Kubernetes StatefulSet: In-Depth Guide and Practical Differences from Deployment

Kubernetes has revolutionized the way we deploy and scale applications, offering robust abstractions like Deployments, ReplicaSets, and StatefulSets.
While most developers are comfortable deploying stateless applications with Deployments, confusion often arises when dealing with stateful workloads such as databases.

In this article, we’ll break down what a StatefulSet is, why it exists, and how it differs from a Deployment — using clear examples like MySQL and MongoDB to illustrate how stateful applications behave inside Kubernetes.

What Is a StatefulSet?

A StatefulSet is a Kubernetes controller designed specifically for managing stateful applications — those that maintain data or “state” across sessions and restarts.

Examples of stateful applications include:

• Databases: MySQL, PostgreSQL, MongoDB, Cassandra
• Search engines: Elasticsearch
• Messaging systems: Kafka, RabbitMQ

These systems persist and manage data, meaning they must maintain awareness of previous operations. Losing or corrupting that state can have severe consequences.