Kubernetes CRDs — Custom Resource Definitions

Cloud Intermediate

Kubernetes CRDs — Custom Resource Definitions¶

KubernetesCRDAPIExtensibility 5 min read

Extending the Kubernetes API with custom resource types. CRD definitions, validation, versioning and best practices.

What Are CRDs¶

Custom Resource Definitions extend the Kubernetes API with custom object types.

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: databases.example.com
spec:
  group: example.com
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              required: [engine, version]
              properties:
                engine:
                  type: string
                  enum: [postgres, mysql, mongodb]
                version:
                  type: string
                replicas:
                  type: integer
                  minimum: 1
                  maximum: 7
                  default: 3
  scope: Namespaced
  names:
    plural: databases
    singular: database
    kind: Database
    shortNames: [db]

Using a Custom Resource¶

apiVersion: example.com/v1
kind: Database
metadata:
  name: orders-db
  namespace: production
spec:
  engine: postgres
  version: "16"
  replicas: 3

# kubectl get databases
# kubectl describe db orders-db

Versioning¶

CRDs support multiple versions with a conversion webhook.

versions:
  - name: v1
    served: true
    storage: true
  - name: v1beta1
    served: true
    storage: false
conversion:
  strategy: Webhook
  webhook:
    conversionReviewVersions: ["v1"]
    clientConfig:
      service:
        name: db-conversion
        namespace: system
        path: /convert

Summary¶

CRDs are the foundation of Kubernetes extensibility. They enable declarative management of any infrastructure via kubectl and the standard K8s API.

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