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Re: Wormholes

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On 04.09.2012 15:18, Steven Simpson wrote:
> On 04/09/12 02:12, Roedy Green wrote:
>> Somehow I must pass information down the long chain from x to y.  This
>> means changing the signatures of all the intermediate methods, and
>> adjusting code to the new way.  This can cause ripples incommensurate
>> with the triviality of the change.
>
> Assuming that you can't improve your structure or refactor, a
> ThreadLocal might be appropriate as your wormhole.
>
> I've had to do this lately.  The intermediate steps were recursive calls
> through a hierarchy.  y was a custom implementation of a node nested in
> that hierarchy.  x started off the recursive call.
>
> If I was only using the hierarchy once, for one call by x, I would have
> simply embedded the context in y.  But the hierarchy was to be re-used,
> and y needed a different context for each call by x.  Also, calls by x
> could be concurrent.
>
> However, I could guarantee that any single call by x would invoke y by
> the same thread.  That allowed the context to be passed by a
> ThreadLocal, set by x just before the call.

I knew finally someone would suggest ThreadLocal for this.  This might 
be even worse than global variables, especially since you pass hidden 
state which usually makes testing more difficult.  The proper approach 
would be to pass the state down the call chain.

IMHO the best usage for ThreadLocal is to cache state *inside a class* 
if calls may be concurrent and the cost of creating the state is 
significantly high.  But using it to pass information between classes 
because one wants to avoid adding method parameters is asking for trouble.

Also, you need to be aware that the lifetime of these objects can be 
quite long (there was a discussion about various aspects of ThreadLocal 
in light of thread pools here earlier).

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

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Wormholes Roedy Green <see_website@mindprod.com.invalid> - 2012-09-03 18:12 -0700
  Re: Wormholes eric@invalid.com (EricF) - 2012-09-04 02:06 +0000
    Re: Wormholes Roedy Green <see_website@mindprod.com.invalid> - 2012-09-04 10:08 -0700
    Re: Wormholes Wanja Gayk <brixomatic@yahoo.com> - 2012-09-09 13:55 +0200
  Re: Wormholes Arne Vajhøj <arne@vajhoej.dk> - 2012-09-03 22:34 -0400
    Re: Wormholes Stuart <DerTopper@web.de> - 2012-09-04 12:15 +0200
  Re: Wormholes markspace <-@.> - 2012-09-03 21:00 -0700
    Re: Wormholes Robert Klemme <shortcutter@googlemail.com> - 2012-09-04 07:08 +0200
  Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-04 08:47 +0200
    Re: Wormholes Lew <noone@lewscanon.com> - 2012-09-04 03:10 -0700
      Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-04 16:09 +0200
        Re: Wormholes Gene Wirchenko <genew@ocis.net> - 2012-09-04 08:34 -0700
          Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-04 17:46 +0200
            Re: Wormholes Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-09-04 09:31 -0700
        Re: Wormholes Lew <lewbloch@gmail.com> - 2012-09-04 11:14 -0700
        Re: Wormholes Arne Vajhøj <arne@vajhoej.dk> - 2012-09-04 19:59 -0400
    Re: Wormholes Roedy Green <see_website@mindprod.com.invalid> - 2012-09-04 13:20 -0700
      Re: Wormholes Arne Vajhøj <arne@vajhoej.dk> - 2012-09-04 20:00 -0400
      Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-05 09:01 +0200
      Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-05 09:02 +0200
      Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-05 09:02 +0200
        Re: Wormholes Zermelo <zermelo@teletu.it.nospam.invalid> - 2012-09-05 09:03 +0200
      Re: Wormholes markspace <-@.> - 2012-09-05 17:34 -0700
  Re: Wormholes Steven Simpson <ss@domain.invalid> - 2012-09-04 14:18 +0100
    Re: Wormholes Robert Klemme <shortcutter@googlemail.com> - 2012-09-05 21:51 +0200
      Re: Wormholes Steven Simpson <ss@domain.invalid> - 2012-09-05 23:26 +0100
        Re: Wormholes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2012-09-06 09:32 +0000
          Re: Wormholes Steven Simpson <ss@domain.invalid> - 2012-09-06 11:47 +0100
            Re: Wormholes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2012-09-06 15:10 +0000
              Re: Wormholes Steven Simpson <ss@domain.invalid> - 2012-09-06 20:12 +0100
                Re: Wormholes Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2012-09-06 21:08 +0000
  Re: Wormholes Nigel Wade <nmw@ion.le.ac.uk> - 2012-09-04 17:26 +0100
  Re: Wormholes Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-09-04 09:28 -0700
  Re: Wormholes David Lamb <dalamb@cs.queensu.ca> - 2012-09-04 16:54 -0400

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