Category Archives: OWL

http://www.w3.org/2004/OWL/

(Almost) No More Comments

FOAF, Identity, WordPress

I haven’t been writing much here lately, but others have.

Or rather: They’ve tried.

Some time ago I turned off trackbacks and pingbacks completely, and changed the options for comments to require moderation before anything went online.

That hasn’t stopped the spammers though — I get at least ten mails about spam comments each day, and frankly that’s just not fun.

I should be upgrading to the lastest version of WordPress, as that includes the clever Akismet plugin for catching spam, which seems to work quite well on the other blogs I run. Also, I really should finish the plugin that leverages my social network through FOAF, that’d make for a really great combination, methinks.

Instead I’ve activated a plugin that turns off comments and pings within 30 days of the posting date.

I’m sorry for the inconvience for those who wish to comment on older posts — for now it has to be like that (but do send me a mail if you’d like to comment anyway).

Describing Source Content for Redland/MySQL

Databases, OWL, Redland, SPARQL

I mentioned SADDLE (which used to be a part of the SPARQL Protocol draft, but is no longer) in passing the other day, when describing OWL-S Maker and talking about service description in general.

Service description in this context — and in the context of Dion Hinchcliffe’s OWL-S-less overview of SDLs — is mostly about the interface, the inputs and outputs, not what’s in between.

In contrast, SADDLE originally entered that territory with its properties like saddle:vocabulary, and the other day on dev@gargonza Damian Steer announced a nice little javascript hack for using source content descriptions — this is not about I/O, but about what a “service” contains information about.

Central to Damian’s hack is a source content description, containing OWL statements about which classes and properties are present in the SPARQL source. For example, his description shows that all objects of foaf:name statements (in this particular store) are literals.

While the above example was handmade, I realized this was getting close to what I’ve been meaning to do for generating simpler and cleaner UIs for triplestores (asking for a foaf:Person? It’s likely you’d also want a foaf:name then…), so I figured I should try to generate such an SCD — Source Content Description — automagically, as Damian hints to himself: Ideally this information would mined from the store.

I’ve managed to come up with a single query that returns all the information necessary to construct an SCD, but since it’s quite complex, I’ll explain the steps I took on the way there.

Continue reading Describing Source Content for Redland/MySQL →

OWL-S Maker

OWL, RDF/XML, SPARQL

There’s lots of talk about service descriptions these days, most notably around Tim Bray’s WSDL discussions (which includes SMEX-D), Norm Walsh’s NSDL for WITW, and the DAWG’s Saddle, to be a part of the SPARQL protocol. Phew, acronym overload…

Also in that space is OWL-S, an OWL-based Web service ontology that Danny Ayers has looked at. I though it would be fitting to describe semantic web services with RDF, so I tried it out but ended up feeling quite like Norm after his WSDL endeavour.

I did however manage to get something working, OWL-S Maker, which only uses a subset (and possibly in a wrong way), but is self-powered through its own service description, a PHP class (which currently has too many local dependencies to be worth anything to anyone but myself), and some XSLT. The descriptions could use some more work regarding the “typing” of inputs, but I thank Saddle may eventually provide some insights there. As for the general modelling of services, input, and output, I think I may have to look elsewhere — OWL-S seems too complicated.

As a usage example, see the Sparqlette service description input and the description of nearestAirport for a partially handcrafted example with two profiles.

42

Identity, Vocabularies

According to The Hitchhiker’s Guide to the Galaxy (which is still on my to-read list), “42” is The Answer to Life, the Universe, and Everything. However, I think there may be an exception when it comes to the question about httpRange-14, a long debated issue in web circles (see also http-range-14 no comment).

Earlier today, the Semantic Web Best Practices and Deployment Working Group sent word to the TAG, that it has resolved that an http URI without a hash MAY be used to identify an RDF property. Thus, we may have a partial answer to httpRange-14, one that isn’t “42”, but one that doesn’t answer other questions (not including “Life, the Universe, and Everything”).

I think the conclusion by the SWBPD WG is sound — while in an ideal world it could be useful to know just by looking at a URI whether it identified a document or something else, it isn’t strictly necessary. Sure, it might help a human out now and then, but in the long run, the machines will be doing the analyzing part, and according to the TAG’s finding on what URIs identify, URIs are opaque. It all comes down to semantics assigned by humans.

At the same time, it is often practical and useful to use fragment identifiers for RDF vocabulary terms, so wording that uses “MAY” is perfect in my book — even if it doesn’t answer or solve everything…

Redland Smushing

Identity, Redland, Release

Some time — actually more than a year — ago i wrote a smusher for Redland that works by rewriting nodes based on identity inference.

To begin with, it handled only IFP’s, owl:InverseFunctionalProperty, but the other day I needed it to be able to handle FP’s, owl:FunctionalProperty, as well.

A classic example of an IFP is foaf:homepage — only one resource can have some specific URI as its homepage, which is handy for identity reasoning across the Web. Just as useful is the somewhat recently added property foaf:primaryTopic, which is an FP — if a page is described in more than one place, each with a seemingly different primary topic, it can be inferred that the two “topics” are actually just one, handy when identifying movies, since almost all movies have a page describing it at the Internet Movie Data Base.

The smusher is written in C, isn’t heavily commented, has been used elsewhere without problems, and works by finding IFP’s and FP’s in the model it is smushing or by being passed a specific property to smush on — a nasty way of testing is to pass it rdf:type…

Building it should be somewhat straight forward, but the accompanying Makefile might help out here and there.

Note: This entry — as all entries in the Release category — will serve as a changelog (you can subscribe to its RSS feed if you want to make sure you don’t miss out on any updates).

The current version is 0.21 (released 2005-03-27).

Changes since 0.20:
  • Reworked rewriting process to avoid database deadlocks.