Deploy ReplicaSet in Kubernetes Cluster

The Nautilus DevOps team is gearing up to deploy applications on a Kubernetes cluster for migration purposes. A team member has been tasked with creating a ReplicaSet outlined below: Create a ReplicaSet using nginx image with latest tag (ensure to specify as nginx:latest) and name it nginx-replicaset. Apply labels: app as nginx_app, type as front-end. Name the container nginx-container. Ensure the replica count is 4. Solution Step 1: Generate the ReplicaSet YAML First, let's generate a base ReplicaSet manifest: kubectl create replicaset nginx-replicaset \ --image=nginx:latest \ --dry-run=client -o yaml > nginx-replicaset.yaml Step 2: Edit the YAML to Add Requirements Open the file and modify it according to the requirements: nano nginx-replicaset.yaml Update the file with: Replica count: 4 Labels: app: nginx_app, type: front-end Container name: nginx-container Here's the complete YAML: apiVersion: apps/v1 kind: ReplicaSet metadata: name: nginx-replicaset labels: app: nginx_app type: front-end spec: replicas: 4 selector: matchLabels: app: nginx_app type: front-end template: metadata: labels: app: nginx_app type: front-end spec: containers: - name: nginx-container image: nginx:latest ports: - containerPort: 80 Step 3: Apply the ReplicaSet Create the ReplicaSet in your cluster: kubectl apply -f nginx-replicaset.yaml Expected output: replicaset.apps/nginx-replicaset created Step 4: Verify the ReplicaSet Check that the ReplicaSet was created successfully: kubectl get replicasets Expected output: NAME DESIRED CURRENT READY AGE nginx-replicaset 4 4 4 10s Step 5: Verify Pods Check that 4 pods were created: kubectl get pods Expected output: NAME READY STATUS RESTARTS AGE nginx-replicaset-xxxxx 1/1 Running 0 15s nginx-replicaset-yyyyy 1/1 Running 0 15s nginx-replicaset-zzzzz 1/1 Running 0 15s nginx-replicaset-wwwww 1/1 Running 0 15s Step 6: Verify Labels Check that the labels are correctly applied: kubectl get replicaset nginx-replicaset --show-labels Expected output: NAME DESIRED CURRENT READY AGE LABELS nginx-replicaset 4 4 4 30s app=nginx_app,type=front-end Step 7: Verify Pod Labels Check that pods have the correct labels: kubectl get pods --show-labels Expected output: NAME READY STATUS RESTARTS AGE LABELS nginx-replicaset-xxxxx 1/1 Running 0 20s app=nginx_app,type=front-end nginx-replicaset-yyyyy 1/1 Running 0 20s app=nginx_app,type=front-end nginx-replicaset-zzzzz 1/1 Running 0 20s app=nginx_app,type=front-end nginx-replicaset-wwwww 1/1 Running 0 20s app=nginx_app,type=front-end Step 8: Detailed Verification Get detailed information about the ReplicaSet: kubectl describe replicaset nginx-replicaset Alternative Methods Method 1: Using kubectl create Directly Create the ReplicaSet with a single command: kubectl create replicaset nginx-replicaset \ --image=nginx:latest \ --replicas=4 \ --labels="app=nginx_app,type=front-end" \ --dry-run=client -o yaml > nginx-replicaset.yaml Then apply it: kubectl apply -f nginx-replicaset.yaml Method 2: Complete YAML from Scratch Create a YAML file named nginx-replicaset.yaml with the complete specification: apiVersion: apps/v1 kind: ReplicaSet metadata: name: nginx-replicaset labels: app: nginx_app type: front-end spec: replicas: 4 selector: matchLabels: app: nginx_app type: front-end template: metadata: labels: app: nginx_app type: front-end spec: containers: - name: nginx-container image: nginx:latest ports: - containerPort: 80 resources: requests: memory: "64Mi" cpu: "100m" limits: memory: "128Mi" cpu: "200m" Apply it: kubectl apply -f nginx-replicaset.yaml Method 3: Using kubectl run (Not Recommended for ReplicaSets) kubectl run nginx-replicaset \ --image=nginx:latest \ --labels="app=nginx_app,type=front-end" \ --port=80 \ --replicas=4 \ --restart=Always \ --dry-run=client -o yaml > nginx-replicaset.yaml Complete Verification Commands 1. Check ReplicaSet Status # Basic status kubectl get replicaset nginx-replicaset # Detailed with labels kubectl get replicaset nginx-replicaset --show-labels # Wide output kubectl get replicaset nginx-replicaset -o wide # Custom columns kubectl get replicaset nginx-replicaset -o custom-columns=NAME:.metadata.name,DESIRED:.spec.replicas,READY:.status.readyReplicas,AGE:.metadata.creationTimestamp 2. Check Pods # List all pods kubectl get pods # Filter pods by label kubectl get pods -l app=nginx_app # Show pod labels kubectl get pods --show-labels # Get pod details kubectl describe pods -l app=nginx_app 3. Check ReplicaSet Details # Describe ReplicaSet kubectl describe replicaset nginx-replicaset # Get YAML output kubectl get replicaset nginx-replicaset -o yaml # Get JSON output kubectl get replicaset nginx-replicaset -o json 4. Verify Image Version # Check image of pods kubectl get pods -l app=nginx_app -o jsonpath='{.items[*].spec.containers[0].image}' # Check image via ReplicaSet kubectl get replicaset nginx-replicaset -o jsonpath='{.spec.template.spec.containers[0].image}' 5. Check Events # Check events related to ReplicaSet kubectl get events --field-selector involvedObject.name=nginx-replicaset # Check events related to pods kubectl get events --field-selector involvedObject.kind=Pod | grep nginx-replicaset Understanding ReplicaSets What is a ReplicaSet? A ReplicaSet ensures that a specified number of pod replicas are running at any given time. It replaces the older ReplicationController with more advanced label selectors. ┌─────────────────────────────────────────────┐ │ ReplicaSet │ │ (nginx-replicaset) │ │ Desired: 4 pods │ │ Labels: app=nginx_app │ │ type=front-end │ └──────────────────┬──────────────────────────┘ │ ┌──────────┼──────────┐ │ │ │ ▼ ▼ ▼ ┌───────┐ ┌───────┐ ┌───────┐ │Pod 1 │ │Pod 2 │ │Pod 3 │ │Running│ │Running│ │Running│ └───────┘ └───────┘ └───────┘ │ ▼ ┌───────┐ │Pod 4 │ │Running│ └───────┘ ReplicaSet vs Deployment Feature ReplicaSet Deployment Purpose Maintain pod count Manage application lifecycle Rolling Updates ❌ No ✅ Yes Rollbacks ❌ No ✅ Yes Scaling ✅ Yes ✅ Yes Self-healing ✅ Yes ✅ Yes Use Case When you need simple replication Most production applications Why Use a ReplicaSet? High Availability: Ensures your application always has the desired number of replicas Self-Healing: Automatically replaces failed pods Scalability: Easy to scale up or down Load Distribution: Distributes traffic across multiple pods Advanced Operations 1. Scaling the ReplicaSet # Scale to 6 replicas kubectl scale replicaset nginx-replicaset --replicas=6 # Verify kubectl get replicaset nginx-replicaset # Check pods kubectl get pods -l app=nginx_app 2. Update Image # Update the ReplicaSet to use a different image kubectl set image replicaset nginx-replicaset nginx-container=nginx:1.21 # Check the updated image kubectl get replicaset nginx-replicaset -o jsonpath='{.spec.template.spec.containers[0].image}' 3. Edit ReplicaSet # Edit the ReplicaSet directly kubectl edit replicaset nginx-replicaset # After editing, the ReplicaSet will start new pods 4. Delete ReplicaSet # Delete the ReplicaSet (and its pods) kubectl delete replicaset nginx-replicaset # Delete using YAML kubectl delete -f nginx-replicaset.yaml # Delete only the ReplicaSet, not the pods kubectl delete replicaset nginx-replicaset --cascade=false Troubleshooting Issue 1: Pods Not Creating Error: ReplicaSet shows 0 pods Solutions: # Check ReplicaSet events kubectl describe replicaset nginx-replicaset # Check