2 - Docker Compose

Docker Compose is a useful tool for defining applications that span multiple containers. The configuration for these Docker applications in written in a YAML file, entitled docker-compose.yml invoking the docker-compose up command will run the entire application within their individual containers. Docker Compose comes in handy when developing for multiple different environments (ex. staging, production, development, testing, even CI).

If you've ever heard the term to dockerize an application, then this is likely what they were referring to. Specifying containers for each component of your app to run inside to create your isolated environment (as specified by your Dockerfile). For automation/QA purposes, docker-compose can make black-box tests a breeze, since you can specify everything without worrying about caches or external factors.

One important thing to note is that docker !== docker-compose. You need to install Compose separately, probably using this helpful guide.

Boiled down, there only a few main steps to create a basic docker-compose workflow:

1. Define your app’s environment with a Dockerfile so it can be reproduced anywhere.
2. Define the services that make up your app in docker-compose.yml so they can be run together in an isolated environment.

3. Run docker-compose up and Compose starts and runs your entire app.

   • The first time you'll do it, --build is a useful flag, since it will build the images before actually starting the container
   • Usually you'll run this command with the -d flag, meaning detached mode. This runs the containers in the background and will print their names on execution.
4. If you want to status of your containers, you can run docker ps.
5. To stop your containers, you guessed it, docker-compose down. This will kill them and remove them, clearing up your precious resources.

The following is an example of a simple docker-compose.yml file which will launch a WordPress/MySQL backend:

version: "3"

services:
  db:
    image: mysql:5.7
    volumes:
      - db_data:/var/lib/mysql
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: somewordpress
      MYSQL_DATABASE: wordpress
      MYSQL_USER: wordpress
      MYSQL_PASSWORD: wordpress

  wordpress:
    depends_on:
      - db
    image: wordpress:latest
    ports:
      - "8000:80"
    restart: always
    environment:
      WORDPRESS_DB_HOST: db:3306
      WORDPRESS_DB_USER: wordpress
      WORDPRESS_DB_PASSWORD: wordpress
volumes:
  db_data: ...

Depending on your application you can even choose which of your containers you want to build so that you don't overwork your computer. This can extremely useful for debugging purposes. To do so simply call the usual start command, but specify the name of the container as the last parameter.

docker-compose up -d [SERVICE...]

Using the above example:

docker-compose up -d wordpress

Honestly most of the confusion surrounding Docker is handled through their expansive documentation, even the Command Line stuff. In case you ever get lost, visit the docker docs, the docker-compose docs, or just attach the --help flag onto any of your commands.