How does updating Pods in Deployment and StatefulSet work, and what is the difference?

Updating Pods in Deployment and StatefulSet in Kubernetes occurs differently due to differences in their architecture and purpose.

Deployment:

• Used for stateless applications.
• When updated, a new ReplicaSet with new Pods is created.
• Old Pods are gradually removed (rolling update) to ensure service continuity.
• Pods are identical and interchangeable; the order of creation/deletion does not matter.

StatefulSet:

• Designed for stateful applications where the stability of Pod identifiers and startup order are important.
• When updated, Pods are updated one by one, strictly in order (from the highest index to the lowest or vice versa).
• Each Pod has a unique name and stable storage.
• The update occurs sequentially to avoid disrupting the application's state.

Thus, the main difference:

• Deployment updates Pods in parallel and without regard to order.
• StatefulSet updates Pods sequentially, maintaining order and stability of identifiers.

This is important to consider when choosing a controller for the application and update strategy.