What is Server-side apply

Server-side apply is a new feature that is meant to replace client-side apply feature implemented by kubectl apply. The feature is a new merging algorithm for declarative configuration of objects in kubernetes and was graduated to GA in 1.22

Client-side apply

Client-side apply is used when you run kubectl apply -f somefile.yaml. The declarative configuration you specified is stored in the field metadata.annotations.kubectl.kubernetes.io/last-applied-configuration on the kubernetes object.

If you change something in your yaml file and do kubectl diff -f somefile.yaml you get a diff of whats changed. But if someone has run an imperative command you want get a diff because that wont be saved in to last-applied-configuration field. Because of this it’s hard to know where a value came from.

When runnning kubectl apply -f somefile.yaml on an existing object kubectl does a strategic merge patch and just updates the values that changed from last-applied-configuration. It doesn’t replace the configuration.

Server-side apply

The idea with server-side apply(SSA) is to have a simpler mechanism for tracking and see who’s the owner of a field so that multiple appliers can update the same object.

When using SSA the kubernetes api server adds the metadata.managedFields field that tracks metadata about how the field is managed.

apiVersion: v1
kind: ConfigMap
  name: test-cm
  namespace: default
    test-label: test
  - manager: kubectl
    operation: Apply
    apiVersion: v1
    time: "2010-10-10T0:00:00Z"
    fieldsType: FieldsV1
          f:test-label: {}
        f:key: {}
  key: some value

With the help of the managedFields field SSA enables conflict detection so kubernetes knows when multiple appliers try to edit the same field. An applier could be a user using kubectl or a kubernetes controller.

SSA also removes the need to write hand-rolled patches. Instead you can just apply a partial object which the Apply Api will use update the existing object.

The Apply Api can be used from any language without the need to call kubectl

To use SSA with kubectl you need to add the argument --server-side

kubectl apply -f somefile.yaml --server-side