Replica sets

In Kubernetes, a ReplicaSet is a resource that templates the creation of pods. The definition of a replica set contains a template definition of the pods that it creates, a desired count of replicas, and a selector to discover the pods under its management.

The ReplicaSet is used to ensure that the desired number of pods is always running. If the count of pods matching the selector drops below the desired count, then Kubernetes will schedule another.

Because the life of a pod is tied to that of the node that it is running on, a pod can be considered ephemeral. There are a number of reasons why the life of a particular pod could come to an end. Perhaps it was removed by the operator or an automated process. Kubernetes could have evicted the pod to better utilize the resources of the cluster or prepare the node for shutdown or restart. Or perhaps the underlying node failed.

A ReplicaSet allows us to manage our application by asking the cluster to ensure that the correct number of replicas is running across the cluster as a whole. This is a strategy that Kubernetes embraces across many of its APIs.

As a cluster operator, Kubernetes takes some of the complexity of running applications away from the user. When I decide that I need three instances of my application running, I no longer need to think about the underlying infrastructure: I can just tell Kubernetes to carry out my wishes. And if the worst happens and one of the underlying machines that my application is running on fails, Kubernetes will know how to self-heal my application and launch a new pod. No more pager calls and trying to recover or replace failed instances in the middle of the night.

Often, we want to update the software we run on our cluster. Because of this, we don't normally directly use ReplicaSet but, instead, manage them with a Deployment object. Deployments are used in Kubernetes to gracefully roll out new versions of a ReplicaSet. You will learn more about deployments in Chapter 4, Managing Change in Your Applications.