Elsevier

Journal of Web Semantics

Volume 6, Issue 4, November 2008, Pages 309-322
Journal of Web Semantics

OWL 2: The next step for OWL

https://doi.org/10.1016/j.websem.2008.05.001Get rights and content

Abstract

Since achieving W3C recommendation status in 2004, the Web Ontology Language (OWL) has been successfully applied to many problems in computer science. Practical experience with OWL has been quite positive in general; however, it has also revealed room for improvement in several areas. We systematically analyze the identified shortcomings of OWL, such as expressivity issues, problems with its syntaxes, and deficiencies in the definition of OWL species. Furthermore, we present an overview of OWL 2—an extension to and revision of OWL that is currently being developed within the W3C OWL Working Group. Many aspects of OWL have been thoroughly reengineered in OWL 2, thus producing a robust platform for future development of the language.

Introduction

Since the inception of the Semantic Web, the development of languages for modeling ontologies—conceptualizations of a domain shared by a community of users—has been seen as a key task. The initial proposals focused on RDF and RDF Schema; however, these languages were soon found to be too limited in expressive power [18]. The World Wide Web Consortium (W3C) therefore formed the Web Ontology Working Group, whose goal was to develop an expressive language suitable for application in the Semantic Web. The result of this endeavor was the OWL Web Ontology Language, which became a W3C recommendation in February 2004. OWL is actually a family of three language variants (often called species) of increasing expressive power: OWL Lite, OWL DL, and OWL Full [28].

The standardization of OWL has sparked the development and/or adaption of a number of reasoners, including FacT++ [37], Pellet [32], RACER [11], and HermiT [26], and ontology editors, including Protégé1 and Swoop [21]. OWL ontologies are being developed in areas as diverse as e-Science, medicine, biology, geography, astronomy, defense, and the automotive and aerospace industries. OWL is extensively used in the life sciences community, where it has rapidly become a de facto standard for ontology development and data interchange; for example, see BioPAX,2 NASA’s SWEET ontologies,3 and the National Cancer Institute Thesaurus.4

Despite the success story surrounding OWL, the numerous contexts in which the language has been applied have revealed some deficiencies in the original design. In Section 2, we present a systematic analysis of problems identified by OWL users and the designers of OWL tools such as editors and reasoners. For example, ontology engineers developing ontologies for biomedical applications have identified significant expressivity limitations of the language. Also, the designers of OWL APIs have identified several practical limitations such as difficulties in parsing OWL ontologies or the inability to check for obvious errors, such as mistyped names.

In response to users’ comments and requests, the idea was born to address some of these needs via an incremental revision of OWL, provisionally called OWL 1.1. The initial goal of OWL 1.1 was to exploit recent developments in DL research in order to address some of the expressivity limitations of the language. After extensive discussions at the 2005 OWL Experiences and Directions Workshop,5 a consensus was reached regarding the new features to be provided by OWL 1.1. This set of new features roughly corresponds to the intersection of what users wanted, what theoreticians said was possible, and what implementors believed was practicable. As the design of OWL 1.1 progressed, it was decided to also take the opportunity to “clean up” the language and its specification, so as to provide a more robust platform for future development.

The development of OWL 1.1 was initially undertaken by an informal group of language users and developers. After the original specification reached a mature state and first implementations were released, the OWL 1.1 proposal was submitted to the W3C as a Member Submission6 with the intention of using it as a starting point for a new W3C Working Group. The Working Group was officially formed in September 2007. As the work on the new language progressed, the initial Member Submission evolved significantly. Consequently, the Working Group eventually decided in April 2008 to call the new language OWL 2 and thus indicate a substantial step in the evolution of the language.

In Section 3, we present the design of OWL 2 and discuss how it addresses the drawbacks of OWL 1 that we identified in Section 2. We discuss different aspects of the language, such as its expressivity, syntax, specification style, and various metalogical features. In Section 4 we discuss the current state of implementation, and conclude in Section 5 with a discussion of possible future extensions. To avoid any ambiguity, we refer to the initial version of OWL as OWL 1 in the rest of this paper.

Section snippets

Why go beyond OWL 1?

Although, or even perhaps because, OWL 1 has been successful, certain problems have been identified in its design. None of these problems are severe, but, taken together, they indicate a need for a revision of OWL 1. In this section, we discuss what these problems are.

The design of OWL 2

To address the problems with OWL 1 that we identified in Section 2, the design of OWL 2 has departed from that of OWL 1 in several ways.

Implementation and adoption

Considerable progress has been achieved in the development of tool support for OWL 2. This is due to the improvements made in the specification which have facilitated the implementors’ work, as well as to the pressing need in practice for the additional functionality provided by OWL 2. The development activity can be grouped into two major categories: the extension of existing APIs and ontology management systems to the new syntax and the MOF metamodel, and the extension of reasoning systems to

The future of OWL

Apart from addressing acute problems with expressivity, a goal in the development of OWL 2 was to provide a robust platform for future development. We conclude this paper by speculating about potential future extensions.

Syntactic extensibility. Since the first OWL-ED workshop, many proposals for syntactic sugar were put forward. In particular, extending OWL with a “macro” system, allowing users to define their syntactic shortcuts, might be very useful. For example, one could define

Acknowledgements

Major groups involved in the development of OWL 2 include the World Wide Web Consortium and the OWL Working Group. There are far too many people involved to list them individually, even if limited to the major participants. The views on the design decisions of OWL expressed in this paper are those of the authors, and do not necessarily reflect those of other members of the OWL Working Group. Finally, by the time this paper was completed, the Working Group was still active, so some of the

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