 	Was a public wikimediastatus.net entry created? 	No 	
Process 	Is there a phabricator task for the incident? 	Yes	
Process 	Are the documented action items assigned? 	No 	
Process 	Is this incident sufficiently different from earlier incidents so as not to be a repeat occurrence? 	No 	
Tooling 	To the best of your knowledge was the open task queue free of any tasks that would have prevented this incident? Answer “no” if there are
open tasks that would prevent this incident or make mitigation easier if implemented.	No 	While they might not have fully prevented this incident, there are (long) outstanding performance issues with syntaxhighlight: ,
Tooling 	Were the people responding able to communicate effectively during the incident with the existing tooling? 	Yes	
Tooling 	Did existing monitoring notify the initial responders? 	Yes	
Tooling 	Were the engineering tools that were to be used during the incident, available and in service? 	Yes	
Tooling 	Were the steps taken to mitigate guided by an existing runbook? 	No 	
Total score (count of all “yes” answers above)	Total score (count of all “yes” answers above) 	9