What is a ReplicaSet?

Learn that a ReplicaSet ensures Pod replicas.

What is a ReplicaSet?

A ReplicaSet's only job is to make sure that, at every moment, exactly replicas pods matching its selector exist. If some are missing it creates new ones from the template; if there are too many it deletes some. It knows nothing of version transitions or rollbacks; the Deployment adds those.

$ kubectl get rs
NAME            DESIRED   CURRENT   READY   AGE
web-7c5b9f6d8   3         3         3       2h
web-6b4d8c7f5   0         0         0       3h     # old version, for rollback

That is why ReplicaSets are used not directly but through a Deployment.

The trouble with a bare ReplicaSet

If you change the image in the template of a directly created ReplicaSet, the existing pods aren't updated; only pods created from then on get the new image. A Deployment in this case opens a new ReplicaSet and replaces all the pods in an orderly way.

kubectl set image rs/api api=api:2.0
kubectl get pods -l app=api -o jsonpath='{.items[*].spec.containers[0].image}'
# api:1.0 api:1.0 api:1.0   <- still old

Exercise Inspect the ReplicaSets

Write two commands: list the ReplicaSets labelled app=web; then show the history of the web deployment.

Show example solution

kubectl get rs -l app=web
kubectl rollout history deployment/web

Test yourself

Why isn't the old ReplicaSet with DESIRED 0 deleted?
- It can't be deleted
- It remained because of an error
- It creates new pods
- It is kept in the Deployment history for rollback
Show answer

It is kept in the Deployment history for rollback — revisionHistoryLimit determines how many are kept.
List the ReplicaSets.
```
kubectl get ____
```
Show answer
```
kubectl get rs
```
Match.

DESIRED

Desired replicas
CURRENT

Existing replicas
READY

Ready replicas
DESIRED 0

Old version

Next lesson: ReplicaSet YAML

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies