Mastering Kubernetes ReplicaSets and Deployments

In this hands-on guide, we’ll explore Kubernetes ReplicaSets and Deployments, two crucial concepts for managing containerized applications at scale. We’ll go through practical exercises and troubleshooting scenarios to deepen our understanding of these Kubernetes objects.

ReplicaSet Exercises

Let’s start with some exercises involving ReplicaSets.

1. Create a ReplicaSet

First, we’ll create a ReplicaSet based on the nginx image with 3 replicas:

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

Save this as nginx-replicaset.yaml and apply it:

kubectl apply -f nginx-replicaset.yaml

2. Update Replicas in YAML

To update the replicas to 4, modify the replicas field in the YAML file:

spec:
  replicas: 4

Apply the changes:

kubectl apply -f nginx-replicaset.yaml

3. Update Replicas via Command Line

To scale the ReplicaSet to 6 replicas using the command line:

kubectl scale replicaset nginx-replicaset --replicas=6

Deployment Exercises

Now, let’s move on to Deployments, which provide declarative updates for Pods and ReplicaSets.

1. Create a Deployment

Create a Deployment named nginx with 3 replicas:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx
  labels:
    tier: backend
spec:
  replicas: 3
  selector:
    matchLabels:
      app: v1
  template:
    metadata:
      labels:
        app: v1
    spec:
      containers:
        - name: nginx
          image: nginx:1.23.0

Save this as nginx-deployment.yaml and apply it:

kubectl apply -f nginx-deployment.yaml

2. List the Deployment

Verify the Deployment:

kubectl get deployments

3. Update the Image

Update the image to nginx:1.23.4:

kubectl set image deployment/nginx nginx=nginx:1.23.4

4. Verify the Rollout

Check the rollout status:

kubectl rollout status deployment/nginx

5. Assign Change Cause

Record the change cause:

kubectl annotate deployment/nginx kubernetes.io/change-cause="Pick up patch version"

6. Scale the Deployment

Scale to 5 replicas:

kubectl scale deployment/nginx --replicas=5

7. View Rollout History

Check the rollout history:

kubectl rollout history deployment/nginx

8. Rollback to Revision 1

Rollback to the first revision:

kubectl rollout undo deployment/nginx --to-revision=1

9. Verify Image Version

Check the current image version:

kubectl describe deployment nginx | grep Image:

Troubleshooting Exercises

Let’s troubleshoot some common issues with Deployments.

Issue 1: Invalid Kind

Apply the following YAML:

apiVersion: v1
kind: Deployment
metadata:
  name: nginx-deploy
  labels:
    env: demo
spec:
  template:
    metadata:
      labels:
        env: demo
      name: nginx
    spec:
      containers:
        - image: nginx
          name: nginx
          ports:
            - containerPort: 80
  replicas: 3
  selector:
    matchLabels:
      env: demo

You’ll encounter an error because the apiVersion and kind don’t match. To fix this:

1. Change apiVersion: v1 to apiVersion: apps/v1
2. Apply the corrected YAML

Issue 2: Mismatched Labels

Apply the following YAML:

apiVersion: v1
kind: Deployment
metadata:
  name: nginx-deploy
  labels:
    env: demo
spec:
  template:
    metadata:
      labels:
        env: demo
      name: nginx
    spec:
      containers:
        - image: nginx
          name: nginx
          ports:
            - containerPort: 80
  replicas: 3
  selector:
    matchLabels:
      env: dev

This will create a Deployment, but it won’t be able to manage any Pods due to mismatched labels. To fix this:

1. Change the selector.matchLabels.env from dev to demo to match the Pod template labels
2. Change apiVersion: v1 to apiVersion: apps/v1
3. Apply the corrected YAML

Conclusion

Through these exercises, we’ve explored the creation and management of ReplicaSets and Deployments in Kubernetes. We’ve also practiced troubleshooting common issues that can arise when working with these objects. Remember, proper label management and correct API versions are crucial for the smooth operation of your Kubernetes resources.

References

1. Kubernetes ReplicaSet Documentation
2. Kubernetes Deployment Documentation
3. Kubernetes Cheat Sheet