Mastering Zonal Shift Support for EKS Auto Mode and Karpenter

Zonal Shift exists to tackle the critical issue of availability during zone failures in Amazon EKS. When an Availability Zone goes down, the last thing you want is for your applications to suffer. Zonal Shift, part of the Amazon Application Recovery Controller (ARC), automates the recovery process, ensuring your applications remain resilient and available even when infrastructure issues arise.

When Zonal Shift is enabled, it automatically cordons affected worker nodes, preventing new pods from being scheduled in the impaired zone. It deregisters IP addresses of pods in that zone from load balancers and removes their endpoints from endpoint slices, effectively cutting off traffic to those pods. Karpenter plays a crucial role here; it detects shifts—whether manual or automatic—by polling the ARC GetManagedResources API every 30 seconds. If a zone is impaired, Karpenter halts provisioning in that zone until service is restored, ensuring that your resources are allocated efficiently and your applications stay responsive.

In production, enabling Zonal Shift requires an EKS Auto Mode cluster and Karpenter version 1.12.0 or higher. You must also configure kubectl and AWS CLI v2. Be aware that forceful terminations, like EC2 Spot interruptions, will still occur. Additionally, enabling Zonal Shift makes your cluster eligible for manual shifts, which adds complexity to your resource management. Always test your configurations in a controlled environment before rolling them out to production to avoid unexpected behavior.