Custom Resource Definitions

What a CRD Is

CRDs extend the K8s API with your own resource types:

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: crontabs.stable.example.com
spec:
  group: stable.example.com
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                cronSpec:
                  type: string
                image:
                  type: string
                replicas:
                  type: integer
  scope: Namespaced
  names:
    plural: crontabs
    singular: crontab
    kind: CronTab
    shortNames: [ct]

Working with Unstructured Data

For quick prototyping, use the dynamic client:

import (
    "k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
    "k8s.io/apimachinery/pkg/runtime/schema"
    "k8s.io/client-go/dynamic"
)

func listCronTabs(dynClient dynamic.Interface) error {
    gvr := schema.GroupVersionResource{
        Group:    "stable.example.com",
        Version:  "v1",
        Resource: "crontabs",
    }

    list, err := dynClient.Resource(gvr).Namespace("default").List(
        context.Background(), metav1.ListOptions{},
    )
    if err != nil {
        return err
    }

    for _, item := range list.Items {
        name := item.GetName()
        spec, _, _ := unstructured.NestedMap(item.Object, "spec")
        fmt.Printf("%s: %v\n", name, spec)
    }
    return nil
}

Typed Clients (Production)

For production operators, generate typed clients with controller-gen or code-generator. This gives you:

• Compile-time type safety
• Auto-generated Listers, Informers, and Clientsets
• DeepCopy methods