StatefulSets

StatefulSets are workload API objects used to manage stateful applications. Unlike Deployments, StatefulSets maintain a sticky identity for each pod and provide guarantees about the ordering and uniqueness of these pods.

StatefulSets are valuable for applications that require one or more of the following:

Stable, unique network identifiers
Stable, persistent storage
Ordered, graceful deployment and scaling
Ordered, automated rolling updates

How StatefulSets Work

StatefulSets create pods with predictable names following the pattern {statefulset-name}-{ordinal}. For example, a StatefulSet named mysql with 3 replicas creates pods: mysql-0, mysql-1, mysql-2.

Key Characteristics:

Pods are created sequentially (0, 1, 2...) and terminated in reverse order
Each pod gets a stable DNS hostname: {pod-name}.{service-name}.{namespace}.svc.cluster.local
PersistentVolumeClaims are retained when pods are deleted (data persists)
Scaling down does not delete associated PVCs

When to Use StatefulSets

Use Case	Examples
Databases	MySQL, PostgreSQL, MongoDB
Message Queues	Kafka, RabbitMQ
Distributed Systems	Elasticsearch, Cassandra, Zookeeper
Applications requiring stable network identity	Leader election scenarios

Resources

OpenShiftKubernetes

StatefulSets

StatefulSet Basics

References

Headless Service for StatefulSet (required for stable network identities)

apiVersion: v1
kind: Service
metadata:
  name: mysql
  labels:
    app: mysql
spec:
  ports:
    - port: 3306
      name: mysql
  clusterIP: None  # Headless service
  selector:
    app: mysql

StatefulSet Definition

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: mysql
spec:
  serviceName: "mysql"
  replicas: 3
  selector:
    matchLabels:
      app: mysql
  template:
    metadata:
      labels:
        app: mysql
    spec:
      containers:
        - name: mysql
          image: mysql:8.0
          ports:
            - containerPort: 3306
              name: mysql
          env:
            - name: MYSQL_ROOT_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: mysql-secret
                  key: password
          volumeMounts:
            - name: data
              mountPath: /var/lib/mysql
  volumeClaimTemplates:
    - metadata:
        name: data
      spec:
        accessModes: ["ReadWriteOnce"]
        storageClassName: "standard"
        resources:
          requests:
            storage: 10Gi

OpenShiftKubernetes

Create StatefulSet

oc apply -f statefulset.yaml

Get StatefulSets

oc get statefulsets

Describe StatefulSet

oc describe statefulset mysql

Scale StatefulSet

oc scale statefulset mysql --replicas=5

Delete StatefulSet (keeps PVCs)

oc delete statefulset mysql

Delete StatefulSet and PVCs

oc delete statefulset mysql --cascade=foreground
oc delete pvc -l app=mysql