Session 02

📦 Docker Compose – Manage Multi-Container Applications

📌 What is Docker Compose?

Docker Compose is a tool used to define and run multi-container Docker applications.

You define the services, networks, and volumes your app needs in a docker-compose.yml file, and bring everything up with a single command.

✅ Benefits of Docker Compose

Easy to manage complex applications
Reproducible environments
Centralized configuration
Supports custom networks and volumes
Ideal for local development and CI pipelines

Docker Compose Installation

🧱 Example Project Structure

myapp/
├── backend/
│   └── app.py
├── frontend/
│   └── index.html
├── db/
│   └── init.sql
├── docker-compose.yml
└── Dockerfile (for backend)

Run this script to have the structure created automatically

#!/bin/bash

# Create directories
mkdir -p myapp/backend myapp/frontend myapp/db
cd myapp

# Create backend/app.py
cat <<EOF > backend/app.py
from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello from the Backend Flask App!"

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000)
EOF

# Create frontend/index.html
cat <<EOF > frontend/index.html
<!DOCTYPE html>
<html>
<head>
    <title>Frontend</title>
</head>
<body>
    <h1>Hello from Frontend</h1>
</body>
</html>
EOF

# Create db/init.sql
cat <<EOF > db/init.sql
CREATE TABLE IF NOT EXISTS messages (
    id SERIAL PRIMARY KEY,
    text VARCHAR(255) NOT NULL
);
EOF

# Create Dockerfile (for backend)
cat <<EOF > Dockerfile
FROM python:3.10-slim

WORKDIR /app

COPY backend/ .

RUN pip install flask

CMD ["python", "app.py"]
EOF

# Create docker-compose.yml
cat <<EOF > docker-compose.yml
version: "3.9"

services:
  backend:
    build: .
    ports:
      - "5000:5000"
    depends_on:
      - db

  frontend:
    image: nginx:alpine
    ports:
      - "80:80"
    volumes:
      - ./frontend:/usr/share/nginx/html:ro

  db:
    image: postgres:13
    environment:
      POSTGRES_USER: myuser
      POSTGRES_PASSWORD: mypass
      POSTGRES_DB: mydb
    volumes:
      - db_data:/var/lib/postgresql/data
      - ./db/init.sql:/docker-entrypoint-initdb.d/init.sql:ro

volumes:
  db_data:
EOF

echo "✅ Project structure created successfully in ./myapp"

📜 Sample `docker-compose.yml`

version: "3.9"

services:
  backend:
    build: ./backend
    ports:
      - "5000:5000"
    depends_on:
      - db

  frontend:
    image: nginx:alpine
    ports:
      - "80:80"
    volumes:
      - ./frontend:/usr/share/nginx/html

  db:
    image: postgres:13
    environment:
      POSTGRES_USER: myuser
      POSTGRES_PASSWORD: mypass
      POSTGRES_DB: mydb
    volumes:
      - db_data:/var/lib/postgresql/data

volumes:
  db_data:

🚀 Common Docker Compose Commands

docker-compose up               # Start all services
docker-compose up -d            # Run in detached mode
docker-compose down             # Stop and remove all containers
docker-compose ps               # List running services
docker-compose logs             # View logs for all services
docker-compose build            # Build services
docker-compose restart backend  # Restart a specific service

🔄 Rebuild and Restart

docker-compose down --volumes   # Remove all volumes too
docker-compose up --build       # Rebuild and start

🧪 Compose for Local Development

Use Compose to simulate a full application stack locally:

Backend (e.g., Flask/Django/Node)
Frontend (e.g., React/Vue/Angular)
Database (e.g., PostgreSQL/MySQL)
Proxy/Load balancer (e.g., NGINX)

🧠 Tips and Best Practices

Use .env files for secrets and environment config
Use depends_on for startup order (not readiness checks)
Bind volumes for live code changes
Version-lock your base images

✅ Docker Compose is a must-have for developing, testing, and deploying multi-container apps with consistency.