What is a StatefulSet?

Learn that a StatefulSet manages stateful applications.

What is a StatefulSet?

A StatefulSet is the Deployment's counterpart for stateful applications: databases, message queues, Kafka, Elasticsearch. It differs from a Deployment in three ways:

Stable identity: pod names are ordinal (db-0, db-1) and get the same name when recreated.
Persistent storage: each pod has its own PersistentVolumeClaim and is attached to the same disk when recreated.
Ordered operations: pods start in order and shut down in reverse.

$ kubectl get pods -l app=db
NAME   READY   STATUS    RESTARTS   AGE
db-0   1/1     Running   0          10m
db-1   1/1     Running   0          9m
db-2   1/1     Running   0          8m

When to use a StatefulSet?

For applications whose replicas are interchangeable (web, API), a Deployment is used. A StatefulSet is needed where the replicas' identity matters: databases with a primary/replica distinction, nodes joining a cluster with their own identity (etcd, ZooKeeper), systems where each replica holds its own data.

Deployment:   web-7c5b9f-abcde  web-7c5b9f-fghij   (interchangeable)
StatefulSet:  db-0 (primary)     db-1 (replica)     (each different)

In production, an operator (e.g. CloudNativePG) is often used instead of writing a StatefulSet by hand.

Exercise Inspect the StatefulSet

Write three commands: list the StatefulSets; list the pods labelled app=db; show the db StatefulSet in detail.

Show example solution

kubectl get sts
kubectl get pods -l app=db
kubectl describe sts db

Test yourself

What happens if the db-1 pod is deleted?
- db-3 is created
- Its data is deleted
- It is recreated with the same name (db-1) and attached to the same disk
- A new pod with a random name is created
Show answer

It is recreated with the same name (db-1) and attached to the same disk — The identity and storage are kept independently of the pod.
List the StatefulSets with the abbreviation.
```
kubectl get ____
```
Show answer
```
kubectl get sts
```
Match.

db-0

Ordinal stable name
a PVC per pod

Persistent storage
ordered startup

db-0, then db-1
Deployment

Interchangeable replicas

Next lesson: StatefulSet YAML

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