The Myth: Deployments Lose Data

Most developers assume StatefulSets exist because Deployments can't persist data. That's false. A Deployment paired with a Persistent Volume Claim (PVC) retains data across pod restarts just fine.

Experiment 1: Deployment + PVC

Clone the repo and apply the secrets and ConfigMap:

git clone https://github.com//K8s_with_Pravesh.git
cd K8s_with_Pravesh/part-02-statefulsets/configs
kubectl apply -f secrets-and-config.yml

Apply the Deployment manifest (includes PVC, Deployment, Service):

kubectl apply -f deployment.yml

Connect to MySQL, insert a record:

kubectl exec -it mysql-XXXXXX-XXXX -- mysql -uroot -p
# password: Pravesh
mysql> USE crud_app;
mysql> INSERT INTO users(name,email,password) VALUES('Pravesh','pravesh@example.com','secret');
mysql> exit

Delete the pod:

kubectl delete pod mysql-XXXXXX-XXXX

Wait for the new pod, connect, and query:

kubectl exec -it mysql-XXXXXX-XXXX -- mysql -uroot -p
mysql> USE crud_app;
mysql> SELECT * FROM users;

The data is still there. The PVC preserved it — not the Deployment.

Experiment 2: StatefulSet

Apply the StatefulSet manifest:

kubectl apply -f statefulset.yml

Notice the pod name is predictable: mysql-0. Insert another record, delete the pod, and watch it recreate with the same name mysql-0. The data survives.

The Real Difference

Inspect PVCs:

kubectl get pvc

With StatefulSets, each replica gets its own dedicated PVC (e.g., mysql-data-mysql-0, mysql-data-mysql-1, mysql-data-mysql-2). This creates a stable, one-to-one relationship between pod and storage.

StatefulSet pods are not interchangeable. Each has:

When to Use StatefulSets

StatefulSets are for stateful distributed systems like MySQL, PostgreSQL, Kafka, ZooKeeper, Redis Clusters, and Elasticsearch. They need:

Deployments are for stateless apps: frontends, REST APIs, microservices.

Key Differences Table

FeatureDeploymentStatefulSet
Data Persistence✅ With PVC✅ With PVC
Stable Pod Name❌✅
Stable Network Identity❌✅
Dedicated Storage Per Replica❌✅
Ordered Startup/Shutdown❌✅

Conclusion

Don't fall for the myth. A Deployment with a PVC persists data just as well as a StatefulSet. The real value of StatefulSets is the stable identity and dedicated storage per pod. Choose StatefulSets when your app needs those guarantees. For everything else, Deployments are simpler and sufficient.

Next steps: Try the experiments yourself from the GitHub repo and see the difference firsthand.