Cases6 cases

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1. IL-0459 MediaWiki on Kubernetes terminated TLS with a wrong certificate after a deployment, and 1% of requests failed

   Wikimedia Foundation2023

   Symptoms

   On 9 August 2023, right after a MediaWiki configuration deployment at 11:41 UTC, the MediaWiki deployments on Kubernetes terminated TLS with a wrong certificate, and about 1% of Wikimedia web and API traffic, around 120 requests per second, received errors for about 30 minutes. The issue was detected by a page for failed service probes on mw-api-ext and mw-api-int.

   High error rateTLS certificate errorsControl plane lag

   Trigger

   At 09:37 a change to the global Kubernetes defaults was merged that switched supported charts to cert-manager certificates. The next MediaWiki on Kubernetes deployment, a configuration change at 11:41, picked it up.

   Root cause

   Configuration change

   Component

   cert-managerHelm

   Contributing factor

   Change with a delayed effectNo configuration tests

   Detection

   A page from the ProbeDown alerts for mw-api-ext:4447 and mw-api-int:4446 at 11:47; a KubernetesAPILatency alert had fired at 11:46.

   Remediation

   • Identified the issue and prepared a patch, https://gerrit.wikimedia.org/r/947331
   • Redeployed the MediaWiki deployments on Kubernetes

   Action items

   • Review whether deployment procedures and testing should be strengthened, for example against surprising changes on the next deployment and with canary deployments for Kubernetes
   • Look at metrics that become unavailable or unhealthy during deployments, or at reducing the impact of deployments

   Lessons

   The cause was found quickly because the person who made the breaking change was responding. The change should have been caught by CI, but the repository it was made in (hieradata) does not run the CI of the deployment charts. Only 1% of traffic going to Kubernetes kept the impact small, and the low error volume made it slower to link the errors to Kubernetes.

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2. IL-0460 Vault served an expired TLS certificate and almost every GoCardless service pod crash-looped, a 22-minute full outage

   GoCardless2020

   Symptoms

   On Sunday 25 October 2020 the GoCardless API, dashboard, payment pages and partner integrations were all unavailable for 22 minutes, from 07:32 to 07:54 GMT. Automated monitoring paged the on-call engineer at 07:32, a minute after every request to the API had started failing because no healthy backend was left.

   Service outageContainer crash loopTLS certificate errorsCredentials unavailable

   Trigger

   At 07:22 the TLS certificate Vault was still serving expired. cert-manager had already renewed it, but Vault does not load a new certificate without a restart or a SIGHUP.

   Root cause

   Certificate or credential expiry

   Component

   cert-managerSecrets store

   Contributing factor

   Single point of failureMonitoring blind spotUnverified assumptionHarmful manual action

   Detection

   Automated monitoring detected the failing requests and paged the on-call engineer at 07:32. Certificate expiry monitoring existed for the Layer-7 load balancers but did not cover the TCP load balancer that Vault uses.

   Remediation

   • Forced cert-manager to recreate the Vault certificate
   • Restarted the Vault cluster by hand so it loaded the new certificate
   • Deleted some pods of critical workloads to skip their 5-minute exponential backoff

   Action items

   • Check the certificate of the TCP load balancer in front of Vault with the Blackbox exporter, as for the Layer-7 load balancers
   • Deploy the Reloader controller to roll Vault when its certificate data changes
   • Change theatre-envconsul so envconsul no longer manages the child process and only injects secrets at container startup

   Lessons

   A system that almost every service depends on, such as the secrets store, can take everything down at once, and failure scenarios like Vault becoming unreachable had not been tested enough. Assuming the certificate had expired, when it had been renewed but not reloaded, added a slow and unnecessary recreation step to the recovery.

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