Cases4 cases

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1. IL-0298 Wikimedia edits failed for 38 minutes when sessionstore Cassandra nodes in both data centres crashed on full disks

   Wikimedia Foundation2025

   Symptoms

   At 02:58 UTC on 31 March 2025, the sessionstore service in both Wikimedia data centres began returning 500 errors and edits failed. MediaWikiEditFailures (session loss) fired as critical at 03:03, and SessionStoreErrorRateHigh paged at 03:04.

   High error rateService outageNode disk fullData loss

   Trigger

   A rise in session writes, made up of session overwrites at a high rate, filled the storage of the sessionstore Cassandra nodes. The review says the rollout of SUL3 correlates strongly with it.

   Root cause

   Capacity exhaustion

   Component

   Database

   Contributing factor

   No alert on limitUnverified assumption

   Detection

   MediaWikiEditFailures (session loss) fired as critical at 03:03 UTC, and SessionStoreErrorRateHigh, a high 5xx rate alert, paged the batphone at 03:04. Users also reported the problem in task T390512.

   Remediation

   • Wiped the sessionstore storage, removing all sessions, and restarted Cassandra

   Action items

   • Page when the disk utilisation trend becomes unreasonable
   • Increase sessionstore storage capacity
   • Add namespacing to sessionstore
   • Add observability of the sessionstore workload
   • Identify the cause of the high rate of storage growth in sessionstore Cassandra
   • Consider an alert, without paging, on the Cassandra error rate
   • Consider paging or alerting when Cassandra nodes enter a down state

   Lessons

   The storage growth before the node failures made their timing predictable, so an alert on high storage utilisation well in advance would have given a chance to act before users were affected, and an alert on Cassandra node failures would have told responders more than an HTTP error rate alert. A cluster seen as heavily over-provisioned can still grow quickly and unsustainably under an unusual workload.

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2. IL-0324 Three MediaWiki deployments lost Memcached after a PHP 8.1 rollout and overloaded the External Storage database cluster

   Wikimedia Foundation2025

   Symptoms

   On 2025-03-12 from 10:58 UTC, alerts for high backend response times came in, and the MediaWiki deployments mw-api-int, mw-parsoid and mw-jobrunner, which serve MediaWiki for non-user-facing purposes, showed high latency and database errors. The External Storage cluster, whose hosts store the compressed text of wiki page revisions, had a sharp rise in load and connections. The same happened again on 2025-03-13 from 11:15 UTC.

   Database overloadHigh latencyHigh error rateLog floodMonitoring outage

   Trigger

   At 10:44 UTC on 12 March a Scap deployment of the PHP 8.1 rollout for mw-api-int, mw-parsoid and mw-jobrunner was run with the flag -Dbuild_mw_container_image:False; the rollout was deployed with Scap again at 11:15 UTC on 13 March.

   Root cause

   Faulty deployment

   Component

   DatabaseIn-memory cacheConfigMaps and SecretsHelmCI system

   Contributing factor

   Unverified assumptionDependence on a warm cacheSignal buried in noise

   Detection

   Alerts for high backend response times started at 10:58 UTC on 12 March, 14 minutes after the Scap deployment; 2 on-call engineers were paged. The MediaWikiMemcachedHighErrorRate alert that pointed at the cause fired only briefly because its log pipeline was overloaded.

   Remediation

   • Deployed a patch to cirrus-streaming-updater that reduced SUP parallelism
   • Reduced the concurrency of the categoryMembershipChange job in changeprop-jobqueue
   • On 12 March, reverted 1126607 and 1126650 and re-ran Scap
   • On 13 March, rolling-restarted changeprop-jobqueue
   • Rolling-restarted mw-api-int
   • Disabled the categorymembership job in changeprop-jobqueue
   • Reverted 1127476 and 1127478
   • Redeployed mw-api-int to pick up the revert
   • Increased the number of mw-api-int replicas
   • Redeployed mw-jobrunner to pick up the revert
   • Redeployed mw-parsoid to pick up the revert

   Action items

   • Reduce the number of messages sent through channel:Memcached during failures
   • Add helm rollback functionality to Scap
   • Decide whether Scap should update helmfile-defaults when -Dbuild_mw_container_image:False is set
   • Consider removing envvars.inc from MediaWiki images
   • Consider "Why" dashboards that give an overview of a group of related components together with relevant links to follow
   • Coordinate incident responders better
   • Be able to work with MediaWiki developers during incidents

   Lessons

   The write-up puts the weight not on the technical root cause but on why the team could not see, even after the 12 March incident, that three MediaWiki deployments were running without Memcached. Alert dashboards showed what was happening, such as many busy PHP workers, but not why; dashboards linking components that affect each other, such as SQLBlobStore keys and External Storage, would have helped. The alert that pointed in the right direction did not stay firing and would probably have been lost among 78 others. The incident coordinator could not tell who was working on what or who had joined or left, the channels were noisy, the external impact was judged more critical than it was, and trace.wikimedia.org was not used although it might have helped.

