It is interesting that you are going from Java --> Rails rather than the other way around.

It's actually more of a move away from MySQL -> Mongo, than Java to Rails

All of our other apps are on a Rails/Mongo stack. Most of our dev team is Rails and figured it'd be easier to hire Senior Rails hackers than Java folks.

That's what I was thinking. Or at least not Java to Scala.

Ditto, did you consider Java to Javascript? It may make sense to you given that you are using AngularJS on the front-end as well as for consolidating skills.

I am a Ruby/Rails dev that switched to using Clojure and Node.js, depending on the use cases. I wouldn't want to use Ruby for anything else than static sites and tooling now. Some people think that Javascript is a gimmick, but is it vastly superior to Ruby when it comes to concurrency, ability to share back-end and front-end code, and the tools have gotten really good too.

PS: I wouldn't necessarily recommend switching from Java to Scala, the benefits may be low or negative for most people, see Yammer's experience: http://www.infoq.com/news/2011/11/yammer-scala

I actually joined about a year ago and the decision had already been made to use Rails to run our API and server-side code.

We have 4 different apps; one of the major ones is already using Rails/EventMachine to power it and we've learned how to scale it.

I'm not sure why Node.js wasn't seriously considered for this rewrite, but if I had to guess, it's because we didn't want to take the time to learn a new thing, we've got a legacy system that badly needs customers migrated off it, and shipping something soon is paramount.

I'm sure at some later date, we'll move all our stuff to JS. It seems inevitable with the way things are currently headed.