Introduction

Welcome to this tutorial where we will explore how to define and run multi-container applications using Docker Compose. Docker Compose is a crucial tool in the Docker ecosystem and provides an efficient method to orchestrate multiple containers and create multi-container apps.

By the end of this tutorial, you will learn:
- What Docker Compose is and why it's beneficial.
- How to write a docker-compose.yml file.
- How to manage your application with Docker Compose.

Before we start, you should have Docker installed on your system. Basic knowledge of Docker and YAML syntax will be helpful but not mandatory.

Step-by-Step Guide

Concept of Docker Compose

Docker Compose is a tool for defining and managing multi-container Docker applications. It uses a YAML file (docker-compose.yml) to configure your application's services, which allows you to start all services with a single command.

Writing a docker-compose.yml file

A docker-compose.yml file is a YAML file that defines services, networks, and volumes. Here's an example:

version: '3'
services:
  web:
    build: .
    ports:
      - "5000:5000"
  redis:
    image: "redis:alpine"

In this example, we have two services: web and redis. The web service builds from the Dockerfile in the current directory and maps port 5000 on the host to port 5000 on the container. The redis service uses the redis image from Docker Hub.

Running docker-compose up

To start your application, navigate to the directory with your docker-compose.yml file and run the command docker-compose up. This command will start your application and its services.

Code Examples

Let's look at a practical example where we have an application with a web server (Node.js) and a database (MongoDB).

Dockerfile for Node.js app

First, let's create a Dockerfile for our Node.js application:

# Dockerfile
FROM node:14
WORKDIR /usr/src/app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 8080
CMD [ "node", "server.js" ]

Docker Compose file

Next, we will write our docker-compose.yml file:

version: '3'
services:
  web:
    build: .
    ports:
      - "8080:8080"
    links:
      - db
  db:
    image: mongo

In this example, we have two services: web (Node.js server) and db (MongoDB). The web service builds from the Dockerfile in the current directory, maps port 8080 from the host to the container, and links to the db service. The db service uses the mongo image from Docker Hub.

Running Docker Compose

Run docker-compose up to start your application:

$ docker-compose up

You should see output indicating that both the web and db services have started.

Summary

In this tutorial, we've covered the basics of Docker Compose, including writing a docker-compose.yml file and running multi-container applications. We've also walked through a practical example with a Node.js server and a MongoDB database.

For further learning, consider exploring more complex docker-compose.yml files and using Docker Compose in conjunction with Docker Swarm for production-level applications.

Practice Exercises

1. Simple Exercise: Create a docker-compose.yml file for an application with a Python Flask server and a MySQL database.
2. Intermediate Exercise: Add a Redis cache service to the application from the first exercise.
3. Advanced Exercise: Create a multi-container application with a load balancer (e.g., Nginx), a web application (e.g., Node.js), and a database (e.g., PostgreSQL).

Remember that practice is key in mastering Docker Compose. Happy learning!