That just refers to running multiple processes instead of multiple threads. Rails wasn't thread safe until recently and many ruby libraries still aren't (inclu

That just refers to running multiple processes instead of multiple threads. Rails wasn't thread safe until recently and many ruby libraries still aren't (including the mysql driver). So it's common practice to run a "pack of mongrels" (a few processes simultaneously) and then load balance between them in case a long request ties one up for a while. Mongrel2 will let you effectively design part of your app to benefit from Rails (for example) and part to use 0mq and do a more evented style of design. All within one configuration / approach.