Getting started with docker-compose

Remarks#

Compose is a tool for defining and running multi-container Docker applications. With Compose, you use a Compose file to configure your application’s services. Then, using a single command, you create and start all the services from your configuration. To learn more about all the features of Compose see the list of features.

Using Compose is basically a three-step process.

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. Lastly, run docker-compose up and Compose will start and run your entire app.

Installation

If you are running Docker on OS X or Windows, docker-compose should be included in your Docker for Windows or Docker Toolbox installation.

On Linux you can get the latest binaries straight from the GitHub release page: https://github.com/docker/compose/releases

You can install the specific release with the following commands:

curl -L https://github.com/docker/compose/releases/download/1.7.1/docker-compose-`uname -s`-`uname -m` > /usr/local/bin/docker-compose
chmod +x /usr/local/bin/docker-compose

For more info please refer to documentation page

Create a simple application

Run command in docker-compose service

Install Docker Compose

Docker Compose hello world

A very basic docker-compose.yml looks like this:

version: '2'
services:
  hello_world:
    image: ubuntu
    command: [/bin/echo, 'Hello world']

This file is making it so that there’s a hello_world service, that’s initialized from the ubuntu:latest image and that, when it’s run, it just runs echo 'Hello world'

If you’re on the folder directory (and it contains this docker-compose.yml file), you can do docker-compose up and you should see

Creating folder_hello_world_1
Attaching to folder_hello_world_1
hello_world_1 | Hello world
folder_hello_world_1 exited with code 0

This created the container from the ubuntu image, and ran the command that was specified on the docker-compose.yml

Docker-Compose uses the folder name as the project name to prefix containers and networks. To set another project name, you can either call docker-compose --project-name NAME {up|down|...} or you suppy a file called .env next to your docker-compose.yml and write COMPOSE_PROJECT_NAME=name in it. Better avoid long project names with hyphens (-) because docker compose bahaves strange with this kind of names.

Note: docker-compose allows you to run multiple docker containers on a single host. If you want to run multiple containers on more than one node, please refer to solution such as swarm / kubernetes.

Ruby on Rails with docker-compose

If you want to use docker for rails app, and use database, you need to know that all the data in the docker container will be destroyed (unless you configure the container specifically for keeping data) Sometimes, you need to create a docker container with an application and attach it to an old container with a database.

As an example of rails application, I used a simple app. You can create it from command:

rails new compose-app --database=postgresql

Of course, you need to install rails, ruby, etc. beforehand.

Then, create Dockerfile in your project, and set this data to it:

FROM ruby:2.3.1
RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs
RUN mkdir /compose-app
WORKDIR /compose-app
ADD Gemfile /compose-app/Gemfile
ADD Gemfile.lock /compose-app/Gemfile.lock
RUN bundle install
ADD . /compose-app

Next step - create docker-compose.yml with the data:

version: '2'
services:
  db:
    image: postgres
  web:
    build: .
    command: bundle exec rails s -e development -p 80 -b '0.0.0.0'
    volumes:
      - .:/compose-app
    ports:
      - "80:80"
    depends_on:
      - db

You can replace 80 port (-p 80 ) with another.

Develop section of database.yml config must be changed to:

development: &default
  adapter: postgresql
  encoding: unicode
  database: postgres
  pool: 5
  username: postgres
  password:
  host: db

Now you can build images from command:

docker-compose build

(Run this in project directory)

And start all from:

docker-compose up

If everything is done correctly, you will be able to see logs from rails in the console.

Close console. It will be working.

If you want to delete only the container with the rails application without the database, you need to run then in project directory:

docker-compose stop web
docker-compose build web
docker-compose up -d --no-deps web

New container with rails app will be created and launched.