Cluster API provides support for three kinds of webhooks: validating webhooks, defaulting webhook and conversion webhooks.

Validating webhooks

Validating webhooks are an implementation of a Kubernetes validating webhook. A validating webhook allows developers to test whether values supplied by users are valid. e.g. the Cluster webhook ensures the Infrastructure reference supplied at the Cluster’s .spec.infrastructureRef is in the same namespace as the Cluster itself and rejects the object creation or update if not.

Defaulting webhooks

Defaulting webhooks are an implementation of a Kubernetes mutating webhook. A defaulting webhook allows developers to set default values for a type before they are placed in the Kubernetes data store. e.g. the Cluster webhook will set the Infrastructure reference namespace to equal the Cluster namespace if .spec.infrastructureRef.namespace is empty.

Conversion webhooks

Conversion webhooks are what allow Cluster API to work with multiple API types without requiring different versions. It does this by converting the incoming version to a Hub version which is used internally by the controllers. To read more about conversion see the Kubebuilder documentation

For a walkthrough on implementing conversion webhooks see the video in the Developer Guide

Implementing webhooks with Controller Runtime and Kubebuilder

The webhooks in Cluster API are offered through tools in Controller Runtime and Kubebuilder. The webhooks implement interfaces defined in Controller Runtime, while generation of manifests can be done using Kubebuilder.

For information on how to create webhooks refer to the Kubebuilder book.

Webhook manifests are generated using Kubebuilder in Cluster API. This is done by adding tags to the webhook implementation in the codebase. Below, for example, are the tags on the the Cluster webhook:

// +kubebuilder:webhook:verbs=create;update;delete,path=/validate-cluster-x-k8s-io-v1beta1-cluster,mutating=false,failurePolicy=fail,matchPolicy=Equivalent,groups=cluster.x-k8s.io,resources=clusters,versions=v1beta1,name=validation.cluster.cluster.x-k8s.io,sideEffects=None,admissionReviewVersions=v1;v1beta1
// +kubebuilder:webhook:verbs=create;update,path=/mutate-cluster-x-k8s-io-v1beta1-cluster,mutating=true,failurePolicy=fail,matchPolicy=Equivalent,groups=cluster.x-k8s.io,resources=clusters,versions=v1beta1,name=default.cluster.cluster.x-k8s.io,sideEffects=None,admissionReviewVersions=v1;v1beta1

// Cluster implements a validating and defaulting webhook for Cluster.
type Cluster struct {
    Client client.Reader

A detailed guide on the purpose of each of these tags is here.