How does k8s know which pod to update?

Question

I'm currently getting started with Kubernetes, and so far, I have a question that I could not find answered anywhere.

Until know, I have learned what containers, pods, and replica sets are. I basically understand the things, but one thing I did not get is: If I update a manifest of a pod (or of a replica set), and re-POST it to k8s - how does k8s know which existing manifest this refers to?

Is this matching done by the manifest's name, ie by the name of the pod or the replica set? Or …?

In other words: If I update a manifest, how does k8s know that it is an updated one, and how does it detect which one is the one with the previous version?

Answer 1

You are right, k8s uses metadata.name for identifying resources. That name is unique per resource type (Pod/ReplicaSet/...) and namespace.

Answer 2

Well, for starters lets get things straight. When you update manifest, it is obvious what to update in the first place - the object you updated - ie. Deployment or ReplicaSet. Now, when that is updated, the RollingUpdate kicks in, and this is what I assume you wonder about as well as in general how ownership of pod is established. If you make a kubectl get pod -o yaml you can find a keys like ownerReferences , pod-template-hash and kubernetes.io/created-by which should be rather obvious when you see the content. In the other direction (so not from the Pod but from Deployment) you have a selector field which defines what labels are used to filter pods to find the right ones.