What is Kubernetes StatefulSet?

Question

Accepted Answer

**StatefulSet** — manages stateful applications with stable identity and storage.

```yaml
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: postgres
spec:
  serviceName: postgres
  replicas: 3
  selector:
    matchLabels:
      app: postgres
  template:
    metadata:
      labels:
        app: postgres
    spec:
      containers:
      - name: postgres
        image: postgres:15
        ports:
        - containerPort: 5432
        env:
        - name: POSTGRES_PASSWORD
          valueFrom:
            secretKeyRef:
              name: postgres-secret
              key: password
        volumeMounts:
        - name: data
          mountPath: /var/lib/postgresql/data
        resources:
          requests:
            memory: "256Mi"
            cpu: "250m"
          limits:
            memory: "1Gi"
            cpu: "1000m"
  volumeClaimTemplates:
  - metadata:
      name: data
    spec:
      accessModes: ["ReadWriteOnce"]
      storageClassName: fast
      resources:
        requests:
          storage: 10Gi
---
# Headless Service for stable DNS
apiVersion: v1
kind: Service
metadata:
  name: postgres
spec:
  clusterIP: None
  selector:
    app: postgres
  ports:
  - port: 5432
    targetPort: 5432
```

**Stable Identity:**
- Pod names: postgres-0, postgres-1, postgres-2.
- DNS: postgres-0.postgres.namespace.svc.cluster.local.
- Ordered deployment and scaling.

**Use cases:**
- Databases (PostgreSQL, MySQL).
- Message queues (Kafka, RabbitMQ).
- Distributed systems (ZooKeeper, etcd).

DevOps: What is Kubernetes StatefulSet?