Multi-Container Pod Design Patterns

Auteur: Fabrice JAMMES (LinkedIn). Date: Nov 24, 2025 · 10 min read

Quick Exercise: Fix the Bugs! 🐛

Objective

Create a pod with an init container and a sidecar - There are 2 bugs to fix!

Scenario

  • Init container: generates an index.html with system info
  • Main container: nginx 1.25.3 web server
  • Sidecar: counts requests in access logs every 10 seconds

Task

Deploy the following pod and fix the errors:

apiVersion: v1
kind: Pod
metadata:
  name: webapp-demo
spec:
  initContainers:
  - name: setup-content
    image: busybox:1.37.0
    command:
    - sh
    - -c
    - |
      echo "Creating /web/index.html"
      echo "<h1>Welcome!</h1>" > /web/index.html
      echo "<p>Pod: $HOSTNAME</p>" >> /web/index.html
      echo "<p>Generated at: $(date)</p>" >> /web/index.html
    volumeMounts:
    - name: html-content
      mountPath: /web

  containers:
  - name: nginx
    image: nginx:1.25.3
    ports:
    - containerPort: 80
    volumeMounts:
    - name: web-content
      mountPath: /usr/share/nginx/html
    - name: logs
      mountPath: /var/log/nginx

  - name: request-counter
    image: busybox:1.37.0
    command:
    - sh
    - -c
    - |
      while true; do
        echo "=== $(date) ==="
        echo "Total requests: $(wc -l < /logs/access.log 2>/dev/null || echo 0)"
        sleep 10
      done
    volumeMounts:
    - name: nginx-logs
      mountPath: /logs

  volumes:
  - name: web-content
    emptyDir: {}
  - name: logs
    emptyDir: {}

Questions

  1. Find the 2 bugs (hint: check volume names carefully!)
  2. What happens when you try to deploy this pod?
  3. How do you debug init container failures?
  4. Once fixed, how can you verify:
    • The init container completed successfully?
    • The nginx server is running?
    • The sidecar is counting requests?

Debugging Commands

# Deploy the pod
kubectl apply -f webapp-demo.yaml

# Check pod status
kubectl get pod webapp-demo
Troubleshooting 
# Check init container logs
kubectl logs webapp-demo -c setup-content

# Check nginx logs
kubectl logs webapp-demo -c nginx

# Check sidecar logs
kubectl logs webapp-demo -c request-counter

# Describe the pod to see events
kubectl describe pod webapp-demo

# Port-forward to test nginx (once fixed)
kubectl port-forward webapp-demo 808<ID>:80

# Test the web server
curl localhost:808<ID>

Expected Behavior (After Fix)

  1. Init container runs first and creates /web/index.html
  2. Nginx starts and serves the HTML file
  3. Sidecar continuously counts requests every 10 seconds
  4. All containers share volumes properly
Solution

Bugs Found

  1. Init container bug: Uses volume name html-content but the volume is defined as web-content
  2. Sidecar bug: Uses volume name nginx-logs but the volume is defined as logs

Corrected YAML

apiVersion: v1
kind: Pod
metadata:
  name: webapp-demo
spec:
  initContainers:
  - name: setup-content
    image: busybox:1.37.0
    command:
    - sh
    - -c
    - |
      echo "Creating /web/index.html"
      echo "<h1>Welcome!</h1>" > /web/index.html
      echo "<p>Pod: $HOSTNAME</p>" >> /web/index.html
      echo "<p>Generated at: $(date)</p>" >> /web/index.html
    volumeMounts:
    - name: web-content          # ✅ Fixed: was html-content
      mountPath: /web

  containers:
  - name: nginx
    image: nginx:1.25.3
    ports:
    - containerPort: 80
    volumeMounts:
    - name: web-content
      mountPath: /usr/share/nginx/html
    - name: logs
      mountPath: /var/log/nginx

  - name: request-counter
    image: busybox:1.37.0
    command:
    - sh
    - -c
    - |
      while true; do
        echo "=== $(date) ==="
        echo "Total requests: $(wc -l < /logs/access.log 2>/dev/null || echo 0)"
        sleep 10
      done
    volumeMounts:
    - name: logs                 # ✅ Fixed: was nginx-logs
      mountPath: /logs

  volumes:
  - name: web-content
    emptyDir: {}
  - name: logs
    emptyDir: {}

Key Learning Points

  • Volume names must match exactly between volumeMounts and volumes definitions
  • Init containers must complete before main containers start
  • Multiple containers in a pod share the same volumes
  • Use kubectl describe pod to identify volume mount errors
  • Init container failures prevent the pod from starting