Most applications should be scalable and all must be fault tolerant. Pods do not provide those features, ReplicaSets do.
We learned that Pods are the smallest unit in Kubernetes. We also learned that Pods are not fault tolerant. If a Pod is destroyed, Kubernetes will do nothing to remedy the problem. That is, if Pods are created without Controllers.
The first Controller we'll explore is called ReplicaSet. Its primary, and pretty much only function, is to ensure that a specified number of replicas of a Pod matches the actual state (almost) all the time. That means that ReplicaSets make Pods scalable.
We can think of ReplicaSets as a self-healing mechanism. As long as elementary conditions are met (for example, enough memory and CPU), Pods associated with a ReplicaSet are guaranteed to run. They provide fault-tolerance and high availability.
It...
