How to Run a Web App in Two and Three and Roll Backables?

Yelp was able to run app simultaneously in two and three versions. They were able to use URL reverse proxy, like nginx or something to say, this part of the web app runs Python three. And this went over here, run Python two and and filtered the traffic. If you didn't have that compatibility, it would be like, all right, today we pull this way, it's junk.