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Re: Comments on GRDDL (using 3rd-party XML schemas with GRDDL) [OK?]

From: Chimezie Ogbuji <ogbujic@ccf.org>
Date: Tue, 24 Jul 2007 15:14:35 -0400
To: "C. M. Sperberg-McQueen" <cmsmcq@acm.org>
cc: "Booth, David (HP Software - Boston)" <dbooth@hp.com>, "Dan Connolly" <connolly@w3.org>, "Andrew Eisenberg" <andrew.eisenberg@us.ibm.com>, public-grddl-comments@w3.org, w3c-xsl-query@w3.org
Message-ID: <1185304475.20722.39.camel@otherland>

I have the same disclaimer as David's (this is not an 'official'
response), etc..

On Tue, 2007-07-24 at 12:12 -0600, C. M. Sperberg-McQueen wrote:
> On 18 Jul 2007, at 22:26 , Booth, David (HP Software - Boston) wrote:
>  > 1. A key concept of GRDDL is that the XML document author has
>  > *authorized* the resulting RDF as a "faithful rendition" of the
>  > original XML document: http://www.w3.org/TR/grddl/#sec_rend
>  > The central idea is that there must be a clear chain of authority
>  > leading from the XML document to the resulting RDF, either via
>  > direct annotations or by explicit reference to some a namespace or
>  > profile document.
> It seems to me to be a surprising and unfortunate lurch toward a
> closed-world assumption, to require that the RDF representation of a
> document originate with, or be authorized by, the creator of the
> document.  I think that's a pretty severe design error, and one that
> surprises me, coming from the Semantic Web activity.

We should be careful not to conflate closed-world assumption (which is
specifically a feature of inference - GRDDL has nothing to do with
inference) with functional data transformations.  Consider that XSLT
also doesn't rely on any other input to the transformation other than
what is explicitly included in the source markup.

The RDF representation is the result of a transformation function and as
most functions go they need well-defined inputs for them to be
'functional'.  Otherwise you don't really have a 'faithful' RDF

> The problem reported to you from the XML Schema, XML
> Query, and XSL Working Groups is that if GRDDL works on some schema
> documents (by which I mean: GRDDL can locate transformations described
> or mentioned in some schema documents), but only on schema documents
> dereferenceable from a namespace name, then GRDDL is imposing a
> restriction which has nothing to do with guaranteeing a clear chain of
> authority from the XML document.

I think there is some confusion about authority and the relationship
between GRDDL and schema documents.  GRDDL treats XML schema documents
same as any other XML document: it simply tries to extract RDF, some of
which will provide a trail to follow in order to extract subsequent RDF.

The crux of the issue is that the *only* general (linking) mechanism for
XML understood by a GRDDL aware agent is via Namespace URI dereference.
The kind of schema association being discussed here doesn't fall under
either of these categories, but this has nothing to do with any specific
way GRDDL handles XML schema documents.  

Since the charter targeted XML in general and XHTML specifically, there
are (in addition) XHTML-specific linking mechanisms which GRDDL
understands.  However, it should be clear that to a GRDDL-aware agent,
an XML schema is simply an XML document.  XHTML is the only specific XML
dialect it has native interpretations for.

So it's not that GRDDL is imposing any restrictions on how it locates
XML schema documents, but rather it restricts the means by which
additional XML content is considered in calculating a 'faithful' RDF

>    (7) Schema documents for a namespace which are not at the namespace
>        name and not pointed to from the document, but found elsewhere
>        (e.g. in a local repository), are not authorized by the
>        author of the document.
>        (Not true.)

The problem here isn't so much whether they are 'authorized' but the
lack of a well-defined mechanism for resolution outside of those
mentioned above (all of which are 'authorized' via linkage in web space
- the most pervasive mechanism for linkage in this space).  

>    (8) Since document authors have not authorized GRDDL transforms
>        pointed to from schema documents not findable at the namespace
>        name and not pointed to from the document itself, GRDDL should
>        not support such schema documents.
>        (Since the premise is untrue, the conclusion is unsound.)

The premise seems contrived.  In the absence of alternative,
well-defined resolution mechanisms, the GRDDL specification simply gives
*a* normative means for locating additional XML content (a subset of
which are XML schema documents - but GRDDL does not make this
distinction!) whose GRDDL results are relevant to the source document.

> If I have understood your logic, the problem is that GRDDL is
> attributing properties to schema documents and their locations which
> don't seem to have any basis in the relevant specs (namely XML Schema
> 1.0 and 1.1).

There is nothing mentioned in the specification along these lines.
Notice GRDDL has no normative dependencies on XML Schema 1.* (it would
have to if it handled XML schemas in such a fashion).

>   Nothing in the XML Schema spec licenses the inference
> that schema documents dereferenceable from a namespace name have
> more authority than others.  The choice of schema documents is not
> made by the author of the document but by the user or user agent
> asking for validation to be performed.

Again, GRDDL treats XML schemas same as any other XML document (with the
exception of XHTML).

>  > 2. The use (or non-use) of XML Schema is irrelevant to GRDDL.
> That's one reason it so mystifies me to see GRDDL making rules about
> which schema documents are to be trusted and which are not.  Isn't
> that something that should be out of scope for the GRDDL spec?

It *is* out of scope.  So much so that there is no such mention of rules
about schema documents :)

> How on earth do you get from the proposition "Transformation T is not
> indicated directly by the XML document" to "Transformation T is a
> third-party transformation" ?!  

I don't think it is useful to consider 'authority' as the defining
criteria here.  This isn't a matter of authority, but of reproducible,
deterministic, data transformations and well-defined mechanisms that can
support this.

[1] http://www.w3.org/TR/grddl/#rule_GRDDL_transformation
Chimezie Ogbuji
Lead Systems Analyst
Thoracic and Cardiovascular Surgery
Cleveland Clinic Foundation
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Cleveland, Ohio 44195
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Received on Tuesday, 24 July 2007 19:15:08 UTC

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