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From Sylvain Wallez <>
Subject Re: Substractive view labels (long)
Date Sun, 31 Mar 2002 09:19:58 GMT
Stefano Mazzocchi wrote:


>When I was designing the view concept, I was looking at resources from
>the 'user-agent' perspective and wanted to provide a way for them to
>'unlock' the resources and have access to specific 'views' of them.
>This looking into the resource from the outside (looking into the
>serializer, so to speak), triggered the solution of having the pipeline
>stop at the 'last' encoutered label, which is the "first" label that the
>'user-agent' would encounter if it was scanning the pipe from the
>Why so?
>Well, the server uses the pipelines to *augment* the information and
>shape it until it's ready for consumption (in case of POST/PUT requests)
>or production (in case of GET/HEAD requests).
>The 'user-agent' looks at the pipelines from the outside-in and wants to
>connect to 'different' or less processed 'pipeline stages'.
>With this vision in mind, it looks very elegant to provide this 'exit
>from last label' behavior, because is the behavior that a 'user-agent'
>would expect from decomposing the pipeline from the outside looking in.

Stefano, the above "outside-in" sentence shed a completely new light on 
my understanding of views, and I'm now convinced that views should start 
from the last label !

The point that made me wait before answering was about the execution of 
indirect components and their possible side-effects (actions). I now 
think that side effects of indirect components are part of a pipeline 
semantics. And views should not change this semantic, but only provide a 
different mmmh... view ;) on the pipeline result.

So branching on a view should not change the actions taken in the 
pipeline. If we need different side-effects, then we should consider 
having different pipelines. What do you think ?

Ah, and since you gave us so brilliant explanations, would you please 
(finally ;) consider answering my other question about views and 
aggregation ? In short, the question is about the behaviour associated 
to <map:part> labels ? Should they cause view branching, or only filter 
those parts that belong to the view, branching being decided by the 
labels on <map:aggregate>. My (current) opinion is filtering, but you 
may again have an explanation that will make me change my mind.

Again, see for 
more details on this.

Thanks for being such a great mind and sharing it with us.


Sylvain Wallez
 Anyware Technologies                  Apache Cocoon 

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