Managing resources from custom resource definitions

This guide describes how developers can manage custom resources (CRs) that come from custom resource definitions (CRDs).

Custom resource definitions

In the Kubernetes API, a resource is an endpoint that stores a collection of API objects of a certain kind. For example, the built-in Pods resource contains a collection of Pod objects.

A custom resource definition (CRD) object defines a new, unique object type, called a kind, in the cluster and lets the Kubernetes API server handle its entire lifecycle.

Custom resource (CR) objects are created from CRDs that have been added to the cluster by a cluster administrator, allowing all cluster users to add the new resource type into projects.

Operators in particular make use of CRDs by packaging them with any required RBAC policy and other software-specific logic.

Creating custom resources from a file

After a custom resource definition (CRD) has been added to the cluster, custom resources (CRs) can be created with the CLI from a file using the CR specification.

Procedure

Create a YAML file for the CR. In the following example definition, the cronSpec and image custom fields are set in a CR of Kind: CronTab. The Kind comes from the spec.kind field of the CRD object:
Example YAML file for a CR
```
apiVersion: "stable.example.com/v1" 
kind: CronTab 
metadata:
  name: my-new-cron-object 
  finalizers: 
  - finalizer.stable.example.com
spec: 
  cronSpec: "* * * * /5"
  image: my-awesome-cron-image
```
1. Specify the group name and API version (name/version) from the CRD.
2. Specify the type in the CRD.
3. Specify a name for the object.
4. Specify the finalizers for the object, if any. Finalizers allow controllers to implement conditions that must be completed before the object can be deleted.
5. Specify conditions specific to the type of object.
After you create the file, create the object:
```
$ oc create -f <file_name>.yaml
```

Inspecting custom resources

You can inspect custom resource (CR) objects that exist in your cluster using the CLI.

Prerequisites

A CR object exists in a namespace to which you have access.

Procedure

To get information on a specific kind of a CR, run:
```
$ oc get <kind>
```
For example:
```
$ oc get crontab
```
Example output
```
NAME                 KIND
my-new-cron-object   CronTab.v1.stable.example.com
```
Resource names are not case-sensitive, and you can use either the singular or plural forms defined in the CRD, as well as any short name. For example:
```
$ oc get crontabs
```
```
$ oc get crontab
```
```
$ oc get ct
```

You can also view the raw YAML data for a CR:

$ oc get <kind> -o yaml

For example:

$ oc get ct -o yaml

Example output

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

Custom data from the YAML that you used to create the object displays.