Cases4 cases

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1. IL-0234 An onboarding operator deployed without its scope file installed dev applications on other IONOS Kubernetes clusters

   IONOS Cloud2026

   Symptoms

   On 8 May 2026 IONOS Cloud reported "K8s Control Planes not available": some Managed Kubernetes customers in DE/TXL, DE/FRA and DE/FRA/2 had trouble connecting to the control plane and saw degraded Kubernetes functionality. Between 15:22 and 16:12 (UTC+2) applications meant for a new internal development cluster were installed on other customer clusters, which were also modified.

   Control plane unresponsiveImage pull failures

   Trigger

   A routine internal operation onboarded a new Managed Kubernetes development cluster. The scoping configuration file for the onboarding operator failed to reach the deployment branch because of a merge conflict, and at the same time a second change removed the operator from the exclusion list, which deployed it at once.

   Root cause

   Faulty deployment

   Component

   CI systemContainer registry and its credentials

   Contributing factor

   No configuration testsExcessive blast radiusMissed in review

   Remediation

   • Corrected the onboarding operator so it needs an explicit namespace configuration before acting on any cluster
   • Reconfigured the deployment system to fail deployments when required configuration files are missing
   • Rolled back the identified changes on the affected resources
   • Ran a recovery script, validated in staging, that removed all unintended installations and updates from the affected clusters

   Action items

   • Add a mandatory CI/CD step that confirms all required configuration files reached the target branch before any deployment starts
   • Require additional peer review and change management approval for every deployment that touches production, whatever its size
   • Give teams a way to inform each affected customer in time about the status of post-incident cleanup

   Lessons

   IONOS treats the unintended change to cluster configurations as a failure of the controls that should keep internal operations from affecting customer environments.

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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-0467 Boskos crash-looped after a node drain started it from a buggy StatefulSet image, failing Kubernetes PR jobs for 9 hours

   Kubernetes project2019

   Symptoms

   Overnight into 26 March 2019 (PDT), every Prow test job that needed a GCP project from Boskos, the Kubernetes test-infra service that hands out GCP projects to test jobs, failed. The required job pull-kubernetes-e2e-gce failed on every run, so no pull request could be merged in kubernetes/kubernetes for just over nine hours, and other projects saw similar failures.

   Container crash loopService outage

   Trigger

   At 23:04 PDT the node running the only Boskos pod was drained to be replaced with a 1.13 node during the upgrade of the Prow clusters from Kubernetes 1.11 to 1.13. The replacement pod started from the StatefulSet's current spec, which named a newer Boskos image containing the bug.

   Root cause

   Faulty deployment

   Component

   CI systemNode pool

   Contributing factor

   Change with a delayed effectUnverified assumptionSingle point of failureNo alert on limitChange started before off-hours

   Detection

   Users reported the failure on GitHub at 02:50 and on Slack at 05:16 and 06:27, and an engineer saw the Slack reports on waking at 07:16. The Boskos monitoring dashboard showed the failure, but no alert caught it.

   Remediation

   • Built and deployed a new Boskos image with the bug fixed
   • Cleaned up the GCP projects that had been left open

   Action items

   • Change the Boskos configuration so that deployments actually take effect (#11956)
   • Automate Boskos deploys (#11957)
   • Improve alerting on Boskos status (#11953)

   Lessons

   The write-up lists what went poorly: a ten-hour upgrade was started shortly after 17:00 when on-call coverage ended at 18:00; deployments were not checked to confirm that something had actually rolled out; the Boskos StatefulSet was not set up to perform updates, so changing its spec left the old pod running; the December change appears never to have been tested; and Boskos was deployed by hand, unlike most of Prow, which left more room for error. Once people were available, the problem was fixed quickly.

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4. IL-0466 Prow scheduled test job pods onto its service cluster, leaving hundreds Pending and starving Prow's own services

   Kubernetes project2019

   Symptoms

   On 8 February 2019 the Kubernetes project's Prow instance at prow.k8s.io became very slow: its controllers took about five minutes to respond to the hook, bot commands such as the cat command stopped working, and the prow.k8s.io site was hard to reach. GitHub status was normal, and the Prow service nodes were found running above 100% of CPU and memory.

   Pods PendingSustained high node CPUHigh latency

   Trigger

   At 13:20 PST an auto-generated bump PR for Prow was merged and deployed, and Prow started scheduling workloads onto the service cluster.

   Root cause

   Faulty deployment

   Component

   CI system

   Contributing factor

   Unrepresentative stagingNo resource requests or limitsLimited debug access

   Detection

   While working on a PR, a maintainer noticed the bot's cat command failing, found GitHub status normal, learned on Slack that the Prow nodes were above 100% CPU and memory, and then found 600 pending pods in the test-pods namespace.

   Remediation

   • Rolled Prow back to v20190206-b01b288 and cleaned up the pods and ProwJobs in the service cluster
   • Applied the revert by hand with kubectl apply after the deployment permission was missing
   • Sent a fix, PR 11209, so the client no longer overwrites a provided literal default client
   • Rolled Prow back to v20190125-2aca69d, from before the bug was introduced
   • Cleaned up the wrongly scheduled pods and ProwJobs again and confirmed Prow was healthy

   Action items

   • Find out why the bug did not show deterministically
   • Be more careful with sweeping Prow changes, with more end-to-end tests or a real staging Prow

   Lessons

   The behaviour was not deterministic, and the change was deployed on a Friday afternoon. Prow instances are set up differently, for example in how build clusters are defined, so another project's Prow running a version fine does not show it is safe for the Kubernetes Prow, which needs a staging environment that looks like it.

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