For which application should StatefulSet be used instead of Deployment and why?

StatefulSet should be used for applications that require:

1. Stable network identity. Pods receive a unique, persistent hostname in the format <statefulset-name>-<pod-index>.
2. Stable, persistent storage. Each Pod replica is assigned its own PersistentVolume. Even upon Pod restart, its data remains attached.
3. Ordered deployment and scaling. Pods are created and deleted in a strictly defined order (ascending for creation, descending for deletion).
4. Ordered or parallel updates. Supports both gradual (rolling update) and parallel (in-place or parallel) updates, with the ability to control the order.

Examples of such applications:

• Databases (MySQL, PostgreSQL, MongoDB)
• Message brokers (Kafka, RabbitMQ)
• Cache systems (Redis, Memcached)
• Distributed file systems (e.g., Ceph, if running in Kubernetes)
• Any application that needs to preserve state across restarts and have a stable identity.

Unlike Deployment, where Pods are interchangeable and stateless, StatefulSet guarantees uniqueness and state preservation for each replica, which is critical for applications dependent on persistent storage and stable network identity.

An example of a simple StatefulSet for a database:

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: mysql
spec:
  selector:
    matchLabels:
      app: mysql
  serviceName: "mysql" # Necessary for stable network identity of Pods
  replicas: 3
  template:
    metadata:
      labels:
        app: mysql
    spec:
      containers:
      - name: mysql
        image: mysql:5.7
        ports:
        - containerPort: 3306
        env:
        - name: MYSQL_ROOT_PASSWORD
          value: "password" # Use Secrets in production
        volumeMounts:
        - name: mysql-persistent-storage
          mountPath: /var/lib/mysql # Path where persistent storage is mounted
  volumeClaimTemplates:
  - metadata:
      name: mysql-persistent-storage # PVC template name
    spec:
      accessModes: [ "ReadWriteOnce" ] # Access mode
      resources:
        requests:
          storage: 10Gi # Requested storage size