What are Kubernetes Custom Resource Definitions (CRDs) and how do you create custom controllers?

Custom Resource Definitions extend Kubernetes API to create domain-specific resources, while custom controllers implement the logic to manage these resources.\n\nCRD Components:\n• Schema definition: OpenAPI v3 specification\n• Validation rules: Field constraints and requirements\n• Subresources: Status and scale endpoints\n• Multiple versions: API evolution support\n• Conversion webhooks: Version compatibility\n\nController Pattern:\n• Watch: Monitor resource changes via API\n• Reconcile: Compare desired vs actual state\n• Update: Make necessary changes\n• Requeue: Handle errors and retries\n\nExample CRD:\napiVersion: apiextensions.k8s.io/v1\nkind: CustomResourceDefinition\nmetadata:\n name: webapps.example.com\nspec:\n group: example.com\n versions:\n - name: v1\n served: true\n storage: true\n schema:\n openAPIV3Schema:\n type: object\n properties:\n spec:\n type: object\n properties:\n replicas:\n type: integer\n\nBest Practices:\n• Use controller-runtime framework\n• Implement proper error handling\n• Add comprehensive validation\n• Support multiple API versions\n• Follow Kubernetes conventions