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Extend the Kubernetes API with CustomResourceDefinitions

This page shows how to install a custom resource into the Kubernetes API by creating a CustomResourceDefinition.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube, or you can use one of these Kubernetes playgrounds:

To check the version, enter kubectl version.

Create a CustomResourceDefinition

When you create a new CustomResourceDefinition (CRD), the Kubernetes API Server reacts by creating a new RESTful resource path, either namespaced or cluster-scoped, as specified in the CRD’s scope field. As with existing built-in objects, deleting a namespace deletes all custom objects in that namespace. CustomResourceDefinitions themselves are non-namespaced and are available to all namespaces.

For example, if you save the following CustomResourceDefinition to resourcedefinition.yaml:

apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  # name must match the spec fields below, and be in the form: <plural>.<group>
  name: crontabs.stable.example.com
spec:
  # group name to use for REST API: /apis/<group>/<version>
  group: stable.example.com
  # version name to use for REST API: /apis/<group>/<version>
  version: v1
  # either Namespaced or Cluster
  scope: Namespaced
  names:
    # plural name to be used in the URL: /apis/<group>/<version>/<plural>
    plural: crontabs
    # singular name to be used as an alias on the CLI and for display
    singular: crontab
    # kind is normally the CamelCased singular type. Your resource manifests use this.
    kind: CronTab
    # shortNames allow shorter string to match your resource on the CLI
    shortNames:
    - ct

And create it:

kubectl create -f resourcedefinition.yaml

Then a new namespaced RESTful API endpoint is created at:

/apis/stable.example.com/v1/namespaces/*/crontabs/...

This endpoint URL can then be used to create and manage custom objects. The kind of these objects will be CronTab from the spec of the CustomResourceDefinition object you created above.

Please note that it might take a few seconds for the endpoint to be created. You can watch the Established condition of your CustomResourceDefinition to be true or watch the discovery information of the API server for your resource to show up.

Create custom objects

After the CustomResourceDefinition object has been created, you can create custom objects. Custom objects can contain custom fields. These fields can contain arbitrary JSON. In the following example, the cronSpec and image custom fields are set in a custom object of kind CronTab. The kind CronTab comes from the spec of the CustomResourceDefinition object you created above.

If you save the following YAML to my-crontab.yaml:

apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * * */5"
  image: my-awesome-cron-image

and create it:

kubectl create -f my-crontab.yaml

You can then manage your CronTab objects using kubectl. For example:

kubectl get crontab

Should print a list like this:

NAME                 AGE
my-new-cron-object   6s

Note that resource names are not case-sensitive when using kubectl, and you can use either the singular or plural forms defined in the CRD, as well as any short names.

You can also view the raw YAML data:

kubectl get ct -o yaml

You should see that it contains the custom cronSpec and image fields from the yaml you used to create it:

apiVersion: v1
items:
- apiVersion: stable.example.com/v1
  kind: CronTab
  metadata:
    clusterName: ""
    creationTimestamp: 2017-05-31T12:56:35Z
    deletionGracePeriodSeconds: null
    deletionTimestamp: null
    name: my-new-cron-object
    namespace: default
    resourceVersion: "285"
    selfLink: /apis/stable.example.com/v1/namespaces/default/crontabs/my-new-cron-object
    uid: 9423255b-4600-11e7-af6a-28d2447dc82b
  spec:
    cronSpec: '* * * * */5'
    image: my-awesome-cron-image
kind: List
metadata:
  resourceVersion: ""
  selfLink: ""

Delete a CustomResourceDefinition

When you delete a CustomResourceDefinition, the server will uninstall the RESTful API endpoint and delete all custom objects stored in it.

kubectl delete -f resourcedefinition.yaml
kubectl get crontabs
Error from server (NotFound): Unable to list "crontabs": the server could not find the requested resource (get crontabs.stable.example.com)

If you later recreate the same CustomResourceDefinition, it will start out empty.

Advanced topics

Finalizers

Finalizers allow controllers to implement asynchronous pre-delete hooks. Custom objects support finalizers just like built-in objects.

You can add a finalizer to a custom object like this:

apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  finalizers:
  - finalizer.stable.example.com

Finalizers are arbitrary string values, that when present ensure that a hard delete of a resource is not possible while they exist.

The first delete request on an object with finalizers merely sets a value for the metadata.deletionTimestamp field instead of deleting it. Once this value is set, entries in the finalizer list can only be removed.

This triggers controllers watching the object to execute any finalizers they handle. This will be represented via polling update requests for that object, until all finalizers have been removed and the resource is deleted.

The time period of polling update can be controlled by metadata.deletionGracePeriodSeconds.

It is the responsibility of each controller to removes its finalizer from the list.

Kubernetes will only finally delete the object if the list of finalizers is empty, meaning all finalizers are done.

