The extra ReplicaSet layer is what makes rolling updates and instant rollbacks possible.

You edit the Deployment. It creates a new ReplicaSet for each pod-template change and keeps old ones around, scaled to zero.

Deployment checkout

replicas: 3, image checkout:1.4

ReplicaSet -7d9f8

image 1.4, scaled to 3

podpodpod

ReplicaSet -5c4b1

image 1.3, scaled to 0 (kept for rollback)

---

rollback is just flipping scale

$ kubectl get rs -l app=checkout
NAME              DESIRED  CURRENT  READY
checkout-7d9f8    3        3        3
checkout-5c4b1    0        0        0
$ kubectl rollout undo deployment/checkout
$ kubectl rollout history deployment/checkout

History limit

revisionHistoryLimit (default 10) controls how many old ReplicaSets are kept.

Pod template hash

The suffix in a ReplicaSet name is a hash of the pod template, so any template change makes a new one.

Deployment owns a ReplicaSet, which owns the Pods

A Deployment is the standard way to run a stateless application. It does not create Pods directly. It creates a ReplicaSet, whose only job is to keep a given number of identical Pods running, and the ReplicaSet creates the Pods. Reveal the layers with the right arrow.

Why the extra layer? When you change the Deployment's pod template (a new image, a changed environment variable), the Deployment creates a brand-new ReplicaSet for the new template and gradually shifts replicas from the old ReplicaSet to the new one. The old ReplicaSet is kept, scaled to zero. kubectl rollout undo works by simply scaling the old ReplicaSet back up and the new one down; no configuration needs to be reconstructed.

The ReplicaSet name suffix is a hash of the pod template, so identical templates reuse the same ReplicaSet, and any change creates a new one. Changing only the replica count does not create a new ReplicaSet because replicas is not part of the template.

Gotcha: kubectl rollout undo only reverts the pod template, not other fields such as replicas, and GitOps tools will fight a manual undo because Git still says the new version. In a GitOps workflow, revert in Git.