KodeCloud Kubernetes Deployments
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Deployment vs Replica Set
Components of the deployment definition file are exactly same as the Replica Set definition file except for the kind.
What are the additional features provided by Deployment over ReplicaSet considering they have the same definition file.
- You don't want to upgrade all the instances instantly. You would want rolling updates.
- Roll back the changes.
- Make multiple changes to your environment. You wouldn't want to apply it immediately. You would like to pause the environment make the changes and resume so that all the changes are rolled out together.
- The deployment automatically creates a ReplicaSet
- Pod naming convention when created by deployment:
- We can see the status of roll out using:
k rollout status deployment/<deployment-name>
- Get the history and revisions of the rollout:
k rollout history deployment/<deployment-name>
- Get the rollout history of a particular revision:
k rollout history deployment/<deployment-name> --revision=3
- Rolling update is the default deployment strategy.
Another deployment strategy is Recreate in which all the pods are first destroyed and then the new revision is added.
Difference between Recreate & Rolling Update:
- See how the ReplicaSet replicas change in Recreate (reduced to 0) and Rolling Update (few units at a time)
- When we apply a deployment definition a new rollout is triggered and a new revision is created.
How do upgrades happen in Deployments?
- When we upgrade our application a new ReplicaSet is created under the hood and starts deploying the containers there.
- At the same time pods are being taken down in the old ReplicaSet in a rolling update strategy.
- This can be verified when we list the ReplicaSets. We will see one ReplicaSet with 0 pods and other one with x pods.
- We can easily rollback using
k rollout undo deployment/<deployment-name>
- This will destroy the pods in the new ReplicaSet and bring the older ones up in the old ReplicaSet.
- We can notice the difference in ReplicaSets before and after the rollback.
Rollback to a particular revision:
k rollout undo deployment/<deployment-name> --to-revision=1
Whenever we apply a deployment file we can record the cause of rollout using the
k apply -f deployment.yaml --record
- Using this would mean that the cause of change is tracked in the revision history.
Scenario: In the deployment you change the image to a non existent image
- When you apply this configuration you will have some working pods (from the older ReplicaSet) and some non working pods (from the new ReplicaSet)
- In the above example image we see that only one pod was terminated (from the older ReplicaSet) and it failed to create 3 new pods since it couldn't pull the image.
- You can notice the different ReplicaSet by comparing the hash.
- Even though we have specified the wrong image, the application is not impacted since we have 5 working pods.
- This is because of the default deployment strategy of rolling deployment.
- Kubernetes will only delete the older pods once all the new pods are up and running.
Last updated: 2022-10-13