How to Use Docker Compose

If your Docker application includes more than one container (for example, a webserver and database running in separate containers), building, running, and connecting the containers from separate Dockerfiles is cumbersome and time-consuming. Docker Compose solves this problem by allowing you to use a YAML file to define multi-container apps. You can configure as many containers as you want, how they should be built and connected, and where data should be stored. When the YAML file is complete, you can run a single command to build, run, and configure all of the containers.

This guide will explain how the docker-compose.yml file is organized, and show how to use it to create several basic app configurations.

Note

Generally the containers in an application built using Docker Compose will all run on the same host. Managing containers running on different hosts usually requires an additional tool, such as Docker Swarm or Kubernetes.

You will need a Linode with Docker CE installed to follow along with the steps in this guide.

To install Docker CE (Community Edition), follow the instructions within one of the guides below:

• Installing and Using Docker on Ubuntu and Debian

• Installing and Using Docker on CentOS and Fedora

For complete instructions on even more Linux distributions, reference the Install Docker Engine section of Docker’s official documentation.

1. Download the latest version of Docker Compose. Check the releases page and replace 1.25.4 in the command below with the version tagged as Latest release:

   sudo curl -L https://github.com/docker/compose/releases/download/1.25.4/docker-compose-`uname -s`-`uname -m` -o /usr/local/bin/docker-compose

2. Set file permissions:

   sudo chmod +x /usr/local/bin/docker-compose

This section will review an example Docker Compose file taken from the Docker official documentation.

1. Open docker-compose.yml in a text editor and add the following content:

   File: docker-compose.yml

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   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:

2. Save the file and run Docker Compose from the same directory:

   docker-compose up -d
   

   This will build and run the db and wordpress containers. Just as when running a single container with docker run, the -d flag starts the containers in detached mode.

3. You now have a WordPress container and MySQL container running on your host. Navigate to 192.0.8.1:8000/wordpress in a web browser to see your newly installed WordPress application. You can also use docker ps to further explore the resulting configuration:

   docker ps
   

4. Stop and remove the containers:

   docker-compose down
   

A docker-compose.yml file is organized into four sections:

Most of this guide will focus on setting up containers using the services section. Here are some of the common directives used to set up and configure containers:

Many other configuration directives are available. See the Compose File reference for details.

Caution

The example docker-compose.yml above uses the environment directive to store MySQL user passwords directly in the YAML file to be imported into the container as environment variables. This is not recommended for sensitive information in production environments. Instead, sensitive information can be stored in a separate .env file (which is not checked into version control or made public) and accessed from within docker-compose.yml by using the env_file directive.

Create a docker-compose.yml file one section at a time to illustrate the steps of building a multi-container application.

1. Create a new docker-compose.yml in a text editor and add the following content:

   File: docker-compose.yml

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   version: '3' services: distro: image: alpine restart: always container_name: Alpine_Distro entrypoint: tail -f /dev/null

   Each entry in the services section will create a separate container when docker-compose is run. At this point, the section contains a single container based on the official Alpine distribution:

   • The restart directive is used to indicate that the container should always restart (after a crash or system reboot, for example).
   • The container_name directive is used to override the randomly generated container name and replace it with a name that is easier to remember and work with.
   • Docker containers exit by default if no process is running on them. tail -f is an ongoing process, so it will run indefinitely and prevent the container from stopping. The default entrypoint is overridden to keep the container running.

2. Bring up your container:

   docker-compose up -d
   

3. Check the status of your container:

    docker ps
   

   The output should resemble the following:

   CONTAINER ID        IMAGE               COMMAND               CREATED             STATUS              PORTS               NAMES
   967013c36a27        alpine              "tail -f /dev/null"   3 seconds ago       Up 2 seconds                            Alpine_Distro

4. Bring down the container:

    docker-compose down
   

From here you can begin to build an ecosystem of containers. You can define how they work together and communicate.

