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 orig-
inal 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 rea-
soners. For example, ontology engineers developing
ontologies for biomedical applications have identi-
fied significant expressivity limitations of the lan-
guage. Also, the designers of OWL APIs have iden-
tified 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 ex-
ploit 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 de-
cided 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 under-
taken by an informal group of language users and
developers. After the original specification reached
a mature state and first implementations were re-
leased, the OWL 1.1 proposal was submitted to the
W3C as a Member Submission
6
with the inten-
tion 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 Submis-
sion evolved significantly. Consequently, the Work-
ing Group eventually decided in April 2008 to call
the new language OWL 2 and so indicate a substan-
tial 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 differ-
4
http://www.nci.nih.gov/
5
http://www.mindswap.org/2005/OWLWorkshop/
6
http://www.w3.org/Submission/2006/10/
ent 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.
2. Why Go Beyond OWL 1?
Although, or even perhaps because, OWL 1 has
been successful, certain problems have been identi-
fied in its design. None of these problems are severe,
but, taken together, they indicate a need for a re-
vision of OWL 1. In this section, we discuss what
these problems are.
2.1. Expressivity Limitations
Practical experience with OWL 1 has shown that
OWL 1 DL—the most expressive but still decidable
language of the OWL 1 family—lacks several con-
structs that are often necessary for modeling com-
plex domains [31,30]. As a response, the community
of OWL 1 users and application designers developed
various patterns for approximating the missing con-
structs.
7
Since the actual expressive power is miss-
ing, these workarounds are often unsound or incom-
plete with respect to the intended semantics. Fur-
thermore, it is usually difficult, if not impossible, to
identify all the cases in which these patterns yield in-
correct results. Such patterns are therefore not only
cumbersome but also of limited utility, and extend-
ing the language with the missing constructs seems
to be the only satisfactory solution to the problem.
2.1.1. Qualified Cardinality Restrictions
The existential restrictions of OWL 1 DL allow
the restriction to be qualified with a class; for ex-
ample, one can define a class such as “persons that
have at least one child who is male.” Cardinality re-
strictions, however, cannot be qualified with a class;
thus, OWL 1 DL allows for the definition of a person
with at least three children, but not of a person with
at least three children who are male. Expressing the
latter class requires the restriction to be qualified
with respect to the class (i.e., male) to which the
objects being counted (i.e., children) belong. This
7
http://www.w3.org/2001/sw/BestPractices/OEP/
2