A single pod in an environment with high availability requirements is insufficient. What if the pod crashes? What if we need to update the application running inside the pod but cannot afford any service interruption? These questions and more indicate that pods alone are not enough and we need a higher-level concept that can manage multiple instances of the same pod. In Kubernetes, the ReplicaSet is used to define and manage such a collection of identical pods that are running on different cluster nodes. Among other things, a ReplicaSet defines which container images are used by the containers running inside a pod and how many instances of the pod will run in the cluster. These properties and many others are called the desired state.
The ReplicaSet is responsible for reconciling the desired state at all times...