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StatefulSet

https://medium.com/avmconsulting-blog/deploying-statefulsets-in-kubernetes-k8s-5924e701d327

StatefulSet Components

  • A Headless Service

  • A StatefulSet

  • A PersistentVolume

Scaling StatefulSet

https://kubernetes.io/docs/tutorials/stateful-application/basic-stateful-set/

kubectl scale sts web --replicas=5                 #scale up
kubectl patch sts web -p '{"spec":{"replicas":3}}' #scale down

https://kubernetes.io/docs/tasks/run-application/scale-stateful-set/

  • You cannot scale down a StatefulSet when any of the stateful Pods it manages is unhealthy.

  • Scaling down only takes place after those stateful Pods become running and ready.

rollout restart StatefulSet

Stateful set are removed following their ordinal index with the highest ordinal index first.

kubectl rollout restart is applicable for deployments, daemonsets and statefulsets.

  • partitions if in StatefulSet we have updateStrategy.rollingUpdate.partition: 1 it will restart all pods with index 1 or higher. 
    kubectl rollout restart statefulset demo -n dev
kubectl rollout status statefulset demo -n dev

Force delete StatefulSet pod

https://kubernetes.io/docs/tasks/run-application/force-delete-stateful-set-pod/ graceful pod deletion

kubectl delete pods <pod>

A Pod is not deleted automatically when a node is unreachable. The Pods running on an unreachable Node enter the Terminating or Unknown state after a timeout. Pods may also enter these states when the user attempts graceful deletion of a Pod on an unreachable Node.

The only ways in which a Pod in such a state can be removed from the apiserver are as follows:

  • The Node object is deleted (either by you, or by the Node Controller).

  • The kubelet on the unresponsive Node starts responding, kills the Pod and removes the entry from the apiserver.

  • Force deletion of the Pod by the user.

Force delete StatefulSet pod

kubectl delete pods <pod> --grace-period=0 --force
#if the pod is stuck on Unknown state after delete, remove the pod from the cluster using
kubectl patch pod <pod> -p '{"metadata":{"finalizers":null}}'