Controllers

Kubernetes is built with the notion that you tell it what you want, and it knows how to do it. When you interact with Kubernetes, you are asserting you want one or more resources to be in a certain state, with specific versions, and so forth. Controllers are where the brains exist for tracking those resources and attempting to run your software as you described. These descriptions can include how many copies of a container image are running, updating the software version running within a Pod, and handling the case of a Node failure where you unexpectedly lose part of your cluster.

There are a variety of controllers used within Kubernetes, and they are mostly hidden behind two key resources that we will dig into further: Deployments and ReplicaSets.