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Re: RDFa + RDF/XML Considered Harmful? (was RE: Ordnance Survey data as Linked Data)

From: Richard Cyganiak <richard@cyganiak.de>
Date: Mon, 14 Jul 2008 11:15:00 +0100
Cc: "Tom Heath" <Tom.Heath@talis.com>, "Kingsley Idehen" <kidehen@openlinksw.com>, public-lod@w3.org, semantic-web@w3.org
Message-Id: <06623516-E438-4520-BB13-D3970D3701D0@cyganiak.de>
To: "Mark Birbeck" <mark.birbeck@webbackplane.com>

Hi Mark,

On 14 Jul 2008, at 10:22, Mark Birbeck wrote:
> I would say that RDFa has made the situation an order of magnitude
> _less_ complicated, since authors and developers now have an easier
> way to publish metadata;

Well, RDFa has made life simpler for those publishers whose  
requirements are met well by RDFa. It has made life more complicated  
for client developers, since they have to support yet another RDF  
syntax.

I think RDFa is an important piece of the SemWeb technology puzzle,  
but your claim that it “makes the situation an order of magnitude less  
complicated” is unfounded, IMO.

> as Kinglsey said, increasing the number of
> ways to publish metadata increases the number of possible clients that
> might consume the data:

Kingsley was talking about a situation where the publisher offers  
*all* different methods of publishing RDF. Sure this increases the  
number of theoretically possible clients, but it also increases the  
cost of publishing RDF.

>>> I also forgot to mention obvous use of RDFa in the HTML doc
>>> which broadens the range of rdf aware user agents tha
>>> commence RDF discovery from HTML
>
>> Does it not
>> also complicate the picture of making provenance statements using  
>> named
>> graphs, if the subject of the triple could be both an HTML document  
>> and
>> an RDF graph?
>
> Is it possible to distinguish a graph URI? I hadn't realised that. It
> would certainly be a good idea to  have an rdf:Graph type, but I
> hadn't realised that there was one.

There is no rdf:Graph type, and Tom didn't say there is one. Tom used  
the words “RDF document” and “RDF graph” synonymously, which is a bit  
sloppy.

> However, is not an rdf:Graph a type of information resource?

Depends on how you squint at it. Technically speaking, a graph, as a  
mathematical entity, is a fixed, immutable thing. An information  
resource, on the other hand, can (and often does) vary over time. That  
is, tomorrow it might have a different representation than today.

One could say that, in the case where we look at the Web through RDF  
goggles, the concrete representation that we get back at any time from  
an information resource is an RDF graph. The information resource  
itself is not an RDF graph, but rather a function that returns an RDF  
graph.

Now, if we ignore time and pretend that we just deal with a static,  
frozen-in-time snapshot of the Web, then it's probably okay to pretend  
that information resources are RDF graphs (because the function is  
constant). This is what the RDF browsers out there do in practice,  
they treat the Web as a set of named graphs, where the URIs of RDF  
documents are the graph names.

> An
> RDF/XML document delivered from a web server is both a document and a
> graph,

Yes and no, see above. It's true if you ignore time, but  
architecturally speaking, Web documents (information resources) change  
over time, while graphs are immutable.

> but we have chosen to ignore that in the RDF architecture; it's
> not possible to say 'this graph was published by', in RDF/XML, i.e.,
> to talk about the information resource itself, because you will always
> be talking about whatever the RDF/XML itself is about.

Huh? Of course it is possible to talk about information resources in  
RDF. Assume that this is the content of an RDF document published at http://example.com/my_rdf_document.rdf 
  :

<rdf:Description rdf:about="http://example.com/my_rdf_document.rdf">
     <dc:publisher>Richard</dc:publisher>
     <dc:date>2008-07-14</dc:date>
     ...
</rdf:Description>

This is an RDF document talking about itself. This is standard  
practice, you can find examples like this in the RDF spec, and  
everywhere in the wild.

(Note that in the example, I could have used rdf:about="", because an  
empty URI is expanded to the URI of the document.)

> But there is no reason that we could not enable this, and if we wanted
> to go this route, RDFa+HTML allows it.

It's equally possible in RDFa and RDF/XML, today.

Best,
Richard


>
>
> Regards,
>
> Mark
>
> -- 
> Mark Birbeck, webBackplane
>
> mark.birbeck@webBackplane.com
>
> http://webBackplane.com/mark-birbeck
>
> webBackplane is a trading name of Backplane Ltd. (company number
> 05972288, registered office: 2nd Floor, 69/85 Tabernacle Street,
> London, EC2A 4RR)
>
Received on Monday, 14 July 2008 10:15:52 UTC

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