mbp: I reckon Erlang's concurrency is the best thing since sliced bread -- and it's very explicit, a long way from the lazy functional languages. I like it because it gives me a very high level programming interface with very lightweight processes, and neatly avoids both the hairiness of shared-everything threading and the inconvenience of explicit state machines.

The unit of concurrency in Erlang is the process, which is the central idea in the language. An Erlang process is more like a Unix process than a thread, but is very lightweight -- creating a process takes only a few microseconds and a few hundred bytes of memory. Erlang processes share no mutable memory or variables and only communicate explicitly, using send and receive primitives. When a process receives messages, it specifies which types of message are acceptable, causing others to be queued in a mailbox -- this gives a very beautiful synchronization-through-communication programming style from CSP. Erlang processes own files and sockets in the same way as Unix processes, which helps to cleanup after errors, and can detect each others' termination (kinda like SIGCHLD) and so on.

The Erlang runtime system itself is a C program implemented, AFAIK, very much like Squid, except that the main loop is an Erlang interpreter instead of a HTTP proxy. If you ported Squid to Erlang, with one Erlang process per connection, I think it would execute in much the same way wrt. performance and system calls (though I'm no expert on Squid or the Erlang runtime system.) Overall you get the efficiency of select/poll/...()-loop state machines plus a programming interface easier than unix processes, which I say is just great :-).

That's the best late-night summary I can come up with :-). I skipped some other interesting bits about Erlang: that all data structures are immutable, the ways for writing robust programs, nicely integrated distributed programming, and the funky stuff borrowed from functional languages.

Here are some nice links for more about Erlang:

• Open Source Erlang website (apparently some DNS trouble just now), where you can download the book Concurrent Programming in Erlang as a PDF. The book is highly recommended - light and enjoyable reading.
• Lightweight Languages Workshop #2 homepage, where you can watch a recent talk of Joe Armstrong introducing Erlang (he's one of the inventors.)
• The Erlang publications page, which I link so that I can give this thought provoking quote, from the original Erlang paper:
  In programming large systems, many small programming errors will be made - we view this as inevitable. Formal systems and exhaustive test procedures are currently not capable of ensuring fault free software for systems of the size and complexity of modern telecomms applications. Given that errors will be made, we are interested in the problem of detecting and handling those errors in such a manner that the system as a whole exhibits satisfactory behaviour in the presence of errors.