What is the difference between StatefulSet and ReplicaSet/Deployment?

ReplicaSet is a controller in Kubernetes that ensures a specified number of identical pods (replicas) are running. It monitors and maintains the desired number of pods.

Deployment is a higher-level abstraction that manages ReplicaSet. Deployment allows easy updates (rolling updates), rollbacks, scaling, and lifecycle management of ReplicaSet.

StatefulSet is a controller for managing stateful applications that require stable identities, ordered deployment and scaling, and persistent storage. Unlike ReplicaSet/Deployment, StatefulSet guarantees:

• Unique and stable pod names (e.g., pod-0, pod-1) that persist across restarts.
• Sequential creation, update, and deletion of pods.
• Binding to persistent volumes (PersistentVolume) that retain data across restarts.

Summary:

• ReplicaSet maintains a specified number of identical pods.
• Deployment manages ReplicaSet, adding update and rollback capabilities.
• StatefulSet is for stateful applications where stable identity and order are important (e.g., databases, clusters).

This allows choosing the appropriate controller depending on application requirements.