Custom Resources and Operators

• 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

Access the Cluster

Before continuing, make sure you can access your kubernetes cluster. Either through the cloud shell or through the web-terminal

kubectl config current-context

kubectl get nodes

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,

kubectl create -f guestbook-crd.yaml

When run in the terminal, the output would look like this:

$ kubectl 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

kubectl create -f my-guestbook.yaml

When run in the terminal,the output would look like this:

$ kubectl 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.

$ kubectl get all
NAME    TYPE    CLUSTER-IP    EXTERNAL-IP    PORT(S)    AGE
service/kubernetes    ClusterIP    172.21.0.1    <none>    443/TCP    5d14h

$ kubectl get guestbook
NAME    AGE
my-guestbook    8m32s

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

$ kubectl 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,

$ kubectl 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 Kubernetes Dashboard web console, you can browse to Custom Resource Definitions and find the Guestbook CRD.

Administration > Custom Resource Definitions

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.

Cleanup

kubectl delete guestbook my-guestbook

kubectl delete customresourcedefinition guestbooks.apps.ibm.com

Operators

https://kubernetes.io/docs/concepts/extend-kubernetes/operator/

Operators are clients of the Kubernetes API that act as controllers for a Custom Resource.

To write applications that use the Kubernetes REST API, you can use one of the following supported client libraries:

• Go,

• Python,

• Java,

• CSharp dotnet,

• JavaScript,

• Haskell.

In addition, there are many community-maintained client libraries.

Ready made operators

At the OperatorHub.io, you find ready to use operators written by the community.

OperatorHub.io

Create a Custom Resource and Operator using the Operator SDK

To write your own operator you can use existing tools:

• KUDO (Kubernetes Universal Declarative Operator),

• kubebuilder,

• Metacontroller using custom WebHooks,

• the Operator Framework.

The Operator SDK provides the following workflow to develop a new Operator:

The following workflow is for a new Go operator:

1. Create a new operator project using the SDK Command Line Interface(CLI)

2. Define new resource APIs by adding Custom Resource Definitions(CRD)

3. Define Controllers to watch and reconcile resources

4. Write the reconciling logic for your Controller using the SDK and controller-runtime APIs

5. Use the SDK CLI to build and generate the operator deployment manifests

Install sdk-operator

The following section uses the operator-sdk cli, which depends on Go to be installed. Make sure you have completed the operator-sdk-setup instructions to esnure both tools are installed in your terminal or cloud shell.

operator-sdk version

Create the Operator

1. Create a New Project

Create a new Operator project,

export DOCKER_USERNAME=<your-docker-username>

and

export OPERATOR_NAME=guestbook-operator

export OPERATOR_PROJECT=guestbook-project

export OPERATOR_GROUP=guestbook.ibm.com

export OPERATOR_VERSION=v1

export CRD_KIND=Guestbook

operator-sdk new $OPERATOR_PROJECT --type go --repo github.com/$DOCKER_USERNAME/$OPERATOR_NAME

cd $OPERATOR_PROJECT

The scaffolding of a new project will create an operator, an api and a controller.

new project directory structure

2. Create a new API

Add a new API definition for a new Custom Resource under pkg/apis and generate the Custom Resource Definition (CRD) and Custom Resource (CR) files under deploy/crds.

operator-sdk add api --api-version=$OPERATOR_GROUP/$OPERATOR_VERSION --kind=$CRD_KIND

The command will create a new API, a Custom Resource (CR), a Custom Resource Definition (CRD).

new project directory structure

One file is created in pkg/apis called addtoscheme_guestbook_v1.go that registers the new schema. One new file is created in pkg/apis/guestbook called group.go that defines the package. Four new files are created in pkg/apis/guestbook/v1:

• doc.go,

• guestbook_types.go,

• register.go,

• zz_generated.deepcopy.go.

The guestbook_types.go file,

package v1

import (
    metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
)

// GuestbookSpec defines the desired state of Guestbook
type GuestbookSpec struct {
}

// GuestbookStatus defines the observed state of Guestbook
type GuestbookStatus struct {
}

type Guestbook struct {
    metav1.TypeMeta   `json:",inline"`
    metav1.ObjectMeta `json:"metadata,omitempty"`

    Spec   GuestbookSpec   `json:"spec,omitempty"`
    Status GuestbookStatus `json:"status,omitempty"`
}

// GuestbookList contains a list of Guestbook
type GuestbookList struct {
    metav1.TypeMeta `json:",inline"`
    metav1.ListMeta `json:"metadata,omitempty"`
    Items           []Guestbook `json:"items"`
}

func init() {
    SchemeBuilder.Register(&Guestbook{}, &GuestbookList{})
}

The Custom Resource (CR) in file deploy/crds/guestbook.remkoh.dev_v1_guestbook_cr,

apiVersion: guestbook.remkoh.dev/v1
kind: Guestbook
metadata:
  name: example-guestbook
spec:
  # Add fields here
  size: 3

The Custom Resource Definition (CRD) in file deploy/crds/guestbook.remkoh.dev_guestbooks_crd.yaml,

