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From giacomo <giac...@apache.org>
Subject Re: Sitemap parameters object & Threads
Date Thu, 20 Dec 2001 20:38:22 GMT
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.

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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