Kubernetes: Stateless vs Stateful

apiVersion: apps/v1
kind: Deployment                   # Stateless!
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3                       # 3 identical copies
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.25
        ports:
        - containerPort: 80
        resources:
          requests: { memory: "64Mi", cpu: "100m" }
          limits:   { memory: "128Mi", cpu: "250m" }

apiVersion: v1
kind: Service
metadata:
  name: nginx-service
spec:
  type: ClusterIP
  selector:  { app: nginx }
  ports:
  - port: 80
    targetPort: 80

$ kubectl apply -f nginx-deployment.yaml
$ kubectl apply -f nginx-service.yaml
$ kubectl get pods
NAME                                READY   STATUS    AGE
nginx-deployment-7d9f8b7945-ab12c   1/1     Running   30s
nginx-deployment-7d9f8b7945-cd34e   1/1     Running   30s
nginx-deployment-7d9f8b7945-ef56g   1/1     Running   30s
# Pod names are RANDOM!

apiVersion: v1
kind: Service
metadata:
  name: mysql-headless
spec:
  clusterIP: None                    # THIS makes it headless
  selector:  { app: mysql }
  ports:
  - port: 3306

apiVersion: apps/v1
kind: StatefulSet                  # NOT Deployment!
metadata:
  name: mysql
spec:
  serviceName: mysql-headless        # REQUIRED
  replicas: 3
  selector:
    matchLabels:  { app: mysql }
  template:
    metadata:
      labels:  { app: mysql }
    spec:
      containers:
      - name: mysql
        image: mysql:8.0
        ports:
        - containerPort: 3306
        env:
        - name: MYSQL_ROOT_PASSWORD
          value: "my-secret-password"
        volumeMounts:
        - name: mysql-data
          mountPath: /var/lib/mysql
  # KEY DIFFERENCE: auto-create disk per pod
  volumeClaimTemplates:
  - metadata:
      name: mysql-data
    spec:
      accessModes: ["ReadWriteOnce"]
      storageClassName: standard
      resources:
        requests:
          storage: 10Gi

$ kubectl apply -f mysql-headless-svc.yaml
$ kubectl apply -f mysql-statefulset.yaml
$ kubectl get pods -w
mysql-0   1/1   Running   45s    # First
mysql-1   1/1   Running   30s    # After mysql-0 ready
mysql-2   1/1   Running   15s    # After mysql-1 ready

$ kubectl get pvc
mysql-data-mysql-0    Bound    10Gi
mysql-data-mysql-1    Bound    10Gi
mysql-data-mysql-2    Bound    10Gi

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast-ssd
provisioner: kubernetes.io/aws-ebs
parameters:
  type: gp3                          # Fast SSD
reclaimPolicy: Retain               # Keep disk if PVC deleted

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-web
spec:
  replicas: 3
  strategy:
    type: RollingUpdate
    rollingUpdate: { maxUnavailable: 1, maxSurge: 1 }
  selector:
    matchLabels:  { app: nginx-web }
  template:
    metadata:
      labels:  { app: nginx-web }
    spec:
      containers:
      - name: nginx
        image: nginx:1.25-alpine
        ports:
        - containerPort: 80
        resources:
          requests: { cpu: "100m", memory: "64Mi" }
          limits:   { cpu: "250m", memory: "128Mi" }
        readinessProbe:
          httpGet: { path: /, port: 80 }
        livenessProbe:
          httpGet: { path: /, port: 80 }
---
apiVersion: v1
kind: Service
metadata:
  name: nginx-web-svc
spec:
  type: LoadBalancer
  selector:  { app: nginx-web }
  ports:
  - port: 80
    targetPort: 80

# Secret + Headless Service + StatefulSet
---
apiVersion: v1
kind: Secret
metadata:  { name: mysql-secret }
type: Opaque
data:
  root-password: bXktc2VjcmV0LXBhc3M=
---
apiVersion: v1
kind: Service
metadata:  { name: mysql-headless }
spec:
  clusterIP: None
  selector:  { app: mysql }
  ports:  [{ port: 3306 }]
---
apiVersion: apps/v1
kind: StatefulSet
metadata:  { name: mysql }
spec:
  serviceName: mysql-headless
  replicas: 3
  selector:
    matchLabels:  { app: mysql }
  template:
    metadata:
      labels:  { app: mysql }
    spec:
      terminationGracePeriodSeconds: 30
      containers:
      - name: mysql
        image: mysql:8.0
        ports:  [{ containerPort: 3306 }]
        env:
        - name: MYSQL_ROOT_PASSWORD
          valueFrom:
            secretKeyRef:
              name: mysql-secret
              key: root-password
        resources:
          requests: { cpu: "500m",  memory: "512Mi" }
          limits:   { cpu: "1000m", memory: "1Gi"   }
        volumeMounts:
        - name: mysql-data
          mountPath: /var/lib/mysql
        readinessProbe:
          exec:
            command: ["mysqladmin","ping","-h","127.0.0.1"]
          initialDelaySeconds: 30
  volumeClaimTemplates:
  - metadata:  { name: mysql-data }
    spec:
      accessModes: ["ReadWriteOnce"]
      storageClassName: standard
      resources:
        requests:  { storage: 10Gi }

# ===== STATELESS =====
kubectl create deployment nginx --image=nginx
kubectl get deployments
kubectl scale deployment nginx --replicas=5
kubectl rollout status deployment/nginx
kubectl rollout undo deployment/nginx

# ===== STATEFUL =====
kubectl get statefulsets
kubectl get pvc
kubectl scale statefulset mysql --replicas=5
kubectl delete statefulset mysql      # PVCs remain!

# ===== GENERAL =====
kubectl get pods -w                   # Watch live
kubectl describe pod mysql-0
kubectl logs mysql-0
kubectl exec -it mysql-0 -- bash
kubectl get all