If there’s one thing guaranteed to spark a huge amount of e-mail on any of our mailing lists, it’s raising the issue of the difference between an information resource (IR) and a non-information resource (NIR). The poster boy example is that of a toucan*. If I put http://example.com/toucan in my browser I might receive all manner of things down the wire. An image of a toucan, an audio clip of its call, a video of it in flight. Who knows, I might receive the virtual reality experience of having a toucan sitting on my desk. But a toucan cannot be transmitted as a series of bytes.
The most recent version of this debate began with a blog post by Ian Davis, CTO of (W3C Member) Talis, in which he argued for a new solution to the IR/NIR debate for linked data. After several days of intense e-mail discussion, he posted an updated version that relies on the Content-location HTTP header to disambiguate between the two. Whether the proposal survives and becomes an accepted alternative way of publishing an electronic document describing a real world object, both of which have URI identifiers, remains to be seen. But it looks hopeful and well worth considering.
It was during the debate that I realised that there was a mistake in the POWDER specifications, work on which I lead between 2007 and 2009. Admitting a mistake is always embarrassing but not to do so would only prolong the agony.
As part of POWDER, we specified a link relationship type of describedby. Now included in RFC 5988, this is a very general relationship that is defined thus:
The relationship A ‘describedby’ B asserts that resource B provides a description of resource A. There are no constraints on the format or representation of either A or B, neither are there any further constraints on either resource.
So far so good. But POWDER can be serialized as RDF/OWL and so we wanted to create a semantically identical RDF property. The relevant prefix used is wdrs so the property in question is wdrs:describedby. The error made was that we had written into the namespace document and one part of the spec an inference that wdrs:describedby pointed specifically to POWDER documents. This was a mistake as evidenced by other parts of the text.
So, an erratum has been published, with full details of the error, the (unlikely but) possible implications and the steps taken already to rectify the situation. Having done this, we can say that all of the following are legitimate and semantically identical:
- XHTML, ATOM
<link rel="describedby" href="/doc" type="foo/bar" />
Link: </doc>; rel="describedby" type="foo/bar";
@prefix wdrs: <http://www.w3.org/2007/05/powder-s#> .
<> wdrs:describedby </doc> .
* Anyone know where the toucan meme came from originally?