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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 creates a new RESTful resource path for each version you specify. The CRD can be 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
  # list of versions supported by this CustomResourceDefinition
  versions:
    - name: v1
      # Each version can be enabled/disabled by Served flag.
      served: true
      # One and only one version must be marked as the storage version.
      storage: true
  # 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.

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

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:
    creationTimestamp: 2017-05-31T12:56:35Z
    generation: 1
    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 {"stable.example.com" "v1" "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.

Serving multiple versions of a CRD

See Custom resource definition versioning for more information about serving multiple versions of your CustomResourceDefinition and migrating your objects from one version to another.

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 sets a value for the metadata.deletionTimestamp field but does not delete it. Once this value is set, entries in the finalizer list can only be removed.

When the metadata.deletionTimestamp field is set, controllers watching the object execute any finalizers they handle, by polling update requests for that object. When all finalizers have been executed, the resource is deleted.

The value of metadata.deletionGracePeriodSeconds controls the interval between polling updates.

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

Kubernetes only finally deletes the object if the list of finalizers is empty, meaning all finalizers have been executed.

Validation

FEATURE STATE: Kubernetes v1.13 beta
This feature is currently in a beta state, meaning:

  • The version names contain beta (e.g. v2beta3).
  • Code is well tested. Enabling the feature is considered safe. Enabled by default.
  • Support for the overall feature will not be dropped, though details may change.
  • The schema and/or semantics of objects may change in incompatible ways in a subsequent beta or stable release. When this happens, we will provide instructions for migrating to the next version. This may require deleting, editing, and re-creating API objects. The editing process may require some thought. This may require downtime for applications that rely on the feature.
  • Recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases. If you have multiple clusters that can be upgraded independently, you may be able to relax this restriction.
  • Please do try our beta features and give feedback on them! After they exit beta, it may not be practical for us to make more changes.

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

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
  versions:
    - name: v1
      served: true
      storage: true
  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

Additional printer columns

Starting with Kubernetes 1.11, kubectl uses server-side printing. The server decides which columns are shown by the kubectl get command. You can customize these columns using a CustomResourceDefinition. The following example adds the Spec, Replicas, and Age columns.

  1. 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
    additionalPrinterColumns:
    - name: Spec
      type: string
      description: The cron spec defining the interval a CronJob is run
      JSONPath: .spec.cronSpec
    - name: Replicas
      type: integer
      description: The number of jobs launched by the CronJob
      JSONPath: .spec.replicas
    - name: Age
      type: date
      JSONPath: .metadata.creationTimestamp

  2. Create the CustomResourceDefinition:

      kubectl create -f resourcedefinition.yaml

  3. Create an instance using the my-crontab.yaml from the previous section.

  4. Invoke the server-side printing:

      kubectl get crontab my-new-cron-object

    Notice the NAME, SPEC, REPLICAS, and AGE columns in the output:

      NAME                 SPEC        REPLICAS   AGE
  my-new-cron-object   * * * * *   1          7s

The NAME column is implicit and does not need to be defined in the CustomResourceDefinition.

Priority

Each column includes a priority field for each column. Currently, the priority differentiates between columns shown in standard view or wide view (using the -o wide flag).

Type

A column’s type field can be any of the following (compare OpenAPI v3 data types):

If the value inside a CustomResource does not match the type specified for the column, the value is omitted. Use CustomResource validation to ensure that the value types are correct.

Format

A column’s format field can be any of the following:

The column’s format controls the style used when kubectl prints the value.

Subresources

FEATURE STATE: Kubernetes v1.13 beta
This feature is currently in a beta state, meaning:

  • The version names contain beta (e.g. v2beta3).
  • Code is well tested. Enabling the feature is considered safe. Enabled by default.
  • Support for the overall feature will not be dropped, though details may change.
  • The schema and/or semantics of objects may change in incompatible ways in a subsequent beta or stable release. When this happens, we will provide instructions for migrating to the next version. This may require deleting, editing, and re-creating API objects. The editing process may require some thought. This may require downtime for applications that rely on the feature.
  • Recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases. If you have multiple clusters that can be upgraded independently, you may be able to relax this restriction.
  • Please do try our beta features and give feedback on them! After they exit beta, it may not be practical for us to make more changes.

Custom resources support /status and /scale subresources.

You can disable this feature using the CustomResourceSubresources feature gate on the kube-apiserver:

--feature-gates=CustomResourceSubresources=false

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
  versions:
    - name: v1
      served: true
      storage: true
  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
  versions:
    - name: v1
      served: true
      storage: true
  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

What's next

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