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Lab 3.2 - Deploying with ReplicaSets

Objectives​

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

  • Create and manage ReplicaSets
  • Understand self-healing
  • Scale ReplicaSets
  • Observe ReplicaSet behavior

Exercise 1: Create a ReplicaSet​

Step 1: Create the YAML file​

Create replicaset.yaml:

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: web-replicaset
  labels:
    app: web
spec:
  replicas: 3
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
      - name: nginx
        image: nginx:1.20
        ports:
        - containerPort: 80

Step 2: Apply​

kubectl apply -f replicaset.yaml

Step 3: Verify​

# View the ReplicaSet
kubectl get replicaset

# View the created Pods
kubectl get pods -l app=web

Exercise 2: Self-Healing​

Step 1: Delete a Pod​

# Delete a Pod
kubectl delete pod <pod-name>

# Observe immediately
kubectl get pods -l app=web -w

Question: What happens? A new Pod is automatically created.

Step 2: Check the ReplicaSet​

kubectl describe replicaset web-replicaset

Exercise 3: Scaling​

Step 1: Increase replicas​

# Scale to 5 replicas
kubectl scale replicaset web-replicaset --replicas=5

# Verify
kubectl get pods -l app=web

Step 2: Decrease replicas​

# Scale to 2 replicas
kubectl scale replicaset web-replicaset --replicas=2

# Observe the deletion
kubectl get pods -l app=web -w

Exercise 4: Modify the Template​

Step 1: Edit the ReplicaSet​

kubectl edit replicaset web-replicaset

Change the image from nginx:1.20 to nginx:1.21.

Question: What happens? Existing Pods do not change. Why?

Step 2: Delete the Pods​

# Delete all Pods
kubectl delete pods -l app=web

# Observe the recreation
kubectl get pods -l app=web

The new Pods use the new image.

Cleanup​

kubectl delete replicaset web-replicaset

Reflection Questions​

  1. Why does the ReplicaSet not update existing Pods?
  2. How does the ReplicaSet maintain the number of replicas?
  3. What is the difference between a ReplicaSet and a Deployment?

Next Steps​

Lab 3.3: Deployments and Rolling Updates
Chapter 3.6: Deployments

Lab created: December 2024