On Mon, 06 Dec 2004 22:12:35 -0500, Frank W. Zammetti <[EMAIL PROTECTED]> wrote: > Interesting answer Craig... > > I'm sure we've all heard the admonishment that we should not spawn > threads inside a servlet container, the container should be "handling > all threading issues". In fact, I think I even remember something about > it in the spec, or at least being told there was something about it in > the spec :) >
You might be thinking of the J2EE platform spec, which identifies some of the issues with threads in EJB containers, or when you're trying to use transactions. The servlet spec describes a bunch of things that are either likely to or guaranteed to not work when you deal with multiple threads in the context of a single request. > I've always felt that "rule" was a bit ambiguous, to say the least... > > I agree that the solution outlined here is the typical pattern, but if > for no other reason than to foster debate on a potentially interesting > point, why isn't this contrary to that usual bit of advice about threading? > It is. Who ever said that "popular" and "a good idea" were always synoymous? :-) It's also the case that a very large number of webapps are installed on standalone servlet containers like Tomcat, which don't enforce the "no threads" restriction unless you configure them to run a security manager, and then provide a policy that prohibits them. (For example, if I was running an ISP hosting service, that's something I would almost certainly restrict.) My concern about web developers has more to do with all the ways newbies can get themselves in trouble than it is whether starting background threads is a technically elegant idea or not. It is really easy to shoot yourself in the foot. > The same question arises with daemon threads. For instance I have one > app that does some periodic processing, and for various reasons it made > the most sense for it to be tied to the app instance. So, when the app > starts up, I spawn a couple of daemon threads, set them to low priority, > and they do their thing every few hours. I've been told this is also a > Bad Thing(tm) within a servlet container, but I've always felt it really > wasn't. Certainly I've observed no ill effects to do such a thing. > Just out of curiousity, how do ensure that the threads are ever shut down correctly? Are there any controls to ensure that other parts of your app won't hang forever waiting for a response from such a thread? Are there any places where you pass in a servlet request or session object to another thread (even as a parameter to a method that returns synchronously)? Are you assuming that you can propogate transaction context across threads (most implementations use a thread local variable to store this context, so passing a message to a new thread will typically *not* have the same transaction privileges as the old one). If you're not having any of those problems, you're probably fine ... that's only the things to watch out for I thought of in the time it took me to type the paragraph (there's undoubtedly a bunch more). Threads are a powerful problem solving technique, for the right kinds of problems. It's just that the exercise of power also comes with associated responsibility to understand what you're doing, and what you shouldn't do, with your powers :-). > What I'm really getting to here, and I suspect it would be for the > benefit of a great many people reading this, is the simple question, > what is actually OK to do with threads in a servlet container and what > isn't? Perhaps more importantly, what is the reasoning behind the > answers? Any thoughts? (not necessarily just from Craig :) ) > There isn't going to be universally applicabe answers to that question -- it's going to depend a lot on things like whether you're running in a container that supports this (some containers might support appication-spawned threads as a value-add feature with extra help to avoid or solve some of teh problems), what your use case is, what else is going on in the same container, what your security policies are, how skilled your developers are, ... It's hard to give relevant general answers on a question like this. But I would tend to worry less when the number of users is small, the amount of available CPU and memory is large, and your developers have done some basic study on threading and/or you have somebody who understands this stuff doing code reviews. Craig > -- > Frank W. Zammetti > Founder and Chief Software Architect > Omnytex Technologies > http://www.omnytex.com > > > > > Craig McClanahan wrote: > > You're using the typical pattern for this use case (although it's also > > feasible you could use something like JMS to accomplish the > > asynchronicity). The most important thing to do, though, is to ensure > > that you eventually kill the thread no matter what actually happens, > > so that you don't have something needlessly consuming