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3. IL-0392 MariaDB replication of section x1 between eqiad and codfw broke on a missing heartbeat row during a cookbook run

   Wikimedia Foundation2024

   Symptoms

   On 18 September 2024, while a cookbook was reconfiguring Wikimedia's core MariaDB databases from one-way eqiad -> codfw replication to circular replication ahead of a datacenter switchover, replication in the codfw -> eqiad direction broke on the x1 and es6 sections. Engineers watching the run noticed at 21:28 UTC that the x1 primary was not replicating.

   Replication lag

   Trigger

   Running the cookbook sre.switchdc.databases.prepare, which set up codfw -> eqiad replication and started pt-heartbeat, on the row-based x1 and es sections.

   Root cause

   Configuration change

   Component

   DatabaseDatabase replication

   Contributing factor

   Monitoring blind spotGap in automated testsHarmful manual action

   Detection

   Engineers watching the first run of the automation saw on orchestrator that the x1 primary was not replicating, because monitoring of circular replication was not yet enabled. The break in the eqiad -> codfw direction was reported by Icinga.

   Remediation

   • Skipped replication of the heartbeat tables with Replication_wild_ignore_table='heartbeat.%'
   • Inserted the missing heartbeat row with binary logging off on the es hosts
   • Inserted the missing heartbeat row with binary logging off on the x1 codfw hosts and removed the ignore rule

   Action items

   • Avoid the same issue again, probably by inserting the missing row when it is detected
   • Add a post-maintenance health check that replication works in both directions
   • Add options to run the maintenance in another order or on each section individually
   • Update the Phabricator task for each section rather than only at the end
   • Fix the monitoring gaps for circular replication
   • Made pt-heartbeat-wikimedia use STATEMENT format for its session on the x1 and row-based read-write es hosts, fixed in a later deploy

   Lessons

   Circular replication is not a normal working mode, so the first fix only moved the break to the other side; TRUNCATE would have worked better than DELETE because it always replicates as a statement, but it was not considered because it is more dangerous. Running sql_skip_slave_counter was avoided, since it could have made things worse or lost data. The check of the replication channel should also run once all tasks are complete, not only right after setup. The combination of row-based replication, pt-heartbeat and differing heartbeat rows was not described in earlier maintenance or documentation.

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4. IL-0323 A RAID battery backup unit failure crashed db1075, the MariaDB primary of section s3, and every s3 wiki went read-only

   Wikimedia Foundation2019

   Symptoms

   At 18:40 UTC db1075, the primary database of section s3, stopped answering, and all s3 wikis went read-only because no writes could reach it. Reads kept working from the replicas. At 18:42:45 Icinga reported "PROBLEM - Host db1075 is DOWN: PING CRITICAL - Packet loss = 100%".

   Writes rejectedReplication lagHigh latency

   Trigger

   The RAID battery backup unit (BBU) of db1075, the s3 primary database host, failed and the whole host crashed.

   Root cause

   Hardware failure

   Component

   DatabaseDatabase replication

   Contributing factor

   Pages not deliveredHarmful manual action

   Detection

   Icinga reported db1075 HOST DOWN on IRC at 18:42:45, but that alert does not page; replica Slave IO alerts followed, users reported problems on #wikimedia-operations, and the first SMS page went out at 18:56. Volunteers and staff noticed the failure before the alerts did.

   Remediation

   • Power-cycled db1075 from the iDRAC after finding the BBU problem
   • Let MySQL finish InnoDB recovery on db1075
   • Removed read_only=ON from db1075 by hand
   • Ran puppet by hand a second time so pt-heartbeat started

   Action items

   • Implement or refactor a script that moves replicas when the primary is unavailable
   • Fix the MediaWiki heartbeat model so pt-heartbeat does not use a super-user, avoids a single point of failure and follows the real primary without depending on puppet
   • Decide what to do with the batch of hosts that already had BBU issues
   • Buy a new BBU for db1075
   • Remove db1075 from the primary role
   • Address MediaWiki spam while the database is read-only or the primary is unavailable
   • Make primary database hosts page on HOST DOWN
   • Track hardware errors better in Netbox

   Lessons

   A BBU failure should not crash a whole host, yet it had happened before on HP hosts. HOST DOWN on a primary database should page instead of only alerting on IRC. Starting pt-heartbeat through puppet races with MySQL startup, and the unnoticed failure kept lag reported and made the outage 8 minutes longer.

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