CRDs & Operators Basics

This page is a hands-on intro to Custom Resource Definitions and the controllers that act on them. By the end you can define a new API type, create objects of that type, and understand what a controller adds.

Prerequisites

• kubectl matching your cluster (Kubernetes 1.36.2 here; keep within one minor of the API server).
• A cluster you can apply CRDs to (kind, minikube, or a cloud cluster). CRDs are cluster-scoped, so you need appropriate RBAC.
• Optional for building a controller: Go 1.23+, kubebuilder, and Docker Engine 29.x to build the controller image.

# Quick sanity check
kubectl version
kubectl api-resources | head

Basic Examples

1. Define a minimal CRD

A CRD registers a new kind. This one adds a namespaced Widget resource in group example.com.

apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: widgets.example.com
spec:
  group: example.com
  scope: Namespaced
  names:
    plural: widgets
    singular: widget
    kind: Widget
    shortNames: ["wdg"]
  versions:
    - name: v1
      served: true
      storage: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                size:
                  type: string

• apiextensions.k8s.io/v1 is the stable, required API for CRDs.
• scope: Namespaced means each Widget lives in a namespace, like a Pod.
• Exactly one version must set storage: true; that is the version persisted to etcd.
• served: true means the API server will accept and return that version.
• The openAPIV3Schema is mandatory in v1 and controls validation.

2. Apply the CRD and see the new type

Once applied, the new kind behaves like a built-in one.

kubectl apply -f widget-crd.yaml
kubectl api-resources | grep widgets
kubectl explain widget.spec

• kubectl apply registers the type cluster-wide.
• kubectl api-resources now lists widgets, proving the API server knows the kind.
• kubectl explain reads the schema you supplied, so documentation is generated from the CRD.

3. Create a custom resource

A CR is an instance of the type, written like any manifest.

apiVersion: example.com/v1
kind: Widget
metadata:
  name: blue-widget
spec:
  size: large

• The apiVersion combines the group and version from the CRD.
• kubectl apply -f blue-widget.yaml stores the object; kubectl get widgets lists it.
• At this point nothing acts on the object; it is just stored, validated data.

4. Add validation to the schema

Structural schemas can enforce allowed values and required fields.

spec:
  type: object
  required: ["size"]
  properties:
    size:
      type: string
      enum: ["small", "medium", "large"]
    replicas:
      type: integer
      minimum: 1
      maximum: 10

• required rejects objects missing the field at admission time.
• enum limits size to known values, catching typos before they persist.
• minimum and maximum bound numeric fields without any controller code.

5. Add printer columns

Printer columns customize what kubectl get shows.

additionalPrinterColumns:
  - name: Size
    type: string
    jsonPath: .spec.size
  - name: Age
    type: date
    jsonPath: .metadata.creationTimestamp

• Columns are defined per version under the CRD versions entry.
• jsonPath selects the field to display.
• kubectl get widgets now shows a Size column, improving day-to-day usability.

6. Scope RBAC to your resource

Custom resources use standard RBAC by group and resource name.

apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  name: widget-editor
  namespace: default
rules:
  - apiGroups: ["example.com"]
    resources: ["widgets"]
    verbs: ["get", "list", "watch", "create", "update", "patch", "delete"]

• apiGroups matches the CRD spec.group.
• resources matches the plural name.
• Bind this Role to a ServiceAccount so a controller or user can act on Widget objects.

7. Inspect and delete

Custom resources support the same lifecycle verbs as built-ins.

kubectl get widgets -o yaml
kubectl describe widget blue-widget
kubectl delete widget blue-widget

• -o yaml shows the stored object including any status the controller writes.
• describe surfaces events, useful once a controller emits them.
• Deleting the CRD deletes all its custom resources, so treat CRD deletion with care.

Intermediate Examples

8. Add a status subresource

The status subresource splits spec (user intent) from status (controller-reported state).

versions:
  - name: v1
    served: true
    storage: true
    subresources:
      status: {}
    schema:
      openAPIV3Schema:
        type: object
        properties:
          spec: { type: object }
          status:
            type: object
            properties:
              phase: { type: string }

• With subresources.status: {}, updates to .status go through a separate endpoint.
• Controllers update status without racing user edits to spec.
• This is the idiomatic split for any resource a controller manages.

9. What a controller adds

A CRD stores data; a controller makes it mean something.

// Reconcile is called whenever a Widget changes.
func (r *WidgetReconciler) Reconcile(ctx context.Context, req ctrl.Request) (ctrl.Result, error) {
	var w examplev1.Widget
	if err := r.Get(ctx, req.NamespacedName, &w); err != nil {
		return ctrl.Result{}, client.IgnoreNotFound(err)
	}
	// Drive real state toward w.Spec, then update w.Status.
	return ctrl.Result{}, nil
}

• The Reconcile function is the control loop for your type.
• It reads the object, does the work to match the spec, and returns.
• A CRD plus a reconciling controller is exactly the operator pattern; see The Operator Pattern Explained.

10. Scaffold a controller with kubebuilder

Kubebuilder generates the project, CRD manifests, and reconciler skeleton.

kubebuilder init --domain example.com --repo example.com/widget-operator
kubebuilder create api --group example --version v1 --kind Widget
make manifests

• init creates the controller-runtime project layout.
• create api scaffolds both the CRD types and a Reconcile stub.
• make manifests regenerates CRD YAML from your Go types, keeping them in sync.

Stack versions: This page was written for Kubernetes 1.36.2, Docker Engine 29.6.1 (BuildKit default), containerd (CRI runtime on nodes), Helm 3, Compose v2, Argo CD (latest - verify at build), and Gateway API (GA - verify controller support at build).

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