Cases62 cases

1 to 5 of 62

1. IL-0179 With DEBUG logging and synchronous log output, a high-traffic service filled the 64 KB stdout pipe, so request threads blocked in write() while holding database connections and HikariCP ran dry, failing requests in 30-second bursts

   MUSINSA2026

   Symptoms

   The on-call engineer for a high-traffic service was alerted that requests were failing in large batches every 30 seconds.

   High error rateIntermittent timeoutsConnection pool exhaustion

   Trigger

   DEBUG level logging was on in production when traffic surged, raising log volume to about 100 times normal while the service logged synchronously.

   Root cause

   Configuration change

   Component

   Application logging libraryContainer runtimeTelemetry agentDatabase

   Contributing factor

   Unverified assumptionExcessive blast radius

   Detection

   The on-call engineer was alerted that requests were failing in large batches every 30 seconds.

   Action items

   • Replace synchronous logging with TraceAwareAsyncAppender, which captures trace_id and span_id on the calling thread and drops logs instead of blocking when the queue is full (neverBlock=true)
   • Keep the amount of log output at a sustainable level, since an asynchronous queue still fills under sustained overload

   Lessons

   Removing AsyncAppender to keep trace IDs also removed the protection that kept request threads away from stdout back-pressure. A company-wide standard applies the same side effect to dozens of services, so check what a standard rests on and what working behaviour it takes away. ThreadLocal context does not cross into an asynchronous worker, but capturing it on the calling thread before handing the event off keeps both the IDs and the asynchronous output. Under extreme load, dropping log lines is preferable to stopping request handling.

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2. IL-0471 GitHub Actions runs failed or sat queued for 9 hours after a routine deployment's pod replacement saturated capacity

   GitHub2026

   Symptoms

   From 15:05 UTC on 6 August 2026 GitHub Actions workflow runs failed to start, failed partway through or stayed queued for a long time, on both GitHub-hosted and self-hosted runners. Some requests to the Actions REST API returned errors, and some customers saw unexpected rate limiting in their workflows.

   High error rateHigh latency

   Trigger

   A routine deployment to an internal Actions service that processes events and generates Actions jobs replaced pods, and the remaining capacity became saturated.

   Root cause

   Capacity exhaustion

   Component

   CI system

   Contributing factor

   No headroomExcessive blast radiusHarmful manual action

   Remediation

   • Expanded capacity of the affected services
   • Throttled incoming webhook-triggered work
   • Increased processing capacity for the backlog of affected events
   • Deployed changes that stopped runners from repeatedly trying to acquire invalid jobs
   • Deployed a fix for self-hosted runners that were not picking up jobs and enabled it incrementally
   • Paused migrations with GitHub Enterprise Importer
   • Restored webhook-triggered Actions workflows to full throughput
   • Rolled back the mitigation that left some ARC runners offline, and recovered those runners by hand
   • Shared CLI and UI steps for customers to clear jobs that could not be retried or canceled

   Action items

   • Improve deployment and capacity safeguards for the affected services
   • Strengthen monitoring for the conditions that preceded the incident
   • Improve the resiliency and recovery of queued work and runner assignment
   • Add automatic recovery for self-hosted runners in upcoming Runner and ARC releases
   • Reduce the risk of cascading failures and speed up recovery during large Actions disruptions

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3. IL-0438 Builder Work became almost entirely unreachable with 504 errors when the Amazon CloudFront VPC Origins feature failed

   ANDPAD2026

   Symptoms

   At about 17:00 JST on Thursday 16 July 2026, Datadog alerted, and at almost the same time 5xx errors began to rise on the Amazon CloudFront distributions that used VPC Origins connections. Builder Work, ANDPAD's job and career service for the construction industry, became almost entirely unreachable.

   Service outageHigh error rate

   Trigger

   The Amazon CloudFront VPC Origins feature failed; AWS Health later reported a capacity constraint in an internal shared subsystem that manages connections for VPC Origins.

   Root cause

   Cloud provider outage

   Component

   CDN and edge networkCloud load balancerIngress controllerWeb application firewall rules

   Contributing factor

   Single point of failure

   Detection

   Datadog raised an alert, and at about the same time 5xx errors were seen rising on the CloudFront distributions that used VPC Origins.

