K8S StatefulSet

StatefulSet is an object that manages a set of K8S PodK8S Pod
Pod is a [[K8S Object]] used to run one or more tightly coupled containers and is rarely used directly, especially in production because of the limitations on the number of properties that can be u...
s and provides guarantees on their ordering and uniqueness.

It is pretty similar to K8S Deployment, but has a few important differences which make it a much better candidate for managing stateful applications:

  • instead of pods getting a random name suffix, StatefulSet always gives the same suffix to the pod it creates
    • for example, first pod created by StatefulSet called test will always be called test-0, second one test-1 and so on
  • since pods maintain their identity, StatefulSets can reuse existing volumes, so that if test-1 pod is recreated, new test-1 pod will be able to keep using the same volume as the old one
  • if we delete a pod of a StatefulSet, its PVC is not deleted by default so it can be reused by a new pod (this can be overridden by changing persistentVolumeClaimRetentionPolicy)
  • they are rolled out and rolled back in order (assuming default pod management policy OrderedReady is used):
    • when creating pods, StatefulSet will ensure the pod test-0 is ready before moving on to provision test-1
    • when destroying pods, StatefulSet will ensure the pod with the highest index is destroyed first, before moving on to the pod with the next lowest index
    • when updating pods, StatefulSet will first recreate the pod with the highest index, and once it's ready it will move on to the pod with the next highest index (assuming default update strategy RollingUpdate is used)
  • StatefulSets need a Headless ServiceHeadless Service
    A [[01 Inbox/Kubernetes Service]] will expose an IP address, and when you reach it, it will forward your request to one of the underlying pods. This is good if you want to connect to one pod, but w...
    which needs to be manually created

Status: #💡