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From Stefano Mazzocchi <stef...@apache.org>
Subject Re: accessing the pipeline structure
Date Fri, 10 Sep 2004 16:08:18 GMT
Jorg Heymans wrote:

> Hi,
> 
> Recently there were 2 requests on the users list about accessing the 
> current pipeline structure.
> 
> http://marc.theaimsgroup.com/?t=109448337100002&r=1&w=2
> http://marc.theaimsgroup.com/?l=xml-cocoon-users&m=109473546831881&w=2
> 
> What does the devlist think about this? Certainly cocoon is being 
> stretched in all areas possible, having the need for metadata about the 
> executing pipeline structure is just one example of this.
> 
> Having this extra metadata would allow for components that can produce 
> different output depending on how they are used in a pipeline (but this 
> probably breaks a few cocoon design rules right?). In my cocoon video 
> rendering application i got around this by creating an xml control 
> structure that was used by all components, so a component could find out 
> what all previous components did and what the results were.
> 
> Thoughts? Is this difficult to implement? A 2.2 feature perhaps?

This smells awefully close to "dynamic pipelines" which is something 
that is considered an anti-pattern in cocoon (because it has been tried 
in the 1.x generation and miserably failed).

People see the pipeline machinery and think they can do things like SOAP 
validation with it... well, it's not designed for that.

A while ago there was the proposal of "content-aware selectors" that 
would allow you to implement what these guys want, I know that there is 
also an implementation floating around even if we never reached 
consensus on whether or not that was a good thing to have.

What you are proposing is "context-dependent behavior"... well, this is 
*exactly* what we are trying to avoid with pipelines: reusability of the 
component is focused to be completely independent of the place where it 
is used in the pipeline. Make the component "location dependent" and 
kiss goodbye to orthogonality, and pretty soon you need a language that 
indicates the potential 'neightbours' of each component... and maybe 
people would want to add conditionals to that language... and pretty 
soon people will ask you to deparate the cross-cutting pipeline concerns 
into pipeline interceptors... and so on.

we *already* have clear separation, let's not ruining it.

Now, the guy needs a SOAP validator transformer. My suggestion would be 
*NOT* to reuse the cocoon pipeline machineries for that but to just call 
something else that does it for you, like Axis, or write his own 
reactor-based valication code in flowscript calling Axis stuff.

-- 
Stefano.


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