Kubernetes StatefulSets: deploying databases and stateful applications

StatefulSets Provide Identity to Pods

Deployments treat Pods as interchangeable. StatefulSets give each Pod a stable identity: a persistent name (pod-0, pod-1), a stable DNS hostname, and its own dedicated PersistentVolume. Pods start and stop in order. This is essential for databases and clustered applications.

StatefulSet Manifest

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: postgres
spec:
  serviceName: postgres-svc  # must match a headless service
  replicas: 3
  selector:
    matchLabels:
      app: postgres
  template:
    metadata:
      labels:
        app: postgres
    spec:
      containers:
      - name: postgres
        image: postgres:15
        env:
        - name: POSTGRES_PASSWORD
          valueFrom:
            secretKeyRef:
              name: pg-secret
              key: password
        volumeMounts:
        - name: pgdata
          mountPath: /var/lib/postgresql/data
  volumeClaimTemplates:           # each pod gets its own PVC
  - metadata:
      name: pgdata
    spec:
      accessModes: ["ReadWriteOnce"]
      resources:
        requests:
          storage: 20Gi

Each Pod gets a PVC named pgdata-postgres-0, pgdata-postgres-1 etc. These PVCs persist even if the StatefulSet is deleted — you must delete them manually to free storage.