Managing Multi-Container Applications with Docker Compose

Introduction

Docker Compose is a powerful tool that simplifies the management of multi-container applications. This guide will walk you through creating and managing applications using Docker Compose, providing practical examples to get you started.

What Is Docker Compose?

Docker Compose is a tool for defining and running multi-container Docker applications. It uses a YAML file (docker-compose.yml) to configure application services, making it easy to manage complex environments.

Benefits of Docker Compose

• Simplified Configuration: Manage all containers in a single YAML file.
• Reproducibility: Ensure consistent environments across development, testing, and production.
• Networking: Automatically creates networks for container communication.
• Scaling: Easily scale services up or down.

Installing Docker Compose

Docker Compose comes bundled with Docker Desktop. To install it on Linux:

sudo apt-get update
sudo apt-get install docker-compose-plugin

Verify the installation:

docker compose version

Step 1: Writing a docker-compose.yml File

The docker-compose.yml file defines your application services. Here's an example for a simple web application:

version: '3.8'
services:
  web:
    image: nginx
    ports:
      - '8080:80'
  app:
    build: ./app
    volumes:
      - ./app:/usr/src/app
    command: npm start

Explanation:

• version: Defines the Compose file format version.
• services: Lists the application services.
  • web: Uses the official Nginx image and maps port 8080 to 80.
  • app: Builds a custom application from the ./app directory and mounts it.

Step 2: Starting the Application

Run the application with:

docker compose up

This command starts all services defined in docker-compose.yml.

Add the -d flag to run in detached mode:

docker compose up -d

Step 3: Managing Services

Stop Services

docker compose down

Stops and removes all containers, networks, and volumes defined by the Compose file.

Restart Services

docker compose restart

Restarts all running services.

View Logs

docker compose logs

Shows logs for all services. Use -f for real-time logs.

Scale Services

docker compose up --scale app=3

Starts three instances of the app service.

Step 4: Adding a Database Service

Here’s an updated docker-compose.yml with a database:

version: '3.8'
services:
  web:
    image: nginx
    ports:
      - '8080:80'
  app:
    build: ./app
    volumes:
      - ./app:/usr/src/app
    command: npm start
    depends_on:
      - db
  db:
    image: postgres
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password

Explanation:

• depends_on: Ensures the db service starts before app.
• environment: Sets environment variables for the db service.

Conclusion

Docker Compose simplifies the management of multi-container applications, enabling you to define, run, and scale services effortlessly. Whether you're setting up a simple project or a complex stack, Compose ensures a seamless and efficient workflow.