Incidents/2022-12-13 sessionstore

== Summary ==
A wrong configuration change caused sessionstore pods to be unschedulable in our WikiKube cluster. This resulted in failed edits across all projects for a period of 9 minutes.
==Timeline==
All times in UTC.
12:15 OUTAGE BEGINS.  Alexandros becomes IC.
12:15 (ProbeDown) firing: Service sessionstore:8081 has failed probes (http_sessionstore_ip4) #page -
12:16 It becomes apparent that sessionstore is no longer serving requests, after an increase and some pod restarts
12:18 A correlation is made to a change for MatchNodeSelector for sessionstore
12:24 nodeAffinity for specific rack rows was removed manually from sessionstore deployments in k8s (basically https://gerrit.wikimedia.org/r/c/operations/deployment-charts/+/867572 was done manually via kubectl edit) OUTAGE ENDS
12:25 Incident created in wikimediastatus.net
12:27 Status page updated
12:30 Resolved
==Detection==
Automated monitoring detected the issue
(ProbeDown) firing: Service sessionstore:8081 has failed probes (http_sessionstore_ip4) #page -
==Conclusions ==
===What went well?===
* The incident was immediately detected by automated monitoring and the problem was quickly identified and fixed.
* Multiple people responded
* An IC was appointed quickly
===What went poorly?===
* Incident status page was update at the end of the rather short incident.
===Where did we get lucky?===
* Nowhere?
==Links to relevant documentation==
Change that caused the issue:
https://gerrit.wikimedia.org/r/c/operations/puppet/+/791597/9
The CR changed the typology zone annotation for Kubernetes Nodes running on Ganeti from something like “row-a” to “ganeti-eqiad-b”, referencing the Ganeti cluster name and group. With sessionstore Pods being only scheduled to specific rows it made them unschedulable in the cluster.
Grafana
==Actionables==
Fix for sessionstore deployments:
https://gerrit.wikimedia.org/r/c/operations/deployment-charts/+/867572
==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	2 vs 2, let's say it's sufficient
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 actively updated during the incident? 	yes	
Process 	Was the public status page updated? 	yes	
Process 	Is there a phabricator task for the incident? 	yes	
Process 	Are the documented action items assigned? 	yes	
Process 	Is this incident sufficiently different from earlier incidents so as not to be a repeat occurrence? 	yes	
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.	yes	
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) 	14