A Little XLink Here and There

by Bob DuCharme



While it's generally accepted that the W3C XLink isn't going anywhere and never got very far to begin with, I've stumbled across it in a few new places lately. I recently heard that the on-line help development system for
Longhorn, Microsoft's next generation of Windows, uses XLink to link
from one help topic to another. Like SVG, (from what href='http://www.longhornblogs.com/rdawson/'>little I can find about
the Longhorn help markup) it doesn't seem to use any more of XLink
than the xlink:href attribute. Without using any other XLink
attributes, this doesn't buy you anything that you can't get from
HTML's a element, although keeping all HTML out of the Longhorn
on-line help markup keeps them off the slippery slope that RSS and
other vocabularies stepped onto when they allowed some HTML in, so I
suppose that's a positive aspect to using such a small bit of XLink. With its
section, title, and especially para element
types, the Longhorn help markup looks suspiciously like DocBook,
although DocBook certainly has no element type called
"conceptual".



Yesterday, as part of my search
for RDF that can play a role in a semantic web
, I accidentally
stumbled across the most significant use of XLink I've seen outside of
XBRL. (It reminded me of how, when you give up looking for your keys and
start searching for your sunglasses, you're more likely to find your
keys.) The metadata
accompanying this collection
of pictures of New Zealand cultural
artifacts is RDF/XML, and includes elements like this:




<dc:identifier rdf:resource="http://collection.aucklandartgallery.govt.nz/results.jsp?view=detail&id=13430"
xlink:arcrole="http://www.natlib.govt.nz/dr/role#AccessFormat"
xlink:role="http://www.isi.edu/in-notes/iana/assignments/media-types/text/html"
xlink:title="View: Achilles carries Penthesilea through Doubtful Sound"
xlink:type="simple"/>

<dcterms:hasFormat rdf:resource="http://collection.aucklandartgallery.govt.nz/images/thumbs/2001-2010/2003_39_4.jpg"
xlink:arcrole="http://www.natlib.govt.nz/dr/role#Thumbnail"
xlink:role="http://www.isi.edu/in-notes/iana/assignments/media-types/image/jpeg"
xlink:title="Thumbnail: Achilles carries Penthesilea through Doubtful Sound"
xlink:type="simple"/>


The use of XLink attributes such as title, role, and
arcrole takes good advantage of the added value of what XLink was
supposed to provide: useful metadata about the links themselves. It's
ironic to see this in an RDF context, when I've claimed
that one reason that XLink got so little traction is that the evangelical
energy that might have been spent on it was diverted to RDF (and Topic
Maps) and that RDF's ability to express relationships between resources can cover some of the original goals of XLink. I never pictured XLink attributes serving as RDF predicates, but it makes perfect sense to me now. (If you're interested in a closer analysis of the relationship, see Ron Daniel's September 2000 W3C Note Harvesting RDF Statements from XLinks.)





Are there any other new uses of XLink, especially within RDF/XML?