Re: Want GRDDL intro for WG-dummies :)

Michael,

Rinke and Bijan has already answered; just let me add the very practical 
side to it.

There are already a bunch of GRDDL implementations out there. What they 
do is, roughly: look at the XML document or look at the namespace 
document of the XML document, find pointers to a transformation (or 
several), execute that transformation with some sort of an engine, and 
reap the result in RDF/XML. What GRDDL tells you is how to achieve these 
steps automatically by just following the various URI-s placed at some 
standardized positions. So you are right that the core is the XSLT 
transformation because that is what does the real work, but the issue is 
to find the right transformation automatically.

As Bijan said, the GRDDL specification itself does not mandate the 
transformation to be in XSLT. That is the theory. The practice is that 
all GRDDL implementations that I know about (in Jena, in OpenLink, in 
the Tabulator, and probably others that I do not know about) implement 
XSLT, and XSLT only. So if we want existing GRDDL implementations to 
work with OWL/XML in practice, we should face this reality (in my view).

The question is whether the WG should develop, or 'bless' an 
implementation, and I think that is the core of the disagreement among 
some WG members. There is one more aspect that we have to consider in 
this discussion. The really nice way of using GRDDL would be to add the 
relevant pointers to the relevant transformation to the namespace 
document of OWL/XML (I say the 'really nice'; it is also possible to add 
such a pointer to each individual OWL/XML document, but that is an extra 
burden on the user). However, the namespace document is owned by this 
Working Group as long as it is around, ie, adding any pointer into this 
namespace document is, essentially, a blessing of that XSLT transform.

By the way, this would not be unheard of. The RDFa task force of the SWD 
and XHTML2 Working Groups has decided to add an XSLT reference to the 
XHTML namespace document, so that all XHTML documents should be 
GRDDL-able using this XSLT transformation. The XSLT stuff itself was 
written by Fabien Gandon, member of that group. A similar work is done 
by the POWDER working group that defines an XML format for what they 
want to do, plus an XSLT transformation that is used in a GRDDL mechanism.

I hope this helps...

Ivan

P.S.1.: a very practical application that shows why I believe it would 
be good to have this mechanism work in practice asap: The promise is 
that OWL/XML makes it much simpler to write down an ontology than 
RDF/XML. On the other hand if, for example, I want to use OWL-R-Full, 
and I want to feed it into an OWL-R engine, I have to write it in RDF. 
Well, if the GRDDL transformation works, I can write down my OWL-R 
ontology in OWL/XML, and let the rest be done automatically without 
forcing the OWL-R implementation to implement a dedicated OWL/XML 
parser. I am not sure Oracle implements GRDDL already or not, but I can 
see that mechanism working very well with OpenLink (that has a built in 
rule engine, too, so implementing OWL-R there would be a breeze) or Jena 
with an upcoming OWL-R implementation...

Michael Schneider wrote:
> Ivan Herman wrote:
> 
>>>> O.k. Another possibility (but that is really the other extreme):
>>>>
>>>> http://www.w3.org/People/Ivan/CorePresentations/SWTutorial/Slides.pdf
>>>>
>>>> and look at slides #88-#92
>>> Yes, much better! :)
>>>
>>> I will have a deeper look on it later this day. Be prepared to answer
>> concrete
>>> questions w.r.t. our OWL/XML GRDDL issue. :)
>>>
>> I will do my best...
> 
> Ok, then I will put a deliberately provocative question: 
> What *is* GRDDL actually?
> 
> I can define an XSLT between two XML formats, anyway. 
> Is GRDDL more than just a hint to people that they can 
> use XSLT to transform arbitrary XML to *RDF/XML* in particular?
> 
> Or putting it differently: What would be missing, 
> if GRDDL wouldn't exist, given that there is already XSLT?
> 
> Or putting it even more differently: What would be wrong with 
> just defining *some* XSLT from OWL/XML to RDF/XML, 
> simply forgetting about the *word* "GRDDL"?
> 
> Cheers,
> Michael
> 

-- 

Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
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