Cases3 cases

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1. IL-0188 Rolling updates on EKS returned 502 errors for about 30 seconds because Kubernetes deleted old pods as soon as new pods passed readiness, before the ALB had marked the new pod targets healthy

   Yonghyun Kim2025

   Symptoms

   Every rolling update of the team's microservices on Amazon EKS, behind an Application Load Balancer, was followed by intermittent 502 and 503 errors for users. A Slack message at 3 a.m. read "배포는 성공했는데, 서비스는 죽었다" (the deployment succeeded, but the service died). Every pod was Ready and Datadog was green while users saw errors.

   High error rateService outage

   Trigger

   A rolling update of a Deployment served through an ALB that registers pod IPs as targets, with no pod readiness gates.

   Root cause

   Faulty deployment

   Component

   Cloud load balancerIngress controllerPod termination

   Contributing factor

   Unverified assumptionUnreviewed defaultsMonitoring blind spot

   Detection

   A Slack message at 3 a.m. reported that the deployment had succeeded but the service was down; Datadog dashboards were green at the time.

   Remediation

   • labelled the namespace with elbv2.k8s.aws/pod-readiness-gate-inject=enabled so the AWS Load Balancer Controller injects pod readiness gates and Kubernetes waits for the ALB target to be healthy before deleting old pods

   Lessons

   A pod being Ready in Kubernetes does not mean the load balancer will send it traffic. Without a readiness gate, a rolling update can delete the old pods while the ALB still considers the new targets unhealthy, and dashboards that track pod status stay green during the errors. Making pods wait longer on shutdown reduced the errors but did not fix the start-up gap and made every deployment much slower; the readiness gate ties the pod's Ready state to the ALB's view of the target.

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2. IL-0433 Shifting API traffic from EC2 to EKS on Fargate overloaded two CoreDNS pods and caused mass name-resolution errors

   MonotaRO2023

   Symptoms

   When MonotaRO raised the share of requests for the remaining endpoints of its backend API sent to EKS from 1% to 25%, Sentry filled with errors saying the API could not reach the database and outside services. The errors were about DNS name resolution, and the traffic had to be shifted back to the EC2 version.

   DNS resolution failuresHigh error rate

   Trigger

   MonotaRO raised the share of requests for the remaining API endpoints routed by the ALB to the EKS version from 1% to 25%.

   Root cause

   Capacity exhaustion

   Component

   CoreDNSClient-side DNS resolver and cacheContainer CPU limitHorizontal pod autoscalerPod termination

   Contributing factor

   Unreviewed defaultsNo resource requests or limitsUnrepresentative stagingUnverified assumption

   Detection

   Sentry showed a large number of errors about connecting to the database and outside services right after the traffic share was raised.

   Remediation

   • Shifted the requests back to the EC2 version to stop the errors
   • Added an HPA to autoscale the CoreDNS pods as a stopgap, which let the migration to EKS finish

   Action items

   • Set ndots to 1 through dnsConfig in the application Deployments
   • Set cache 600 in the Corefile in the coredns ConfigMap to lengthen the CoreDNS cache TTL
   • Add lameduck 30s to the CoreDNS health plugin so CoreDNS keeps serving while it is scaled in
   • Consider reducing the differences between production and staging, weighed against cost

   Lessons

   MonotaRO assumed that a managed service would not cause problems and so skipped researching other companies' migrations, load testing and monitoring. Staging differed from production in its network layout, so ndots differed between them, and staging databases could not take a high-load test. Looking at the CoreDNS logs and resolv.conf instead of guessing found that ndots:5 made lookups of outside names try every search domain first.

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3. IL-0062 Intermittent 502 errors behind a GKE Ingress because pods shut down before the load balancer stopped sending them traffic

   Ravelin2019

   Symptoms

   After Ravelin moved its API layer from VMs into its GKE cluster behind a Kubernetes Ingress, the integration tests occasionally received 502 errors.

   Intermittent connection failuresStale endpoint routing

   Trigger

   Pods behind the Ingress being removed by their controller, as happens during rolling deploys and scaling.

   Root cause

   Software defect

   Component

   Ingress controllerPod termination

   Contributing factor

   Unverified assumption

   Detection

   Integration tests occasionally received 502 errors.

   Lessons

   Removing a pod's endpoint and signalling the pod happen at the same time, and an ingress can take a long time to catch up. A pod behind an ingress should keep serving after SIGTERM until its grace period ends, and the grace period should be longer than the load balancer's reprogramming time.

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