apiextensionsk8sio

apiextensions.k8s.io is a Kubernetes API group used to extend the Kubernetes API surface with custom resources and related extension objects. The primary resource in this group is CustomResourceDefinition (CRD), which allows cluster operators to define new resource kinds that behave like native Kubernetes objects and are managed through the standard API machinery.

CRDs define a custom resource’s API group, version, kind, and semantics. A CRD specifies the resource’s names (plural, singular, kind) and scope (Namespaced or Cluster), the versions served and stored, and a validation schema (OpenAPI v3). When a CRD is applied and Established, clients can create, read, update, and delete custom resources of that kind via the Kubernetes API path /apis/<group>/<version>/<plural>, and through kubectl or client libraries. CRDs may also declare subresources, status fields, and conversion strategies for versioning.

Versions and evolution: the apiextensions.k8s.io group provides a v1 API, which is the stable, recommended version. Earlier Kubernetes releases exposed a v1beta1 version, which has been deprecated in favor of v1. The v1 API enforces structural schemas and provides stronger validation, defaulting, and tooling support. This group is central to the operator pattern, as CRDs enable controllers to manage domain-specific resources without modifying the core Kubernetes code.

Usage considerations: CRDs enable dynamic extension of the Kubernetes API but require careful lifecycle management and governance. Installing and upgrading CRDs affects the API surface and may interact with custom controllers that reconcile the defined resources. The apiextensions.k8s.io group is maintained by Kubernetes API machinery and the broader ecosystem of extension tooling.