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DAML is a Semantic Web ontology language that was developed as part of the DARPA DAML program, which originated in 2000 and continues to the present. Soon after the initial U.S.-based DAML language version had emerged, DAML researchers and the comparable European Union-based OIL language researchers became aware of each other's effort.21 There have subsequently been two versions of the combined language, now called DAML+OIL: December 2000 and March 2001. More recently, the DAML-Service (DAML-S) extension has emerged.22 DAML-S is really a collection of ontologies represented in DAML+OIL that address the semantics of Web services, including services modeled as processes, resources, service profiles, service models, and service groundings (i.e., the concrete realization of the abstractly specified service components, and comparable to the Web Service Description Language's notion of binding).
20For a good additional tutorial on RDF/S, see Manola and Miller (2002).
21The first official version of DAML (DAML-ONT) can be found at http://www.daml.org/ 2000/10/daml-ont.html. Also see OIL http://www.ontoknowledge.org/oil/, and Bechhofer et al. (2000).
22DAML-S v0.7: http://www.daml.org/services/daml-s/0.7/. For a good introduction, see http://www.daml.org/services/daml-s/0.7/daml-s.html.
One important point that you should understand is that all the Semantic Web languages take advantage of the other languages beneath them in the so-called layer cake or stack diagram of the Semantic Web. All the languages use XML syntax, at least for interchange purposes. Figure 8.12 displays a stack used in a particular domain namespace (the namespace itself can be composed of additional namespaces). We see that XML is at the bottom of the stack. XML furnishes the base syntax for interoperability on the Web. Above it is XML Schema, which provides a database-like structuring capability for Web objects, comparable to database schemas.
The next layer is the RDF/S layer, which provides a simple language for expressing ontology concepts and relations and their instances, and again is in XML syntax. Above it is DAML+OIL or OWL, which enable defining a much more expressive ontology and which in turn use the RDF/S level for representing instances of the ontology constructs. Both DAML+OIL and OWL also directly use XML Schema data types. It should be emphasized that although all of these layers are expressed in XML syntax, you still need to use specific interpreters to understand the particular language in order to really take advantage of what that language offers. For example, though all RDF/S, DAML+OIL, and OWL files can be validated as being in legitimate XML syntax, only RDF/S, DAML+OIL, or OWL interpreters can interpret those respective layers, with this slight qualification: In general, the higher language interpreters can correctly interpret every layer below its language level. So, an OWL interpreter will be able to use any embedded or referenced RDF/S or XML Schema data type construct, in addition to OWL-specific code.23
Finally, at the top are reasoning and proof methods, and the so-called "web of trust" layer, which uses automated proof, as well as security and identity features that are still relatively less understood and so, less mature as technologies. At the very top of the stack, we see "Intelligent" domain applications; these are applications that can utilize all of the Semantic Web layers and hence display more "intelligent" behavior or offer more "intelligent" services.
We will not say much more about DAML+OIL, since it is a language that is fairly comparable to OWL and that is expected to be superseded by OWL. Instead, we focus our discussion on OWL. For a feature comparison of XML, RDF/S, DAML+OIL, and portions of OWL, we refer the interested reader to the DAML site (http://www.daml.org/language/features.html) and to Gil and Ratnakar (2002).
23This is not quite the whole story, since as we will see in the section on OWL, which has three levels of language representation, some language levels of OWL do not treat the underlying RDF/S level in the same way.
1 Trust: Proof + Security + Identity 1 Reasoning/Proof Methods OWL, DAML+OIL: Ontologies 1 RDF Schema: Ontologies 1 RDF: Instances
1 XML Schema: Encodings of Data Elements & Descriptions via:
- Define Types, Elements, Content Models, Structures, Local Usage Constraints: structural, cardinality, datatyping
1 XML: Base Documents
"Intelligent" Domain Applications |
Security + Identity
Reasoning/Proof Inference Engine
Higher Semantics DAML+OIL, OWL
Figure 8.12 Stack architecture for the Semantic Web.
XML Schema I
XML I /
Web Ontology Language (sometimes referred to as Ontology Web Language) is the most expressive of the ontology languages currently defined or being defined for the Semantic Web. Unlike DAML+OIL, OWL is originating as a World Wide Web Consortium (W3C) sponsored language (http://www.w3 .org/2001/sw/WebOnt/). The W3C's Web Ontology Working Group was formed in November 2001, and the first official version of OWL is anticipated to be available in early 2003.