Introduction

Kubernetes is a powerful container orchestration platform that automates application deployment, scaling, and management. In this blog, I will walk through an exercise covering ReplicaSets, Deployments, updating pods, rollout history, and troubleshooting issues in Kubernetes. These concepts are crucial for maintaining high availability and reliability in production environments.

Step 1: Creating and Updating a ReplicaSet

A ReplicaSet ensures that a specified number of pod replicas are running at any given time.

Creating a ReplicaSet with 3 Replicas

To create a ReplicaSet with 3 replicas of nginx, apply the following YAML:

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
        ports:
        - containerPort: 80

Apply the file:

kubectl apply -f nginx-replicaset.yaml

Updating the Replicas

Update replicas to 4 from the YAML file:

Modify the replicas field to 4 and apply the change:

kubectl apply -f nginx-replicaset.yaml

Update replicas to 6 from the command line:

kubectl scale rs nginx-replicaset --replicas=6

Verify the changes:

kubectl get rs

Step 2: Creating and Managing a Deployment

A Deployment provides declarative updates for Pods and ReplicaSets. It helps in rolling updates and rollbacks, ensuring zero downtime.

Creating a Deployment with 3 Replicas

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    tier: backend
spec:
  replicas: 3
  selector:
    matchLabels:
      app: v1
  template:
    metadata:
      labels:
        app: v1
    spec:
      containers:
      - name: nginx
        image: nginx:1.23.0
        ports:
        - containerPort: 80

Apply the Deployment:

kubectl apply -f nginx-deployment.yaml

Verify that the correct number of replicas are running:

kubectl get deployments

Updating the Image Version

We now update the image to nginx:1.23.4:

kubectl set image deployment/nginx-deployment nginx=nginx:1.23.4

Confirm that all replicas have updated successfully:

kubectl rollout status deployment/nginx-deployment

Assign a change cause:

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

Scaling the Deployment to 5 Replicas

kubectl scale deployment nginx-deployment --replicas=5

Checking Deployment Rollout History

kubectl rollout history deployment/nginx-deployment

Rolling Back to Revision 1

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

Ensure the rollback by verifying the image version:

kubectl describe deployment nginx-deployment | grep Image

Step 3: Troubleshooting Deployment Issues

Issue 1: Missing selector Field

Problem YAML:

apiVersion: apps/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

Fix: The selector field must match the labels used in the template:

  selector:
    matchLabels:
      env: demo

Issue 2: Mismatched Labels

Problem YAML:

apiVersion: apps/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

Fix: The labels inside the selector must match the ones in template.metadata.labels:

  selector:
    matchLabels:
      env: demo

Key Takeaways

1. ReplicaSets maintain a stable set of pods.

2. Deployments allow declarative updates and rollbacks.

3. Scaling can be done via YAML or command line.

4. Rollouts ensure smooth updates.

5. Rollbacks help in reverting faulty updates.

6. Selectors and Labels must always match to avoid issues.

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