Cases6 cases

1 to 5 of 6

1. IL-0019 An EKS Terraform module upgrade shrank node root disks from 100 GB to 20 GB and image pulls failed with no space left on device

   UK Ministry of Justice2023

   Symptoms

   At 13:42 the RootVolUtilisation-Critical alert fired in the high-priority alerts channel. At 14:03 a user reported image pull errors with the message "no space left on device".

   Node disk fullImage pull failuresPod evictionPods stuck in ContainerCreatingIP address exhaustion

   Trigger

   Upgrading the EKS Terraform module to version 18, which changed the node launch template, and draining workloads onto the new nodes it created.

   Root cause

   Configuration change

   Component

   Node root diskNode launch templateNode poolAWS VPC CNICoreDNS

   Contributing factor

   Missed in reviewNo configuration testsNo alert on limit

   Detection

   The RootVolUtilisation-Critical alert fired in the high-priority alerts channel at 13:42, and a user reported "no space left on device" image pull errors at 14:03.

   Remediation

   • Uncordoned the old default node group and drained the new nodes to move workloads back
   • Identified the launch template change that cut the root volume to 20 GB and applied a fix on the manager and live clusters
   • Drained the old node group onto the fixed nodes

   Action items

   • Update the runbook to compare launch template changes during an EKS module upgrade
   • Create a test setup that pulls images of different sizes, like live
   • Update the RootVolUtilisation alert runbook to check the disk space configuration
   • Scale CoreDNS dynamically based on the number of nodes
   • Investigate whether IPv6 can solve the IP prefix starvation problem
   • Add drift testing to identify when a terraform plan shows a change to the launch template
   • Set up logging for the CNI and ipamd logs and alerts on errors related to IP prefix starvation

   Lessons

   The write-up lists review actions rather than lessons. They aim at two gaps: the launch template changes brought by a module upgrade were not compared or drift-tested before rollout, and there were no CNI or ipamd logs or alerts to show the IP prefix starvation the team suspected during the drain.

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2. IL-0017 AWS VPC CNI stopped allocating pod IP addresses on six nodes of the live cluster, leaving new pods in ContainerCreating

   UK Ministry of Justice2023

   Symptoms

   At 08:15 a user posted in the #ask-cloud-platform channel that new deployments were not starting: their pods stayed in ContainerCreating.

   Pods stuck in ContainerCreatingIP address exhaustion

   Root cause

   Configuration change

   Component

   AWS VPC CNINode pool

   Contributing factor

   No configuration tests

   Detection

   A user reported new deployments stuck in ContainerCreating in the #ask-cloud-platform channel at 08:15; the incident was declared at 09:31.

   Remediation

   • Added new nodes and cordoned and drained the six affected nodes
   • Enabled prefix delegation in the VPC CNI on the live cluster after comparing its CNI settings with a 1.23 test cluster

   Action items

   • Add a test or check that confirms IP address allocation is working as expected (#4669)

   Lessons

   The write-up states no lessons beyond its review action, which is to add a check that IP address allocation works.

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3. IL-0109 A retried node rollover on a DFDS EKS cluster set the Auto Scaling group back to 15 instances, and the default termination policy removed every healthy worker node, leaving the cluster with no nodes

   DFDS2023sev1

   Symptoms

   At 09:33 UTC on 14 February 2023 end users told the platform team that Kubernetes logs were not arriving in CloudWatch as expected. The investigation went through fluentd and found that the nodes were running containerd as their container runtime instead of docker-shim. A scheduled node rollover the day before, at 07:30 UTC on 13 February, had set containerd as the default runtime by accident.

   Service outageResources deletedMonitoring outage

   Trigger

   A scheduled node rollover had accidentally made containerd the default container runtime, which stopped log shipping to CloudWatch; the unscheduled rollover to revert it timed out, and the pipeline run that followed reduced the Auto Scaling group from 30 to 15 instances.

   Root cause

   Configuration change

   Component

   Container runtimeNode launch templateNode poolTerraformTelemetry agent

   Contributing factor

   Automation amplificationUnreviewed defaultsUnrepresentative stagingNo configuration testsNo runbookHarmful manual action

   Detection

   End users reported the missing CloudWatch logs that started the work. The postmortem lists 2 minutes as the time to detection of the node failure.