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: guestbooks.guestbook.remkoh.dev
spec:
  group: guestbook.remkoh.dev
  names:
    kind: Guestbook
    listKind: GuestbookList
    plural: guestbooks
    singular: guestbook
  scope: Namespaced
  versions:
  - name: v1
    schema:
      openAPIV3Schema:
        description: Guestbook is the Schema for the guestbooks API
        properties:
          apiVersion:
            description: 'APIVersion defines the versioned schema of this representation
              of an object. Servers should convert recognized schemas to the latest
              internal value, and may reject unrecognized values. More info: https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#resources'
            type: string
          kind:
            description: 'Kind is a string value representing the REST resource this
              object represents. Servers may infer this from the endpoint the client
              submits requests to. Cannot be updated. In CamelCase. More info: https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#types-kinds'
            type: string
          metadata:
            type: object
          spec:
            description: GuestbookSpec defines the desired state of Guestbook
            type: object
          status:
            description: GuestbookStatus defines the observed state of Guestbook
            type: object
        type: object
    served: true
    storage: true
    subresources:
      status: {}

3. Create a new Controller

Add a new controller under pkg/controller/<kind>.

operator-sdk add controller --api-version=$OPERATOR_GROUP/$OPERATOR_VERSION --kind=$CRD_KIND

This command creates two files in pkg/controller:

• add_guestbook.go, which registers the new controller, and

• guestbook/guestbook_controller.go, which is the actual custom controller logic.

The file guestbook/guestbook_controller.go defines the Reconcile function,

// Reconcile reads state of the cluster for a Guestbook object and makes changes based on the state read and what is in the Guestbook.Spec
// TODO(user): User must modify this Reconcile function to implement their own Controller logic.  This example creates a Pod as an example
func (r *ReconcileGuestbook) Reconcile(request reconcile.Request) (reconcile.Result, error) {
    ...
    // Fetch the Guestbook instance
    instance := &guestbookv1.Guestbook{}
    ...
    // Define a new Pod object
    pod := newPodForCR(instance)
    ...
}

4. Compile and Build the Code

The operator-sdk build command compiles the code and builds the executables. fter you built the image, push it to your image registry, e.g. Docker hub.

operator-sdk build docker.io/$DOCKER_USERNAME/$OPERATOR_NAME

And push the build image to your image repository,

docker login docker.io -u $DOCKER_USERNAME

docker push docker.io/$DOCKER_USERNAME/$OPERATOR_NAME

5. Deploy the Operator

First replace the image attribute in the operator resource with the built image,

sed -i "s|REPLACE_IMAGE|docker.io/$DOCKER_USERNAME/$OPERATOR_NAME|g" deploy/operator.yaml

Make sure you are connected to the OpenShift cluster (see above how to connect), and deploy the operator with the following template code.

kubectl create sa $OPERATOR_PROJECT

kubectl create -f deploy/role.yaml

kubectl create -f deploy/role_binding.yaml

kubectl create -f deploy/crds/${OPERATOR_GROUP}_${CRD_KIND,,}s_crd.yaml

kubectl create -f deploy/operator.yaml

kubectl create -f deploy/crds/${OPERATOR_GROUP}_${OPERATOR_VERSION}_${CRD_KIND,,}_cr.yaml

Verify the deployment,

kubectl get deployment $OPERATOR_PROJECT

kubectl get pod -l app=example-${CRD_KIND,,}

kubectl describe ${CRD_KIND,,}s.${OPERATOR_GROUP} example-${CRD_KIND,,}

For our example Guestbook project the above templates should resolve as follows,

$ kubectl create sa guestbook-project
$ kubectl create -f deploy/role.yaml
$ kubectl create -f deploy/role_binding.yaml
$ kubectl create -f deploy/crds/guestbook.remkoh.dev_guestbooks_crd.yaml
$ kubectl create -f deploy/operator.yaml
$ kubectl create -f deploy/crds/guestbook.remkoh.dev_v1_guestbook_cr.yaml
$ kubectl get deployment guestbook-project
$ kubectl get pod -l app=example-guestbook
$ kubectl describe guestbooks.guestbook.remkoh.dev example-guestbook

Cleanup

kubectl delete sa $OPERATOR_PROJECT
kubectl delete role $OPERATOR_PROJECT
kubectl delete rolebinding $OPERATOR_PROJECT
kubectl delete customresourcedefinition ${CRD_KIND,,}s.${OPERATOR_GROUP}
kubectl delete deployment $OPERATOR_PROJECT

Application CRD

The Application CRD (Custom Resource Definition) and Controller provide the following:

• Describe an applications metadata.

• A point to connect the infrastructure, such as Deployments, to as a root object.

• Application level health checks.

This could be used by:

• Application operators.

• Tools, such as Helm.

• Dashboards.

apiVersion: app.k8s.io/v1beta1
kind: Application
metadata:
  name: "guestbook"
  labels:
    app.kubernetes.io/name: "guestbook"
spec:
  selector:
    matchLabels:
     app.kubernetes.io/name: "guestbook"
  componentKinds:
    - group: v1
      kind: Deployment
    - group: v1
      kind: Service
  descriptor:
    type: "guestbook"
    keywords:
      - "gb"
      - "guestbook"
    links:
      - description: Github
        url: "https://github.com/IBM/guestbook"
    version: "0.1.0"
    description: "The Guestbook application is an example app to demonstrate key Kubernetes functionality."
    maintainers:
      - name: IBM Developer
        email: [e-mail]
    owners:
      - name: IBM Developer
        email: [e-mail]