Cases4 cases

1 to 4 of 4

1. IL-0409 AssemblyAI's async and LLM Gateway APIs returned 5xx after its auth service could not get ENIs and scaled to zero

   AssemblyAI2026

   Symptoms

   On 29 January 2026 AssemblyAI's Async Transcription API and LLM Gateway on the NA API endpoints went down for 25 minutes, returning 500 errors, and then ran degraded for two hours. The status page first reported the investigation at 17:54 UTC.

   Service outageHigh error rateHigh latencyCloud API rate limit exhausted

   Trigger

   An influx of batch transcription jobs higher than usual made autoscaling add workers until spot instances ran out, on-demand instances were used, and the AWS Elastic Network Interface limit was reached.

   Root cause

   Capacity exhaustion

   Component

   Amazon ECS

   Contributing factor

   Excessive blast radiusMonitoring blind spotAutomation amplificationIgnored early signal

   Remediation

   • Worked through the backlog of requests until turnaround times were back to normal

   Action items

   • Add alerts on ENI utilisation
   • Run the authentication service on dedicated capacity, separate from batch workers
   • Fix the Lambda function that rate-limited the AWS API during recovery
   • Run everything in multiple regions

   Lessons

   AssemblyAI calls the outage preventable: ENI utilisation was not monitored, and because the authentication service shared resources with batch workers, a traffic spike in one starved the other. The warning signs were there but were not connected, and a tool built for recovery ended up blocking recovery.

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2. IL-0552 CircleCI's archival database ran out of storage, slowing workflow and job starts and the display of workflows in the UI

   CircleCI2025

   Symptoms

   At about 23:00 UTC on 21 January 2025, an automated alert reported that the CircleCI database instance holding archived data was almost out of free storage space. From 23:50 UTC customers saw slow starts and cancellations of workflows and jobs, and delays and difficulty viewing workflows in the CircleCI UI, although pipelines for a specific project could still be viewed.

   High latencyIntermittent timeoutsLog floodNode disk fullCloud API rate limit exhausted

   Trigger

   A code change to a function in the archival database on 16 January 2025 unintentionally made it write an excessive number of log messages, and the database filled its storage.

   Root cause

   Faulty deployment

   Component

   DatabaseCI system

   Contributing factor

   Change with a delayed effectExcessive blast radiusOverlapping failures

   Detection

   An automated alert at about 23:00 UTC warned that the archival database was almost out of free storage, before customers were affected. The growing workflows queue was not linked to it at first because a separate alert about request volume fired at about the same time.

   Remediation

   • Scaled up the event consumer that processes the workflows queue at 23:44 UTC
   • Promoted a read replica of the archival database to a standalone primary at 23:55 UTC

   Action items

   • Fix the database function so it no longer writes an excessive number of log messages
   • Add a max duration to the workflows retry policy for archiving so it fails earlier than the default timeout
   • Move workflow archival to an event-based model that decouples it from the workflows service
   • Add forecast and anomaly monitoring for databases that alerts on unusual activity before it reaches critical levels

   Lessons

   The storage alert on the archival database did fire, but the database filled much faster than forecast, so there was little time to act before customers were affected. Because the workflows service called the archival service synchronously, a problem in the archival database slowed the whole workflows service.

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3. IL-0328 A field type change in the database of CircleCI's job distribution service left all jobs stuck in "not running"

   CircleCI2021

   Symptoms

   From about 18:50 to 19:43 UTC on Monday, November 8, 2021, CircleCI customer jobs on all executors were blocked in a "not running" state. Errors and alerts appeared right after a deploy that changed a field type in the database of the job distribution service.

   Service outageHigh latencyScale-up failureCloud API rate limit exhausted

   Trigger

   At 18:50 UTC a deploy changed the type of a field in the PostgreSQL database of the job distribution service, in a way that was not backwards compatible.

   Root cause

   Faulty deployment

   Component

   DatabaseNode pool

   Contributing factor

   Ineffective rollbackExcessive blast radiusGap in automated tests

   Detection

   Errors and alerts related to the distribution failure appeared immediately after the 18:50 UTC deploy.

   Remediation

   • Rolled back the field type change, which did not clear the errors
   • Wrote, built and manually deployed a change that made the distribution system ignore the field
   • Manually scaled the primary compute cluster for the expected thundering herd of work
   • Monitored until processing was normal and resolved the incident

   Action items

   • Fix the gaps found in the testing strategy
   • Change how the distributor reacts to unexpected data so that this kind of deploy should not affect all jobs
   • Make recovery faster when unforeseen failures occur

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4. IL-0294 CircleCI Docker and machine jobs stalled for nearly 12 hours after a VM cleanup queue lost all its RabbitMQ consumers

   CircleCI2021

   Symptoms

   From 13:30 UTC on 21 May 2021, CircleCI jobs on Docker, the Machine executor (including Windows, Mac, Arm and GPU) and remote Docker were delayed or blocked. Engineers had been paged at 13:00 for excessive delays in starting jobs, and the alerts showed CircleCI could not provision some types and classes of VM because it was hitting its CPU quotas in GCP.

   Throughput degradationCloud API rate limit exhaustedDatabase overload

   Trigger

   The upgrade of the RabbitMQ cluster from 3.8.9 to 3.8.16 on 20 May 2021, which brought in a 15 minute consumer acknowledgement timeout for all queue types.

   Root cause

   Breaking change in an upgrade

   Component

   Message brokerDatabase

   Contributing factor

   Change with a delayed effectIgnored early signalRetry stormExcessive blast radiusOverlapping failures

   Detection

   Engineers were paged at 13:00 UTC on 21 May for excessive delays in starting jobs. Earlier, a 02:31 notification that a VM destroyer queue had no consumers was handled by restarting pods, and a 10:11 alert that a VM destroy queue was backing up notified once at its threshold and did not page.

   Remediation

   • Blocked Linux machine jobs from entering the work queue
   • Had GCP raise the CPU quotas
   • Deleted old VMs by hand
   • Cancelled some workflows to free resources
   • Reset the VM services deployment so the VM destroy queues had consumers again
   • Scaled up the RDS database instance, at 18:20 and again at 19:15
   • Turned down VM provisioning and blocked remote-docker jobs to shed load
   • Scaled down all pods that created and destroyed VMs
   • Deployed a revised distribution backend that kept remote-docker jobs out of the provisioning queues
   • Brought provisioning and distribution back one component at a time and reversed the mitigation code changes

   Action items

   • Make VM provisioning more robust to changes in load so it can recover from thundering herds
   • Make VM provisioning independent so its failure does not affect Docker jobs
   • Review and improve observability of RabbitMQ and the production queues
   • Batch requests to avoid hitting GCP API rate limits
   • Improve incident response for long-running incidents

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