Pieter-- Last year, I read the book, Programming Erlang, by Joe Armstrong, and I was fascinated by the ideology behind the general thread-per-object approach, where each "object" is managed by its own thread, via message passing.
Erlang has a really "involved" message passing scheme, involving pattern matching, a "mailbox", recieve timer, a "save queue". Needless to say, all this makes a very powerful way of prioritizing messages, so a busy object manager can pull high-priority requests from the mailbox and act on them immediately, saving lower priority requests for later. I see in a paper at http://zeromq.org/blog:multithreading-magic, the same sort of admiration for Erlang's methodology. But... I'm not seeing the cure-all, end-all, solution to concurrency problems, and it bothers me, because I'm probably missing something fundamental, something I should have picked up on, but didn't. Erlang allows other processes/threads to drop requests in an object's mailbox, but it also has a mechanism for waiting until the action is complete, as the "object" can send a response. It's this response wait that is the killer. Now, I've done a lot of work on/with Asterisk, and it is heavily mulithreaded, in the old-school way, and has a ton of critical sections, and locks, and mutiple locks for a single action. They have evolved some fairly involved strategies to avoid deadlocks, including putting a timer on the lock, and if it times out, giving up the lock they already have, and starting over, allowing the contending party to obtain the lock they need, finish their "thing", which allows you to continue and obtain the lock you need to do your "thing". And on and on it goes. Now, I didn't go and look up the particulars about "N-party dance", etc., but the classic resource deadlock situations still seem in play when you have to wait for completion. A asks B to complete a task, and waits for him to respond. In order to get that done, B requests A for something, and waits forever for A to finish. And so on. Perhaps C or even D are also involved. I keep thinking that such basic situations aren't solved by switching to the Erlang methods. There must be some architectural, perhaps hierarchical organizing, some sort of general design practice, that can overcome these kinds of problems, I'm just blind to it at the moment. Situations like 'atomic' changes on two or more objects at once, etc. and I don't see in the fog, how Erlang solves these problems in general. Can someone point me to some literature that might make things clear? murf -- Steve Murphy ParseTree Corporation 57 Lane 17 Cody, WY 82414 ✉ murf at parsetree dot com ☎ 307-899-5535
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