Replication controllers and replica sets

Replication controllers (RCs), as the name suggests, manage the number of nodes that a pod and included container images run on. They ensure that an instance of an image is being run with the specific number of copies.

As you start to operationalize your containers and pods, you'll need a way to roll out updates, scale the number of copies running (both up and down), or simply ensure that at least one instance of your stack is always running. RCs create a high-level mechanism to make sure that things are operating correctly across the entire application and cluster.

RCs are simply charged with ensuring that you have the desired scale for your application. You define the number of pod replicas you want running and give it a template for how to create new pods. Just like services, we will use selectors and labels to define a pod's membership in a replication controller.

Kubernetes doesn't require the strict behavior of the replication controller, which is ideal for long-running processes. In fact, job controllers can be used for short lived workloads which allow jobs to be run to a completion state and are well suited for batch work.

Replica sets, are a new type, currently in Beta, that represent an improved version of replication controllers. Currently, the main difference consists of being able to use the new set-based label selectors as we will see in the following examples.

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