Docker Compose for Multi-Container Applications

Table of Contents

1. What is Docker Compose?
2. Why Use Docker Compose?
3. Installing Docker Compose
4. Understanding docker-compose.yml
5. Creating a Multi-Container Application
6. Managing Services with Compose
7. Networking in Docker Compose
8. Volumes and Data Persistence
9. Best Practices for Writing Compose Files
10. Conclusion

What is Docker Compose?

Docker Compose is a tool that allows you to define and run multi-container Docker applications. Using a single YAML configuration file (docker-compose.yml), you can specify the services, networks, and volumes required for your app and spin them up with one command.

It is especially useful in microservice-oriented applications, where different components (e.g., API, database, cache) run in separate containers.

Why Use Docker Compose?

• Simplified Configuration: All container definitions in one file.
• Easy Environment Replication: Consistent development, staging, and production setups.
• One-Command Setup: Bring up all services using docker-compose up.
• Supports Volumes and Networks: Preconfigure how containers communicate and store data.

Installing Docker Compose

If you have Docker Desktop (on macOS or Windows), Docker Compose is already included.

On Linux (CLI installation):

sudo curl -L "https://github.com/docker/compose/releases/download/v2.22.0/docker-compose-$(uname -s)-$(uname -m)" -o /usr/local/bin/docker-compose
sudo chmod +x /usr/local/bin/docker-compose
docker-compose --version

Understanding docker-compose.yml

A basic docker-compose.yml file looks like this:

version: '3.8'
services:
  web:
    build: .
    ports:
      - "3000:3000"
  redis:
    image: "redis:alpine"

Breakdown:

• version: Specifies the Compose file format version.
• services: Defines each container.
• build: Builds the image from the current directory’s Dockerfile.
• image: Pulls an image from Docker Hub.
• ports: Maps container port to host port.

Creating a Multi-Container Application

Let’s build a basic app with a Node.js API and a Redis cache.

Directory Structure

/multi-app
  ├── app
  │   ├── Dockerfile
  │   ├── index.js
  │   └── package.json
  └── docker-compose.yml

index.js

const express = require('express');
const redis = require('redis');
const app = express();
const client = redis.createClient({ url: 'redis://redis:6379' });

client.connect();

app.get('/', async (req, res) => {
  const count = await client.incr('visits');
  res.send(`Visit count: ${count}`);
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

package.json

{
  "name": "docker-compose-app",
  "dependencies": {
    "express": "^4.18.2",
    "redis": "^4.6.7"
  }
}

Dockerfile

FROM node:18-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["node", "index.js"]

docker-compose.yml

version: '3.8'
services:
  web:
    build: ./app
    ports:
      - "3000:3000"
    depends_on:
      - redis
  redis:
    image: redis:alpine

Run the App

docker-compose up --build

Navigate to http://localhost:3000 — Each refresh increments the counter using Redis.

Managing Services with Compose

• Start services: docker-compose up -d
• Stop services: docker-compose down
• Rebuild services: docker-compose up --build
• View logs: docker-compose logs -f

Networking in Docker Compose

All services defined in a Compose file share a common default network. This allows containers to refer to each other by their service names (redis, web, etc.) without needing IP addresses.

You can define custom networks:

networks:
  backend:

Then assign services to them:

services:
  web:
    networks:
      - backend

Volumes and Data Persistence

To persist Redis data:

services:
  redis:
    image: redis:alpine
    volumes:
      - redis-data:/data

volumes:
  redis-data:

This ensures Redis data isn’t lost when the container stops.

Best Practices for Writing Compose Files

1. Use .env Files for environment configuration.
2. Use Specific Image Versions: Avoid using latest tag blindly.
3. Keep Services Modular: Break monolith services into distinct containers.
4. Use Health Checks to monitor container readiness.
5. Avoid Hardcoding Secrets: Use secret management tools or Docker secrets.

Conclusion

Docker Compose enables you to orchestrate and manage multi-container applications effortlessly. It’s foundational for microservices and essential for any DevOps pipeline involving local development, staging, or testing.