if there's a label selector mismatch kubectl get replicaset nginx-replicaset -o yaml | grep -A 5 selector Issue 2: ImagePullBackOff Error: Pods stuck in ImagePullBackOff Solutions: # Check pod details kubectl describe pod -l app=nginx_app # Verify image name kubectl get replicaset nginx-replicaset -o jsonpath='{.spec.template.spec.containers[0].image}' # Update to correct image kubectl set image replicaset nginx-replicaset nginx-container=nginx:latest Issue 3: Pods Not Running Error: Pods in Pending state Solutions: # Check node resources kubectl describe nodes # Check pod events kubectl describe pod -l app=nginx_app # Check if there are resource constraints kubectl get replicaset nginx-replicaset -o yaml | grep -A 5 resources Issue 4: Wrong Container Name Error: Container name doesn't match Solutions: # Check container name kubectl get replicaset nginx-replicaset -o jsonpath='{.spec.template.spec.containers[0].name}' # Edit to correct name kubectl edit replicaset nginx-replicaset Best Practices ✅ DO's Use Deployments Instead of ReplicaSets # Unless you have a specific reason to use ReplicaSet directly # Use Deployments for most use cases Properly Define Selectors selector: matchLabels: app: nginx_app type: front-end Set Resource Limits resources: requests: memory: "64Mi" cpu: "100m" limits: memory: "128Mi" cpu: "200m" Add Health Checks livenessProbe: httpGet: path: / port: 80 initialDelaySeconds: 30 periodSeconds: 10 Use Meaningful Labels labels: app: nginx_app type: front-end environment: production version: "1.0" Document Your ReplicaSet metadata: annotations: description: "Nginx web server for front-end applications" owner: "devops-team" ❌ DON'Ts Don't Use ReplicaSets for Stateful Applications Use StatefulSets instead ReplicaSets are for stateless applications Don't Create ReplicaSets Without Selectors Selectors are required They determine which pods the ReplicaSet manages Don't Directly Modify Pods Managed by ReplicaSets # BAD - Pod will be recreated kubectl delete pod nginx-replicaset-xxxxx # GOOD - Scale the ReplicaSet kubectl scale replicaset nginx-replicaset --replicas=3 Don't Use :latest in Production # BAD - Unpredictable image: nginx:latest # GOOD - Specific version image: nginx:1.21.6 Lab Completion Summary ✅ Task Requirements Checklist [x] ReplicaSet created: nginx-replicaset [x] Image: nginx:latest [x] Labels: app: nginx_app, type: front-end [x] Container name: nginx-container [x] Replica count: 4 [x] All pods running: 4/4 ready Final Verification Commands echo "=== ReplicaSet Status ===" kubectl get replicaset nginx-replicaset echo -e "\n=== Pod Status ===" kubectl get pods -l app=nginx_app echo -e "\n=== Labels ===" kubectl get replicaset nginx-replicaset --show-labels echo -e "\n=== Container Image ===" kubectl get replicaset nginx-replicaset -o jsonpath='{.spec.template.spec.containers[0].image}' echo -e "\n\n=== Container Name ===" kubectl get replicaset nginx-replicaset -o jsonpath='{.spec.template.spec.containers[0].name}' echo -e "\n\n=== Replica Count ===" kubectl get replicaset nginx-replicaset -o jsonpath='{.spec.replicas}' echo -e "\n\n=== All Resources ===" kubectl get all -l app=nginx_app Expected output: === ReplicaSet Status === NAME DESIRED CURRENT READY AGE nginx-replicaset 4 4 4 2m === Pod Status === NAME READY STATUS RESTARTS AGE nginx-replicaset-xxxxx 1/1 Running 0 2m nginx-replicaset-yyyyy 1/1 Running 0 2m nginx-replicaset-zzzzz 1/1 Running 0 2m nginx-replicaset-wwwww 1/1 Running 0 2m === Labels === NAME DESIRED CURRENT READY AGE LABELS nginx-replicaset 4 4 4 2m app=nginx_app,type=front-end === Container Image === nginx:latest === Container Name === nginx-container === Replica Count === 4 Success! 🎉