"kubcetl replace replicaset" did not update pods with new image. Had to delete pods

Kubernetes
#1

Hello,
In ReplicaSets practice test of CKA course, there was a task to modify the new-replica-set to use correct busybox image. I saved the yaml file of the existing replica set (kubectl get rs new-replica-set -o yaml > new-replica-set.yml). I modified the image name in the yml file and saved it. I then ran command kubectl replace rs -f new-replica-set.yml. However, the pods did not reinitialize. I had to delete the pods or recreate the replica set (delete and create) to fix the issue.

Question: Why did the replace command not reinitialize the pods? How does the replace command update a replica set when it does not reinitialize the pods? What does the replace operation really do, and what does it not do?

#2

It appears that if you try to apply the yaml, it causes an error. Running k delete -f YAML-FILE; k apply -f YAML-FILE worked, provided you replaced the image in the yaml with a valid name for the busybox image (I used “busybox”).

#3

In this case replace is working a bit like apply.

With replicasets, changing the pod properties does not cause a redeployment of the pods - the RS must be deleted first. That’s one of the issues that Deployment solves.

You should instead run

kubectl replace --force -f new-replica-set.yml

--force forces the existing object to be deleted and then recreated. This is also essential when trying to replace a standalone Pod, otherwise replace would error saying you can’t change properties of a running pod.

#5

Hi Alistair.

In lecture 28 of the CKA course, the slide titled “Scale” should be updated. The k replace command as mentioned in the slide does yields an error without the --force option

#6

The slides are currently all being redesigned for this course. I will ensure the correct command is on the new slide.