TRENDS & CONTROVERSIES The semantic Web and its languages Editor: Dieter Fensel Vrije Universiteit Amsterdam [email protected] The Web has drastically changed the availability of electronic informa- epistemologically rich modeling primitives, provided by the frame com- tion, but its success and exponential growth have made it increasingly munity; formal semantics and efficient reasoning support, provided by difficult to find, access, present, and maintain such information for a wide description logics; and a standard proposal for syntactical exchange nota- variety of users. In reaction to this bottleneck, many new research initia- tions, provided by the Web community. (See the essay by Frank van tives and commercial enterprises have been set up to enrich available Harmelen and Ian Horrocks.) information with machine-processable semantics. Such support is essen- Another candidate for such a Web-based ontology modeling lan- tial for bringing the Web to its full potential in areas such as knowledge guage is DAML-ONT. The DARPA Agent Markup Language is a management and electronic commerce. This semantic Web will provide major, well-funded initiative, aimed at joining the many ongoing intelligent access to heterogeneous and distributed information, enabling semantic Web efforts and focused on bringing ontologies to the Web. software products (agents) to mediate between user needs and available The DAML language inherits many aspects from OIL, and the capabili- information sources. Early steps in the direction of a semantic Web were ties of the two languages are relatively similar. Both initiatives cooper- SHOE1 and later Ontobroker,2 but now many more projects exist. ate in a Joint EU/US ad hoc Agent Markup Language Committee to Originally, the Web grew mainly around HTML, which provided a achieve a joined language proposal. The next step will be DAML- standard for structuring documents so that browsers could translate Logic, a language with sufficient means for expressing axioms and them in a canonical way. On the one hand, it was HTML’s simplicity rules. (See the essay by James Hendler and Deborah L. McGuiness.) that enabled the Web’s fast growth, but on the other, its simplicity seri- Defining languages for the semantic Web is just the first step. Devel- ously hampered more advanced Web applications in many domains and oping new tools, architectures, and applications is the real challenge for many tasks. This was the reason for XML (see Figure 1), which lets that will follow. us define arbitrary domain- and task-specific extensions. Even HTML has been redefined as an XML application—XHTML. Consequently, References we define the semantic Web as an XML application. 1. S. Luke, L. Spector, and D. Rager, “Ontology-Based Knowledge The Resource Description Framework took the first step toward Discovery on the World-Wide Web,” Working Notes Workshop defining the Web in XML terms. RDF defines a syntactical convention Internet-Based Information Systems at the 13th Nat’l Conf. Artifi- and a simple data model for representing data’s machine-processable cial Intelligence (AAAI 96), 1996. semantics. It is a standard for Web metadata developed by the World 2. D. Fensel et al., “Ontobroker: The Very High Idea,” Proc. 11th Wide Web Consortium. (See the essay by Ora Lassila.) Int’l Flairs Conf. (FLAIRS 98), 1998, pp. 131–135. The RDF Schema candidate recommendation that defines basic onto- logical modeling primitives on top of RDF took the second step, fol- lowed by the Ontology Inference Layer, which uses the RDFS as a start- Dieter Fensel is an associate professor at the Division of Mathematics ing point and extends it to a full-fledged ontology modeling language. and Computer Science, Vrije Universiteit, Amsterdam, and he is a new department editor for Trends & Controversies. After studying mathe- OIL unifies three important aspects provided by different communities: matics, sociology, and computer science in Berlin, he joined the Institute AIFB at the University of Karlsruhe. His major subject was knowledge engineering and his PhD DAML-O OIL thesis was about a formal specification DAML-O DARPA Agent Markup Language-Ontology language for knowledge-based systems. RDFS OIL Ontology Inference Layer Currently, his focus is on the use of ontolo- gies to mediate access to heterogeneous XHTML RDF RDF Resource Description Framework knowledge sources and to apply them in HTML XML RDFS Resource Description Framework Schema knowledge management and electronic commerce. Contact him at [email protected]. Figure 1. The layer language model for the Web. nl; www.cs.vu.nl/~dieter. The Resource Description Framework Modeling alog information), RDF is suitable for Ora Lassila, Nokia Research Center describing any Web resource, and as such it All items that RDF expressions describe provides interoperability between applica- are called resources, and, broadly speaking, The Resource Description Framework is tions that exchange machine-understand- anything a Universal Resource Identifier a standard for Web metadata that the World able information on the Web. Its goal is to can name is also