   Remediation

   • Decided to route traffic temporarily through an internet-facing ALB without CloudFront
   • Recreated the internet-facing Ingress, attached it to the Argo Rollouts Rollout resource and promoted it
   • Created a regional WAF for the ALB with the security team
   • Created a new IngressClass with the ACM certificate, the WAF and access log settings
   • Added Ingresses for normal traffic, the admin screens and redirects, which restored routing

   Action items

   • Return to the CloudFront and VPC Origins layout while keeping a way to switch to internet-facing in code
   • Keep the internet-facing ALB, limited to approved CIDRs through spec.inboundCIDRs in IngressClassParams, so commenting out securityGroupInboundCidrs in values.yaml opens it to 0.0.0.0/0
   • Point the Terraform-managed Route53 A record back to CloudFront and keep the ALB alias commented out in code, so uncommenting and applying switches DNS to the ALB

   Lessons

   VPC Origins keeps the ALB off the internet and simplifies the traffic path, but it makes the service depend on an AWS-managed shared subsystem, and when that fails the service stops even though CloudFront is healthy. ANDPAD kept VPC Origins and instead prepared the escape route ahead of time, accepting the cost of an ALB that always runs, because recreating the Ingress and attaching it to the Rollout had taken so long during this incident.

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4. IL-0435 Error rates rose as Aurora MySQL killed connections and declined queries to avoid OOM while load metrics stayed flat

   CyberAgent2026

   Symptoms

   An alert fired one day because an application's error rate had suddenly risen. The error log of the Aurora MySQL RDS instance for that time showed Aurora killing connections and declining queries to avoid running out of memory. The killed queries also made application latency worse.

   High error rateMemory pressure

   Trigger

   An application release added a batch job run every minute by a Kubernetes CronJob. Each run's short-lived pod connected to the database from a new pod IP.

   Root cause

   Capacity exhaustion

   Component

   DatabaseJobs and CronJobs

   Contributing factor

   Unreviewed defaultsChange with a delayed effect

   Detection

   An alert fired on the application's error rate.

   Remediation

   • Set performance_schema to manual management and capped two related Performance Schema parameters, unlimited by default, at 100 in the parameter group, then restarted the RDS instance

   Lessons

   When system alerts fire but the database's load metrics look calm, Performance Schema growth is worth suspecting. The two parameters that were capped are unlimited by default, so connections from ever-changing source addresses keep adding to the records. Capping them has a cost: once the cap is exceeded, per-host figures in Performance Insights are missing. Putting RDS Proxy between the application and Aurora MySQL to keep source hosts stable is another possible measure.

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5. IL-0092 Upgrading ingress-nginx from 1.12.0 to 1.14.3 brought a newer ModSecurity and OWASP Core Rule Set whose false positives blocked HMPPS Auth, so users could not sign in to their applications

   UK Ministry of Justice2026

   Symptoms

   At 08:56 on 30 March 2026, about an hour and three quarters after the rollout of a new ingress-nginx to production began, the MoJ Cloud Platform team noticed messages in the #hmpps-auth-audit-registers Slack channel about ModSecurity unexpectedly blocking requests. Users in HMPPS also raised incidents with the MoJ service desk.

   High error rateService outage

   Trigger

   The Cloud Platform team began deploying ingress-nginx 1.14.3, on images maintained by Chainguard, to the three production ingresses at 07:11 on 30 March 2026.

   Root cause

   Breaking change in an upgrade

   Component

   Ingress controllerWeb application firewall rules

   Contributing factor

   Unrepresentative staging

   Detection

   At 08:56 the Cloud Platform team saw messages in the #hmpps-auth-audit-registers Slack channel showing ModSecurity blocking requests unexpectedly; HMPPS users also raised incidents with the MoJ service desk.

   Remediation

   • rolled the ingress-nginx deployment back to the previous version, from 09:14 to 09:43
   • HMPPS applied ModSecurity rule exclusions for the rules blocking traffic to HMPPS Auth
   • added a temporary global exclusion for the CRS rule triggered by the PostHog cookie
   • redeployed ingress-nginx 1.14.3 with both exclusions in place, completed at 14:00

   Action items

   • Rescope the PostHog cookie so it is no longer sent with requests to all *.justice.gov.uk services
   • Remove the temporary global exclusion once the PostHog cookie has expired from enough users' browsers
   • Remind service teams to run dev and test workloads through the non-production ingress controllers so future ModSecurity and CRS updates are tested
   • Run production and non-production workloads in separate clusters in Cloud Platform 3.0, so non-production workloads must use the non-production ingress

   Lessons

   An ingress controller upgrade that bundles a web application firewall also changes which requests are allowed, not only how they are routed. A beta or non-production ingress only catches false positives for traffic that actually passes through it, and teams that keep their test workloads on the production ingress miss them. A fix on the client side, such as rescoping a cookie, takes effect only when old cookies have expired in users' browsers, so the rule change has to wait for it or ship with an exclusion.

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