Cases11 cases

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1. IL-0470 Buildkite notifications were delayed after Prometheus storage filled and EKS notification workers could not autoscale

   Buildkite2026

   Symptoms

   From 20:01 UTC on 28 May 2026, Buildkite build and job notifications (GitHub commit status, email, Slack and webhook notifications) reached a subset of customers late. Internal queue latency monitoring showed the delay as queues grew while notification-processing workers stayed underprovisioned.

   High latencyAutoscaler misbehaviour

   Trigger

   The Prometheus service used by Buildkite's EKS autoscaling path ran out of storage, so some EKS-based notification workers could not autoscale while their queues were growing.

   Root cause

   Capacity exhaustion

   Component

   Prometheus

   Contributing factor

   No alert on limitPartial rolloutIneffective rollbackUnverified assumptionHarmful manual action

   Detection

   Internal queue latency monitoring detected the rising notification delivery latency.

   Remediation

   • Moved the affected notification-processing workloads from EKS back to the ECS-based infrastructure
   • Adjusted scaling configuration and worker counts by hand
   • Completed the rollback and scaled the remaining affected ECS services
   • Increased Prometheus storage capacity and recorded the change in infrastructure as code

   Action items

   • Alert before Prometheus storage exhaustion can affect autoscaling
   • Fix GitHub notification retry behaviour for a class of errors that could cause repeated retries
   • Make the EKS-to-ECS rollback verify destination capacity, autoscaling configuration and traffic movement
   • Review the CPU, memory and storage allocations of EKS control-plane dependencies such as KEDA and Karpenter
   • Move customer-critical workloads to EKS more gradually, with clearer settling periods
   • Improve customer-level monitoring of notification delivery latency to detect customer-impacting regressions earlier
   • Review which notification types sit on customers' scheduling or CI hot path and need tighter latency, separate queueing or specific alerting
   • Update the status page earlier when notification latency is likely to affect customer workflows
   • Describe the customer-visible impact in status page updates, not only the affected internal service
   • Improve internal escalation for customers who report critical CI impact

   Lessons

   Buildkite said recovery took longer than expected because the rollback path from EKS to ECS did not fully handle this failure, and that its public status page did not show the customer-visible impact as quickly or clearly as it should have, since notification latency affects customers differently depending on how their CI and deployment workflows use notifications.

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2. IL-0548 Buildkite's EKS Prometheus server was killed for memory again and again, so services stopped scaling and jobs lagged

   Buildkite2026

   Symptoms

   On 20 May 2026 a subset of Buildkite customers saw delays in notification delivery, build dispatch and the metrics services. The status page first reported delays to notifications for all customers and later narrowed it to a subset of customers.

   High latencyOOMKilledMonitoring outageAutoscaler misbehaviour

   Trigger

   Autoscaling during Buildkite's migration of production services from AWS Fargate to AWS EKS raised the number of pods that Prometheus tracked, and with it the memory the Prometheus server needed.

   Root cause

   Capacity exhaustion

   Component

   PrometheusContainer memory limitNode pool

   Contributing factor

   No resource requests or limits

   Remediation

   • Moved the load of the affected services back to AWS Fargate by following prepared documentation
   • Scaled up the services of a few customers with very large backlogs further by hand

   Action items

   • Upsize the underlying system nodes
   • Set higher requests and limits for the Prometheus server so it can handle more load
   • Review all new EKS resources and set any missing requests and limits
   • Add observability and monitors for EKS pod and node health

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3. IL-0044 Prometheus OOMKilled in a loop while replaying its write-ahead log after a kube-prometheus-stack upgrade

   Saifeddine Rajhi2024

   Symptoms

   After an upgrade of the kube-prometheus-stack Helm chart, several pods stayed not ready. Among them was prometheus-prometheus-operator-prometheus-0, showing 3/4 Running and a recent termination.

   OOMKilledProbe failuresMonitoring outage

   Trigger

   During the kube-prometheus-stack Helm chart upgrade, Prometheus had to replay its write-ahead log on startup, and the replay needed more memory than its limit allowed.

   Root cause

   Capacity exhaustion

   Component

   PrometheusContainer memory limit

   Contributing factor

   Unknown limitNo headroom

   Detection

   The team noticed pods not ready after the Helm chart upgrade and found the OOMKilled termination with kubectl describe pod.

   Remediation

   • ran promtool tsdb analyze inside the Prometheus pod to find high-cardinality metrics such as haproxy_server_http_responses_total

   Action items

   • Verify that the memory overhead during WAL replay stays within acceptable limits, for example 10 to 15%, and do not run Prometheus close to its memory limit
   • Optimise storage, scraping and remote write configuration and upgrade to newer Prometheus releases
   • Recover automatically from OOM crash loops, for example by deleting the WAL on OOM events
   • Enforce scrape limits so Prometheus stops scraping targets when memory use passes a threshold

   Lessons

   Size Prometheus for its restart, not only for steady state: WAL replay can need far more memory than normal running, and a Prometheus that already runs near its limit can be OOM-killed during the replay. Much of Prometheus memory use comes from cardinality, which promtool tsdb analyze can measure even when Prometheus itself is down.

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4. IL-0046 Prometheus restarted again and again because its WAL replay took longer than the startup probe allowed

   UK Ministry of Justice2024

   Symptoms

   At 12:32 Prometheus on the Cloud Platform was not available after a planned change, and the Prometheus pod had restarted several times. The platform lost its monitoring and had missing metrics for the period.

   Monitoring outageProbe failures

   Trigger

   A planned change restarted Prometheus at a time when the WAL count was high.

   Root cause

   Capacity exhaustion

   Component

   Prometheus

   Contributing factor

   Unreviewed defaultsUnknown limit

   Detection

   The team saw that the Prometheus pod had restarted several times after the planned change.

   Remediation

   • raised the Prometheus startupProbe threshold to 30 minutes

   Action items

   • Perform planned Prometheus restarts when the WAL count is lower
   • Set the default CPU and memory requests to meet maximum usage
   • Create a test setup that recreates the live WAL count
   • Explore the memory-snapshot-on-shutdown and auto-gomaxprocs feature flags
   • Explore storing WAL files remotely in a different location
   • Look into a blue-green Prometheus to test changes on a live-like setup before applying them to live
   • Spike into Amazon Managed Prometheus

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5. IL-0045 Prometheus on the live cluster restarted several times after readiness probe failures, leaving gaps in metrics

   UK Ministry of Justice2023

   Symptoms

   At 10:15 on 1 November 2023 a Pingdom check raised a PagerDuty high-priority alert that the "Prometheus - live healthcheck is DOWN". The same alert fired for the third time within a few minutes, and the incident was declared at 10:41.

   Monitoring outageProbe failures

   Root cause

   unknown

   Component

   Prometheus

   Contributing factor

   –

   Detection

   A Pingdom health check on the live Prometheus failed and raised a PagerDuty high-priority alert.

   Remediation

   • increased the Prometheus readiness probe timeout from 3 to 6 seconds

   Action items

   • Inspect more closely to identify this kind of failure earlier
   • Investigate whether the ingestion of data into the database is too big or too long
   • Check whether executing some queries makes Prometheus work harder and stop responding to the readiness probe
   • Check whether any other service probing Prometheus triggers the restart
   • Check whether regular Velero backups disturb EBS reads and writes and cause the restart

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