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.
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', '22.214.171.124'
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.
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 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.
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 to rebuild.)
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&8212;after a few seconds&8212;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 126.96.36.199 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'
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 (
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 re-start the application with
To restart the application:
docker-compose upin the project directory.
docker-compose run web rake db:create
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
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, then restart the database:
docker-compose run web rake db:create.
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.
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