Getting Started with ReplicaSets

Understanding ReplicaSets#

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 (e.g., enough memory and CPU), Pods associated with a ReplicaSet are guaranteed to run. They provide fault-tolerance and high availability.

If you’re familiar with Replication Controllers, it is worth mentioning that ReplicaSet is the next-generation ReplicationController. The only significant difference is that ReplicaSet has extended support for selectors. Everything else is the same. ReplicationController is considered deprecated, so we’ll focus only on ReplicaSet.

ReplicaSet’s primary function is to ensure that the specified number of replicas of service are (almost) always running.

Let’s explore ReplicaSet in this chapter through examples and see how it works and what it does.