resources for > > the remainder of the lifetime of your server instance. > > > > Craig > > > > > > > > On Tue, 7 Dec 2004 10:50:06 +0800, Yves Sy <[EMAIL PROTECTED]> wrote: > > > >>Here's a follow-up question: > >> > >>I remember creating a thread in one of my Action classes because I > >>needed to show a "Wait while your request is being processed..." page. > >> > >>The flow goes something like: > >> 1. the MAIN thread returns an ActionForward right away that > >>contains the "processing" page; > >> 2. the NEW thread I created goes ahead and makes the back-end call > >>that takes a considerable amount of time to process; > >> 3. After NEW thread returns with the results, it sets a flag in > >>the session that it's done with the processing; > >> 4. Meanwhile, the processing page keeps refreshing itself and > >>sending execution to an action which checks for the session flag set > >>in #3; > >> 5. When it finally finds the session flag, it forwards to the results > >> page. > >> > >>Its working fine for me. No weird behavior on Weblogic or SAP WAS. > >>Although now I'm curious: Is there a better way to approach this > >>problem? > >> > >>Regards, > >>-Yves- > >> > >>On Mon, 6 Dec 2004 15:03:09 -0600, [EMAIL PROTECTED] > >> > >> > >><[EMAIL PROTECTED]> wrote: > >> > >>> > >>>As has been noted by others, JMS would be the better solution for an > >>>asynchronous 'process'. > >>> > >>>But, if you have to use threads then it is probably a better approach to > >>>create a thread pool at appliction initialization and have the actions use > >>>those threads via a common synchronized data structure (hidden behind an > >>>interface). > >>> > >>>Ensure that you have a good unique context for correlating the request and > >>>response (not to be confused with the http req/resp) > >>> > >>>depending upon the volume of traffic you should be able to get away with a > >>>small number of threads. The actual count can be controlled via an extenal > >>>property. > >>> > >>>good luck. > >>> > >>>JC > >>> > >>> "Jim Barrows" > >>> <[EMAIL PROTECTED] To: "Struts Users > >>> Mailing List" <[EMAIL PROTECTED]>, > >>> m> [EMAIL PROTECTED] > >>> cc: > >>> 12/06/2004 02:52 Subject: RE: [OT]Threads > >>> and Servlets Question > >>> > >>> > >>> PM > >>> Please respond to > >>> "Struts Users > >>> Mailing List" > >>> > >>> > >>>>-----Original Message----- > >>>>From: bryan [mailto:[EMAIL PROTECTED] > >>>>Sent: Monday, December 06, 2004 1:15 PM > >>>>To: Struts Users Mailing List > >>>>Subject: Re: [OT]Threads and Servlets Question > >>>> > >>>> > >>>>threads are also a finite resource ( particularly on Linux ). > >>>> > >>>>--b > >>>> > >>>> > >>>>On Mon, 6 Dec 2004 21:13:57 +0100, bryan <[EMAIL PROTECTED]> wrote: > >>>> > >>>>>because you should use a message driven bean to do > >>>> > >>>>something like that. > >>>> > >>>>>--b > >>> > >>>If the brass monkeys upstairs would let me, I would. However, they won't, > >>>and I've used up all of my "oops I did it anyway" cards for a while. So, > >>>while helpful, doesn't really answer my question. > >>> > >>>As for a finite resource...... as someone else said so is memory, disk > >>>space, CPU, etc etc. As for being on linux.... I've done some pretty nasty > >>>multi-threading, in java, on linux and haven't hit that ceiling yet... > >>>ymmmv. > >>> > >>> > >>>>> > >>>>> > >>>>> > >>>>>On Mon, 6 Dec 2004 11:48:15 -0700, Jim Barrows > >>>> > >>>><[EMAIL PROTECTED]> wrote: > >>>> > >>>>>>Okay... I know I've read this somewhere, but can't remember. > >>>>>>Why is it recommended you NOT start a thread inside a > >>>> > >>>>servlet, which would translate to "Why is it a bad idea to > >>>>start a thread inside an action?". > >>>> > >>>>>>And, can you point me at some documentation? > >>>>>> > >>>>>> > >>>> > >>>>--------------------------------------------------------------------- > >>>> > >>>>>>To unsubscribe, e-mail: [EMAIL PROTECTED] > >>>>>>For additional commands, e-mail: [EMAIL PROTECTED] > >>>>>> > >>>>>> > >>>>> > >>>>> > >>>>>-- > >>>>>http://www.revoltingdigits.com > >>>>>https://jestate.dev.java.net > >>>>> > >>>> > >>>> > >>>>-- > >>>>http://www.revoltingdigits.com > >>>>https://jestate.dev.java.net > >>>> > >>>>--------------------------------------------------------------------- > >>>>To unsubscribe, e-mail: [EMAIL PROTECTED] > >>>>For additional commands, e-mail: [EMAIL PROTECTED] > >>>> > >>>> > >>> > >>>--------------------------------------------------------------------- > >>>To unsubscribe, e-mail: [EMAIL PROTECTED] > >>>For additional commands, e-mail: [EMAIL PROTECTED] > >>> > >>> > >>>------------------------------------------------------------------------------ > >>>********** > >>>The information contained in this communication is confidential, private, > >>>proprietary, or otherwise privileged and is intended only for the use of > >>>the addressee. 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