Richard Cyganiak wrote:
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.
If clients can rely on a solid RDF parsing toolkit for each language, I
think this is a bearable (and perhaps healthy) division of labour. We've
a way to go before that's achieved yet (eg. a 'just use x' answer for
each of Ruby, Perl, Prolog ..., where x has rdfa, grddl, rdfxml, turtle
etc support).
http://en.wikipedia.org/wiki/Postel%27s_law seems pertinent:
"Be conservative in what you do; be liberal in what you accept from
others."
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.
Talk of 'orders of magnitude' without a clear metric for what we're
comparing, ... worries me. RDFa is a good thing. A very good thing. But
don't ask me to put numbers on that.
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 think we'll see recipies around eg. OAuth whereby someone publishes in
RDFa (or RDF/XML) and a helper service periodically converts and reposts
(via OAuth/AtomPub) the derrived data back to the site. Perhaps after
doing a bit of OWL reasoning, identity reasoning (smushing) etc.
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.
Not sure I'd even go that far. RDF formally has very little to say about
changes over time. It's like trying to talk to residents of
http://en.wikipedia.org/wiki/Flatland about the mythical 3rd dimension.
Of course RDF-based information systems do need this notion; hence
various graph and named graph constructs.
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.
Yup. When the world stops moving, we can use RDF to describe it. And we
can use RDF to manage the descriptions of the world from different
points in time and perspectives too. And we can sometimes flatten these
diverse descriptions into a single one. Throughout all of this the word
'graph' gets bandied about in slightly different ways, which can make it
very confusing. In parallel, the notion of 'Document' also kinda melts
between our fingers when we try to pin it down (the FRBR work is this
communities best narrative for what's going on there, I think).
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.
Yup. It might be clearer if the .rdf on the URI was missing and the
thing was conneg'd HTML vs XHTML/RDFa vs RDF/XML etc. dc:publisher is
stretchy enough to work in either case.
(Note that in the example, I could have used rdf:about="", because an
empty URI is expanded to the URI of the document.)
Yup. Again a common idiom.
Note that some such styles do work better when it's a distinct
bag-of-bits that's being talked about. For example the wot:assurance
construct,
http://web.archive.org/web/20070814001334/http://usefulinc.com/foaf/signingFoafFiles
http://xmlns.com/wot/0.1/ etc
...
I used to use this to sign the FOAF spec RDF, but it got a bit confusing
when trying to also use conneg.
cheers,
Dan
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
[EMAIL PROTECTED]
http://webBackplane.com/mark-birbeck
webBackplane is a trading name of Backplane Ltd. (company number
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