website/content/en/docs/tasks/access-kubernetes-api/extend-api-custom-resource-definitions.md

---
title: Extend the Kubernetes API with CustomResourceDefinitions
reviewers:
- deads2k
- enisoc
content_template: templates/task
---

{{% capture overview %}}
This page shows how to install a
[custom resource](/docs/concepts/api-extension/custom-resources/)
into the Kubernetes API by creating a
[CustomResourceDefinition](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#customresourcedefinition-v1beta1-apiextensions).
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{{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}

* Make sure your Kubernetes cluster has a master version of 1.7.0 or higher.

* Read about [custom resources](/docs/concepts/api-extension/custom-resources/).

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## 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`:

```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:

```shell
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`:

```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:

```shell
kubectl create -f my-crontab.yaml
```

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

```shell
kubectl get crontab
```

Should print a list like this:

```console
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:

```shell
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:

```console
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**.

```shell
kubectl delete -f resourcedefinition.yaml
kubectl get crontabs
```

```console
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.

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## 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:

```yaml
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](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#schemaObject).
Additionally, the following restrictions are applied to the schema:

- The fields `default`, `nullable`, `discriminator`, `readOnly`, `writeOnly`, `xml`,
`deprecated` and `$ref` cannot be set.
- The field `uniqueItems` cannot be set to true.
- The field `additionalProperties` cannot be set to false.

This feature is __beta__ in v1.9.
You can disable this feature using the `CustomResourceValidation` feature gate on
the [kube-apiserver](/docs/admin/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:

- `spec.cronSpec` must be a string and must be of the form described by the regular expression.
- `spec.replicas` must be an integer and must have a minimum value of 1 and a maximum value of 10.

Save the CustomResourceDefinition to `resourcedefinition.yaml`:

```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:

```shell
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:

- `spec.cronSpec` does not match the regular expression.
- `spec.replicas` is greater than 10.

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

```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:

```shell
kubectl create -f my-crontab.yaml
```

you will get an error:

```console
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`:

```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:

```shell
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](/docs/admin/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.

- The status and the spec stanzas are represented by the `.status` and `.spec` JSONPaths respectively inside of a custom resource.
- `PUT` requests to the `/status` subresource take a custom resource object and ignore changes to anything except the status stanza.
- `PUT` requests to the `/status` subresource only validate the status stanza of the custom resource.
- `PUT`/`POST`/`PATCH` requests to the custom resource ignore changes to the status stanza.
- Any changes to the spec stanza increments the value at `.metadata.generation`.

#### 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.

- `SpecReplicasPath` defines the JSONPath inside of a custom resource that corresponds to `Scale.Spec.Replicas`.

  - It is a required value.
  - Only JSONPaths under `.spec` and with the dot notation are allowed.
  - If there is no value under the `SpecReplicasPath` in the custom resource,
the `/scale` subresource will return an error on GET.

- `StatusReplicasPath` defines the JSONPath inside of a custom resource that corresponds to `Scale.Status.Replicas`.

  - It is a required value.
  - Only JSONPaths under `.status` and with the dotation are allowed.
  - If there is no value under the `StatusReplicasPath` in the custom resource,
the status replica value in the `/scale` subresource will default to 0.

- `LabelSelectorPath` defines the JSONPath inside of a custom resource that corresponds to `Scale.Status.Selector`.

  - It is an optional value.
  - It must be set to work with HPA.
  - Only JSONPaths under `.status` and with the dotation are allowed.
  - If there is no value under the `LabelSelectorPath` in the custom resource,
the status selector value in the `/scale` subresource will default to the empty string.

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

Save the CustomResourceDefinition to `resourcedefinition.yaml`:

```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:

```shell
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`:

```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:

```shell
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:

```shell
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`:

```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:

```shell
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`:

```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:

```shell
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`:

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

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{{% capture whatsnext %}}
* Learn how to [Migrate a ThirdPartyResource to CustomResourceDefinition](/docs/tasks/access-kubernetes-api/migrate-third-party-resource/).
* See [CustomResourceDefinition](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#customresourcedefinition-v1beta1-apiextensions-k8s-io).
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