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Everything in Kubernetes is an object, and every object has a current state, a preferred state, and a specification, in code, of how a state transition is achieved. This specification includes applications, deployments, services to machine configuration, and management of specific hardware resources. When a Kubernetes object is created, the cluster uses the object to transition toward the preferred state for the cluster.
Custom resource definitions (CRDs) specify new resource types which are used to create and add custom resources (CRs). Middleware operators use this extensible mechanism to create resource types that Kubernetes and other middleware with appropriate access can manage and use.
For more information, see Custom Resources.