Deployment: Rollout

A rollout occurs when a Deployment or StatefulSet updates its Pods to a new version. Kubernetes gradually replaces old Pods with new ones, following a defined strategy.

• Ensures Zero Downtime: New Pods are created before terminating old ones.

• Supports Rollback: Can revert to the previous version if needed.

• Controlled Updates: Uses a strategy to limit impact.

By default when a deployment object is applied, kubernetes will do a rollout update to the existing pods.

Rollout Restart

Whenever we want to restart pods because of new latest image is available or to fetch latest configuration from ConfigMap or Secret we can do a rollout restart.

➜ kubectl rollout restart deployment simple-go 
deployment.apps/simple-go restarted

And when you check the pods immediately after rollout restart you will see the process of rollout. The controller manager will spin up 1 new pods and as soon the new pods ready it will take down 1 pods until all new pods are ready.

➜ kubectl get pods                            
NAME                         READY   STATUS              RESTARTS   AGE
simple-go-764bc77644-grfhg   1/1     Running             0          3h31m
simple-go-764bc77644-snb8n   1/1     Running             0          3h31m
simple-go-7cf4d4d6df-hgjv8   1/1     Running             0          5s
simple-go-7cf4d4d6df-hj7sk   0/1     ContainerCreating   0          2s

As we can see below after some time all olds pods already gone replaced by new sets of pods.

➜ kubectl get pods
NAME                         READY   STATUS    RESTARTS   AGE
simple-go-7cf4d4d6df-5ck9n   1/1     Running   0          30s
simple-go-7cf4d4d6df-hgjv8   1/1     Running   0          36s
simple-go-7cf4d4d6df-hj7sk   1/1     Running   0          33s

Rollout History

To show list of deployment history we can use command rollout history deployment <name>.

➜ kubectl rollout history deployment simple-go
deployment.apps/simple-go 
REVISION  CHANGE-CAUSE
1         <none>
2         <none>
3         <none>

In here the CHANGE-CAUSE is empty because we need to explicitly specify the message in metadata.annotations.

Rollout Undo

rollout undo will roll back existing deployment into previous version.

➜ kubectl rollout undo deployment simple-go

We can also also specify the revision based on the rollout history using --to-revision.

➜ kubectl rollout undo deployment simple-go --to-revision=2

References

• https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#updating-a-deployment
• https://kubernetes.io/docs/reference/kubectl/generated/kubectl_rollout/
• https://kubernetes.io/docs/concepts/overview/working-with-objects/annotations/