When to use StatefulSet instead of Deployment? Provide examples.

StatefulSet and Deployment are controllers in Kubernetes for managing pods, but they serve different purposes.

When to use StatefulSet:

• When you need to manage the state of applications that require stable identifiers, persistent storage, and ordered deployment.
• Examples:
  • Databases (PostgreSQL, Cassandra), where each instance has a unique identifier and a persistent disk.
  • Distributed state clusters (e.g., ZooKeeper, Kafka brokers).
  • Applications where the order of starting and stopping pods is important.

Deployment is suitable for stateless applications where pods are interchangeable and do not depend on specific state.

Key differences of StatefulSet:

• Each pod gets a unique stable name (e.g., myapp-0, myapp-1).
• Pods get persistent volumes (PersistentVolumeClaims) attached to them.
• Ensures ordered creation, updating, and deletion of pods.

Example: if you are deploying a MongoDB cluster with 3 replicas, it is better to use StatefulSet so that each replica has a persistent identifier and storage.