[viraptor]

IMNSHO (dealing with both kinds in some cases):

Probably because they weren't popular. And they weren't popular because they were hard to write in "classic" languages. I'd never try to write a complex asynchronous server in a language like C, or Java. But then, I'd never try to write a thread-per-connection server in Python. Limiting ourselves to popular languages only: "old" ones didn't provide good support for this kind of thing. In "new" ones evented servers are trivial with closures, simulated continuations, etc. This might seem silly, but look at the slides 36 from "Paul Tyma" mentioned in another comment. "Found that when switching between clients, the code for saving and restoring values/state was difficult" - I'm not even sure what he means by that... why save / restore? That seems like a seriously difficult way to write the code.

Then again there are places where you really shouldn't choose one over another, unless you've got a really good reason. For example in telecommunication when you're dealing with signalling, doing thread per connection is close to insane. That's one of the reasons there's only a handful of people who understand chan_sip in the Asterisk project and why it's full of DEADLOCK_AVOIDANCE macros.