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Top1 Introduction
Ontology languages are emerging as the de facto standard for capturing semantics on the web. One of the most important ontology languages today is the Web Ontology Language (OWL), standardized and recommended by the World Wide Web Consortium (W3C) (Patel-Schneider et al., 2004). One issue currently addressed in the research community is how to define reusable ontologies or ontology parts. In more general terms, how to construct an ontology from possibly independently developed components.
OWL natively provides some facilities for reusing ontologies and ontology parts. First, a feature inherited from RDF (Hayes et al., 2004), upon which OWL is layered, is linking—loosely referencing distributed web content and other ontologies using URIs. Second, OWL provides an owl:imports construct which syntactically includes the complete referenced ontology into the importing ontology. The linking mechanism is convenient from a modeling perspective, but is semantically not well-defined—there is no guarantee that the referenced ontology or web content exists. Furthermore, the component (usually an ontology class) is small and often hard to detach from the surrounding ontology in a semantically well-defined way. Usually a full ontology import is required since it is unclear which other classes the referenced class depends on. The owl:imports construct can only handle complete ontologies and does not allow for partial reuse. This can lead to inconsistencies in the resulting ontology due to conflicting modeling axioms. Overall, OWL seems to be inflexible in the kind of reuse provided, especially regarding the granularity of components.
Existing approaches addressing these issues often refer to modular ontologies and, in general terms, aim at enabling the reuse of ontology parts or fragments in a well-defined way (for some work in this direction, see Cuenca Grau et al., 2006, 2007a, 2007b). That is, investigate how only certain parts of an ontology can be reused and deployed elsewhere. While it is interesting work and allows for reuse, we believe that such extracted ontological units fail to provide an intuitive meaning of why those units should constitute components—they were not designed as such.
The object-oriented software community has long discussed new ways of modeling software. One interesting result of this research is the notion of role modeling (Reenskaug et al., 1996). The main argument is that today’s class-oriented modeling mixes two related but ultimately different notions: natural types and role types. Natural types capture the identity of its instances, while a role type describes their interactions. Intuitively, an object cannot discard its natural type without losing its identity while a role type can be changed depending on the current context of the object. Person for example, is a natural type while Parent is a role type. Parent is a role that can be played by persons. A role type thus only models one specific aspect of its related natural types. Related role types can be joined together into a role model to capture and separate one specific concern of the modeled whole.
In this paper we introduce role modeling to ontologies. Role modeling can bring several benefits to ontologies and ontological modeling. Roles provide:
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More natural ontological modeling by separating roles from classes
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An appropriate notion and size of reusable ontological components—role models
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Separation of concerns by capturing a single concern in a role model