The em-synchrony project (https://github.com/igrigorik/em-synchrony) has patched versions of the mysql gem and they say they can run most of the rails stack. They use a fiber model to un-event the program flow a bit. Ilya is brilliant really. Still, I would be a little leary of betting LinkedIn on a bunch of monkey-patched client code. The challenge in porting to EM is that so many Ruby network clients will block. With Node.js that is impossible. I think though for a rewrite it wouldn't matter, you'd use the right libraries and design your app around the framework just like they had to do switching to Node.
