Ontology Tools Survey, Revisited
by Michael Denny
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Pages: 1, 2
Language Suitability
One ontology building trend not articulated in the survey responses, but highlighted in a dedicated session of the recent WWW 2004 Conference, is support for ontology languages built on RDF and the use of URIs as identifiers for referring to unique entities. Ontologies for the Semantic Web are characterized as RDF ontologies, and are being built using OWL and other languages based on RDF. Current attention to the Semantic Web and the language standardization it offers has resulted in the single most prominent change in ontology editors since the original survey in 2002. This growth in direct support for RDF and various species of OWL has created some controversy.
The issue arises in consideration of whether RDF is the best base language for implementing ontologies on the Web or elsewhere, and whether it affords the scalability necessary to implement very large ontologies and webs of ontologies, and whether it affords the representational power or expressiveness to build ontologies of the sophistication necessary for demanding applications.
Other ontology languages such as SCL, CycL, and LOOM, for example, arguably offer more power of expression and reasoning, but lack intimate support of RDF. The advantage offered by RDF that remains compelling for ontologies seems to be the universal use of URI and XML namespace protocols on the Web. This unifying aspect, for instance, may make it easier to establish, through collaboration and consensus, the utilitarian vocabularies (as ontologies) needed for far-flung cooperative and integrative applications using the Web.
Wearing the mantle of W3C standardization, OWL enjoys much more attention today than any other ontology language -- in or out of the Web world. Its detractors tend to single out its limits of expression, its inelegant syntax and, of course, its reliance on the RDF model of representation using triples. Some basic language constructs like lists and other collections are deemed cumbersome and in need of extension in new language implementations. These shortcomings, if one chooses to see them as such, clearly add more to the ontology toolmaker's plate. The successful ontology editor may be expected to mask these kind of idiosyncrasies with higher level functionalities.
Tool Usability
Traditionally, an integrated development environment (IDE) for software is language specific and exploits underlying details and native capabilities of the language whether it is a programming language like Java or C++, or a design notational language like UML, or both. Such a consistent focus has not yet emerged in a suite of tools for building ontologies. Indeed, when this does happen it may be as part of a general enterprise level IDE. Conversion from UML to OWL and from OWL to Java is already under development in some tools.
As a precursor to such IDEs, ontology toolmakers are now paying attention to creating a coherent view of the ontology as the software is built. Results of the present survey speak to the advantage of a tool that promotes and maintains the user's apprehension of the ontology. Navigation features for easily drilling down and zooming out in an ontology structure, as well as jumping to semantically related elements in the structure, are important cognitive aids to productivity. Lexical support that helps find and organize terms within and across ontologies similarly buoys the editing task. Other tool features that may contribute to the builder's craftsmanship include:
- Simplified entry of higher level ontology constructs and principled patterns using generic and domain-specific templates/widgets and wizards.
- Diagrammatic presentation and manipulation of the ontology and its axioms.
- Automatic classification and integrity checking of ontology (via an inference engine) as specification statements are entered.
- Loading, viewing and editing multiple ontologies concurrently.
- Ability to modularize and arbitrarily partition parts of the ontology for building, merging and testing.
- Refactoring the ontology via name changes, subclass migration, property type migration, concept-individual migration, tree pruning, etc.
- Automatic and author-specific annotation of the ontology during development
using metadata to record software evolution and provenance including rationales
and proofs, revisions, comments, origins, natural language references, etc.
- For example, tracking the user's ordering of ontology terms and axioms, and allowing definable resorting of terms.
- Full support of the language's capabilities to import external ontologies.
- Software import/export capabilities in abstract syntax or canonical serializations to ensure reliable and complete round-trip development while using other ontology tools.
Achieving most of the ontology editing functionality suggested above is a ways off. For example, simply achieving reasonably complete support of the OWL language in an ontology editor is proving to be no mean feat for the OWL Plugin for Stanford's Protégé editor. As evidenced in the public view of its course of development, this yearlong endeavor has provided a very capable tool, but one that is still very much evolving in terms of both capabilities and the user interface.
