Threaded vs Evented Servers

Paul Tyma discusses some other myths regarding evented vs threaded servers: http://www.mailinator.com/tymaPaulMultithreaded.pdf

A lot of existing libraries, web services and other external resources make eventing more difficult since they don't provide an asynchronous way of using them.

In the case of node.js for example a lot of libraries (mysql for example) have to be wrapped or rewritten since it would block the whole event loop.

This Usenix article argues that threaded servers are a better programming model, though it admits that evented servers with current setups may be more efficient (e.g. if there's no compiler support for minimizing per-thread stack overhead): http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.5.58...

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.

I think the answer is actually pretty straightforward. It's trivial to go from a serial to a threaded server. It's way harder to write an event-based server using poll or select.

Serially:

  int handle_connection(int fd) {
    ...
  }

  int loop() {
    ...
    while(1) {
      fd = accept(listener);
      handle_connection(fd);
    }
  }

Threaded:

  int handle_connection(int fd) {
    ...
  }

  int loop() {
    ...
    while(1) {
      fd = accept(listener);
      thread_start(handle_connection, fd);
    }
  }