Heroku uses cowboy for their routing layer. If you inspect the headers in a response from an app served on Heroku, cowboy is the server. I would guess Heroku is

Heroku uses cowboy for their routing layer. If you inspect the headers in a response from an app served on Heroku, cowboy is the server. I would guess Heroku is the largest cowboy deployment. As for largest Phoenix deployment, I'm unsure.