Create a Custom Resource

• Create a Custom Resource
• Operators
• Ready Made Operators
• Create a Custom Resource and Operator using the Operator SDK
• Install sdk-operator
• Create the Operator
• Cleanup
• Application CRD

Create a Custom Resource (CR)

https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/

Custom Resource Definitions (CRD) were added in Kubernetes v1.7 in June 2017. A CRD defines Custom Resources (CR). A CR is an extension of the Kubernetes API that allows you to store your own API Objects and lets the API Server handle the lifecycle of a CR. On their own, CRs simply let you store and retrieve structured data.

For instance, our Guestbook application consists of an object Guestbook with attributes GuestbookTitle and GuestbookSubtitle, and a Guestbook handles objectes of type GuestbookMessage with attributes Message, Sender.

You have to ask yourself if it makes sense if your objects are added as a Custom Resource to Kubernetes or not. If your API is a Declarative API you can consider adding a CR.

• Your API has a small number of small objects (resources).
• The objects define configuration of applications or infrastructure.
• The objects are updated relatively infrequently.
• Users often need to read and write the objects.
• main operations on the objects are CRUD (create, read, update and delete).
• Transactions between objects are not required.

It doesn't immediately make sense to store messages by Guestbook users in Kubernetes, but it might make sense to store meta-data about a Guestbook deployment, for instance the title and subtitle of a Guestbook deployment, assigned resources or replicas.

Another benefit of adding a Custom Resource is to view your types in the Kubernetes Dashboard.

If you want to deploy a Guestbook instance as a Kubernetes API object and let the Kubernetes API Server handle the lifecycle events of the Guestbook deployment, you can create a Custom Resource Definition (CRD) for the Guestbook object as follows. That way you can deploy multiple Guestbooks with different titles and let each be managed by Kubernetes.

cat <<EOF >>guestbook-crd.yaml
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: guestbooks.apps.ibm.com
spec:
  group: apps.ibm.com
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                guestbookTitle:
                  type: string
                guestbookSubtitle:
                  type: string
  scope: Namespaced
  names:
    plural: guestbooks
    singular: guestbook
    kind: Guestbook
    shortNames:
    - gb
EOF

• You can see that the apiVersion is part of the apiextensions.k8s.io/v1 API Group in Kubernetes, which is the API that enables extensions, and the kind is set to CustomResourceDefinition.
• The served flag can disable and enable a version.
• Only 1 version can be flagged as the storage version.
• The spec.names.kind is used by your resource manifests and should be CamelCased.

Create the Custom Resource for the Guestbook witht he command,

oc create -f guestbook-crd.yaml

When run in the terminal,

$ oc create -f guestbook-crd.yaml
customresourcedefinition.apiextensions.k8s.io/guestbooks.apps.ibm.com created

You have now added a CR to the Kubernetes API, but you have not yet created a deployment of type Guestbook yet.

Create a resource specification of type Guestbook named my-guestbook,

cat <<EOF >>my-guestbook.yaml
apiVersion: "apps.ibm.com/v1"
kind: Guestbook
metadata:
  name: my-guestbook
spec:
  guestbookTitle: "The Chemical Wedding of Remko"
  guestbookSubtitle: "First Day of Many"
EOF

And to create the my-guestbook resource, run the command

oc create -f my-guestbook.yaml

When run in the terminal,

$ oc create -f my-guestbook.yaml
guestbook.apps.ibm.com/my-guestbook created

If you list all Kubernetes resources, only the default Kubernetes service is listed. To list your Custom Resources, add the extended type to your command.

$ oc get all
NAME    TYPE    CLUSTER-IP    EXTERNAL-IP    PORT(S)    AGE
service/kubernetes    ClusterIP    172.21.0.1    <none>    443/TCP    5d14h
service/openshift    ExternalName    <none>    kubernetes.default.svc.cluster.local    <none>    5d14h
service/openshift-apiserver    ClusterIP    172.21.6.8    <none>    =443/TCP    5d14h

$ oc get guestbook
NAME    AGE
my-guestbook    8m32s

To read the details for the my-guestbook of type Guestbook, describe the instance,

$ oc describe guestbook my-guestbook

Name:         my-guestbook
Namespace:    default
Labels:       <none>
Annotations:  <none>
API Version:  apps.ibm.com/v1
Kind:         Guestbook
Metadata:
  Creation Timestamp:  2020-06-30T20:31:36Z
  Generation:          1
  Resource Version:    1081471
  Self Link:           /apis/apps.ibm.com/v1/namespaces/default/guestbooks/my-guestbook
  UID:                 dcbdcafc-999d-4051-9244-0315093357e7
Spec:
  Guestbook Subtitle:  First Day of Many
  Guestbook Title:     The Chemical Wedding of Remko
Events:                <none>

Or retrieve the resource information by specifying the type,

$ oc get Guestbook -o yaml
apiVersion: v1
items:
- apiVersion: apps.ibm.com/v1
  kind: Guestbook
  metadata:
    creationTimestamp: "2020-07-02T04:41:57Z"
    generation: 1
    name: my-guestbook
    namespace: default
    resourceVersion: "1903244"
    selfLink: /apis/apps.ibm.com/v1/namespaces/default/guestbooks/my-guestbook
    uid: 3f774899-3070-4e00-b74c-a6a14654faeb
  spec:
    guestbookSubtitle: First Day of Many
    guestbookTitle: The Chemical Wedding of Remko
kind: List
metadata:
  resourceVersion: ""
  selfLink: ""

In the OpenShift web console, you can browse to Administration > Custom Resource Definitions and find the Guestbook CRD at /k8s/cluster/customresourcedefinitions/guestbooks.apps.ibm.com.

You have now created a new type or Custom Resource (CR) and created an instance of your new type. But just having a new type and a new instance of the type, does not add as much control over the instances yet, we can basically only create and delete a static type with some descriptive meta-data. With a custom controller or Operator you can over-write the methods that are triggered at certain lifecycle events.