What Is a Custom Controller?
CRDs and CRs help you define the desired state for your CRs. There is a need for a component that makes sure that the state of the Kubernetes system matches the desired state as defined by the CR. As you have seen in earlier chapters, the Kubernetes components that do this are called controllers. Kubernetes comes up with many of these controllers whose job is to make sure that the desired state (for example, the number of replicas of Pods defined in a Deployment) is equal to the value defined in the Deployment object. In summary, a controller is a component that watches the state of resources through the Kubernetes API server and attempts to match the current state with the desired state.
The built-in controllers that are included in a standard setup of Kubernetes are meant to work with built-in objects such as Deployments. For our CRDs and their CRs, we need to write our own custom controllers.
