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Extend the Kubernetes API with ThirdPartyResources

DEPRECATION NOTICE: As of Kubernetes 1.7, this has been deprecated

• What is ThirdPartyResource?
• Structure of a ThirdPartyResource
• Creating a ThirdPartyResource
• Creating Custom Objects

What is ThirdPartyResource?

ThirdPartyResource is deprecated as of Kubernetes 1.7 and may be removed in version 1.8 in accordance with the deprecation policy for beta features.

To avoid losing data stored in ThirdPartyResources, you must migrate to CustomResourceDefinition before upgrading to Kubernetes 1.8 or higher.

Kubernetes comes with many built-in API objects. However, there are often times when you might need to extend Kubernetes with your own API objects in order to do custom automation.

ThirdPartyResource objects are a way to extend the Kubernetes API with a new API object type. The new API object type will be given an API endpoint URL and support CRUD operations, and watch API. You can then create custom objects using this API endpoint. You can think of ThirdPartyResources as being much like the schema for a database table. Once you have created the table, you can then start storing rows in the table. Once created, ThirdPartyResources can act as the data model behind custom controllers or automation programs.

Structure of a ThirdPartyResource

Each ThirdPartyResource has the following:

• metadata - Standard Kubernetes object metadata.
• kind - The kind of the resources described by this third party resource.
• description - A free text description of the resource.
• versions - A list of the versions of the resource.

The kind for a ThirdPartyResource takes the form <kind name>.<domain>. You are expected to provide a unique kind and domain name in order to avoid conflicts with other ThirdPartyResource objects. Kind names will be converted to CamelCase when creating instances of the ThirdPartyResource. Hyphens in the kind are assumed to be word breaks. For instance the kind camel-case would be converted to CamelCase but camelcase would be converted to Camelcase.

Other fields on the ThirdPartyResource are treated as custom data fields. These fields can hold arbitrary JSON data and have any structure.

You can view the full documentation about ThirdPartyResources using the explain command in kubectl.

$ kubectl explain thirdpartyresource

Creating a ThirdPartyResource

When you create a new ThirdPartyResource, the Kubernetes API Server reacts by creating a new, namespaced RESTful resource path. For now, non-namespaced objects are not supported. As with existing built-in objects, deleting a namespace deletes all custom objects in that namespace. ThirdPartyResources themselves are non-namespaced and are available to all namespaces.

For example, if you save the following ThirdPartyResource to resource.yaml:

apiVersion: extensions/v1beta1
kind: ThirdPartyResource
metadata:
  name: cron-tab.stable.example.com
description: "A specification of a Pod to run on a cron style schedule"
versions:
- name: v1

And create it:

$ kubectl create -f resource.yaml
thirdpartyresource "cron-tab.stable.example.com" created

Then a new RESTful API endpoint is created at:

/apis/stable.example.com/v1/namespaces/<namespace>/crontabs/...

This endpoint URL can then be used to create and manage custom objects. The kind of these objects will be CronTab following the camel case rules applied to the metadata.name of this ThirdPartyResource (cron-tab.stable.example.com)

Creating Custom Objects

After the ThirdPartyResource 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, a cronSpec and image custom fields are set to the custom object of kind CronTab. The kind CronTab is derived from the metadata.name of the ThirdPartyResource object we 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
cronSpec: "* * * * /5"
image: my-awesome-cron-image

and create it:

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

You can then manage our CronTab objects using kubectl. Note that resource names are not case-sensitive when using kubectl:

$ kubectl get crontab
NAME                 KIND
my-new-cron-object   CronTab.v1.stable.example.com

You can also view the raw JSON data. Here you can see that it contains the custom cronSpec and image fields from the yaml you used to create it:

$ kubectl get crontab -o json
{
    "apiVersion": "v1",
    "items": [
        {
            "apiVersion": "stable.example.com/v1",
            "cronSpec": "* * * * /5",
            "image": "my-awesome-cron-image",
            "kind": "CronTab",
            "metadata": {
                "creationTimestamp": "2016-09-29T04:59:00Z",
                "name": "my-new-cron-object",
                "namespace": "default",
                "resourceVersion": "12601503",
                "selfLink": "/apis/stable.example.com/v1/namespaces/default/crontabs/my-new-cron-object",
                "uid": "6f65e7a3-8601-11e6-a23e-42010af0000c"
            }
        }
    ]
    "kind": "List",
    "metadata": {},
    "resourceVersion": "",
    "selfLink": ""
}