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1. IL-0078 Enabling EKS secrets encryption with KMS through Terraform removed worker node IAM permissions and took down all ZenML Pro workspaces

   ZenML2025

   Symptoms

   Between 12 AM and 8 AM CEST on 11 May 2025, every ZenML Pro workspace server was unreachable, so customers could not use workspace UIs or pipelines. The ZenML Pro dashboard was not affected.

   Service outage

   Trigger

   At 23:38 on 10 May, secrets encryption with AWS KMS was enabled through Terraform as part of the EKS module upgrade.

   Root cause

   Software defect

   Component

   TerraformNode pool

   Contributing factor

   Unrepresentative stagingDeferred upgrades

   Detection

   Engineers found the cluster degraded when they resumed the upgrade at 06:00.

   Remediation

   • deleted and recreated the EKS node group with the correct IAM permissions and configuration
   • restored service access across all affected ZenML Pro environments

   Action items

   • Upgrade all Terraform modules (EKS, VPC) to current supported versions
   • Complete the EKS cluster upgrades as planned
   • Review the strategy for periodic Kubernetes and infrastructure upgrades to prevent a recurrence

   Lessons

   A successful upgrade on a much smaller staging cluster understated the risk of the same change in production. Skipping Kubernetes and module versions let upgrade complexity build up until one upgrade carried many changes at once.

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2. 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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