Quickstart: Compose and Rails

Estimated reading time: 8 minutes

This Quickstart guide shows you how to use Docker Compose to set up and run a Rails/PostgreSQL app. Before starting, install Compose.

Define the project

Start by setting up the four files needed to build the app. App will run inside a Docker container containing its dependencies. Defining dependencies is done using a file called Dockerfile. To begin with, the Dockerfile consists of:

FROM ruby:2.5
RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs
RUN mkdir /myapp
WORKDIR /myapp
COPY Gemfile /myapp/Gemfile
COPY Gemfile.lock /myapp/Gemfile.lock
RUN bundle install
COPY . /myapp

That’ll put your application code inside an image that builds 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', '5.2.0'

Create an empty Gemfile.lock 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: '3'
    image: postgres
      - ./tmp/db:/var/lib/postgresql/data
    build: .
    command: bundle exec rails s -p 3000 -b ''
      - .:/myapp
      - "3000:3000"
      - db

Tip: You can use either a .yml or .yaml extension for this file.

Build the project

With those four files in place, you can now generate the Rails skeleton app using docker-compose run:

docker-compose run web rails new . --force --database=postgresql

First, Compose builds the image for the web service using the Dockerfile. Then it runs rails new inside a new container, using that image. Once it’s done, you should have generated a fresh app.

List the files.

$ ls -l
total 72
-rw-r--r-- 1 vmb staff 223 5 26 14:20 Dockerfile
-rw-r--r-- 1 vmb staff 2223 5 26 14:24 Gemfile
-rw-r--r-- 1 vmb staff 5300 5 26 14:25 Gemfile.lock
-rw-r--r-- 1 vmb staff 374 5 26 14:24 README.md
-rw-r--r-- 1 vmb staff 227 5 26 14:24 Rakefile
drwxr-xr-x 10 vmb staff 320 5 26 14:24 app
drwxr-xr-x 9 vmb staff 288 5 26 14:25 bin
drwxr-xr-x 16 vmb staff 512 5 26 14:24 config
-rw-r--r-- 1 vmb staff 130 5 26 14:24 config.ru
drwxr-xr-x 3 vmb staff 96 5 26 14:24 db
-rw-r--r-- 1 vmb staff 266 5 26 14:22 docker-compose.yml
drwxr-xr-x 4 vmb staff 128 5 26 14:24 lib
drwxr-xr-x 3 vmb staff 96 5 26 14:24 log
-rw-r--r-- 1 vmb staff 63 5 26 14:24 package.json
drwxr-xr-x 9 vmb staff 288 5 26 14:24 public
drwxr-xr-x 3 vmb staff 96 5 26 14:24 storage
drwxr-xr-x 11 vmb staff 352 5 26 14:24 test
drwxr-xr-x 6 vmb staff 192 5 26 14:24 tmp
drwxr-xr-x 3 vmb staff 96 5 26 14:24 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.

Now that you’ve got a new Gemfile, you need to build the image again. (This, and changes to the Gemfile or the Dockerfile, 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:

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

  <<: *default
  database: myapp_development

  <<: *default
  database: myapp_test

You can now boot the app with docker-compose up:

docker-compose up

If all’s well, you should see some PostgreSQL output, and then — after a few seconds — the familiar refrain:

Starting rails_db_1 ...
Starting rails_db_1 ... done
Recreating rails_web_1 ...
Recreating rails_web_1 ... done
Attaching to rails_db_1, rails_web_1
db_1   | LOG:  database system was shut down at 2017-06-07 19:12:02 UTC
db_1   | LOG:  MultiXact member wraparound protections are now enabled
db_1   | LOG:  database system is ready to accept connections
db_1   | LOG:  autovacuum launcher started
web_1  | => Booting Puma
web_1  | => Rails 5.2.0 application starting in development
web_1  | => Run `rails server -h` for more startup options
web_1  | Puma starting in single mode...
web_1  | * Version 3.11.4 (ruby 2.5.1-p57), codename: Love Song
web_1  | * Min threads: 5, max threads: 5
web_1  | * Environment: development
web_1  | * Listening on tcp://
web_1  | Use Ctrl-C to stop

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'

View the Rails welcome page!

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 Machine, then docker-machine ip MACHINE_VM returns the Docker host IP address, to which you can append the port (<Docker-Host-IP>:3000).

Rails example

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 executed the 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 restart the application with docker-compose up.

Restart the application

To restart the application run docker-compose up in the project directory.

Rebuild the application

If you make changes to the Gemfile or the Compose file to try out some different configurations, you need to rebuild. Some changes 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 Gemfile.lock to 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 3000 in our first example to 3001. Make the change to the Compose file to expose port 3000 on the container through a new port, 3001, on the host, and save the changes:

ports: - "3001:3000"

Now, rebuild and restart the app with docker-compose up --build.

Inside the container, your app is running on the same port as before 3000, but the Rails Welcome is now available on http://localhost:3001 on your local host.

More Compose documentation

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