Mastering Docker: A Complete Guide to Containerization and Modern DevOps

📘 Chapter 3: Docker Compose and Multi-Container Apps (Part 1 of 3)

🧠 What is Docker Compose?

Docker Compose is a tool for defining and managing multi-container Docker applications using a single file, typically named docker-compose.yml.

Instead of running multiple docker run commands manually for each service (web app, database, cache, etc.), Compose lets you configure everything in one place and start the entire stack with a single command:

bash

docker-compose up

⚙️ When and Why to Use Docker Compose

📂 Structure of a Docker Compose File

The docker-compose.yml file is written in YAML and defines:

• The services (containers) to run
• Their networks, volumes, and environment
• Port mappings, dependencies, and build contexts

🔹 Basic Syntax Structure

yaml

version: '3'

services:

  service_name:

    image: image_name

    build:

      context: .

    ports:

      - "host_port:container_port"

    environment:

      - VAR=value

    volumes:

      - ./data:/app/data

    depends_on:

      - another_service

🛠️ Example: Node.js App with MongoDB

Let’s say we have:

• A Node.js backend
• A MongoDB database
• We want both to run together in isolated containers

🔹 Folder Structure

pgsql

project/

├── docker-compose.yml

├── app/

│   ├── Dockerfile

│   ├── server.js

│   └── package.json

🔹 Dockerfile (inside app/ folder)

Dockerfile

FROM node:18

WORKDIR /app

COPY package*.json ./

RUN npm install

COPY . .

CMD ["node", "server.js"]

🔹 docker-compose.yml (root level)

yaml

version: '3.8'

services:

  web:

    build: ./app

    ports:

      - "3000:3000"

    depends_on:

      - mongo

    environment:

      - MONGO_URL=mongodb://mongo:27017/mydb

  mongo:

    image: mongo

    volumes:

      - mongo-data:/data/db

volumes:

  mongo-data:

🔍 Breakdown of Services

▶️ Running the Stack

In the terminal:

bash

docker-compose up

This will:

• Build the Node.js image
• Pull the MongoDB image
• Start both services
• Set up a private network so web can talk to mongo

You can visit http://localhost:3000 (assuming your app serves on port 3000).

📍 Verifying Service Status

To see running containers:

bash

docker-compose ps

To stop and remove all containers:

bash

docker-compose down

To rebuild containers:

bash

docker-compose up --build

🌐 Networking in Docker Compose

By default, Docker Compose creates a single network for all defined services. This enables containers to communicate with each other using service names as hostnames.

🔹 Example: web talks to mongo

In our earlier setup:

yaml

environment:

  - MONGO_URL=mongodb://mongo:27017/mydb

The web service can access MongoDB at mongo:27017 (service name = hostname), thanks to Docker’s internal DNS.

🔹 Inspecting the Network

bash

docker network ls

docker network inspect project_default

This reveals which containers are connected and how they resolve names.

🔹 Defining Custom Networks (Optional)

yaml

networks:

  mynet:

    driver: bridge

services:

  web:

    ...

    networks:

      - mynet

  mongo:

    ...

    networks:

      - mynet

🔐 Custom networks allow advanced isolation, shared services, or inter-project linking.

🌱 Using Environment Variables and .env Files

Instead of hardcoding variables in your docker-compose.yml, you can externalize them.

🔹 Creating a .env File

ini

PORT=3000

MONGO_HOST=mongo

DB_NAME=mydb

🔹 Reference in Compose File

yaml

environment:

  - MONGO_URL=mongodb://${MONGO_HOST}:27017/${DB_NAME}

ports:

  - "${PORT}:${PORT}"

📌 Docker Compose automatically reads .env from the same directory as your YAML file.

💾 Managing Persistent Data with Volumes

Containers are ephemeral. Volumes allow data to persist even after container restarts.

🔹 Named Volumes in Compose

yaml

services:

  mongo:

    image: mongo

    volumes:

      - mongo-data:/data/db

volumes:

  mongo-data:

🔹 Bind Mounts (Local Folder to Container)

yaml

services:

  web:

    volumes:

      - ./app:/app

🔍 View Volumes

bash

docker volume ls

docker volume inspect mongo-data

📈 Scaling Services

Docker Compose lets you scale any service that runs the same container image.

🔹 Example: Scale 3 web containers

bash

docker-compose up --scale web=3

All web containers will share the same network and load can be distributed (e.g., with Nginx or Traefik as reverse proxy).

🧠 Tips on Scaling

• Only works for stateless services (e.g., web servers)
• Use reverse proxy or service discovery for balancing
• Stateful services like databases should NOT be scaled this way

🚨 Restart Policies

You can make services restart automatically after crashes:

yaml

restart: always

Options:

• no (default)
• on-failure
• always
• unless-stopped

🌍 Real-World Example #1: WordPress + MySQL

Let’s run a full WordPress site locally with just Docker Compose!

🔹 docker-compose.yml

yaml

version: '3.8'

services:

  db:

    image: mysql:5.7

    restart: always

    environment:

      MYSQL_DATABASE: wordpress

      MYSQL_USER: user

      MYSQL_PASSWORD: secret

      MYSQL_ROOT_PASSWORD: rootpass

    volumes:

      - db_data:/var/lib/mysql

  wordpress:

    image: wordpress:latest

    restart: always

    ports:

      - "8080:80"

    environment:

      WORDPRESS_DB_HOST: db:3306

      WORDPRESS_DB_USER: user

      WORDPRESS_DB_PASSWORD: secret

      WORDPRESS_DB_NAME: wordpress

    depends_on:

      - db

volumes:

  db_data:

▶️ Start Everything

bash

docker-compose up -d

Visit http://localhost:8080 to set up WordPress.

🧪 Real-World Example #2: MEAN Stack (MongoDB + Express + Angular + Node.js)

Using Compose, you can define all services in one file and connect them seamlessly:

• api → Node.js + Express
• frontend → Angular
• db → MongoDB

📌 This setup is ideal for teams collaborating on full-stack projects.

🧰 Common Docker Compose Commands

🐛 Troubleshooting Tips

🔒 Best Practices for Compose Projects

📁 Dev vs Prod: Compose Override Strategy

Split environments into multiple files:

bash

docker-compose -f docker-compose.yml -f docker-compose.prod.yml up -d

docker-compose.prod.yml may define different environment variables, optimized images, or restrict volumes.

✅ Summary: Chapter 3 Wrap-Up