Chapter 6.4 - StatefulSets and Persistent Storage

Learning Objectives

By the end of this chapter, you will be able to:

• Understand StatefulSets and their characteristics
• Deploy stateful applications with persistent storage
• Understand stable Pod identities
• Use volumeClaimTemplates
• Understand the differences with Deployments
• Manage ordered scaling

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Introduction

Deployments are perfect for stateless applications. For stateful applications like databases, you need StatefulSets.

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What Is a StatefulSet?

A StatefulSet manages deployments and scaling of a set of Pods with:

• Stable identities: Predictable Pod names (web-0, web-1, web-2)
• Persistent storage: Each Pod has its own volume
• Guaranteed ordering: Ordered deployment and scaling
• Headless service: Service without ClusterIP for DNS discovery

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Key Characteristics

1. Stable Identities

Pod Names:

• web-0
• web-1
• web-2

Advantages:

• Stable identity even after restart
• Stable DNS for service discovery
• Storage tied to identity

2. Individual Persistent Storage

Each Pod has its own persistent volume created via volumeClaimTemplates.

3. Guaranteed Ordering

Deployment: Ascending order (0, 1, 2)
Deletion: Descending order (2, 1, 0)
Scaling: Ordered

4. Headless Service

A headless service (without ClusterIP) enables stable DNS discovery.

apiVersion: v1
kind: Service
metadata:
  name: web-service
spec:
  clusterIP: None  # Headless service
  selector:
    app: web
  ports:
  - port: 80

DNS:

• web-0.web-service.default.svc.cluster.local
• web-1.web-service.default.svc.cluster.local
• web-2.web-service.default.svc.cluster.local

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Complete Example

Step 1: Create the Headless Service

apiVersion: v1
kind: Service
metadata:
  name: web-service
spec:
  clusterIP: None
  selector:
    app: web
  ports:
  - port: 80
    name: http

Step 2: Create the StatefulSet

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web-statefulset
spec:
  serviceName: "web-service"  # Must match the headless service
  replicas: 3
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
      - name: nginx
        image: nginx:1.20
        ports:
        - containerPort: 80
          name: http
        volumeMounts:
        - name: www
          mountPath: /usr/share/nginx/html
  volumeClaimTemplates:
  - metadata:
      name: www
    spec:
      accessModes: [ "ReadWriteOnce" ]
      storageClassName: "fast-ssd"
      resources:
        requests:
          storage: 10Gi

Result:

• 3 Pods: web-0, web-1, web-2
• 3 PVCs: www-web-0, www-web-1, www-web-2
• Each Pod has its own volume

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VolumeClaimTemplates

volumeClaimTemplates automatically create a PVC for each Pod.

Characteristics:

• Automatic PVC creation
• Name: {template-name}-{pod-name}
• Each Pod has its own PVC
• PVCs are deleted only if the Pod is deleted

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Differences: Deployment vs StatefulSet

Characteristic	Deployment	StatefulSet
Identity	Random (random-abc123)	Stable (web-0, web-1)
Storage	Shared or ephemeral	Individual persistent
Scaling	Random	Guaranteed order
Service	Normal ClusterIP	Headless (DNS)
Use Case	Stateless apps	Stateful apps (DB, etc.)
Restart	New name	Same name

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Use Cases

1. Databases

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: postgres
spec:
  serviceName: postgres
  replicas: 3
  template:
    spec:
      containers:
      - name: postgres
        image: postgres:14
        env:
        - name: PGDATA
          value: /var/lib/postgresql/data/pgdata
        volumeMounts:
        - name: postgres-data
          mountPath: /var/lib/postgresql/data
  volumeClaimTemplates:
  - metadata:
      name: postgres-data
    spec:
      accessModes: [ "ReadWriteOnce" ]
      storageClassName: "fast-ssd"
      resources:
        requests:
          storage: 50Gi

2. Stateful Applications

• Distributed file systems
• Caches with persistent data
• Applications with stable identities

3. Services Requiring Ordering

• Master-slave replication
• Applications with quorum
• Services with leader election

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Scaling

Scaling Up

kubectl scale statefulset web-statefulset --replicas=5

Result: Creates web-3 then web-4 in order.

Scaling Down

kubectl scale statefulset web-statefulset --replicas=2

Result: Deletes web-2 then web-1 in descending order.

Important: PVCs are not automatically deleted. Delete them manually if needed.

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Updates

Rolling Update

spec:
  updateStrategy:
    type: RollingUpdate
  template:
    spec:
      containers:
      - name: nginx
        image: nginx:1.21  # New version

Process:

1. Update web-2 (last)
2. Update web-1
3. Update web-0 (first)

OnDelete

spec:
  updateStrategy:
    type: OnDelete

Process: Updates only when a Pod is manually deleted.

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Useful Commands

Create and Manage

# Create a StatefulSet
kubectl apply -f statefulset.yaml

# View StatefulSets
kubectl get statefulset
kubectl get sts

# View Pods
kubectl get pods -l app=web

# Details
kubectl describe statefulset web-statefulset

Scaling

# Increase replicas
kubectl scale statefulset web-statefulset --replicas=5

# Decrease replicas
kubectl scale statefulset web-statefulset --replicas=2

View Volumes

# View created PVCs
kubectl get pvc

# View details
kubectl describe pvc www-web-0

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Best Practices

1. Use for Stateful Applications

StatefulSets only for stateful applications. Use Deployments for stateless.

2. Headless Service Required

Always create a headless service for DNS discovery.

3. Appropriate Storage

Choose the right access mode (RWO for databases).

4. Backups

Set up regular volume backups.

5. Monitoring

Monitor the state and performance of each Pod.

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Summary

In this chapter, you learned:

StatefulSet: Manages stateful applications with stable identities
Characteristics: Predictable names, individual volumes, guaranteed ordering, headless service
volumeClaimTemplates: Automatic PVC creation per Pod
Use cases: Databases, stateful applications, ordered services
Differences: Stable vs random identity, individual vs shared storage
Scaling: Guaranteed ordering (ascending for up, descending for down)
Updates: RollingUpdate (reverse) or OnDelete

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Next Steps

Module 7: Advanced Workloads (in-depth StatefulSets, DaemonSets, Jobs)
Lab 6.4: StatefulSet with Persistent Storage

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Chapter created: December 2024