CustomResourceDefinition

A CustomResourceDefinition, or CRD, is a Kubernetes API extension that enables users to introduce new custom resource types to the cluster. By defining a CRD, you register a new resource kind that behaves like built‑in Kubernetes objects, allowing clients to create, read, update, and delete instances of that type. CRDs are stored as objects in the Kubernetes API server under the apiextensions.k8s.io API group.

A CRD specifies the shape and behavior of the custom resource. Its spec includes the resource group, one or more versions with details such as served and storage flags, and an OpenAPI v3 schema for validation. It also defines the resource’s scope (Namespaced or Cluster), and the names used to refer to the resource (plural, singular, kind, and shortNames). Optional features include subresources like status and scale, additionalPrinterColumns for kubectl output, and conversion settings for multiple versions.

The lifecycle of a CRD is integral to its use. A CRD must be established by the API server before any custom resources of that type can be created. Once established, users can create Custom Resources that conform to the CRD’s schema. Typically, a controller or operator watches those resources and acts to reconcile the current state with the desired state described by the custom resources.

CRDs support versioning and validation, and, in newer configurations, can include schema validation and subresources. They are primarily a mechanism to extend Kubernetes with domain‑specific resources without modifying the core API.