Exercise 6: Custom Resource Definitions (CRDs)

Task 1 - Understanding Custom Resource Definitions

  1. Custom Resource Definitions (CRDs) allow you to extend the Kubernetes API with your own custom resources.

  2. CRDs define:

    • The structure of your custom resource (schema)
    • The validation rules for the resource
    • How the resource is presented in the API

  3. Create a CRD manifest:

    ​
    $applicationCrd = @"
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: applications.workshop-example.io
spec:
  group: workshop-example.io
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                deploymentName:
                  type: string
                replicas:
                  type: integer
                  minimum: 1
                  maximum: 10
                image:
                  type: string
                containerPort:
                  type: integer
                resources:
                  type: object
                  properties:
                    memoryRequest:
                      type: string
                    cpuRequest:
                      type: string
                    memoryLimit:
                      type: string
                    cpuLimit:
                      type: string
              required:
                - deploymentName
                - replicas
                - image
  scope: Namespaced
  names:
    plural: applications
    singular: application
    kind: Application
    shortNames:
    - app
"@

# Output the CRD to review it
$applicationCrd
    cat <<EOF > application-crd.yaml
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: applications.workshop-example.io
spec:
  group: workshop-example.io
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                deploymentName:
                  type: string
                replicas:
                  type: integer
                  minimum: 1
                  maximum: 10
                image:
                  type: string
                containerPort:
                  type: integer
                resources:
                  type: object
                  properties:
                    memoryRequest:
                      type: string
                    cpuRequest:
                      type: string
                    memoryLimit:
                      type: string
                    cpuLimit:
                      type: string
              required:
                - deploymentName
                - replicas
                - image
  scope: Namespaced
  names:
    plural: applications
    singular: application
    kind: Application
    shortNames:
    - app
EOF

# Output the CRD to review it
cat application-crd.yaml

    This manifest defines an Application resource with properties like deploymentName, replicas, image, etc.

Task 2 - Create a Custom Resource Definition

  1. First, check if any previous version of the CRD exists and delete it if needed:

    ​
    kubectl delete crd applications.workshop-example.io --ignore-not-found
    kubectl delete crd applications.workshop-example.io --ignore-not-found

  2. Apply the Application CRD:

    ​
    $applicationCrd | kubectl apply -f -
    kubectl apply -f application-crd.yaml

  3. Verify the CRD was created:

    ​
    kubectl get crd applications.workshop-example.io
    kubectl get crd applications.workshop-example.io

  4. Examine the CRD details:

    ​
    kubectl describe crd applications.workshop-example.io
    kubectl describe crd applications.workshop-example.io

    Notice the schema validation rules and the API versions available.

Task 3 - Create a Custom Resource

  1. Create the Application custom resource:

    ​
    $applicationCr = @"
apiVersion: workshop-example.io/v1
kind: Application
metadata:
  name: example-app
spec:
  deploymentName: example-app
  replicas: 3
  image: k8sonazureworkshoppublic.azurecr.io/nginx:latest
  containerPort: 80
  resources:
    memoryRequest: "64Mi"
    cpuRequest: "100m"
    memoryLimit: "128Mi"
    cpuLimit: "200m"
"@

# Output the custom resource to review it
$applicationCr
    cat <<EOF > application-cr.yaml
apiVersion: workshop-example.io/v1
kind: Application
metadata:
  name: example-app
spec:
  deploymentName: example-app
  replicas: 3
  image: k8sonazureworkshoppublic.azurecr.io/nginx:latest
  containerPort: 80
  resources:
    memoryRequest: "64Mi"
    cpuRequest: "100m"
    memoryLimit: "128Mi"
    cpuLimit: "200m"
EOF

# Output the custom resource to review it
cat application-cr.yaml

    This is an instance of the Application custom resource type we just defined.

  2. Create the Custom Resource:

    ​
    $applicationCr | kubectl apply -f -
    kubectl apply -f application-cr.yaml

  3. Verify the Custom Resource was created:

    ​
    kubectl get applications
    kubectl get applications

  4. View the details of the Custom Resource:

    ​
    kubectl describe application example-app
    kubectl describe application example-app

Task 4 - Test Validation Rules

  1. Try to create an invalid Custom Resource (with too many replicas):

    ​
    $invalidApp = @"
apiVersion: workshop-example.io/v1
kind: Application
metadata:
  name: invalid-app
spec:
  deploymentName: invalid-app
  replicas: 20
  image: k8sonazureworkshoppublic.azurecr.io/nginx:latest
  containerPort: 80
"@

$invalidApp | kubectl apply -f -
    cat <<EOF | kubectl apply -f -
apiVersion: workshop-example.io/v1
kind: Application
metadata:
  name: invalid-app
spec:
  deploymentName: invalid-app
  replicas: 20
  image: k8sonazureworkshoppublic.azurecr.io/nginx:latest
  containerPort: 80
EOF

    You should get an error because the replicas field has a maximum value of 10 in our CRD.

  2. Fix the invalid resource and apply again:

    ​
    $validApp = @"
apiVersion: workshop-example.io/v1
kind: Application
metadata:
  name: valid-app
spec:
  deploymentName: valid-app
  replicas: 5
  image: k8sonazureworkshoppublic.azurecr.io/nginx:latest
  containerPort: 80
  resources:
    memoryRequest: "64Mi"
    cpuRequest: "100m"
    memoryLimit: "128Mi"
    cpuLimit: "200m"
"@

$validApp | kubectl apply -f -
    cat <<EOF | kubectl apply -f -
apiVersion: workshop-example.io/v1
kind: Application
metadata:
  name: valid-app
spec:
  deploymentName: valid-app
  replicas: 5
  image: k8sonazureworkshoppublic.azurecr.io/nginx:latest
  containerPort: 80
  resources:
    memoryRequest: "64Mi"
    cpuRequest: "100m"
    memoryLimit: "128Mi"
    cpuLimit: "200m"
EOF

  3. Verify the resource was created:

    ​
    kubectl get applications
    kubectl get applications

Task 5 - Using Kubectl for Custom Resources

  1. Try kubectl commands with your custom resource:

    ​
    # List all applications
kubectl get applications

# Get output in YAML format
kubectl get application example-app -o yaml

# Get specific fields using jsonpath
kubectl get application example-app -o jsonpath='{.spec.replicas}'
    # List all applications
kubectl get applications

# Get output in YAML format
kubectl get application example-app -o yaml

# Get specific fields using jsonpath
kubectl get application example-app -o jsonpath='{.spec.replicas}'

  2. Edit a Custom Resource:

    ​
    kubectl edit application example-app
    kubectl edit application example-app

    Change the number of replicas to 4 and save the file.

  3. Verify the change:

    ​
    kubectl get application example-app -o jsonpath='{.spec.replicas}'
    kubectl get application example-app -o jsonpath='{.spec.replicas}'

Task 6 - Clean up

  1. Remove the Custom Resources:

    ​
    kubectl delete application example-app valid-app
    kubectl delete application example-app valid-app

  2. Remove the Custom Resource Definition:

    ​
    kubectl delete crd applications.workshop-example.io
    kubectl delete crd applications.workshop-example.io