What's in a controller?

Controllers are the core of Kubernetes, and of any operator.

It's a controller's job to ensure that, for any given object, the actual state of the world (both the cluster state, and potentially external state like running containers for Kubelet or loadbalancers for a cloud provider) matches the desired state in the object. Each controller focuses on one root Kind, but may interact with other Kinds.

We call this process reconciling.

In controller-runtime, the logic that implements the reconciling for a specific kind is called a Reconciler. A reconciler takes the name of an object, and returns whether or not we need to try again (e.g. in case of errors or periodic controllers, like the HorizontalPodAutoscaler).

{{#literatego ./testdata/emptycontroller.go}}

Now that we've seen the basic structure of a reconciler, let's fill out the logic for CronJobs.