Cases2 cases

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1. IL-0232 Calico pods in the codfw WikiKube cluster hit their resource limits and crash-looped, taking nodes out one by one

   Wikimedia Foundation2024

   Symptoms

   At 05:01 UTC on 9 October 2024 a page fired for GatewayBackendErrorsHigh, elevated 5xx errors from page-analytics_cluster in codfw, followed by a growing number of KubernetesCalicoDown alerts for WikiKube nodes not running the calico-node pod and by MediaWikiMemcachedHighErrorRate. For about an hour uncached MediaWiki requests, such as those of logged-in users, edits and API calls, saw high latency or errors.

   High error rateHigh latencyContainer crash loopProbe failuresControl plane lag

   Trigger

   An operator used the new mwscript-k8s utility to run a batch of video transcode maintenance, the TimedMediaHandler requeueTranscodes.php script. mwscript-k8s deploys a whole helm release for every script invocation, so the batch created close to 2k Calico NetworkPolicies along with many other resources over several hours.

   Root cause

   Capacity exhaustion

   Component

   CalicoHelmJobs and CronJobs

   Contributing factor

   Change with a delayed effectNo runbook

   Detection

   Automated monitoring detected the issue and paged the on-call SRE: GatewayBackendErrorsHigh at 05:01:36 UTC, 16 KubernetesCalicoDown alerts at 05:03:32 UTC and MediaWikiMemcachedHighErrorRate at 05:04:15 UTC.

   Remediation

   • Raised the resource limits of the Calico and Typha pods and applied the change with helmfile, which redeployed all Calico pods

   Action items

   • Stop mwscript-k8s from creating so many resources

   Lessons

   The incident came close to a full outage. Automated monitoring caught it and many responders found the cause, but end users were affected and responders were unsure whether they could switch over to eqiad. The problem was noticed just before the hand-off from Americas to EMEA SRE shifts, a period with a long-known lack of coverage, and it helped that the transcodes were not queued on a Friday evening in the US.

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2. IL-0216 Wikimedia on-wiki search was slow or rejected after a datacenter switchover overloaded one Elasticsearch node in codfw

   Wikimedia Foundation2023

   Symptoms

   Right after the primary datacenter was switched from EQIAD to CODFW at 14:00 UTC on 20 September 2023, on-wiki search on all Wikimedia wikis became delayed or unavailable for a subset of users. At 14:08 the alert "CirrusSearch codfw 95th percentile latency - more_like" fired and reached IRC and the Search Platform email list as "PROBLEM alert - graphite1005/CirrusSearch codfw 95th percentile latency - more_like is CRITICAL".

   High latencyHigh error rateLoad imbalanceMisrouted requests

   Trigger

   At 14:00 UTC the primary datacenter was switched from EQIAD to CODFW. more_like queries that were expected to go to eqiad, the non-primary datacenter, went to codfw instead, apparently because RESTBase sent them as POST rather than GET.

   Root cause

   Software defect

   Component

   Elasticsearch or OpenSearchIn-memory cache

   Contributing factor

   Unverified assumptionDependence on a warm cacheNo runbook

   Detection

   Automated monitoring caught the problem, but not quickly enough to prevent user-facing impact: the Grafana alert "CirrusSearch codfw 95th percentile latency - more_like" went to IRC and the Search Platform email list at 14:08. No one was paged; SREs were already watching closely because of the datacenter switchover. At 14:14 Hashar pinged #wikimedia-search and engineers began troubleshooting.

   Remediation

   • Banned the overloaded node elastic2044 from the Elasticsearch cluster

   Action items

   • Work out which Elasticsearch steps are needed before a datacenter switchover
   • Update the Elasticsearch section of the datacenter switchover documentation
   • Review the suggested switchover improvements in T285347 and decide whether to implement them
   • Make the RESTBase /related/{article} endpoint send GET instead of POST so the CirrusSearch query cache stays warm in both datacenters
   • Review the shard balance of busy wikis, especially enwiki_content

   Lessons

   Monitoring caught the problem, but not quickly enough to prevent user-facing impact. The heightened attention during the switchover meant the impact was recognised immediately, and banning a single node was quick and easy. The mitigation was not guided by an existing runbook, and the shards on the overloaded host were not checked before it was banned.

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