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Re^8: Help designing a threaded service

by Tommy (Chaplain)
on Jan 26, 2014 at 23:11 UTC ( #1072153=note: print w/replies, xml ) Need Help??


in reply to Re^7: Help designing a threaded service
in thread Help designing a threaded service

Your response is exactly why I want to go with threading: because I can solve all my hard problems via shared memory vars, easily reconnecting a request to it's output stream. But I have to face some unhappy facts: I don't know enough about threading to know what's necessary to keep the thread memory consumption from ballooning out of control. I also don't know how to handle corner cases that I've yet to identify with the creation of a highly-available multi-threaded network service. And finally I don't yet know gracefully kill off threads that get "stuck" without resorting to a SIGKILL (which isn't exactly a showstopper, but the other issues are).

The multi-threaded IRC chat bot code that was shared earlier in this discussion is too bare-boned to inspire confidence that extending it could handle the problems I've outlined above. If I venture down this path, I'd need a map and a guide. And frankly the latter and more essential of the two is hard to come by in *nix land where forking is king and threading has a bad reputation for, from what I can tell, all stupid reasons.

Tommy
A mistake can be valuable or costly, depending on how faithfully you pursue correction
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Re^9: Help designing a threaded service
by BrowserUk (Patriarch) on Jan 27, 2014 at 05:14 UTC
    But I have to face some unhappy facts: I don't know enough about threading ...

    But you do know enough about multiplexing processes via pipes or unix-domain sockets; or attaching shared memory segments in order to solve those problems?

    With regard to the potential barriers you perceive to a threaded solution:

    1. what's necessary to keep the thread memory consumption from ballooning out of control

      Why do you think this would happen?

      There are two ways memory consumption grows out of control:

      • You program the server to spawn lots of threads.

        So don't do that. Re-use existing ones.

      • You allow queues to grow unconstrained.

        So, don't do that. Constrain them.

    2. how to handle corner cases that I've yet to identify with the creation of a highly-available multi-threaded network service.

      I don't know how to handle unidentified problems either.

      But generally, if and when a problem is identified, it usually has an obvious and simple solution.

    3. I don't yet know gracefully kill off threads that get "stuck" without resorting to a SIGKILL

      There is no such thing as a "graceful kill".

      Needing to kill a thread (rather than either asking it to die or allowing it to come to a 'natural death') is a really bad design technique. One of absolutely last resort when all other options fail. Design properly and it should never be necessary.

      With a server, the primary problem is one of ensuring that you never enter a read state to a socket unless there is data available.

      Under windows, where Perl doesn't provide access to the appropriate low-level IO routines to allow reads to be interrupted, that pretty much forces a polling (select or IO::Select) server design. That's a pain because it means implementing line or block buffering yourself; but it's not terribly hard. And if you strictly limit your select groups to just one or two handles per thread, each thread can still deal with just one client and its remote proxy thus avoiding the need to put all your pipes through a single central loop.

      Under *nix, I've been told that read & readline are interruptible using alarm, so you should be able to avoid polling whilst still retaining the guarantee that if a client goes away, you won't leave a thread stuck. (But I don't have the experience to back that up, nor the environment to test it.)

    But, it's your project and your call. You have to decide which set of skills are going to be easiest for you to acquire; which probably comes down to which route to your goal you will get most support in pursuing.

    From my perspective, the most daunting part of your task is the webserver/AJAX client bits, of which I have little to no knowledge, and for which I am not set up nor have any desire to be so.


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    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.

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