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.

Create a Rack app

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

$ mkdir hello
$ cd hello

Create a config.ru file with the following contents:

# config.ru
run lambda { |env| [200, {'Content-Type'=>'text/plain'}, StringIO.new("Hello World!\n")] }

Create a Gemfile to declare you dependencies:

source "https://rubygems.org"

gem "rack"
gem "rackup"
gem "puma"

Install dependencies:

$ bundle install

Boot the server

$ bundle exec rackup -p 9292 config.ru

Test with curl or by visiting http://localhost:9292 in the browser:

$ curl http://localhost:9292
Hello World!

Deploy a Rack app to Heroku

Add the contents to git:

$ git init
$ git add .
$ git commit -m 'pure rack app'

Create a new heroku application and push to it:

$ 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!.

Configure web process

The Ruby officially supported buildpack automatically configured the command to boot your Rack app. If you want to configure it, you can do so by creating a file named Procfile at the root of your project with a web key:

# Procfile
web: bundle exec puma -C config/puma.rb

You can learn more about Procfile here.

Frameworks

Any framework that uses Rack is supported on Heroku, including (but not limited to) Ruby on Rails, Sinatra, and Hanami.

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 it 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 web server like Nginx or Apache be running on the same machine as your app. This is not how Heroku is architected. Using theRack::Sendfilemiddleware **will cause your file downloads to fail because it sends a body with aContent-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.

If you want to serve files from your Rack application, you can use Rack::Static to have Ruby serve them directly:

# config.ru
require "rack/static"

use Rack::Static, urls: ["/images", "/js", "/css"], root: "public"