Module 6: Multi-Container Applications

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🔹 Why Multi-Container Setups Are Needed

In real-world applications, one container is rarely enough.

• A web app might need a database.

• A backend service might rely on a cache like Redis.

• Splitting services makes applications more scalable, modular, and easier to maintain.

Instead of packing everything into one container, we run multiple containers that work together.

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🔹 Linking Services Manually

Before Docker Compose, containers were linked manually:

# Run MySQL container
docker run -d --name mysql-db -e MYSQL_ROOT_PASSWORD=root mysql:8.0

# Run WordPress container linked to MySQL
docker run -d --name wordpress --link mysql-db:mysql -p 8080:80 wordpress

👉 But this approach doesn’t scale well for large projects.

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🔹 Writing a Simple docker-compose.yml

docker-compose makes managing multi-container applications easy.
Here’s an example docker-compose.yml:

version: '3.8'
services:
  db:
    image: mysql:8.0
    restart: always
    environment:
      MYSQL_DATABASE: wordpress
      MYSQL_USER: wp_user
      MYSQL_PASSWORD: wp_pass
      MYSQL_ROOT_PASSWORD: root
    volumes:
      - db_data:/var/lib/mysql

  wordpress:
    image: wordpress:latest
    restart: always
    ports:
      - "8080:80"
    environment:
      WORDPRESS_DB_HOST: db:3306
      WORDPRESS_DB_USER: wp_user
      WORDPRESS_DB_PASSWORD: wp_pass
      WORDPRESS_DB_NAME: wordpress
    depends_on:
      - db

volumes:
  db_data:

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🔹 Example: WordPress + MySQL Using Docker Compose

1. Save the above YAML file as docker-compose.yml.

2. Run:

    docker-compose up -d
   

3. Open browser → http://localhost:8080

4. You’ll see WordPress setup screen, connected to MySQL automatically.

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✅ Key Benefits of Multi-Container Apps

• Clear separation of concerns

• Easier scaling of individual services

• Faster debugging & upgrades

• Production-ready with minimal changes

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