Last updated 04 January 2021

Heroku supports Rack and Rack-based web frameworks like Sinatra, Ramaze, and Camping.

To run a Rack-based app, include a Gemfile, as well as a rackup file named config.ru in the app’s root directory. The config.ru file convention has become common, so most existing Rack applications should not require changes to deploy to Heroku.

Pure Rack apps

First, create a new directory and write a simple config.ru file:

$ mkdir hello
$ cd hello
$ cat <<EOF> config.ru
run lambda { |env| [200, {'Content-Type'=>'text/plain'}, StringIO.new("Hello World!\n")] }
EOF
$ cat <<EOF> Gemfile
source 'https://rubygems.org'
gem 'rack'
EOF

Test it locally:

$ bundle install
$ bundle exec rackup -p 9292 config.ru &
$ curl http://localhost:9292
Hello World!
$ kill %1

Deploy to Heroku:

$ git init
$ git add .
$ git commit -m 'pure rack app'
$ heroku create
$ git push heroku main

The app is now deployed to Heroku. Test by executing heroku open or by visiting your app’s URL in your browser. You should see Hello, World!.

Frameworks

Sinatra

hello.rb:

require 'sinatra'

get '/' do
  "Hello World!"
end

config.ru:

require './hello'
run Sinatra::Application

Gemfile:

source 'https://rubygems.org'
gem 'sinatra'

Ramaze

hello.rb:

require 'ramaze'

class MainController < Ramaze::Controller
  def index
     "Hello World!"
  end
end

config.ru:

require ::File.expand_path('./../hello', __FILE__)
Ramaze.start(:file => __FILE__, :started => true)
run Ramaze

Gemfile:

source 'https://rubygems.org'
gem 'ramaze'

Camping

hello.rb:

require 'camping'

Camping.goes :Hello

module Hello::Controllers
  class Index < R '/'
     def get
        render :hello
     end
  end
end

module Hello::Views
  def hello
     p  "Hello World!"
  end
end

config.ru:

require './hello'
run Rack::Adapter::Camping.new(Hello)

Gemfile:

source 'https://rubygems.org'
gem 'camping'

Database access

Using ActiveRecord

For non-Rails apps using ActiveRecord standalone, put this code into your application to access the DATABASE_URL:

require 'active_record'

ActiveRecord::Base.establish_connection(ENV['DATABASE_URL'] || 'postgres://localhost/mydb')

The code above uses a default local PostgreSQL database named mydb, but you can change this value to point anywhere you like, or override by running your app with the DATABASE_URL environment variable set in your shell.

Using DataMapper or Sequel

DataMapper and Sequel both use database URLs natively, so configuration is a snap:

For DataMapper:

require 'data_mapper'
DataMapper.setup(:default, ENV['DATABASE_URL'] || 'postgres://localhost/mydb')

For Sequel:

require 'sequel'
Sequel.connect(ENV['DATABASE_URL'] || 'postgres://localhost/mydb')

Rack::Sendfile

Heroku does not support the use of Rack::Sendfile. Rack::Sendfile usually requires that a front-end webserver like nginx or apache is running on the same machine as your app. This is not how Heroku is architected. Using the Rack::Sendfile middleware will cause your file downloads to fail, because it sends a body with a Content-Length of 0.

By default, Rails sets this to nil, but make sure that config.action_dispatch.x_sendfile_header is not set in config/enviroments/production.rb.