What is ReplicationSet in Kubernetes?

A ReplicaSet in Kubernetes is a controller that ensures a specified number of replicas (or copies) of a pod are running at any given time. It is used to ensure availability and scalability of pods in a Kubernetes cluster.

A ReplicaSet is defined by a set of labels and a selector, and it will automatically create or delete pods as needed to ensure the desired number of replicas are running.

If a pod crashes or is deleted, the ReplicaSet will automatically create a new one to take its place. If a new pod is created that matches the ReplicaSet's selector, it will be considered part of the ReplicaSet and will be managed accordingly.

ReplicaSet is older version, it's been replaced by Deployment controller in k8s. Deployment is an extension of ReplicaSet, it provides more features like rolling updates, rollbacks, and canary deployment.

In summary, ReplicaSet is a simple and effective way to ensure that a desired number of replicas of a pod are running in a Kubernetes cluster, providing high availability and scalability.