Bits of .NET

💾 Databases Need Stable Identity

Deployments are for stateless apps. StatefulSets are for stateful apps. Stable network identity, persistent storage, ordered deployment.

📝 StatefulSet Example

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
        env:
        - name: POSTGRES_PASSWORD
          valueFrom:
            secretKeyRef:
              name: postgres-secret
              key: password
        volumeMounts:
        - name: data
          mountPath: /var/lib/postgresql/data
  volumeClaimTemplates:
  - metadata:
      name: data
    spec:
      accessModes: ["ReadWriteOnce"]
      resources:
        requests:
          storage: 10Gi

🎯 Key Features

- Stable network identity: pod-0, pod-1, pod-2
- Stable storage: PVC per pod
- Ordered deployment (0, 1, 2)
- Ordered scaling (last in, first out)
- Persistent volume per replica

Use cases:
- Databases (PostgreSQL, MySQL)
- Kafka, ZooKeeper
- Elasticsearch
- Cassandra
- RabbitMQ

Headless service:
apiVersion: v1
kind: Service
metadata:
  name: postgres
spec:
  clusterIP: None
  selector:
    app: postgres
  ports:
  - port: 5432

💡 Commands

• kubectl get statefulset
• kubectl describe statefulset postgres
• kubectl get pods -l app=postgres
• kubectl delete statefulset postgres
• Scale: kubectl scale statefulset postgres –replicas=5