Quickstart: Compose and RailsEstimated reading time: 5 minutes
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 four 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:2.3.3 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
source 'https://rubygems.org' gem 'rails', '184.108.40.206'
You’ll need an empty
Gemfile.lock in order to build our
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
Tip: You can use either a
.yamlextension for this file.
Build the project
With those four files in place, you can now generate the Rails skeleton app
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 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
If you edit
Gemfile at this point or later, you will need to build the image again. (This,
and changes to the Dockerfile itself, should be the only times you’ll need to
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
Replace the contents of
config/database.yml with the following:
development: &default adapter: postgresql encoding: unicode database: myapp_development pool: 5 username: postgres password: host: db test: <<: *default database: myapp_test
You can now boot the app with:
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 file
tmp/pids/server.pid, and then re-start the application with