EN_CS_StatefulSet

Q7: StatefulSets and Stateless Applications

Question: Explain the difference between StatefulSet and Stateless applications in Kubernetes. Cover deployment methods, scaling strategies, Persistent Volumes, Headless Services, and Pod Identity.

Key Terms

Term	Description
StatefulSet	K8s object for managing stateful applications
Stateless Application	App with no internal state; each request handled independently
Headless Service	Service with clusterIP: None — returns Pod IPs directly
Ordinal Index	Ordered number assigned to each StatefulSet Pod (pod-0, pod-1, ...)
PVC	PersistentVolumeClaim — Pod's request for a PersistentVolume
volumeClaimTemplates	Automatically creates a PVC per Pod in a StatefulSet

Stateless vs Stateful

Stateless (Deployment):                Stateful (StatefulSet):
┌──────┐ ┌──────┐ ┌──────┐            ┌──────────┐ ┌──────────┐ ┌──────────┐
│Pod A │ │Pod B │ │Pod C │            │ mysql-0  │ │ mysql-1  │ │ mysql-2  │
│      │ │      │ │      │            │ (Master) │ │ (Slave)  │ │ (Slave)  │
└──────┘ └──────┘ └──────┘            │   PV-0   │ │   PV-1   │ │   PV-2   │
          ↓                           └──────────┘ └──────────┘ └──────────┘
   Shared External DB                       └────── Replication ──────┘

Stateless vs Stateful Comparison

Aspect	Stateless (Deployment)	Stateful (StatefulSet)
Pod Name	Random (nginx-abc123)	Ordered (mysql-0, mysql-1)
Creation Order	Simultaneous	Sequential (0 → 1 → 2)
Deletion Order	Simultaneous	Reverse (2 → 1 → 0)
Network Identity	Unstable (Pod IP changes)	Stable (DNS preserved)
Storage	Shared or none	Dedicated PVC per Pod
Service Type	ClusterIP / LoadBalancer	Headless + ClusterIP
Use Cases	Web servers, APIs	Databases, messaging, distributed storage

Headless Service

Setting clusterIP: None returns Pod IPs directly instead of a VIP.

apiVersion: v1
kind: Service
metadata:
  name: mysql-headless
spec:
  clusterIP: None
  selector:
    app: mysql
  ports:
  - port: 3306

DNS lookup result:

mysql-headless → 10.244.1.10 (mysql-0)
              → 10.244.2.20 (mysql-1)
              → 10.244.3.30 (mysql-2)

Per-Pod DNS (stable across restarts):
mysql-0.mysql-headless.default.svc.cluster.local → 10.244.1.10

Persistent Volume Behavior

• Deleting a StatefulSet does NOT delete PVCs — data is preserved
• Pod recreated → reattaches to existing PVC
• Manual cleanup required: kubectl delete pvc mysql-data-mysql-0

Update Strategies

Strategy	Behavior
RollingUpdate (default)	Sequential update in reverse order (2 → 1 → 0)
OnDelete	No auto-update; new version applied only when Pod is manually deleted
Parallel	All Pods created/deleted simultaneously (when order doesn't matter)

Reference

• ← Back to EN_CS_Knowledge