What is a CRD?
- Docker & Kubernetes course
- Expert
- Lesson 7/47
- 4 min
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 2026-11-03T08:30:10ZA 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.specPractice 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.
Starter code
# buraya yazSample solution
kubectl get crds
kubectl api-resources --api-group=ornek.com
kubectl explain yedek.specExpected output
NAME CREATED AT yedekler.ornek.com 2026-11-03T08:30:10Z NAME SHORTNAMES APIVERSION NAMESPACED KIND yedekler yd ornek.com/v1 true Yedek GROUP: ornek.com KIND: Yedek FIELD: spec <Object>
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
- Pods are created automatically
- It errors
- Kubernetes restarts
Explanation: CRD = data model; controller = behaviour.
2 List the types of a specific API group.
kubectl api-resources ____=ornek.comkubectl api-resources --api-group=ornek.comExplanation: --api-group filters by group.
3 Match.
| Term | Meaning |
|---|---|
CRD |
New type definition |
custom resource |
An object of that type |
denetleyici |
Behaviour |
ServiceMonitor |
A CRD example |