   Remediation

   • ran the node rollover script to add new nodes, which failed on a timeout from Docker Hub
   • ran the node rollover script a second time, which added new nodes that the load balancer marked healthy at 11:01 UTC

   Action items

   • Document the rollover process, especially what can be done when it goes wrong
   • Investigate options for improving the rollover process
   • Add fluentd log collection to the QA test suite so logs are covered end to end
   • Add alarms for fluentd when too little or no log data is sent
   • Implement a staging environment
   • Implement guards against selecting an invalid container runtime

   Lessons

   When a change goes wrong, rolling back to a known working configuration is safer than rushing a fix, which can make the damage worse. Launching new worker nodes depends on the cloud-init script not failing in any way. The QA configuration differed from production, there was no staging environment, the QA tests did not cover fluentd log collection, and the rollover process had no documentation for when it fails.

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4. IL-0010 A templating change that dropped its {{ }} delimiters made Argo CD delete all 478 Skyscanner services in every region

   Skyscanner2021

   Symptoms

   On 25 August 2021, skyscanner.net was unavailable and the Skyscanner mobile apps could not work, for travellers and business partners everywhere at the same time. The full global outage lasted four and a half hours.

   Service outageResources deleted

   Trigger

   A peer-reviewed change to a rarely changed root-level file of the cells templating configuration, intended as a no-op, that left out the {{ }} delimiters; it was submitted around 15:30 GMT, merged to master and rolled out automatically to every region at once.

   Root cause

   Configuration change

   Component

   Argo CDConfiguration templating

   Contributing factor

   Missed in reviewNo staged rolloutNo configuration testsUntested backupAutomation amplificationExcessive blast radius

   Remediation

   • restored the correct configuration for each cluster and let Argo CD reconcile the cluster back to it
   • restored services through the backup and restoration process
   • brought one region back first with the critical services and served all traffic from it from around 20:30 GMT
   • restored and verified the remaining regions and non-critical services over Thursday and Friday

   Action items

   • Avoid global configuration deploys, including rare change paths like this one
   • Treat templated configuration as code and add testing and linting
   • Plan and wargame more drastic failure scenarios in runbooks
   • Verify backup and restore processes, including access to the backups and the loss of a region
   • Refactor runbooks so they are clear and, where appropriate, idempotent for a stressed engineer in the early morning
   • Reconsider how much configuration is templated across regions, weighing configuration drift against automated global rollout

   Lessons

   Global configuration changes had been prevented in many cases, but not in this rarely used path. Configuration that gains templating and logic has become code and needs tests and at least linting. The failure scenarios and runbooks were not pessimistic enough, a backup only counts once it has been restored, and runbooks need care so they work for a stressed engineer at night. Automation that templates configuration across regions trades drift for the risk of rolling one mistake out everywhere. An experienced incident commander who kept the response calm and blameless made a large difference.

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5. IL-0105 Replacing the AWS VPC CNI during a Terraform refactoring detached the ALB target groups, and every service in the Kubernetes cluster returned 503

   DFDS2021

   Symptoms

   At 08:29 UTC on 22 June 2021 alerts fired because all services in the Kubernetes cluster were returning 503. No inbound traffic was reaching the cluster. A customer reported the problem at 08:32.

   Service outageHigh error rate

   Trigger

   The pipeline replaced the AWS VPC CNI as part of a refactoring that moved EKS add-ons from kubectl to native Terraform management.

   Root cause

   Configuration change

   Component

   AWS VPC CNICloud load balancerTerraform

   Contributing factor

   Unverified assumptionNo configuration tests

   Detection

   Alerts fired at 08:29 UTC, eight minutes after the VPC CNI was replaced; a customer reported the issue at 08:32.

   Remediation

   • committed a fix that reverted the infrastructure-modules reference
   • re-triggered the pipeline after it failed on an AWS API timeout
   • let the pipeline reattach the target groups to the ALB

   Action items

   • Revert the ALB de-registration delay to the default of 300 seconds
   • Update the roll-over script to set the ALB de-registration delay to zero temporarily before a roll-over
   • Create an ingress test suite

   Lessons

   The change to the de-registration delay had been weighed mainly for performance. Critical pipeline runs should be watched while they run. Everyone should receive the alerts and react to them, and alert messages that were unclear should be reworded.

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