Thinking Clearly .

Following closely on the heels of the recent release of Pellet 1.5.2, we’ve updated OwlSight. Since OwlSight runs on raw Pellet power, the new version, .52, updates the back-end to take advantage of the recent Pellet release.

If you have not already taken a look at OwlSight, cruise on over to the OwlSight page and take it for a spin. For those not already in the know, OwlSight is a lightweight browser-based ontology browser utilizing both GWT (and GWT-Ext) and Pellet as its core technologies. Until next time, stay classy cyberspace.

Hurrah hurrah! The moment has finally arrived. The W3C has announced today that they are starting up a new OWL working group.

I’m really excited by this, since, after all, I’ve been working toward this for, geez, two years or so? Two and a half? Well, since before the first OWLED. I think it’ll be a big boost for the community and that users will benefit quite a bit.

The working group won’t be the only place where OWL evolution will be happening, of course. OWLED, for example, marches on and I’m hoping for great things from the task forces. There’s lots to do and lots of fun to be had while doing it.

Interesting, tonight, we Mancunians from IMG who are not moving to Oxford threw a bash for those who are. Ian Horrocks (who is the reason I have a job at the University of Manchester) has been successfully wooed by Oxford. Ian will be (co)-chairing the OWL working group.

Interesting times!

By my rather hasty coding, 9 OWLED 2007 papers mentioned SWRL (or DL-Safe rules) as a need, often a critical need. There’s 1 that goes for DLP, and 5 for SPARQL over OWL (which is closely related to rule extensions). There was also a lively panel devoted to rules and queries, and, last but not least, there is an OWLED task force developing a report on DL Safe rules (and one developing a spec for SPARQL/OWL).

So, “we” want some sort of rules, and all the benefits they bring. SWRL is something of a de facto standard for extending OWL with rules with DL Safe SWRL rules (as I shall canonically call them) being the most commonly implemented varient.

However, it’s also clear that most people aren’t too clear on exactly what the difference is between arbitrary SWRL and DL Safe SWRL (remember that DL Safe SWRL is a restricted version of SWRL). It’s very clear that most don’t understand the consequences for modeling (and definitely not for implementation). It’s a complex topic, so I’m going to have a series of posts. Some of the material overlaps with the part I wrote of a chapter for the new edition of the Ontology Handbook on “Rules and Ontologies”.

So, first, what is SWRL? Syntactically, SWRL is an extension to OWL with a new sort of conditional (i.e., if-then statements). OWL already has several sorts of conditional, e.g., (I’m using OWL 1.1’s functional syntax):

    SubClassOf(Person, ObjectUnionOf(Human IntelligentComputer)) 
    SubObjectPropertyOf(parentOf, ancestorOf) 

The first line says that if you are a Person, then you are either a Human or an IntelligentComputer, and the second says that if you have the parent relation then you have the ancestor relation.

However, for a variety of reasons, OWL conditionals are very constrained and specialized. For example, the SubClassOf conditional can only have class expressions in the “if” or the “then” parts. For example, you can’t mix classes and properties (directly) as in:

    SubClassOf(parentOf, ObjectUnionOf(Human IntelligentComputer)) 

That’s just ill formed. These specialized conditionals have several advantages: They allow for variable free syntax; they are more intention revealing; and they help enforce restrictions which make OWL easier to process (e.g., by making it decidable).

However, there is a price: expressivity. This is especially obvious when it comes to the property conditionals. In OWL 1.0, they were restricted to relations between single named properties. Hence the famous “you can’t define uncleOf in OWL” issue.

SWRL generalized OWL conditionals in two ways:

1. It allows for arbitrary patterns of variables


2. It allows for fairly free mixing of expressions (e.g., property and class expressions)

SWRL is like OWL (and unlike many traditional rule languages, e.g., Prolog or Datalog) in that it embraces the open world assumption. There is no negation as failure, among other things, in SWRL. Thus, if anyone says that they “translate” SWRL Rules to Prolog, be wary. This might be a sensible thing to do, but it’s highly unlikely to respect SWRL semantics.

All these facts conspire to make SWRL very expressive. There are lots of built in features of OWL that become redundant in SWRL. Consider transitivity. While OWL has a specific construct to express that a property is transitive (e.g., TransitiveObjectProperty(ancesterOf)), you can encode that directly with SWRL rule:

      ancestorOf(?X, ?Z) :- ancestorOf(?X, ?Y), ancestorOf(?Y, ?Z). 

(I’m using a Prolog like style for the syntax.)

This rule means exactly the same thing as the OWL construct. (Note: Some people think that the rule is “more transparent”, but I think it’s much worse than the simpler OWL construct for a number of reasons, mostly having to do with readability.)

So, (full) SWRL rules are really a kind of OWL axiom. They can make for more expressive property and class axioms (for example, you can’t make classes whose members depend on certain sorts of cyclic structure in OWL, but easily do so with SWRL rules).

In the next post, I’ll explore the DL Safety restriction.

Going through the OWLED 2007 papers, two quotes about OWL 1.1 jump out at me. The first is from Kent Spackman, of SNOMED fame:

Without a solid DL foundation, the Semantic Web would have remained largely irrelevant to health care terminology standardization. Even so, the initial version of OWL was developed without taking adequate account of features of DL that had already been used in both the GALEN [6] and SNOMED [7] efforts. The development of OWL 1.1 eliminated one of the most significant barriers to use of OWL for SNOMED, since it permits the identification of tractable sublanguages capable of handling the size and complexity of SNOMED [8]. Although adding property chain inclusion axioms was reportedly the most difficult step in developing OWL 1.1 [9], this was essential. Without it, adoption of OWL by the SNOMED community would have required awkward workarounds with their attendant complications and complexities – effectively killing movement in that direction. With it, we have a clear path to using OWL 1.1 for further development and integration with other biomedical ontologies. [emphasis added]

It’s always nice to get a super-positive endorsement, esp., one that, given the high density of praise, is remarkably hype-free. (Whether you agree with Kent’s identification of representational requirements is a separate issue.) One really nice bit of his singling out property chain inclusions (think of them as a generalization of transitivity and subpropertying) is that they highlight the synergy between the extensions to OWL in OWL 1.1 and the proposed fragments. Various versions of SNOMED (and other biomedical ontologies) fall into the logic EL++, which has very nice computational properties. However, they also use complex role inclusions, which put them outside the OWL 1.0 part of EL++. Thus we see extending and sensible subsetting marching happily hand in hand.

The second quote is a table:

This table (in spite of the confusing “CWA” column…I think they mean that the features are used in so called “closure axioms”) is really helpful for seeing what various OWL 1.1 features get you. This paper also had one of the few well described use of rules (in particular, of a DL Safe “SWRL” rule) in an OWL ontology. In my coding of features requested/offered in OWLED 2007 papers it was clear that “rules”, in some sense, were very important, but it was typically unclear what sort of rules would suffice. Indeed, it’s interesting that in the standardization session, DL Safe SWRL rules did not end up in “standards track”. I think this is because of the confusion surrounding “rules” (which is a very unhelpful term) and modeling in OWL. This paper gave clear indication of what the ontologists lacked, what they wanted, and what they actually used (they did check their entailments with KAON2). It’s hard to overstress the value of such clarity.

Just a quick update post ESWC (SPARQL tutorial), OWLED (organizing and…everything), and DL (nothing but hanging out; I did jump, fully clothed, into a pool; I expect photos will show up at some point): Lot’s of good and important stuff happened.

One of the most interesting things at OWLED was that Sandro Hawke annouced that the W3C team has started work on a charter for a potential W3C working group to extend OWL along the lines of OWL 1.1. While this is not a done deal, by any means, it is an important step. Thanks to everyone who helped including all the wonderful moral support we’ve received. There are still devils and details, as always, but I’m very happy with how things are going.

More to come as I unpack.