Quickstart: Compose and RailsEstimated reading time: 7 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', '18.104.22.168'
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: '3' 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
First, Compose will build the image for the
web service using the
Dockerfile. Then it will run
rails new inside a new container, using that image. Once it’s done, you should have generated a fresh app:
$ ls -l total 64 -rw-r--r-- 1 vmb staff 223 May 19 11:05 Dockerfile -rw-r--r-- 1 vmb staff 1738 May 19 11:06 Gemfile -rw-r--r-- 1 vmb staff 4306 May 19 11:07 Gemfile.lock -rw-r--r-- 1 vmb staff 374 May 19 11:06 README.md -rw-r--r-- 1 vmb staff 227 May 19 11:06 Rakefile drwxr-xr-x 10 vmb staff 340 May 19 11:06 app drwxr-xr-x 8 vmb staff 272 May 19 11:07 bin drwxr-xr-x 14 vmb staff 476 May 19 11:06 config -rw-r--r-- 1 vmb staff 130 May 19 11:06 config.ru drwxr-xr-x 3 vmb staff 102 May 19 11:06 db -rw-r--r-- 1 vmb staff 212 May 19 11:06 docker-compose.yml drwxr-xr-x 5 vmb staff 170 May 16 15:05 getstart drwxr-xr-x 4 vmb staff 136 May 19 11:06 lib drwxr-xr-x 3 vmb staff 102 May 19 11:06 log drwxr-xr-x 9 vmb staff 306 May 19 11:06 public drwxr-xr-x 20 vmb staff 680 May 19 11:04 rails drwxr-xr-x 2 vmb staff 68 May 19 11:05 rails-new drwxr-xr-x 9 vmb staff 306 May 19 11:06 test drwxr-xr-x 4 vmb staff 136 May 19 11:06 tmp drwxr-xr-x 3 vmb staff 102 May 19 11:06 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. If this is the
case, 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 rebuild.)
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:
web_1 | => Booting Puma web_1 | => Rails 22.214.171.124 application starting in development on http://0.0.0.0:3000 web_1 | => Run `rails server -h` for more startup options web_1 | Puma starting in single mode... web_1 | * Version 3.7.1 (ruby 2.3.3-p222), codename: Snowy Sagebrush web_1 | * Min threads: 5, max threads: 5 web_1 | * Environment: development web_1 | * Listening on tcp://0.0.0.0:3000
Finally, you need to create the database. In another terminal, run:
docker-compose run web rake db:create
Here is an example of the output from that command:
vmb at snapair in ~/sandbox/rails $ docker-compose run web rake db:create Starting rails_db_1 ... done Created database 'myapp_development' Created database 'myapp_test'
That’s it. Your app should now be running on port 3000 on your Docker daemon. On
Docker for Mac and Docker for Windows, go to
http://localhost:3000 on a web
browser to see the Rails Welcome. If you are using Docker
docker-machine ip MACHINE_VM returns the
Docker host IP address, to which you can append the port
Stop the application
To stop the application, run
docker-compose down in your project directory.
You can use the same terminal window in which you started the database, or
another one where you have access to a command prompt. This is a clean way to
stop the application.
vmb at snapair in ~/sandbox/rails $ docker-compose down Stopping rails_web_1 ... done Stopping rails_db_1 ... done Removing rails_web_run_1 ... done Removing rails_web_1 ... done Removing rails_db_1 ... done Removing network rails_default
You can also stop the application with
Ctrl-C in the same shell in which you
docker-compose up. If you stop the app this way, and attempt to
restart it, you might get the following error:
web_1 | A server is already running. Check /myapp/tmp/pids/server.pid.
To resolve this, delete the file
tmp/pids/server.pid, and then re-start the
Restart the application
To restart the application:
docker-compose upin the project directory.
- Run this command in another terminal to restart the database:
docker-compose run web rake db:create
Rebuild the application
If you make changes to the Gemfile or the Compose file to try out some different
configurations, you will need to rebuild. Some changes will require only
docker-compose up --build, but a full rebuild requires a re-run of
docker-compose run web bundle install to sync changes in the
the host, followed by
docker-compose up --build.
Here is an example of the first case, where a full rebuild is not necessary.
Suppose you simply want to change the exposed port on the local host from
in our first example to
3001. Make the change to the Compose file to expose
3000 on the container through a new port,
3001, on the host, and save
ports: - "3001:3000"
Now, rebuild and restart the app with
docker-compose up --build, then restart
docker-compose run web rake db:create.
Inside the container, your app is running on the same port as before
the Rails Welcome is now available on
http://localhost:3001 on your local
More Compose documentation
- User guide
- Installing Compose
- Getting Started
- Get started with Django
- Get started with WordPress
- Command line reference
- Compose file reference