1. Reopen docker-compos.yml and add the database service below:

   File: docker-compose.yml

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   version: '3' services: distro: image: alpine container_name: Alpine_Distro restart: always entrypoint: tail -f /dev/null database: image: postgres:latest container_name: postgres_db volumes: - ../dumps:/tmp/ ports: - "5432:5432"

   There are now two services defined:

   • Distro
   • Database

   The Distro service is the same as before. The Database server contains the instructions for a postgres container, and the directives: volumes: - ../dumps:/tmp and ports:-"5432:5432", the first directive maps the containerd /dumps folder to our local /tmp folder. The second directive maps the containers ports to the local host’s ports.

2. Check the running containers:

   docker ps
   

   This command shows the status of the containers, the port mapping, the names, and the last command running on them. It’s important to note that the postgres container reads “docker-entrypoint…” under commands. The Postgres Docker Entrypoint script is the last thing that launches when the container starts.

   CONTAINER ID        IMAGE               COMMAND                  CREATED              STATUS              PORTS                    NAMES
   ecc37246f6ef        postgres:latest     "docker-entrypoint..."   About a minute ago   Up About a minute   0.0.0.0:5432->5432/tcp   postgres_db
   35dab3e712d6        alpine              "tail -f /dev/null"      About a minute ago   Up About a minute                            Alpine_Distro

3. Bring down both containers:

    docker-compose down
   

1. Add an nginx container so that your application will be able to serve websites:

   File: docker-compose.yml

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   version: '3' services: distro: image: alpine container_name: Alpine_Distro restart: always entrypoint: tail -f /dev/null database: image: postgres:latest container_name: postgres_db volumes: - ../dumps:/tmp/ ports: - "5432:5432" web: image: nginx:latest container_name: nginx volumes: - ./mysite.template:/etc/nginx/conf.d/mysite.template ports: - "8080:80" environment: - NGINX_HOST=example.com - NGINX_port=80 links: - database:db - distro

   This docker-compose file contains some new directives: environment and links. The first directive sets runtime level options within the container. links creates a dependency network between the containers. The nginx container depends on the other two to execute. In addition, the corresponding containers will be reachable at a hostname indicated by the alias. In this case, pinging db from the web container will reach the database service. While you do not need the links directive for the containers to talk with each other, links can serve as a failsafe when starting the docker-compose application.

2. Start Docker Compose and check the container status:

    docker-compose up -d
    docker ps
   

   The output should be similar to:

   CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
   55d573674e49        nginx:latest        "nginx -g 'daemon ..."   3 minutes ago       Up 3 minutes        0.0.0.0:8080->80/tcp     nginx
   ad9e48b2b82a        alpine              "tail -f /dev/null"      3 minutes ago       Up 3 minutes                                 Alpine_Distro
   736cf2f2239e        postgres:latest     "docker-entrypoint..."   3 minutes ago       Up 3 minutes        0.0.0.0:5432->5432/tcp   postgres_db

3. Test nginx by navigating to your Linode’s public IP address, port 8080 in a browser (for example 192.0.2.0:8080). You should see the default nginx landing page displayed.

Storing PostgreSQL data directly inside a container is not recommended. Docker containers are intended to be treated as ephemeral: your application’s containers are built from scratch when running docker-compose up and destroyed when running docker-compose down. In addition, any unexpected crash or restart on your system will cause any data stored in a container to be lost.

For these reasons it is important to set up a persistent volume on the host that the database containers will use to store their data.

1. Add a volumes section to docker-compose.yml and edit the database service to refer to the volume:

   File: docker-compose.yml

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   version: '3' services: distro: image: alpine container_name: Alpine_Distro restart: always entrypoint: tail -f /dev/null database: image: postgres:latest container_name: postgres_db volumes: - data:/var/lib/postgresql ports: - "5432:5432" web: image: nginx:latest container_name: nginx volumes: - ./mysite.template:/etc/nginx/conf.d/mysite.template ports: - "8080:80" environment: - NGINX_HOST=example.com - NGINX_port=80 links: - database:db - distro volumes: data: external: true

2. external: true tells Docker Compose to use a pre-existing external data volume. If no volume named data is present, starting the application will cause an error. Create the volume:

   docker volume create --name=data
   

3. Start the application as before:

   docker-compose up -d
   

Docker Compose is a powerful tool for orchestrating sets of containers that can work together. Things like an app or a development environment can utilize Docker-compose. The result is a modular and configurable environment that can be deployed anywhere.