When you simply deploy a Rails app to Heroku in its initial state, the web server runs on WEBrick.

$ heroku ps
Process  State         Command                            
-------  ------------  ---------------------------------  
web.1    idle for 94h  bundle exec rails server -p $PORT  

Webserver

By default, your app’s web process runs rails server, which uses Webrick. This is fine for testing, but for production apps you’ll want to switch to a more robust webserver. We recommend Thin.

To use Thin with Rails 3, add it to your Gemfile:

gem 'thin'

Run bundle install to set up your bundle locally.

■ Creating/Adding to Procfile

web: bundle exec rails server thin -p $PORT -e $RACK_ENV

echo "RACK_ENV=development" >>.env

Add to .gitignore to ignore .env.

# Ignore Heroku for local config
/.env

■ Installing Foreman

$ gem install foreman

■ Starting Foreman

Start Foreman and access http://localhost:5000/ from your browser to verify.

$ foreman start

■ Deploy to Heroku

$ git add -A
$ git commit -m "use thin via procfile"
$ git push heroku

■ Verify it’s running with thin

$ heroku ps
Process  State        Command                               
-------  -----------  ------------------------------------  
web.1    idle for 4m  bundle exec rails server thin -p $..  

That’s all.

【References】

・Getting Started with Rails 3.x on Heroku/Cedar | Heroku Dev Center

・The Process Model | Heroku Dev Center

・Fiberを使ってem-http-requestとかを同期的に呼び出す - daily gimite

・Issue #13: FiberError with Rails 3.1 on Heroku · mperham/rack-fiber_pool · GitHub

・mperham/rack-fiber_pool · GitHub

That’s all from the Gemba.