Extending Kubernetes with custom resources and operators

In the previous sections, we've learned that the Kubernetes API is not just a single API but also an aggregation of APIs backed by cooperative services called operators and controllers. Operators are extensions to Kubernetes that make use of custom resources to manage systems and applications via controllers. Controllers are components of operators that execute control loops for a kind of resource. A control loop for a custom resource is an iterative process that observes a desired state of the resource and works, possibly over several loops, to drive the state of a system to that desired state.

Those previous sentences are rather abstract. I like to sum it up differently. Kubernetes is a platform for automation. An automation is a series of steps and decision trees that drives to reach an end goal. I like to think of operators in a similar way. I think of writing operators as taking a runbook, the human steps for completing...