Cases2 cases

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1. IL-0136 A deployment that cleared an internal service's caches at peak traffic sent 20 times the normal load to the Azure OpenAI metadata backend in Sweden Central, and health checks that depended on its database restarted instances in a loop for almost six hours

   Microsoft Azure2026

   Symptoms

   From 10:03 UTC on 29 September 2026, requests to Azure OpenAI Service, Foundry Agent Service, Foundry Models and Azure AI services in Sweden Central failed intermittently, took longer, or returned HTTP 500 and 503 errors. At 10:17 UTC service monitoring raised an alert on elevated failure rates for Azure OpenAI Service requests in the region.

   High error rateHigh latencyIntermittent connection failuresIntermittent timeoutsProbe failuresOutbound connection exhaustion

   Trigger

   At about 09:38 UTC a planned deployment of an internal service in Sweden Central cleared that service's caches during the region's peak traffic hours.

   Root cause

   Faulty deployment

   Component

   In-memory cacheApplication database

   Contributing factor

   Change during peak hoursRetry stormAutomation amplificationMonitoring blind spotNo runbookHealth check tied to a dependency

   Detection

   Service monitoring detected elevated failure rates for Azure OpenAI Service requests in the region at 10:17 UTC and alerted the engineering teams; engineers were engaged at 10:28 UTC.

   Remediation

   • reconfigured the backend service's automated health check so it no longer depended on remote connectivity, which stopped the restart loop

   Action items

   • Reconfigure the backend service's automated health check so it no longer depends on connectivity to its database layer
   • Add monitoring and alerting on the load and latency of the caching layer used by the backend service
   • Update troubleshooting and procedural guidance for connection exhaustion scenarios for on-call engineers
   • Add throttling and rate limiting to protect the backend service from sudden traffic increases (estimated October 2026)
   • Increase the resource capacity of the caching layer used by the backend service across regions (estimated October 2026)
   • Improve diagnostic metrics and automated detection to identify such issues more quickly (estimated October 2026)
   • Partition the caching layer used by the backend service to increase its throughput (estimated December 2026)

   Lessons

   A health check that depends on a remote dependency turns a dependency problem into a restart loop: instances that cannot reach the database are killed, the remaining ones take more load and retries, and adding instances or resources does not help until the check itself is changed. Clearing a cache during peak traffic can multiply load on the backend behind it, especially when cache-read defects let lookups bypass the cache. Connection exhaustion was hard to see because the existing network and diagnostic metrics did not show it.

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2. IL-0086 A deployment meant for a CircleCI test Kubernetes cluster also removed Traefik and Calico resources from the production cluster, taking down the CircleCI UI and runner.circleci.com

   CircleCI2021

   Symptoms

   At 15:35 EST on 11 March 2021 the CircleCI web UI and runner.circleci.com became unavailable. The API, which uses a separate ingress point, stayed reachable, and builds kept running.

   Service outageResources deleted

   Trigger

   An update deployed to a test Kubernetes cluster also removed Traefik and Calico resources from CircleCI's primary production cluster.

   Root cause

   Faulty deployment

   Component

   Ingress controllerCalico

   Contributing factor

   Excessive blast radiusNo runbookMonitoring blind spot

   Detection

   CircleCI engineers were alerted and began investigating. The status page posted its first notice at 20:45 UTC.

   Remediation

   • restored Traefik in the production cluster
   • restored Calico in the production cluster, after which pods served customer requests again

   Action items

   • Improve network-level documentation and runbooks to cover every layer of the Kubernetes networks
   • Improve monitoring and alerting for the network infrastructure
   • Add guardrails that isolate the production and staging clusters so they cannot interfere with each other

   Lessons

   A deployment path that can reach both a test cluster and production can remove shared networking components from production. When the ingress controller and the CNI are both affected, restoring the ingress alone is not enough: engineers had to find the link between Calico and Traefik and restore Calico before pods served traffic again. Time went to DNS and AWS red herrings because nothing linked the outage to the test deployment; CircleCI's answer is better network runbooks, network alerting and guardrails between clusters.

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