Kubernetes — Difference between Deployment and StatefulSet in K8s

# Summary

Kubernetes Deployments are optimized for stateless applications, whereas StatefulSets are tailored for stateful applications, providing unique identity and persistent storage for each replica.

# Abstract

In Kubernetes, Deployments are primarily used for applications that do not require consistent state, such as REST APIs or single-page applications (SPAs) like Angular or React. These deployments manage identical and replaceable pods, with no particular order or state preservation. On the other hand, StatefulSets are designed for applications that necessitate stable and unique network identifiers and persistent storage, such as databases or data-oriented services like ElasticSearch and Redis. StatefulSets ensure the ordered creation and scaling of pods, each with its own Persistent Volume Claim (PVC), and maintain the state across crashes or restarts using volumeClaimTemplates. Unlike Deployments, StatefulSets do not support rollbacks to previous versions and are scaled up or down without creating intermediate ReplicaSets.

# Opinions

- Deployments are suitable for stateless services due to their simplicity and ease of horizontal scaling without the need for persistent state.
- StatefulSets are considered more appropriate for complex applications that require stable storage and network identifiers, ensuring data consistency and reliable communication.
- The unique naming convention for pods in StatefulSets provides a sticky identity, which is crucial for stateful applications where each instance's state and data are critical.
- The inability to roll back to previous versions of StatefulSets is seen as a limitation, which may affect application updates and recovery strategies.
- The article suggests that the choice between Deployment and StatefulSet should be informed by the application's requirements for state management and scaling behavior.

Key Differences

Pods: Pods deployed by Deployment are identical and interchangeable, created in random order with random hashes in their Pod names. In contrast to that, the Pods deployed by StatefulSet component are NOT identical. They each have their own sticky identity, which they keep between restarts and each can be addressed individually. Thus, they can’t be created or deleted at the same time.

ReplicaSet: StatefulSet is also a Controller but unlike Deployments, it doesn’t create ReplicaSet rather itself creates the Pod with a unique naming convention. It manages the deployment and scaling of a set of Pods, and provides guarantees about the ordering and uniqueness of these Pods.

PVC: Every replica of a StatefulSet will have its own state, and each of the pods will be creating its own PVC (Persistent Volume Claim). So StatefulSet with 3 replicas will create 3 pods, each having its own Volume, so total 3 PVCs.

Rollback: Unlike Deployments, you cannot roll back to any previous version of a StatefulSet as it don’t create ReplicaSet or anything of that sort. You can only delete or scale up/down the Statefulset.

States: To save the states StatefulSets use volumeClaimTemplates/claims of persistent volumes in order to keep the state safe in case of crashes or restarts.

Kubernetes — Difference between Deployment and StatefulSet in K8s

TL;DR

Key Differences

Summary