Docker Compose — Troubleshooting — Practice Exercises

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Practice diagnosing and fixing common Docker Compose problems.

Exercise 1: Fix YAML Error

Find and fix the error in this compose file:

services:
  api:
    image: node:20-alpine
    ports:
    - "3000:3000"
    environment:
      NODE_ENV: production
Solution

The ports: list items need proper indentation:

services:
  api:
    image: node:20-alpine
    ports:
      - "3000:3000"
    environment:
      NODE_ENV: production

Use docker compose config -q to validate.

Exercise 2: Port Conflict

You get “address already in use” on port 3000. Diagnose and fix it.

Solution 
# Find what's using port 3000
lsof -i :3000

# Option 1: Kill the process
kill $(lsof -t -i :3000)

# Option 2: Use a different port
# Change compose.yml: "3001:3000"

Exercise 3: DNS Resolution Failure

Container web can’t reach api. Diagnose why.

Solution

Check if they share a network:

docker compose exec web nslookup api

If it fails, ensure both services are on the same network:

services:
  web:
    networks: [app]
  api:
    networks: [app]
networks:
  app:

Exercise 4: Container Keeps Restarting

A service shows “Restarting” in docker compose ps. Diagnose it.

Solution 
# Check logs for crash reason
docker compose logs --tail 50 <service>

# Override entrypoint to investigate
docker compose run --rm --entrypoint sh <service>

# Check if a dependency is missing
docker compose ps -a

Exercise 5: Volume Permission Error

A container can’t write to a mounted volume. Fix it.

Solution 
# Check ownership
ls -la ./data

# Fix permissions
sudo chown -R 1000:1000 ./data

# Or set user in compose

services:
  api:
    user: "1000:1000"
    volumes:
      - ./data:/app/data

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