What is the deference The difference between a Statefulset and a Deployment?

A Statefulset is a Kubernetes controller that is used to manage and maintain one or more Pods. However, so do other controllers like ReplicaSets and, the more robust, Deployments. So what does Kubernetes use StatefulSets for? To answer this question, we need to discuss stateless versus stateful applications.

A stateless application is one that does not care which network it is using, and it does not need permanent storage. Examples of stateless apps may include web servers (Apache, Nginx, or Tomcat).

On the other hand, we have stateful apps. Let’s say you have a Solr database cluster that is managed by several Zookeeper instances. For such an application to function correctly, each Solr instance must be aware of the Zookeeper instances that are controlling it. Similarly, the Zookeeper instances themselves establish connections between each other to elect a master node. Due to such a design, Solr clusters are an example of stateful applications. If you deploy Zookeeper on Kubernetes, you’ll need to ensure that pods can reach each other through a unique identity that does not change (hostnames, IPs...etc.). Other examples of stateful applications include MySQL clusters, Redis, Kafka, MongoDB, and others.

Given this difference, Deployment is more suited to work with stateless applications. As far as a Deployment is concerned, Pods are interchangeable. While a StatefulSet keeps a unique identity for each Pod it manages. It uses the same identity whenever it needs to reschedule those Pods.