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APIs in kcp


kcp supports several built-in Kubernetes APIs, provides extensibility using CustomResourceDefinitions, and adds a new way to export custom APIs for sharing with other workspaces.


kcp, like Kubernetes, allows developers to add new APIs using CustomResourceDefinitions (CRDs). Unlike Kubernetes, kcp allows multiple copies of a CRD (e.g. to be installed in multiple workspaces at the same time. Each copy is entirely independent and isolated from all other copies. This means the API versions and schemas can be entirely different. kcp makes this possible because each workspace is its own isolated "cluster."

There are currently some limitations to be aware of with CRDs in kcp:

  • Conversion webhooks are not supported
  • service-based validating/mutating webhooks are not supported; you must use url-based clientConfigs instead.

CRDs are a fantastic way to add new APIs to a workspace, but if you want to share a CRD with other workspaces, you have to install it in each workspace separately. You also need a controller that can reconcile CRs in all the workspaces where your CRD is installed, which typically means 1 distinct controller per workspace. CRDs are not "cheap" in the API server (each one consumes memory), and kcp offers an improved workflow that significantly reduces overhead.