Custom Resource Definitions

A CRD isn't just the thing you kubectl apply when installing an operator. It's the complete specification for how the operator's API works — what's required, what's validated, what values are rejected before the controller ever sees them. Reading a CRD tells you more about how an operator behaves than its README does.

When you run kubectl get certificates or kubectl get prometheuses, you're querying custom API endpoints that didn't exist in Kubernetes core. A CRD is the declaration that creates those endpoints: it tells the API server what the resource is called, what fields it accepts, and how to validate them.