Quickstart: Compose and RailsEstimated reading time: 7 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. First, since
your app is going to run inside a Docker container containing all of its
dependencies, 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 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
source 'https://rubygems.org' gem 'rails', '18.104.22.168'
Create an empty
Gemfile.lock 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 volumes: - ./tmp/db:/var/lib/postgresql/data 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 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 64 -rw-r--r-- 1 vmb staff 222 Jun 7 12:05 Dockerfile -rw-r--r-- 1 vmb staff 1738 Jun 7 12:09 Gemfile -rw-r--r-- 1 vmb staff 4297 Jun 7 12:09 Gemfile.lock -rw-r--r-- 1 vmb staff 374 Jun 7 12:09 README.md -rw-r--r-- 1 vmb staff 227 Jun 7 12:09 Rakefile drwxr-xr-x 10 vmb staff 340 Jun 7 12:09 app drwxr-xr-x 8 vmb staff 272 Jun 7 12:09 bin drwxr-xr-x 14 vmb staff 476 Jun 7 12:09 config -rw-r--r-- 1 vmb staff 130 Jun 7 12:09 config.ru drwxr-xr-x 3 vmb staff 102 Jun 7 12:09 db -rw-r--r-- 1 vmb staff 211 Jun 7 12:06 docker-compose.yml drwxr-xr-x 4 vmb staff 136 Jun 7 12:09 lib drwxr-xr-x 3 vmb staff 102 Jun 7 12:09 log drwxr-xr-x 9 vmb staff 306 Jun 7 12:09 public drwxr-xr-x 9 vmb staff 306 Jun 7 12:09 test drwxr-xr-x 4 vmb staff 136 Jun 7 12:09 tmp drwxr-xr-x 3 vmb staff 102 Jun 7 12:09 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
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
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:
default: &default adapter: postgresql encoding: unicode host: db username: postgres password: pool: 5 development: <<: *default database: myapp_development 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:
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 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 3.9.1 (ruby 2.3.3-p222), codename: Private Caller web_1 | * Min threads: 5, max threads: 5 web_1 | * Environment: development web_1 | * Listening on tcp://0.0.0.0:3000 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
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 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
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.
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