Kubernetes ReplicaSet 101

This article was originally published at https://www.magalix.com/blog/kubernetes-replicaset-101

What is a Kubernetes ReplicaSet?

A Kubernetes pod serves as a deployment unit for the cluster. It may contain one or more containers. However, containers (and accordingly, pods) are short-lived entities. A container hosting a PHP application, for example may experience an unhandled code exception causing the process to fail, effectively crashing the container. Of course, the perfect solution for such a case is to refactor the code to properly handle exceptions. But, till that happens we need to keep the application running and the business going. In other words, we need to restart the pod whenever it fails. In parallel, developers are monitoring, investigating and fixing any errors that make it crash. At some point, a new version of the pod is deployed, monitored and maintained. It’s an ongoing process that is part of the DevOps practice.

Another requirement is to keep a predefined number of pods running. If more pods are up, the additional ones are terminated. Similarly, of one or more pods failed, new pods are activated until the desired count is reached.

A Kubernetes ReplicaSet resource was designed to address both of those requirements. It creates and maintains a specific number of similar pods (replicas). In this article, we’ll discuss how we can define a ReplicaSet and what are the different options that can be used for fine-tuning it.