Deploy a Multi-Container Application¶

Goal¶

In this tutorial you will deploy an application that runs multiple containers in a single pod.

By the end you will understand:

how containers share a pod
how containers communicate via localhost
how the sidecar pattern works

Problem¶

Some applications require multiple cooperating containers.

Examples include:

application + logging agent
application + metrics exporter
application + proxy
application + file processor

Instead of running these separately, Kubernetes allows multiple containers in one pod.

All containers inside a pod:

share the same network
share volumes
start and stop together

Architecture¶

Traffic flow in this example:

User → Service → Pod
               ├─ App Container
               └─ Logger Container

The application writes logs to a shared directory, and a second container reads those logs.

This pattern is called a sidecar container.

Prerequisites¶

Ensure the following are installed and configured:

Kubernetes cluster
kubectl configured
a working namespace

Verify cluster access:

kubectl get nodes

Expected output:

NAME           STATUS   ROLES
worker-node1   Ready    <none>

Step 1 : Create a Multi-Container Pod¶

Create the file:

multi-container-pod.yaml

apiVersion: v1
kind: Pod
metadata:
  name: multi-container-demo
spec:
  volumes:
    - name: shared-data
      emptyDir: {}

  containers:
    - name: app
      image: busybox
      command: ["/bin/sh", "-c"]
      args:
        - while true; do
            echo "Hello from app container" >> /data/app.log;
            sleep 5;
          done
      volumeMounts:
        - name: shared-data
          mountPath: /data

    - name: logger
      image: busybox
      command: ["/bin/sh", "-c"]
      args:
        - tail -f /data/app.log
      volumeMounts:
        - name: shared-data
          mountPath: /data

This pod contains two containers:

Container	Purpose
app	writes log entries
logger	reads and prints logs

The shared volume allows both containers to access the same data.

Step 2 : Deploy the Pod¶

Apply the configuration.

kubectl apply -f multi-container-pod.yaml

Verify the pod is running.

kubectl get pods

Expected output:

NAME                    READY   STATUS
multi-container-demo    2/2     Running

Notice 2/2 containers are running.

Step 3 : Inspect the Pod¶

View details about the pod.

kubectl describe pod multi-container-demo

Look for the containers section.

You should see:

Containers:
  app
  logger

Step 4 : View Logs from the Logger Container¶

Check logs from the logger container.

kubectl logs multi-container-demo -c logger

Expected output:

Hello from app container
Hello from app container
Hello from app container

The logger container is reading the log file written by the app container.

Step 5 : Access the App Container¶

Open a shell in the app container.

kubectl exec -it multi-container-demo -c app -- sh

View the log file.

cat /data/app.log

You should see entries being written continuously.

What Just Happened¶

The two containers share:

Resource	Description
Network	both containers use localhost
Storage	shared emptyDir volume
Lifecycle	containers start and stop together

The app container writes logs, and the logger container reads them.

This architecture is called a sidecar pattern.

Common Sidecar Use Cases¶

Sidecars are widely used in production clusters.

Examples include:

Use Case	Example
Logging	Fluentd sidecar
Metrics	Prometheus exporters
Proxies	Envoy / Istio
Security	service mesh proxies

Troubleshooting¶

Pod Not Starting

Check pod status.

kubectl get pods

Inspect events.

kubectl describe pod multi-container-demo

Container Logs Not Appearing Ensure the logger container is running.

kubectl get pods

Then check container logs.

kubectl logs multi-container-demo -c logger

Shared Volume Not Working

Confirm both containers mount the same volume.

Look for:

volumeMounts:
  - name: shared-data

Clean Up¶

Delete the pod when finished.

kubectl delete pod multi-container-demo

Key Takeaways¶

You learned how to:

deploy a pod with multiple containers
share data between containers using volumes
inspect individual container logs
implement the sidecar pattern

Multi-container pods are a foundational pattern used in many Kubernetes architectures.

→ Continue to Part 2: Configure App With Configmaps