Kubernetes - Using Custom Resource Definitions (CRDs)

Introduction

Custom Resource Definitions (CRDs) allow you to extend the Kubernetes API to create custom resources. This guide provides an advanced understanding of using CRDs in Kubernetes to define and manage custom resources.

What are Custom Resource Definitions (CRDs)?

Custom Resource Definitions (CRDs) are a way to extend the Kubernetes API by defining new resource types. Once a CRD is created, you can create and manage instances of your custom resource type using the standard Kubernetes API. CRDs are often used in conjunction with custom controllers to implement custom logic and behavior for the custom resources.

# Example of a Custom Resource Definition (CRD)
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: foos.example.com
spec:
  group: example.com
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                bar:
                  type: string
  scope: Namespaced
  names:
    plural: foos
    singular: foo
    kind: Foo
    shortNames:
    - f
                

Creating a Custom Resource Definition (CRD)

To create a CRD, define the custom resource schema and apply it to your Kubernetes cluster. Here is an example of creating a CRD:

# Apply the CRD to the cluster
kubectl apply -f crd.yaml

# Verify the CRD has been created
kubectl get crds
                

Creating Custom Resources

Once the CRD is created, you can create instances of the custom resource. Here is an example:

# Example of a custom resource instance
apiVersion: example.com/v1
kind: Foo
metadata:
  name: my-foo
spec:
  bar: "Hello, World!"

# Apply the custom resource to the cluster
kubectl apply -f foo.yaml

# Verify the custom resource has been created
kubectl get foos
                

Managing Custom Resources

You can manage custom resources using standard Kubernetes commands. Here are some examples:

# Get all custom resources
kubectl get foos

# Get a specific custom resource
kubectl get foo my-foo

# Describe a custom resource
kubectl describe foo my-foo

# Delete a custom resource
kubectl delete foo my-foo
                

Implementing Custom Controllers

Custom controllers watch for changes to custom resources and implement custom logic in response to those changes. Controllers are often implemented using the Kubernetes client libraries. Here is an example of a simple custom controller:

# Example of a custom controller (Python)
from kubernetes import client, config, watch

def main():
    config.load_kube_config()
    v1 = client.CustomObjectsApi()
    w = watch.Watch()
    for event in w.stream(v1.list_cluster_custom_object, group="example.com", version="v1", plural="foos"):
        print(f"Event: {event['type']}, Object: {event['object']}")

if __name__ == "__main__":
    main()
                

Best Practices

Follow these best practices when using Custom Resource Definitions in Kubernetes:

• Define Clear Schemas: Use OpenAPI v3 schemas to define clear and precise specifications for your custom resources.
• Version Your CRDs: Use versioning to manage changes to your CRDs and ensure compatibility with existing custom resources.
• Implement Validation: Implement validation schemas to ensure custom resources conform to expected formats and constraints.
• Monitor Custom Controllers: Ensure your custom controllers are running reliably and monitor their performance and behavior.
• Document Custom Resources: Provide clear documentation and examples for your custom resources to help users understand how to use them.

Conclusion

This guide provided an advanced overview of using Custom Resource Definitions (CRDs) in Kubernetes, including creating CRDs, managing custom resources, and implementing custom controllers. By following best practices and leveraging the power of CRDs, you can extend Kubernetes to handle custom application-specific requirements and logic.