Validation

Validation of custom objects is possible via OpenAPI v3 schema. Additionally, the following restrictions are applied to the schema:

This feature is beta in v1.9. You can disable this feature using the CustomResourceValidation feature gate on the kube-apiserver:

--feature-gates=CustomResourceValidation=false

The schema is defined in the CustomResourceDefinition. In the following example, the CustomResourceDefinition applies the following validations on the custom object:

Save the CustomResourceDefinition to resourcedefinition.yaml:

apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: crontabs.stable.example.com
spec:
  group: stable.example.com
  version: v1
  scope: Namespaced
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
    shortNames:
    - ct
  validation:
   # openAPIV3Schema is the schema for validating custom objects.
    openAPIV3Schema:
      properties:
        spec:
          properties:
            cronSpec:
              type: string
              pattern: '^(\d+|\*)(/\d+)?(\s+(\d+|\*)(/\d+)?){4}$'
            replicas:
              type: integer
              minimum: 1
              maximum: 10

And create it:

kubectl create -f resourcedefinition.yaml

A request to create a custom object of kind CronTab will be rejected if there are invalid values in its fields. In the following example, the custom object contains fields with invalid values:

If you save the following YAML to my-crontab.yaml:

apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * *"
  image: my-awesome-cron-image
  replicas: 15

and create it:

kubectl create -f my-crontab.yaml

you will get an error:

The CronTab "my-new-cron-object" is invalid: []: Invalid value: map[string]interface {}{"apiVersion":"stable.example.com/v1", "kind":"CronTab", "metadata":map[string]interface {}{"name":"my-new-cron-object", "namespace":"default", "deletionTimestamp":interface {}(nil), "deletionGracePeriodSeconds":(*int64)(nil), "creationTimestamp":"2017-09-05T05:20:07Z", "uid":"e14d79e7-91f9-11e7-a598-f0761cb232d1", "selfLink":"", "clusterName":""}, "spec":map[string]interface {}{"cronSpec":"* * * *", "image":"my-awesome-cron-image", "replicas":15}}:
validation failure list:
spec.cronSpec in body should match '^(\d+|\*)(/\d+)?(\s+(\d+|\*)(/\d+)?){4}$'
spec.replicas in body should be less than or equal to 10

If the fields contain valid values, the object creation request is accepted.

Save the following YAML to my-crontab.yaml:

apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * * */5"
  image: my-awesome-cron-image
  replicas: 5

And create it:

kubectl create -f my-crontab.yaml
crontab "my-new-cron-object" created

Subresources

Custom resources support /status and /scale subresources. This feature is alpha in v1.10 and may change in backward incompatible ways.

Enable this feature using the CustomResourceSubresources feature gate on the kube-apiserver:

--feature-gates=CustomResourceSubresources=true

When the CustomResourceSubresources feature gate is enabled, only the properties construct is allowed in the root schema for custom resource validation.

The status and scale subresources can be optionally enabled by defining them in the CustomResourceDefinition.

Status subresource

When the status subresource is enabled, the /status subresource for the custom resource is exposed.

Scale subresource

When the scale subresource is enabled, the /scale subresource for the custom resource is exposed. The autoscaling/v1.Scale object is sent as the payload for /scale.

To enable the scale subresource, the following values are defined in the CustomResourceDefinition.

In the following example, both status and scale subresources are enabled.

Save the CustomResourceDefinition to resourcedefinition.yaml:

apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: crontabs.stable.example.com
spec:
  group: stable.example.com
  version: v1
  scope: Namespaced
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
    shortNames:
    - ct
  # subresources describes the subresources for custom resources.
  subresources:
    # status enables the status subresource.
    status: {}
    # scale enables the scale subresource.
    scale:
      # specReplicasPath defines the JSONPath inside of a custom resource that corresponds to Scale.Spec.Replicas.
      specReplicasPath: .spec.replicas
      # statusReplicasPath defines the JSONPath inside of a custom resource that corresponds to Scale.Status.Replicas.
      statusReplicasPath: .status.replicas
      # labelSelectorPath defines the JSONPath inside of a custom resource that corresponds to Scale.Status.Selector.
      labelSelectorPath: .status.labelSelector

And create it:

kubectl create -f resourcedefinition.yaml

After the CustomResourceDefinition object has been created, you can create custom objects.

If you save the following YAML to my-crontab.yaml:

apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * * */5"
  image: my-awesome-cron-image
  replicas: 3

and create it:

kubectl create -f my-crontab.yaml

Then new namespaced RESTful API endpoints are created at:

/apis/stable.example.com/v1/namespaces/*/crontabs/status

and

/apis/stable.example.com/v1/namespaces/*/crontabs/scale

A custom resource can be scaled using the kubectl scale command. For example, the following command sets .spec.replicas of the custom resource created above to 5:

kubectl scale --replicas=5 crontabs/my-new-cron-object
crontabs "my-new-cron-object" scaled

kubectl get crontabs my-new-cron-object -o jsonpath='{.spec.replicas}'
5

Categories

Categories is a list of grouped resources the custom resource belongs to (eg. all). You can use kubectl get <category-name> to list the resources belonging to the category. This feature is beta and available for custom resources from v1.10.

The following example adds all in the list of categories in the CustomResourceDefinition and illustrates how to output the custom resource using kubectl get all.

Save the following CustomResourceDefinition to resourcedefinition.yaml:

apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: crontabs.stable.example.com
spec:
  group: stable.example.com
  version: v1
  scope: Namespaced
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
    shortNames:
    - ct
    # categories is a list of grouped resources the custom resource belongs to.
    categories:
    - all

And create it:

kubectl create -f resourcedefinition.yaml

After the CustomResourceDefinition object has been created, you can create custom objects.

Save the following YAML to my-crontab.yaml:

apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  cronSpec: "* * * * */5"
  image: my-awesome-cron-image

and create it:

kubectl create -f my-crontab.yaml

You can specify the category using kubectl get:

kubectl get all

and it will include the custom resources of kind CronTab:

NAME                          AGE
crontabs/my-new-cron-object   3s

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