Painless Docker - 2nd Edition

Understanding Docker Compose and How it Works

Docker Compose is already included when you install Docker (or Docker Desktop). You can check if you have it installed by running:

docker compose -h

Docker Compose works by defining a multi-container application in a single file called docker-compose.yml. This file abstracts container management and allows you to define services, networks, and volumes in a declarative way. A service, in development environments, typically represents a container that runs a specific part of your application, such as a web server, database, or cache. To better understand how this tool works, let's create a simple WordPress application using Docker Compose.

Let's start by creating a new folder and a new file called docker-compose.yml:

# Create a new folder
mkdir $HOME/wordpress && cd $HOME/wordpress

# Create a new file called docker-compose.yml
cat << EOF > $HOME/wordpress/docker-compose.yml
services:
   db:
     image: mysql:9.6.0
     volumes:
       - db_data:/var/lib/mysql
     restart: always
     environment:
       MYSQL_ROOT_PASSWORD: mypassword
       MYSQL_DATABASE: wordpress
       MYSQL_USER: user
       MYSQL_PASSWORD: mypassword

   wordpress:
     image: wordpress:6.9.0-apache
     ports:
       - "8000:80"
     restart: always
     environment:
       WORDPRESS_DB_HOST: db:3306
       WORDPRESS_DB_USER: user
       WORDPRESS_DB_PASSWORD: mypassword
volumes:
    db_data:
EOF

Now, let's start the application:

# Make sure you are in the wordpress folder
cd $HOME/wordpress

# Start the application
docker compose up

You should start seeing the logs of the containers:

• db
• wordpress