[bradleyland]

This is a nitpick, but this isn't entirely accurate:

"This is a function of how Rails does concurrency, which is by using multiple processes."

Multi-process vs multi-threaded is an application server architecture matter, not a Rails matter. Yes, your application must be thread safe, but this has been available in some form for Rails apps since a long time now [1]. The biggest problem in recent history has been Gems and their thread-safetyness (is that a word). The number of threaded Ruby application servers has been on the rise for a long while now, and even Passenger (traditionally the multi-process front-runner) is going threaded in version 4.0.

1: An interesting blurg post related to Config.threadsafe! (which was a huge source of confusion): http://tenderlovemaking.com/2012/06/18/removing-config-threa...