Quickstart: Docker Compose and Rails
This Quickstart guide will show you how to use Docker Compose to set up and run a Rails/PostgreSQL app. Before starting, you’ll need to have Compose installed.
Define the project
Start by setting up the three files you’ll need to build the app. First, since your app is going to run inside a Docker container containing all of its dependencies, you’ll need to define exactly what needs to be included in the container. This is done using a file called Dockerfile. To begin with, the Dockerfile consists of:
FROM ruby:2.2.0 RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs RUN mkdir /myapp WORKDIR /myapp ADD Gemfile /myapp/Gemfile ADD Gemfile.lock /myapp/Gemfile.lock RUN bundle install ADD . /myapp
That’ll put your application code inside an image that will build a container with Ruby, Bundler and all your dependencies inside it. For more information on how to write Dockerfiles, see the Docker user guide and the Dockerfile reference.
Next, create a bootstrap Gemfile which just loads Rails. It’ll be overwritten in a moment by rails new.
source 'https://rubygems.org' gem 'rails', '4.2.0'
You’ll need an empty Gemfile.lock in order to build our Dockerfile.
$ touch Gemfile.lock
Finally, docker-compose.yml is where the magic happens. This file describes the services that comprise your app (a database and a web app), how to get each one’s Docker image (the database just runs on a pre-made PostgreSQL image, and the web app is built from the current directory), and the configuration needed to link them together and expose the web app’s port.
version: '2'
services:
db:
image: postgres
web:
build: .
command: bundle exec rails s -p 3000 -b '0.0.0.0'
volumes:
- .:/myapp
ports:
- "3000:3000"
depends_on:
- db
Build the project
With those three files in place, you can now generate the Rails skeleton app using docker-compose run:
$ docker-compose run web rails new . --force --database=postgresql --skip-bundle
First, Compose will build the image for the web service using the Dockerfile. Then it’ll run rails new inside a new container, using that image. Once it’s done, you should have generated a fresh app:
$ ls -l total 56 -rw-r--r-- 1 user staff 215 Feb 13 23:33 Dockerfile -rw-r--r-- 1 user staff 1480 Feb 13 23:43 Gemfile -rw-r--r-- 1 user staff 2535 Feb 13 23:43 Gemfile.lock -rw-r--r-- 1 root root 478 Feb 13 23:43 README.rdoc -rw-r--r-- 1 root root 249 Feb 13 23:43 Rakefile drwxr-xr-x 8 root root 272 Feb 13 23:43 app drwxr-xr-x 6 root root 204 Feb 13 23:43 bin drwxr-xr-x 11 root root 374 Feb 13 23:43 config -rw-r--r-- 1 root root 153 Feb 13 23:43 config.ru drwxr-xr-x 3 root root 102 Feb 13 23:43 db -rw-r--r-- 1 user staff 161 Feb 13 23:35 docker-compose.yml drwxr-xr-x 4 root root 136 Feb 13 23:43 lib drwxr-xr-x 3 root root 102 Feb 13 23:43 log drwxr-xr-x 7 root root 238 Feb 13 23:43 public drwxr-xr-x 9 root root 306 Feb 13 23:43 test drwxr-xr-x 3 root root 102 Feb 13 23:43 tmp drwxr-xr-x 3 root root 102 Feb 13 23:43 vendor
If you are running Docker on Linux, the files rails new created are owned by root. This happens because the container runs as the root user. Change the ownership of the the new files.
sudo chown -R $USER:$USER .
If you are running Docker on Mac or Windows, you should already have ownership of all files, including those generated by rails new. List the files just to verify this.
Uncomment the line in your new Gemfile which loads therubyracer, so you’ve got a Javascript runtime:
gem 'therubyracer', platforms: :ruby
Now that you’ve got a new Gemfile, you need to build the image again. (This, and changes to the Dockerfile itself, should be the only times you’ll need to rebuild.)
$ docker-compose build
Connect the database
The app is now bootable, but you’re not quite there yet. By default, Rails expects a database to be running on localhost - so you need to point it at the db container instead. You also need to change the database and username to align with the defaults set by the postgres image.
Replace the contents of config/database.yml with the following:
development: &default adapter: postgresql encoding: unicode database: postgres pool: 5 username: postgres password: host: db test: <<: *default database: myapp_test
You can now boot the app with:
$ docker-compose up
If all’s well, you should see some PostgreSQL output, and then—after a few seconds—the familiar refrain:
myapp_web_1 | [2014-01-17 17:16:29] INFO WEBrick 1.3.1 myapp_web_1 | [2014-01-17 17:16:29] INFO ruby 2.2.0 (2014-12-25) [x86_64-linux-gnu] myapp_web_1 | [2014-01-17 17:16:29] INFO WEBrick::HTTPServer#start: pid=1 port=3000
Finally, you need to create the database. In another terminal, run:
$ docker-compose run web rake db:create
That’s it. Your app should now be running on port 3000 on your Docker daemon. If you’re using Docker Machine, then docker-machine ip MACHINE_VM returns the Docker host IP address.
Note: If you stop the example application and attempt to restart it, you might get the following error:
web_1 | A server is already running. Check /myapp/tmp/pids/server.pid.One way to resolve this is to delete the filetmp/pids/server.pid, and then re-start the application withdocker-compose up.
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