a resource.4 Consequently, Wide Web Consortium (W3C) devel- add formal semantics to the Web, thus RDF can describe not just things on the oped.1–3 Expanding the traditional notion paving the way for the so-called semantic Web (such as pages, parts of pages, or col- of document metadata (such as library cat- Web. lections of pages) but also things not on the NOVEMBER/DECEMBER 2000 67 Web—as long as they can be XML.” Sure, you could do that, named using some URI scheme. but essentially you would end up (Hypothetically, we could name or reinventing the wheel by building describe any person with a “per- a similar layer on top of XML that son” URI scheme—for example, RDF already introduces. The XML “person:US:123-45-6789.”) The developer community has focused RDF description model uses on RDF’s use (and abuse) of XML, object–attribute–value triples: we sometimes forgetting that RDF is a can view instances of the model as data model whose syntax is largely directed or labeled graphs (which irrelevant. (During development of resemble semantic networks), or the RDF standard, several syntaxes we can take a more object-centric were proposed, some of which view and think of RDF as a frame- were not based on XML.) based representation system. In RDF’s object-oriented extensi- RDF, these triples are known as bility lets developers take pieces of statements. existing RDF schemata and extend Descriptions can be limited to one them as they see necessary. This resource (for example, a library cata- might lead to some type of Dar- log card that names a document’s winian evolution of metadata author and publisher), in which case where the strong solutions will the values of resource attributes (or Illustration by Sally Lee survive and evolve further (as properties, as they are called in RDF) opposed to the all-or-nothing situ- are typically strings. Descriptions can ation that users of XML DTD also span multiple resources: values of proper- RDF model.5 The metaconstructs for the [document type definitions] often face). ties can be other resources—so we can thus type system are terms and concepts that describe arbitrary relationships between multi- URIs name, so RDF effectively represents Applications and future directions ple resources. URIs name properties, which and defines classes and properties. Class In addition to the “syntax wars,” contro- are also resources, and as such they can definitions can be derived from multiple versy surrounds RDF within the knowledge describe a property by asking, “What are a superclasses, and property definitions can representation community. RDF has been particular property’s permitted values, specify domain and range constraints. We criticized for its low expressive power (it does which types of resources can it describe, and can also think of RDFS as a set of ontologi- not have variables, negation, or quantifica- what is its relationship to other properties?” cal modeling primitives on top of RDF. tion—a far cry from first-order predicate Meaning in RDF comes from specific terms calculus, for example). However, RDF is and concepts that URIs define and then Syntax what it is by design. Sometimes you need to name. Because URIs can be unique, two RDF uses XML for the syntactic expres- take baby steps before you can run—Web- systems can define some concept (say, “per- sion of model instances, which is a source of related issues are important (such as simplic- son”) and each use a different URI to name much controversy and confusion. RDF is ity, which enables wide adoption). it to avoid clashes; however, two systems essentially a data model and does not strive Several interesting RDF applications have agreeing on a common concept will use the to replace XML. Instead, it builds a layer on already emerged. Mozilla (also known as same URI and effectively share semantics. top of it, making interoperable exchange of Netscape 6) uses RDF internally as a repre- The RDF model also defines some meta- semantic information possible (for example, sentation format. In 1999, Netscape also level constructs (such as container types for the object-oriented extensibility is intended introduced the RSS formalism (RDF Site describing collections of resources) and to enable a partial understanding of data). Summary, www.egroups.com/group/rss-dev), higher-order statements (statements about RDF lacks primitive data types (such as which has now grown into a broader effort to other statements). Higher-order statements integer, float, and so forth), so strings are build an extensible information description are modeled in RDF and allow the repre- essentially the only literals available; XML and syndication format. sentation of modalities such as beliefs. Fur- atomic datatypes will be used once W3C Dublin Core is—at least initially—a meta- thermore, an extensible, object-oriented completes work on the XML Schema.6 data element set for describing cataloging type system (known as RDF Schema) is We often hear claims such as, “You don’t information, such as that needed by digital introduced as a layer on top of the basic need RDF; you can do everything with libraries.