How does StatefulSet ensure the preservation of container state after restart or reset?

StatefulSet in Kubernetes ensures the persistence of container state after restart or reset by using persistent volumes and unique identifiers for each pod. Each pod in a StatefulSet gets a stable name and is bound to its PersistentVolumeClaim, which allows data to be stored outside the container's lifecycle. When the pod is restarted, Kubernetes mounts the same volume, providing access to saved data and application state.

Example:

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: example
spec:
  serviceName: "example-service"
  replicas: 3
  selector:
    matchLabels:
      app: example
  template:
    metadata:
      labels:
        app: example
    spec:
      containers:
      - name: example-container
        image: example-image
        volumeMounts:
        - name: data
          mountPath: /data
  volumeClaimTemplates:
  - metadata:
      name: data
    spec:
      accessModes: ["ReadWriteOnce"]
      resources:
        requests:
          storage: 1Gi

Here, each pod gets its own volume that preserves data independently of the container's lifecycle.