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Re: issue-dbooth-9a: GRDDL should be usable in a messaging pipeline [OK?]

From: Jeremy Carroll <jjc@hpl.hp.com>
Date: Tue, 12 Jun 2007 18:06:58 +0100
Message-ID: <466ED2B2.8040804@hpl.hp.com>
To: "Booth, David (HP Software - Boston)" <dbooth@hp.com>
CC: public-grddl-comments@w3.org, "McBride, Brian" <brian.mcbride@hp.com>

Dear David,

In addition, the text I just pointed you at:


is intended to address this comment to.

I will add brief in-line commentary to the first part of your comment:

Booth, David (HP Software - Boston) wrote:
> This is a personal comment -- not on behalf of HP.  
> [Incidently, I will be happy to help supply proposed wording changes to
> the GRDDL spec that would address this issue, though this message does
> not specify them.]
> Suppose I wish to use GRDDL in a Unix pipeline in order to make App1
> produce RDF for consumption by App2 by wrapping App0 and transforming
> its XML output of App0:
>                  App1
>   +------------------------------------+
>   |                                    |
>   |  +------+               +-------+  |               +------+
>   |  |      |               |       |  |               |      |
>   |  | App0 |--> XML msg -->| GRDDL |--|--> RDF msg -->| App2 |
>   |  |      |               | xform |  |               |      |
>   |  +------+               +-------+  |               +------+
>   |                                    |
>   +------------------------------------+
> Some observations:
>  - App0 is not "on the web" and does not have a URI.  However, I suppose
> one could consider it an "information resource" in the TAG WebArch
> sense.

The draft text treats the XML msg as the information resource, which 
seems in accordance with RFC 3986 5.1.2, rather than App0.

>  - Each XML msg is a particular XML instance document -- a concrete
> sequence of bytes, or "representation" in the TAG WebArch sense. There
> is nothing vague or abstract about it. No content negotiation is
> involved.

The XML msg is then an information resource that is it's own representation.

>  - Each XML msg is a separate message whose entire semantics are to be
> exposed by GRDDL transforming it into RDF.
>  - It does not make sense to talk about the GRDDL results of App0 in
> general, as though App0 were a static "information resource". It only
> makes sense to talk about the GRDDL results of a particular XML message.

The draft text does not try to make sense of talking about the GRDDL 
results of App0.

> The main reason the spec at present does not adequately address this use
> case is that in multiple places the spec defines GRDDL results in terms
> of "information resources" instead of "representations".  It does not
> always make sense to talk about the GRDDL results of an information
> resource, because that information resource may produce different
> information content at different times or for different consumers.  But
> it *always* makes sense to talk about the GRDDL results of a specific
> representation.

A representation is itself an information resource, some representations 
also have their own URIs and some don't.



Does the new draft text address the use-case of GRDDL in an XML 
messaging pipeline adequately, and, if it does, is this comment 
adequately addressed?



> For example, suppose an information resource, ir,  produces a different
> representation each time it is queried -- the current weather in Oaxaca,
> for example -- and I have two XSLT scripts that I use to glean RDF from
> them: one extracts the temperature (getTemperature.xsl) and the other
> extracts the humidity (getHumidity.xsl).  The final RDF should be the
> combined result of applying getTemperature.xsl and getHumidity.xsl to
> the *same* representation.  But the spec does not define merged GRDDL
> results for a particular representation, it only defines them for an
> information resource as a whole, which could have a jumble of
> temperatures and humidities from different days.
> Actually, to be more specific, the problem is not that the spec *does*
> define results in terms of information resources -- I don't see big harm
> in *also* doing that (except that by doing so it introduces unnecessary
> ambiguity, which I'll discuss separately) and for namespace and profile
> documents there is a need to go from information resource to GRDDL
> results -- the problem is that it *fails* to define results in terms of
> representations.
> Here are some places in the spec where this problem shows up:
> http://www.w3.org/TR/grddl/#rule_result
> http://www.w3.org/TR/grddl/#rule_merge
> http://www.w3.org/TR/grddl/#rule_rdfxbase
> http://www.w3.org/TR/grddl/#rule_profiletrans
> http://www.w3.org/TR/grddl/#rule_txprop
> http://www.w3.org/TR/grddl/#GRDDL_aware_agent
> http://www.w3.org/TR/grddl/#agt_obl
> These should be relatively easy to fix.  In fact, quite tellingly, some
> of the normative rules define a variable for a posited "information
> resource" but never reference that variable.
> Lest anyone assert that this pipeline use case is outside the WG's scope
> because it isn't reflected in the GRDDL use cases document, I will note
> that:
>  - The GRDDL use cases document contains *many* aspects of problem
> context that nobody intends to become a part of the spec.
>  - The use of information resources in the use cases document seemed
> quite natural to me, as part of the *context*, and hence I did not see
> any problem with the use cases document when I reviewed that earlier.
>  - It seemed obvious to me that GRDDL would be used to define the
> semantics of individual *representations*, since one can only really be
> sure of the semantics of a particular representation.
>  - It *always* makes sense to talk about the GRDDL results of a
> particular representation; it does not always make sense to talk about
> the GRDDL results of an information resource.
>  - The GRDDL use cases document does in fact have a wiki example, and
> since wiki content often changes, that is a good example of the need for
> GRDDL results to reflect the semantics of a particular *representation*
> rather than an information resource in general.
> BTW, I would be happy to join the teleconference (if invited) to further
> explain and answer questions if you think that would be helpful.
> Again, thanks for all your work on this, and please let me know how I
> can be most useful in helping to resolve this issue.
> Thanks
> David Booth, Ph.D.
> HP Software
> +1 617 629 8881 office  |  dbooth@hp.com
> http://www.hp.com/go/software

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Received on Tuesday, 12 June 2007 17:07:23 UTC

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