Impending Challenges
While achieving full-range ontology editing functionality is a tall order for toolmakers, the capabilities called out above are not the only demands toolmakers face. The world of ontology application itself is changing in a way that is putting more pressure on ontology language implementations and editing tools to handle new tasks. Some see the gathering demands as an impending crisis for providing editing environments that can accommodate an expanding scope of ontology language responsibilities. Eventually, editors will have to address the ontology language and reasoner functions currently under development, including:
- Ontology rule languages like the Semantic Web Rule Language (SWRL), which combine RuleML with OWL.
- Probabilistic extensions for OWL like those being pursued at The University of Maryland Baltimore County (UMBC).
- Defeasible or alternative logics to support forms of non-monotonic reasoning in closed-world and open-world ontologies that are being investigated.
- Complete reasoning capabilities of OWL Full and RDF, which are being investigated.
- Enforcing formal ontology principles on the design and implementation of ontologies as imposed by development environments like IODE and others.
- Semantic Web services ontology languages like OWL-S.
- Behavior-modeling capabilities of ontology languages like those present in The Discovery Machine and OPCAT.
- Facility for associating an ontology or parts of an ontology with specific problem-solving methods (PSMs).
- Means for ontology processes to request and access web-borne knowledge, irrespective of how that knowledge is organized, such as the URIQA scheme.
- Effective integration of RDF ontologies and XML Schema for full mapping between them using path-query mechanisms analogous to XPath.
- Generation of ontologies specifically suited for use by agents in a multiple-agent system implementation as in the DISCIPLE tool.
- Automatic updating of domain ontologies through built-in processes to acquire and analyze source domain information and identify modifications to the ontology as new or modified concepts and relations.
Choosing your Editor
When comparing the ontology editors described in Table 1, it becomes clear that the tools offer a wide and varied range of capabilities. In the absence of an IDE for ontologies, tried and true or otherwise, the practical approach today is to rely on several ontology building tools to fashion different aspects of an ontology and manage the development process.
Recognizing that fashioning an effective representation of a problem can be a great part of its solution, one objective in choosing an ontology editor is to maximize the match between its potential output (as ontology content and structure) and the character and dynamics of the particular domain problem space that your ontology is intended to address. Thus, tools supporting specific industry vocabularies, linguistic capabilities, modeling styles, or ontology constructs (like instances and datatypes) may be better suited to the demands of particular problem applications. When assembling your own bench of tools to gain the convenience of a coordinated development practice, there are a number of factors to consider. Some of these were enumerated in the foregoing discussion on tool usability. A few more are:
- A common ontology specification interchange language is necessary to ensure that expressivity is not lost and consistency is not compromised when moving between tools. That is, you should require round-trip editing among all your ontology tools.
- Related to the preceding consideration is the rapid growth in widespread adoption of the OWL language. When editors do not natively support OWL import and export, specific translator tools should be identified to seamlessly bridge between the editor's native language(s) and OWL.
- As observed in the original survey, tools may differ markedly in their level of use and maturity. Some tools have very active development and user communities that increase the likelihood that the tool will continue to be available and kept up to date.
- Sometimes independent of a tool's level of community participation, the level of technical support and training available from the software provider is important to forming a productive user team.
- Editors with a software architecture that allows easy extension with addition of functionality and integration with other tools is advantageous. The use of common application frameworks, plug-in facilities, well-implemented and documented APIs, and the like may compensate for the lack of a true IDE.
- Product factors like licensing terms, purchase price, documentation, update policy and upgrade path are also productivity issues.
Regardless of your choices, the ontology-building experience will be challenging, but one that holds the promise of solving real problems when key questions hinge on the operational semantics of the domain.
ResourcesInternational Web Ontology Programs
Recent Conferences Covering Semantic Web Tools
Guidance on Building Web Ontologies
Original Ontology Editor Survey
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- Printing table
2005-05-06 12:37:58 jmartin27 - Using ontologies
2004-08-11 09:31:04 rpg - Table Text Too Small!
2004-07-15 08:22:31 Joshua Lubell - Table Text Too Small!
2004-07-19 09:05:18 ggraham
