Module 4 :Pods, ReplicaSets & Multi-Container Pods

🔹 Content Outline

1. Introduction to Pods

   • Smallest deployable unit in Kubernetes

   • Pod lifecycle & states

   • YAML example of a single pod

2. ReplicaSets

   • Why ReplicaSets are needed

   • Ensuring high availability

   • Scaling pods using ReplicaSets

   • YAML example with replicas

3. Multi-Container Pods

   • Sidecar pattern (logging, monitoring)

   • Ambassador & Adapter patterns

   • Example of 2 containers in one pod (nginx + sidecar container)

4. Hands-On Examples

   • kubectl apply -f pod.yaml

   • kubectl scale rs myapp-rs --replicas=3

   • Example of multi-container pod

5. Troubleshooting Tips

   • Check pod logs: kubectl logs pod-name

   • Describe pod for issues: kubectl describe pod pod-name

   • Common errors (ImagePullBackOff, CrashLoopBackOff)

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🔹 What is a Pod?

• A Pod is the smallest deployable unit in Kubernetes.

• It represents one or more containers that run together on a worker node.

• All containers in a Pod share:

  • The same network namespace (same IP address & port space)

  • Storage volumes (if defined)

  • Communication through localhost

👉 You don’t deploy containers directly in Kubernetes; you deploy Pods.

Example: Simple Nginx Pod

    apiVersion: v1
    kind: Pod
    metadata:
      name: nginx-pod
      labels:
        app: nginx
    spec:
      containers:
        - name: nginx-container
          image: nginx:latest
          ports:
            - containerPort: 80

Apply it:

    kubectl apply -f nginx-pod.yaml
    kubectl get pods

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🔹 What is a ReplicaSet?

A ReplicaSet ensures a specified number of pod replicas are running at all times.

• If a Pod crashes, ReplicaSet automatically replaces it.

• It allows scaling up or down.

Example: ReplicaSet with 3 Nginx Pods

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

Apply it:

    kubectl apply -f nginx-rs.yaml
    kubectl get rs
    kubectl get pods

👉 Scale it dynamically:

    kubectl scale rs nginx-rs --replicas=5

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🔹 Multi-Container Pods

Sometimes you want multiple containers inside a single pod that work closely together. They share the same network & storage, and communicate via localhost.

Common Patterns

• Sidecar – helper container (e.g., log collector with main app)

• Ambassador – proxy container to route traffic

• Adapter – transforms data for the main container

Example: Multi-Container Pod (Nginx + Sidecar Logger)

    apiVersion: v1
    kind: Pod
    metadata:
      name: nginx-multicontainer
    spec:
      containers:
        - name: nginx
          image: nginx:latest
          ports:
            - containerPort: 80
        - name: sidecar
          image: busybox
          command: ["sh", "-c", "while true; do echo Sidecar logging...; sleep 5; done"]

Apply it:

    kubectl apply -f multi-container.yaml
    kubectl logs nginx-multicontainer -c sidecar

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🔹 Troubleshooting Pods

• Check logs:

    kubectl logs pod-name
  

• Describe pod for events:

    kubectl describe pod pod-name
  

• Common issues:

  • ImagePullBackOff → wrong image name or no registry access

  • CrashLoopBackOff → application keeps crashing

  • Pending → not enough resources

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✅ Summary

• Pod = smallest unit, runs 1+ containers.

• ReplicaSet = keeps desired number of Pods running.

• Multi-Container Pods = useful for patterns like Sidecars & Ambassadors.

• Learned hands-on YAML + kubectl commands.

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