I should clarify that I used rdfs:seeAlso specifically because (as I understand it) it is one of the few RDF predicates where it is acceptable for the object to not be another RDF resource. See http://www.w3.org/TR/rdf-schema/#ch_seealso and various other discussions on the web as to why seeAlso was defined this way. Thus, it isn't clear to me that creating a "hasPDF" predicate is a good idea because it might send an uninformed client on a wild-goose chase for non-existent RDF. I believe that the idea behind seeAlso was that clients were on their own in figuring out the meaning of what they get as the result of following the seeAlso link.
Steve

Peter DeVries wrote:

I guess the question is do you want to use a generic seeAlso which most crawlers follow, vs some more specific predicate that says "here is the PDF"

My reluctance was more about minting my own vs. finding some other vocabulary which has a similar predicate.

With the hasPDF predicate it would be pretty easy to query for all species concepts that have a linked original description PDF etc.

I suspect that some standard predicate will eventually become accepted since it is very useful to have something more specific than foaf:Document.

Respectively,

- Pete

On Wed, Jan 5, 2011 at 6:54 PM, Paul Murray <pmurray@anbg.gov.au> wrote:

On 06/01/2011, at 7:48 AM, Peter DeVries wrote:

Also, although I like a lot of what Steve says, I think that most existing crawlers expect that a seeAlso link is to some html, xml, rdf type thing and will

not be able to handle a multi-megabyte PDF.

This is why I reluctantly minted the predicate "hasPDF"

Hmm. This is an issue with linkeddata: when you fetch a URI while crawling the semantic web, if it redirects, then it's an "other resource" and you get RDF. If not, then you are potentially pulling a multimegabyte "information resource" across the wire.

A solution is to use an HTTP "HEAD" request when you do the initial URI fetch. If it's an "other resource", the HEAD return will be a 303 and contain redirect that you want in the "Location" header, and that's all you need. If not, the 200 result will contain the content type and possibly even the size, which is what you need to know before you GET it.

So .. the problem that "hasPDF" is meant to address might be addressable by the crawlers just being a bit smarter about how they browse the semweb.

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