kubectl apply vs kubectl create?

Question

What I understood by the documentation is that:

• kubectl create 

  Creates a new k8s resource in the cluster

• kubectl replace 

  Updates a resource in the live cluster

• kubectl apply 

  If I want to do create + replace (Reference)

My questions are

1. Why are there three operations for doing the same task in a cluster?
2. What are the use cases for these operations?
3. How do they differ from each other under the hood?

Answer 1

Those are two different approaches:

Imperative Management

kubectl create is what we call Imperative Management. On this approach you tell the Kubernetes API what you want to create, replace or delete, not how you want your K8s cluster world to look like.

Declarative Management

kubectl apply is part of the Declarative Management approach, where changes that you may have applied to a live object (i.e. through scale) are "maintained" even if you apply other changes to the object.

You can read more about imperative and declarative management in the Kubernetes Object Management documentation.

In laymans They do different things. If the resource exists, kubectl create will error out and kubectl apply will not error out.

Answer 2

When running in a CI script, you will have trouble with imperative commands as create raises an error if the resource already exists.

What you can do is applying (declarative pattern) the output of your imperative command, by using --dry-run=true and -o yaml options:

kubectl create whatever --dry-run=true -o yaml | kubectl apply -f - 

The command above will not raise an error if the resource already exists (and will update the resource if needed).

This is very useful in some cases where you cannot use the declarative pattern (for instance when creating a docker-registry secret).