Controller manager

The controller manager is the service that runs the core control loops (or controllers) that implement some of core functionality that makes Kubernetes function. Each of these controllers watches the state of the cluster through the API server and then makes changes to try and move the state of the cluster closer to the desired state. The design of the controller manager means that only one instance of it should be running at a given time; however, to simplify deployment in a high-availability configuration, the controller manager has a built-in leader election functionality, so that several instances can be deployed side by side, but only one will actually carry out work at any one time.