What is a CRD?

Learn to define custom resources with a CRD.

What is a CRD?

A CustomResourceDefinition (CRD) adds a new resource type to the Kubernetes API. After the CRD is applied, that type is created and listed with kubectl, protected with RBAC and stored in etcd just like the built-in types. The Prometheus Operator's ServiceMonitor and cert-manager's Certificate are CRDs.

$ kubectl get crds
NAME                                     CREATED AT
certificates.cert-manager.io             2026-10-20T09:12:01Z
servicemonitors.monitoring.coreos.com    2026-10-21T14:03:44Z
yedekler.ornek.com                       2026-11-03T08:30:10Z

A CRD on its own only stores data; the part that does something based on that data is a controller (an operator, next group).

Extending the API

A type added with a CRD appears in kubectl api-resources, and kubectl explain generates documentation from the type definition. That way you can express your own platform concepts (Backup, Database, Environment) in Kubernetes's language:

$ kubectl api-resources --api-group=ornek.com
NAME       SHORTNAMES   APIVERSION     NAMESPACED   KIND
yedekler   yd           ornek.com/v1   true         Yedek
$ kubectl explain yedek.spec

Exercise Explore CRDs

Write three commands: list the CRDs; list the types in the ornek.com API group; show the documentation of the yedek.spec field.

Show example solution

kubectl get crds
kubectl api-resources --api-group=ornek.com
kubectl explain yedek.spec

Test yourself

What happens if you only install a CRD and create a custom resource?
- The object is stored, but without a controller processing it, no action happens
- Pods are created automatically
- It errors
- Kubernetes restarts
Show answer

The object is stored, but without a controller processing it, no action happens — CRD = data model; controller = behaviour.

List the types of a specific API group.

kubectl api-resources ____=ornek.com

Show answer

kubectl api-resources --api-group=ornek.com

Match.

CRD

New type definition
custom resource

An object of that type
denetleyici

Behaviour
ServiceMonitor

A CRD example

Next lesson: CRD YAML

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies