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December 3, 2008

Bridge between SW communities: OWL RL

The W3C OWL Working Group has just published a series of documents for the new version of OWL, most of them being so-called Last Call Working Drafts (which, in the W3C jargon, means that the design is done; after this, it will only change in response to new problems showing up).

There are many aspects of the new OWL 2 that are of a great interest; I would concentrate here on only one of those, namely the so-called OWL RL Profile. OWL 2 defines several “profiles”, which are subsets of the full OWL 2; subsets that have some good properties, e.g., in terms of implementability. OWL RL is one of those. “RL” stands for “Rule Language” and what this means is that OWL RL is simple enough to be implemented by a traditional (say, Prolog-like) rule engine or can be easily programmed directly in just about any programming language. There is of course a price: the possibilities offered by OWL RL are restricted in terms of building a vocabulary, so there is a delicate balance here. Such rule oriented versions of OWL have also precedences: Herman ter Horst published, some years ago, a profile calld pD*; a number of triple store vendors have a similar, restricted versions of OWL implemented in their systems already, referred to as RDFS++, OWLPrime, or OWLIM; and there has been some more theoretical work done by the research community in this direction, too, usually referred to by “DLP”. The goal was common to all of these: find a subset of OWL that is helpful to build simple vocabularies, and that can be implemented (relatively) easily. Such subsets are also widely seen as more easily understandable and usable by communities that work with RDF(S) and need only a “little bit of OWL” for their applications (instead of building more rigorous and complex ontologies which requires extra skills they may not have). Well, this is  the niche of OWL RL.

OWL RL is defined in terms of a functional, abstract syntax (defining a subset of DL) as well as a set of rules of the sort “if that and that triple pattern exists in the RDF Graph then add these and these triples”. The rule set itself is oblivious to the DL restrictions in the sense that it can be used on any RDF graphs, albeit with a possible loss of completeness. (There is a theorem in the document that describes the exact situation if you are interested.)

The number of rules is fairly high (74 in total), which seems to deceive the goal of simplicity. But this is misleading. Indeed, one has to realize that, for example, these rules subsume most of RDFS (e.g., what the meaning of domain, range, or subproperty is). Around 50 out of the 74 rules simply codify such RDFS definitions or their close equivalents in OWL (what it means to be “same as”, to have equivalent/disjoint properties or classes, that sort of things). All of these are simple, obvious, albeit necessary rules. There are only around 20 rules that bring real extra functionality compared to RDFS for building simple vocabularies. Some of these functionalities are:

  • Characterization of properties as being (a)symmetric, functional, inverse functional, inverse, transitive,…
  • Property chains, ie, defining the composition of two or more properties as being the sub property of another one. (Remember the classic “uncle” relationship that cannot be expressed in terms of OWL 1? Well, by chaining “brother” and “parent” one can say that the chain is a subproperty of “uncle” and that is it…)
  • Intersection and union of classes
  • Limited form of cardinality (only maximum cardinality and only with values 0 and 1) and  property restrictions
  • An “easy key” functionality, i.e., deducing the equivalence of two resources if a list of predefined properties have identical values for them (e.g., if two persons have the same name, same email address, and the same home page URI, then the two persons should be regarded as identical)

Some of these features are new in OWL 2 (property chaining, easy keys), others are already been present in OWL 1.

Quick and dirty implementations of OWL RL can be done fairly easily. Either one uses an existing rule engine (say, Jena rules) and lets the rule engine take its course or one encodes the rules directly on top of an RDF environment like Sesame, RDFLib, or Redland, and uses a simple forward chaining cycle. Of course, this is quick and dirty, i.e., not necessary efficient, because it will generate many extra triples. But if the rule engine can be combined with the query system (SPARQL or other), which is the case for most triple store implementations, the actual generation of some of those extra triples (e.g., <r owl:sameAs r> for all resources) may be avoided. Actually, some of the current triple stores already do such tricks with the OWL profiles they implement. (And, well, when I see the incredible evolution on the size and efficiency of triple stores these days, I wonder whether this is really an issue on long term for a large family of applications.) I actually did such quick and dirty implementation in Python; if you are curious what triples are generated via OWL RL for a specific graph, you can try out a small service I’ve set up. (Caveat: it has not been really thoroughly tested yet, i.e., there are bugs. Neither it is particularly efficient. Do not use it for anything remotely serious!).

So what is the possible role of OWL RL in developing SW applications? I think it will become very important. I usually look at OWL RL as some sort of a “bridge” that allows some RDF/SW applications to evolve in different directions. Such as:

  • Some applications may be perfectly happy with OWL RL as is (usually combined with a SPARQL engine to query the resulting, expanded graph), and they do not really need more in term of vocabulary expressiveness. I actually foresee a very large family of applications in this category.
  • Some applications may want to combine OWL RL with some extra, application specific rules. They can rely on a rule engine fed with the OWL RL rules plus the extra application rules. B.t.w., although the details are still to be fleshed out, the goal is that a RIF implementation would accept OWL RL rules and produce what has to be produced. I.e., RIF compatible implementation would provide a nice environment for these types of applications.
  • Some applications may hit, during their evolution, the limitations of OWL RL in terms of vocabulary building (e.g., they might need more precise cardinality restrictions or the full power of property restrictions). In which case they can try expand their vocabulary towards more complex and formal ontologies using, e.g., OWL DL. They may have to accept some more restrictions because they enter the world of DL, and they would require more complex reasoning engines, but that is the price they might be willing to pay. While developers of applications in the other categories would not necessarily care about that, the fact that the language is also defined in terms of a functional syntax makes (i.e., that that version of OWL RL is integral part of OWL 2) this evolution path easier.

Of course, at the moment, OWL RL is still a Draft, albeit in Last Call. Feedbacks and comments of the community as well as the experience of implementers is vital to finalize it. Comments to the Working Group can be sent to public-owl-comments@w3.org (with public archives).

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