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Kubernetes ReplicaSets

What is a ReplicaSet?​

  • Next-generation version of ReplicationController
  • Handles replication and self-healing of applications
  • Supports both equality-based AND set-based selectors (ReplicationControllers only support equality-based)

Why Use ReplicaSets?​

Problem: Single app instance can crash → affects other apps/services

Solution: Run multiple identical instances in parallel = high availability

ReplicaSet oversees the lifecycle of Pods running your application

How It Works​

  1. You set a replica count (example: 3 Pods)
  2. ReplicaSet creates identical Pods from the same template
  3. Each Pod runs the same app but has unique name and IP address
  4. Can scale manually or with autoscaler

Key Features​

  • Scaling - Increase/decrease number of running Pods
  • Self-healing - Replaces crashed Pods automatically
  • Load distribution - Pods can be placed on different worker nodes

YAML Structure​

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: frontend
spec:
  replicas: 3
  selector:
    matchLabels:
      app: guestbook
  template:
    metadata:
      labels:
        app: guestbook
    spec:
      containers:
      - name: frontend
        image: nginx:1.22.1
        ports:
        - containerPort: 80

Common Commands​

Create ReplicaSet​

# Create from YAML file
kubectl create -f redis-rs.yaml
kubectl apply -f redis-rs.yaml

View ReplicaSets​

# List all ReplicaSets
kubectl get replicasets
kubectl get rs  # Short form

# Get detailed view
kubectl get rs -o wide

# Get YAML/JSON output
kubectl get rs frontend -o yaml
kubectl get rs frontend -o json

Scale ReplicaSet​

# Scale to specific number of replicas
kubectl scale rs frontend --replicas=4

# Scale using YAML file
kubectl scale -f redis-rs.yaml --replicas=5

Get Details​

# Describe ReplicaSet
kubectl describe rs frontend

# Get ReplicaSet events
kubectl get events --field-selector involvedObject.name=frontend

Delete ReplicaSet​

# Delete ReplicaSet (and its Pods)
kubectl delete rs frontend

# Delete using YAML file
kubectl delete -f redis-rs.yaml

# Delete ReplicaSet but keep Pods
kubectl delete rs frontend --cascade=orphan

Current State vs Desired State​

  • ReplicaSet maintains desired number of Pod replicas
  • If a Pod crashes, ReplicaSet creates a new one to match desired count

Practical Examples​

Complete ReplicaSet YAML​

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: nginx-replicaset
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.22.1
        ports:
        - containerPort: 80
        resources:
          requests:
            memory: "64Mi"
            cpu: "250m"
          limits:
            memory: "128Mi"
            cpu: "500m"

Set-based Selector Example​

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: advanced-rs
spec:
  replicas: 2
  selector:
    matchExpressions:
    - key: environment
      operator: In
      values:
      - production
      - staging
    - key: tier
      operator: NotIn
      values:
      - frontend
  template:
    metadata:
      labels:
        environment: production
        tier: backend
    spec:
      containers:
      - name: app
        image: nginx:1.22.1

Monitoring ReplicaSet​

# Watch ReplicaSet status in real-time
kubectl get rs -w

# Check ReplicaSet and its Pods
kubectl get rs,pods -l app=nginx

# Monitor scaling operation
kubectl scale rs nginx-replicaset --replicas=5 && kubectl get rs nginx-replicaset -w

Troubleshooting Commands​

# Check why Pods aren't starting
kubectl describe rs nginx-replicaset
kubectl describe pods -l app=nginx

# Check ReplicaSet logs (through its Pods)
kubectl logs -l app=nginx

# Get ReplicaSet status
kubectl get rs nginx-replicaset -o jsonpath='{.status}'

Important Note​

ReplicaSets do important work (maintaining Pod replicas), but you will typically use Deployments which handle ReplicaSets automatically behind the scenes. Deployments provide additional features like rolling updates and rollback capabilities.