kubernetes rollout triggered on scaling

Question

I have a simple kubernetes deployment with 3 replicas of an nginx container.

However when I scale my deployment, it does trigger a rollout. What am I missing here?:

C:\Kubernetes>kubectl scale deployment myapp-deployment --replicas=20
deployment "myapp-deployment" scaled
C:\Kubernetes>kubectl rollout status deployment/myapp-deployment
Waiting for rollout to finish: 3 of 20 updated replicas are available...
Waiting for rollout to finish: 4 of 20 updated replicas are available...
Waiting for rollout to finish: 5 of 20 updated replicas are available...
Waiting for rollout to finish: 6 of 20 updated replicas are available...
Waiting for rollout to finish: 7 of 20 updated replicas are available...
Waiting for rollout to finish: 8 of 20 updated replicas are available...
Waiting for rollout to finish: 9 of 20 updated replicas are available...
Waiting for rollout to finish: 10 of 20 updated replicas are available...
Waiting for rollout to finish: 11 of 20 updated replicas are available...
Waiting for rollout to finish: 12 of 20 updated replicas are available...
Waiting for rollout to finish: 13 of 20 updated replicas are available...
Waiting for rollout to finish: 14 of 20 updated replicas are available...
Waiting for rollout to finish: 15 of 20 updated replicas are available...
Waiting for rollout to finish: 16 of 20 updated replicas are available...
Waiting for rollout to finish: 17 of 20 updated replicas are available...
Waiting for rollout to finish: 18 of 20 updated replicas are available...
Waiting for rollout to finish: 19 of 20 updated replicas are available...
deployment "myapp-deployment" successfully rolled out

What does this rollout mean?

Answer 1

when you update the pod template, the existing pods are brought down and new pods are brought up. When you just change the replica number, the existing pods are left in place and identical ones to the existing are spun up. So it's like adding to the previous rollout, not creating a new one.