kubectl apply vs kubectl create?

Question

What I understood by the documentation is that kubectl apply = kubectl create + kubectl replace. Reference

My understanding is that if I want create new k8s resource in the cluster I should use kubectl create operation. Now If I want to update something in a live k8s resources I should use kubectl replace operation.

If I want to do both operations (create a new k8s resource as well as update the live k8s resources ) then I should use kubectl apply operation

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

At the moment I am using kubectl create operation for creating new resources in the cluster. Thanks

Answer 1

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).

Answer 2

Just to give a more straight forward answer, from my understanding:

apply - makes incremental changes
create - overwrites all changes


Taking this from a DigitalOcean article which was linked by Kubernetes website:

We use apply instead of create here so that in the future we can incrementally apply changes to the Ingress Controller objects instead of completely overwriting them.

Answer 3

Those are two different approaches. 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.

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.

Answer 4

These are imperative commands :

kubectl run = kubectl create deployment

Advantages:

• Simple, easy to learn and easy to remember.
• Require only a single step to make changes to the cluster.

Disadvantages:

• Do not integrate with change review processes.
• Do not provide an audit trail associated with changes.
• Do not provide a source of records except for what is live.
• Do not provide a template for creating new objects.

These are imperative object config:

kubectl create -f your-object-config.yaml

kubectl delete -f your-object-config.yaml

kubectl replace -f your-object-config.yaml

Advantages compared to imperative commands:

• Can be stored in a source control system such as Git.
• Can integrate with processes such as reviewing changes before push and audit trails.
• Provides a template for creating new objects.

Disadvantages compared to imperative commands:

• Requires basic understanding of the object schema.
• Requires the additional step of writing a YAML file.

Advantages compared to declarative object config:

• Simpler and easier to understand.
• More mature after Kubernetes version 1.5.

Disadvantages compared to declarative object configuration:

• Works best on files, not directories.
• Updates to live objects must be reflected in configuration files, or they will be lost during the next replacement.

These are declarative object config

kubectl diff -f configs/

kubectl apply -f configs/

Advantages compared to imperative object config:

• Changes made directly to live objects are retained, even if they are not merged back into the configuration files.
• Better support for operating on directories and automatically detecting operation types (create, patch, delete) per-object.

Disadvantages compared to imperative object configuration:

• Harder to debug and understand results when they are unexpected.
• Partial updates using diffs create complex merge and patch operations.

Answer 5

kubectl create can work with one object configuration file at a time. This is also known as imperative management

kubectl create -f filename|url

kubectl apply works with directories and its sub directories containing object configuration yaml files. This is also known as declarative management. Multiple object configuration files from directories can be picked up. kubectl apply -f directory/

Details :
https://kubernetes.io/docs/tasks/manage-kubernetes-objects/declarative-config/ https://kubernetes.io/docs/tasks/manage-kubernetes-objects/imperative-config/

Answer 6

The explanation below from the official documentation helped me understand kubectl apply.

This command will compare the version of the configuration that you’re pushing with the previous version and apply the changes you’ve made, without overwriting any automated changes to properties you haven’t specified.

kubectl create on the other hand will create (should be non-existing) resources.