Implementing defaulting/validating webhooks

If you want to implement admission webhooks for your CRD, the only thing you need to do is to implement the Defaulter and (or) the Validator interface.

Kubebuilder takes care of the rest for you, such as

  1. Creating the webhook server.
  2. Ensuring the server has been added in the manager.
  3. Creating handlers for your webhooks.
  4. Registering each handler with a path in your server.

First, let’s scaffold the webhooks for our CRD (CronJob). We’ll need to run the following command with the --defaulting and --programmatic-validation flags (since our test project will use defaulting and validating webhooks):

kubebuilder create webhook --group batch --version v1 --kind CronJob --defaulting --programmatic-validation

This will scaffold the webhook functions and register your webhook with the manager in your main.go for you.

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Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

Go imports
package v1

import (
	apierrors ""
	validationutils ""
	ctrl ""
	logf ""

Next, we’ll setup a logger for the webhooks.

var cronjoblog = logf.Log.WithName("cronjob-resource")

Then, we set up the webhook with the manager.

// SetupWebhookWithManager will setup the manager to manage the webhooks
func (r *CronJob) SetupWebhookWithManager(mgr ctrl.Manager) error {
	return ctrl.NewWebhookManagedBy(mgr).

Notice that we use kubebuilder markers to generate webhook manifests. This marker is responsible for generating a mutating webhook manifest.

The meaning of each marker can be found here.

// +kubebuilder:webhook:path=/mutate-batch-tutorial-kubebuilder-io-v1-cronjob,mutating=true,failurePolicy=fail,,resources=cronjobs,verbs=create;update,versions=v1,,sideEffects=None,admissionReviewVersions=v1

We use the webhook.Defaulter interface to set defaults to our CRD. A webhook will automatically be served that calls this defaulting.

The Default method is expected to mutate the receiver, setting the defaults.

var _ webhook.Defaulter = &CronJob{}

// Default implements webhook.Defaulter so a webhook will be registered for the type
func (r *CronJob) Default() {
	cronjoblog.Info("default", "name", r.Name)

	if r.Spec.ConcurrencyPolicy == "" {
		r.Spec.ConcurrencyPolicy = AllowConcurrent
	if r.Spec.Suspend == nil {
		r.Spec.Suspend = new(bool)
	if r.Spec.SuccessfulJobsHistoryLimit == nil {
		r.Spec.SuccessfulJobsHistoryLimit = new(int32)
		*r.Spec.SuccessfulJobsHistoryLimit = 3
	if r.Spec.FailedJobsHistoryLimit == nil {
		r.Spec.FailedJobsHistoryLimit = new(int32)
		*r.Spec.FailedJobsHistoryLimit = 1

This marker is responsible for generating a validating webhook manifest.

// +kubebuilder:webhook:verbs=create;update;delete,path=/validate-batch-tutorial-kubebuilder-io-v1-cronjob,mutating=false,failurePolicy=fail,,resources=cronjobs,versions=v1,,sideEffects=None,admissionReviewVersions=v1

We can validate our CRD beyond what’s possible with declarative validation. Generally, declarative validation should be sufficient, but sometimes more advanced use cases call for complex validation.

For instance, we’ll see below that we use this to validate a well-formed cron schedule without making up a long regular expression.

If webhook.Validator interface is implemented, a webhook will automatically be served that calls the validation.

The ValidateCreate, ValidateUpdate and ValidateDelete methods are expected to validate its receiver upon creation, update and deletion respectively. We separate out ValidateCreate from ValidateUpdate to allow behavior like making certain fields immutable, so that they can only be set on creation. ValidateDelete is also separated from ValidateUpdate to allow different validation behavior on deletion. Here, however, we just use the same shared validation for ValidateCreate and ValidateUpdate. And we do nothing in ValidateDelete, since we don’t need to validate anything on deletion.

// NOTE: The 'path' attribute must follow a specific pattern and should not be modified directly here.
// Modifying the path for an invalid path can cause API server errors; failing to locate the webhook.

var _ webhook.Validator = &CronJob{}

// ValidateCreate implements webhook.Validator so a webhook will be registered for the type
func (r *CronJob) ValidateCreate() (admission.Warnings, error) {
	cronjoblog.Info("validate create", "name", r.Name)

	return nil, r.validateCronJob()

// ValidateUpdate implements webhook.Validator so a webhook will be registered for the type
func (r *CronJob) ValidateUpdate(old runtime.Object) (admission.Warnings, error) {
	cronjoblog.Info("validate update", "name", r.Name)

	return nil, r.validateCronJob()

// ValidateDelete implements webhook.Validator so a webhook will be registered for the type
func (r *CronJob) ValidateDelete() (admission.Warnings, error) {
	cronjoblog.Info("validate delete", "name", r.Name)

	// TODO(user): fill in your validation logic upon object deletion.
	return nil, nil

We validate the name and the spec of the CronJob.

func (r *CronJob) validateCronJob() error {
	var allErrs field.ErrorList
	if err := r.validateCronJobName(); err != nil {
		allErrs = append(allErrs, err)
	if err := r.validateCronJobSpec(); err != nil {
		allErrs = append(allErrs, err)
	if len(allErrs) == 0 {
		return nil

	return apierrors.NewInvalid(
		schema.GroupKind{Group: "", Kind: "CronJob"},
		r.Name, allErrs)

Some fields are declaratively validated by OpenAPI schema. You can find kubebuilder validation markers (prefixed with // +kubebuilder:validation) in the Designing an API section. You can find all of the kubebuilder supported markers for declaring validation by running controller-gen crd -w, or here.

func (r *CronJob) validateCronJobSpec() *field.Error {
	// The field helpers from the kubernetes API machinery help us return nicely
	// structured validation errors.
	return validateScheduleFormat(

We’ll need to validate the cron schedule is well-formatted.

func validateScheduleFormat(schedule string, fldPath *field.Path) *field.Error {
	if _, err := cron.ParseStandard(schedule); err != nil {
		return field.Invalid(fldPath, schedule, err.Error())
	return nil
Validate object name

Validating the length of a string field can be done declaratively by the validation schema.

But the ObjectMeta.Name field is defined in a shared package under the apimachinery repo, so we can’t declaratively validate it using the validation schema.

func (r *CronJob) validateCronJobName() *field.Error {
	if len(r.ObjectMeta.Name) > validationutils.DNS1035LabelMaxLength-11 {
		// The job name length is 63 character like all Kubernetes objects
		// (which must fit in a DNS subdomain). The cronjob controller appends
		// a 11-character suffix to the cronjob (`-$TIMESTAMP`) when creating
		// a job. The job name length limit is 63 characters. Therefore cronjob
		// names must have length <= 63-11=52. If we don't validate this here,
		// then job creation will fail later.
		return field.Invalid(field.NewPath("metadata").Child("name"), r.Name, "must be no more than 52 characters")
	return nil