Adding custom resources to the Kubernetes API server

This document describes the design for implementing the storage of custom API types in the Kubernetes API Server.

Resource Model

The ThirdPartyResource

The ThirdPartyResource resource describes the multiple versions of a custom resource that the user wants to add to the Kubernetes API. ThirdPartyResource is a non-namespaced resource; attempting to place it in a namespace will return an error.

Each ThirdPartyResource resource has the following: * Standard Kubernetes object metadata. * ResourceKind - The kind of the resources described by this third party resource. * Description - A free text description of the resource. * APIGroup - An API group that this resource should be placed into. * Versions - One or more Version objects.

The Version Object

The Version object describes a single concrete version of a custom resource. The Version object currently only specifies: * The Name of the version. * The APIGroup this version should belong to.

Expectations about third party objects

Every object that is added to a third-party Kubernetes object store is expected to contain Kubernetes compatible object metadata. This requirement enables the Kubernetes API server to provide the following features: * Filtering lists of objects via label queries. * resourceVersion-based optimistic concurrency via compare-and-swap. * Versioned storage. * Event recording. * Integration with basic kubectl command line tooling. * Watch for resource changes.

The Kind for an instance of a third-party object (e.g. CronTab) below is expected to be programmatically convertible to the name of the resource using the following conversion. Kinds are expected to be of the form <CamelCaseKind>, and the APIVersion for the object is expected to be <api-group>/<api-version>. To prevent collisions, it’s expected that you’ll use a DNS name of at least three segments for the API group, e.g. mygroup.example.com.

For example mygroup.example.com/v1

‘CamelCaseKind’ is the specific type name.

To convert this into the metadata.name for the ThirdPartyResource resource instance, the <domain-name> is copied verbatim, the CamelCaseKind is then converted using ‘-’ instead of capitalization (‘camel-case’), with the first character being assumed to be capitalized. In pseudo code:

var result string
for ix := range kindName {
  if isCapital(kindName[ix]) {
    result = append(result, '-')
  result = append(result, toLowerCase(kindName[ix])

As a concrete example, the resource named camel-case-kind.mygroup.example.com defines resources of Kind CamelCaseKind, in the APIGroup with the prefix mygroup.example.com/....

The reason for this is to enable rapid lookup of a ThirdPartyResource object given the kind information. This is also the reason why ThirdPartyResource is not namespaced.


When a user creates 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 third party resources in that namespace.

For example, if a user creates:

  name: cron-tab.mygroup.example.com
apiVersion: extensions/v1beta1
kind: ThirdPartyResource
description: "A specification of a Pod to run on a cron style schedule"
- name: v1
- name: v2

Then the API server will program in the new RESTful resource path: * /apis/mygroup.example.com/v1/namespaces/<namespace>/crontabs/...

Note: This may take a while before RESTful resource path registration happen, please always check this before you create resource instances.

Now that this schema has been created, a user can POST:

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

to: /apis/mygroup.example.com/v1/namespaces/default/crontabs

and the corresponding data will be stored into etcd by the APIServer, so that when the user issues:

GET /apis/mygroup.example.com/v1/namespaces/default/crontabs/my-new-cron-object

And when they do that, they will get back the same data, but with additional Kubernetes metadata (e.g. resourceVersion, createdTimestamp) filled in.

Likewise, to list all resources, a user can issue:

GET /apis/mygroup.example.com/v1/namespaces/default/crontabs

and get back:

   "apiVersion": "mygroup.example.com/v1",
   "kind": "CronTabList",
   "items": [
       "metadata": {
         "name": "my-new-cron-object"
       "apiVersion": "mygroup.example.com/v1",
       "kind": "CronTab",
       "cronSpec": "* * * * /5",
       "image":     "my-awesome-cron-image"

Because all objects are expected to contain standard Kubernetes metadata fields, these list operations can also use label queries to filter requests down to specific subsets.

Likewise, clients can use watch endpoints to watch for changes to stored objects.


In order to store custom user data in a versioned fashion inside of etcd, we need to also introduce a Codec-compatible object for persistent storage in etcd. This object is ThirdPartyResourceData and it contains: * Standard API Metadata. * Data: The raw JSON data for this custom object.

Storage key specification

Each custom object stored by the API server needs a custom key in storage, this is described below:


  • resource-namespace: the namespace of the particular resource that is being stored
  • resource-name: the name of the particular resource being stored
  • third-party-resource-namespace: the namespace of the ThirdPartyResource resource that represents the type for the specific instance being stored
  • third-party-resource-name: the name of the ThirdPartyResource resource that represents the type for the specific instance being stored


Given the definitions above, the key for a specific third-party object is:


Thus, listing a third-party resource can be achieved by listing the directory: