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From "Carsten Ziegeler" <cziege...@s-und-n.de>
Subject RE: Sitemap parameters object & Threads
Date Fri, 21 Dec 2001 07:48:28 GMT
Giacomo Pati wrote:
> 
> On Thu, 20 Dec 2001, Marcus Crafter wrote:
> 
> > Hi All,
> >
> > 	Hope all is well.
> >
> > 	On avalon-dev there's been a short discussion about the
> > 	Parameters object that is used in the generated sitemap
> > 	class. I thought I'd bring it up here so we can flesh out a
> > 	solution.
> >
> >         Have a look at this snippet from a generated sitemap:
> >
> > ----------------------
> > public class sitemap_xmap extends AbstractSitemap {
> >   static final String LOCATION = "org.apache.cocoon.www.sitemap_xmap";
> >
> >   static {
> >     dateCreated = 1007984808622L;
> >   }
> >
> >   /** An empty <code>Parameter</code> used to pass to the 
> sitemap components */
> >   private Parameters emptyParam = new Parameters();
> >
> > <snip>...
> >
> >   if (isSelected("userstatus", "permitted", listOfMaps,
> >                emptyParam, objectModel)) {
> > ----------------------
> >
> > 	The problem is 2 fold, and has to do with the 'emptyParam'
> > 	reference when it's passed into the invocation of selectors and
> > 	actions, etc:
> >
> > 	1. The Parameters class itself is not thread safe. Concurrent
> > 	updates to the one object may corrupt data.
> >
> > 	2. There is only one parameter object across the whole sitemap,
> > 	accessed by all threads, and potentially many pipelines.
> >
> > 	The second point is the killer if a developer decides to set a
> > 	parameter in a selector, or action, etc, and read it back elsewhere.
> > 	The problem is that the one object is shared among all 
> threads - if 2
> > 	threads update the object (regardless of whether it's threadsafe
> > 	or not), results can be undefined if they use the same key.
> >
> > 	As far as I can see there's no real need to be able to set
> > 	parameters in this object in selectors and actions, etc, as 
> the Request
> > 	object can handle sharing of data between pipeline components via
> > 	set/getAttribute (or is there some other use for it ?).
> >
> > 	If this is the case, we could make the parameters object read-only
> > 	before it's passed to selectors, etc, which would prevent 
> this problem
> > 	from occuring (for example by wrapping it, or toggling its
> > 	read-only flag).
> 
> If we can vote +1 here I can commit an already patched sitemap.xsl which
> makes the emptyParam read-only.
> 
Hey, you beat me by 15 minutes. Hmpf, next time, I'll be faster.
Ok, keeping it short: +1 to commit the patch.

Carsten

> Giacomo
> 
> >
> > 	Alternatively, we could change the generated sitemap code to
> > 	create a new Parameters object per thread. I'm not sure how much
> > 	this influences other uses of this object though. Perhaps
> > 	someone else a bit closer to that part of the code might be able to
> > 	comment better ?
> >
> > 	I'm wondering what everyone else's thoughts are on a solution
> > 	for this as it seems 'emptyParam' is used almost everywhere in
> > 	the generated sitemap class, and could be a touchy thing to
> > 	change.
> >
> > 	Any comments/thoughts ?
> >
> > 	Cheers,
> >
> > 	Marcus
> >
> >
> 
> 
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