Custom Resource Definition

Exercise 13.1: Create a Custom Resource Definition

Overview

ThirdPartyResource is no longer included with the API in v1.8 and its use will return a validation error. If you have upgraded from a version prior to Kubernetes v1.7, you will need to convert them to CustomResourceDefinitions (CRD). A new resource often requires a controller to manage the resource. Creation of the controller is beyond the scope of this course, basically it is a watch-loop comparing a spec file to the current state and making changes until the states match. A good discussion of creating a controller can be found here: https://coreos.com/blog/introducing-operators.html.

Create a Custom Resource Definition

We will make a simple CRD, but without any particular action. It will be enough to find the object ingested into the API and responding to commands.

1. We will create a new YAML file.

   
   student@lfs458-node-1a0a:~$ vim crd.yaml
   
   apiVersion: apiextensions.k8s.io/v1beta1
   kind: CustomResourceDefinition
   metadata:
   	name: crontabs.training.lfs458.com
   		# This name must match names below.
   		# <plural>.<group> syntax
   	spec:
   	  scope: Cluster #Could also be Namespaced
   	  group: training.lfs458.com
   version: v1
   names:
   	kind: CronTab 		#Typically CamelCased for resource manifest
   	plural: crontabs 	#Shown in URL
   	singular: crontab 	#Short name for CLI alias
   	shortNames:
   	- ct 			#CLI short name
   

2. Add the new resource to the cluster.

   
   student@lfs458-node-1a0a:~$ kubectl create -f crd.yaml
   customresourcedefinition.apiextensions.k8s.io/crontabs.training.lfs458.com
   created
   

3. View and describe the resource. You’ll note the describe output is unlike other objects we have seen so far.

   
   student@lfs458-node-1a0a:~$ kubectl get crd
   NAME 			CREATED AT
   crontabs.training.lfs458.com 2018-08-03T05:25:20Z
   <output_omitted>
   
   student@lfs458-node-1a0a:~$ kubectl describe crd crontab
   Name:	 crontabs.training.lfs458.com
   Namespace:
   Labels: 		<none>
   Annotations: 		<none>
   API Version: 		apiextensions.k8s.io/v1beta1
   Kind: 			CustomResourceDefinition
   <output_omitted>
   

4. Now that we have a new API resource we can create a new object of that type. In this case it will be a crontab-like image, which does not actually exist, but is being used for demonstration.

    
   student@lfs458-node-1a0a:~$ vim new-crontab.yaml
   
   apiVersion: "training.lfs458.com/v1"
   	# This is from the group and version of new CRD
   kind: CronTab
   	# The kind from the new CRD
   metadata:
   	name: new-cron-object
   spec:
   	cronSpec: "*/5 * * * *"
   	image: some-cron-image
   	  #Does not exist
   

5. Create the new object and view the resource using short and long name.

    
   student@lfs458-node-1a0a:~$ kubectl create -f new-crontab.yaml
   crontab.training.lfs458.com/new-cron-object created
   
   student@lfs458-node-1a0a:~$ kubectl get CronTab
   NAME 		AGE
   new-cron-object 22s
   
   student@lfs458-node-1a0a:~$ kubectl get ct
   NAME 		AGE
   new-cron-object 29s
   
   student@lfs458-node-1a0a:~$ kubectl describe ct
   Name: 		new-cron-object
   Namespace:
   Labels: 	<none>
   
   <output_omitted>
   
   Spec:
     Cron Spec: */5 * * * *
     Image: some-cron-image
   Events: <none>
   

6. To clean up the resources we will delete the CRD. This should delete all of the endpoints and objects using it as well.

    
   student@lfs458-node-1a0a:~$ kubectl delete -f crd.yaml
   customresourcedefinition.apiextensions.k8s.io
   "crontabs.training.lfs458.com" deleted
   
   student@lfs458-node-1a0a:~$ kubectl get ct
   Error from server (NotFound): Unable to list "crontabs": the server
   could not find the requested resource
   (get crontabs.training.lfs458.com)
   

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