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Re: Sub-pipelines

From: Rui Lopes <rlopes@di.fc.ul.pt>
Date: Wed, 22 Mar 2006 11:17:11 +0000
Message-ID: <44213237.5030805@di.fc.ul.pt>
To: Alessandro Vernet <avernet@orbeon.com>
Cc: public-xml-processing-model-wg@w3.org

Alessandro Vernet wrote:
> Jeni, Rui, Norm,
> It is a given that we want a pipeline to be able to call another
> pipeline. Now this raises two questions, both of which being discussed
> in this thread:
> 1) Do we want to provide a construction in the pipeline language to
> declare sub-pipelines, that is: a pipeline declared in a pipeline
> document (like some languages allow a function to be declared in a
> function)?
> Norm suggested a way to declare a sub-pipeline. I asked if we really
> need this feature. Rui say he can live with just externally defined
> pipelines. I think that we don't really have a use case or requirement
> that would justifies adding an additional construction to the
> language. So I am not strongly in favor of addressing the issue of how
> to define a sub-pipeline in a pipeline in a first version of pipeline
> language.

Maybe an intermediate solution could be something like pipeline 
aggregation in some root element. This way, we could define several 
pipelines inside the same document without defining sub-pipelines inside 
a pipeline. This may lead to another question: how to trigger the 
execution of one pipeline? Maybe it's up to the environment to figure it 
out, or we can have a special purpose name for the default pipeline 
(like Make's "all", c/c++/java main(), etc.). E.g.:

     <p:pipeline name="main">
         <p:input ... />
         <p:output ... />

         <p:step name="pipeline">
            <p:input name="pipeline" href="#expandandvalidate" />
         <p:step name="xslt" ... />

     <p:pipeline name="expandandvalidate">

> 2) Should invoking a pipeline be any different than invoking, say, an
> XSLT stylesheet? Should we invoke a pipeline with:
> <p:step name="pipeline">
>     <p:input name="pipeline" href="expandandvalidate.xpl"/>
>     <p:input name="schemas" href="a.xsd"/>
>     <p:input name="document" href="mydoc.xml"/>
>     <p:output label="ok1"/>
> </p:step>
> Or:
> <p:call-pipeline name="expandandvalidate">
>     <p:input name="schemas" href="a.xsd"/>
>     <p:input name="document" href="mydoc.xml"/>
>     <p:output label="ok1"/>
> </p:call-pipeline>
> Jeni says that the second way is better because it makes it easier for
> GUI tools and users to identify reusable parts. I tend to like the
> first way, because:
> a) It makes the pipeline language simpler (no need to define anything
> special to call pipeline).
> b) It allows a pipeline to generate another pipeline (say with XSLT)
> and to run it.

We should be careful with dynamically generated pipelines, as that 
feature enables the construction of pipelines with a cyclic digraph 
topology in runtime (i.e. two pipelines invoking each other).

I'm not against cycles, but my feeling is that cycles should be verified 
statically (in compile time).


Received on Wednesday, 22 March 2006 11:17:28 UTC

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