12.4 OWL Specifications

OWL has no shortage of associated documents:

  • Requirements for a Web Ontology Language

  • Web Ontology Language (OWL) Guide Version 1.0

  • OWL Web Ontology Language 1.0 Reference

  • Web Ontology Language (OWL) Abstract Syntax and Semantics

  • Web Ontology Language (OWL) Test Cases

  • Feature Synopsis for OWL Lite and OWL

We just looked at the Requirements document. The other documents somewhat mirror their counterparts over at the RDF Working Group, with the OWL Guide being comparable to the RDF Primer, the OWL Reference to the Syntax document, and the OWL Abstract Syntax and Semantics document to a combination of the RDF Concepts and Semantics documents. The test cases for both are similar; though the OWL feature synopsis doesn't necessarily map to an existing RDF document, it seems similar to a reference card for OWL.

After reviewing the use cases and requirements governing the design of OWL, the next document to review to better understand OWL would be the guide. Just as does its counterpart in RDF, the RDF Primer, the guide provides a general overview of OWL and the associated effort.

12.4.1 OWL Guide 1.0

A further clarification of ontology is provided in the OWL Guide, when it describes how an ontology differs from an XML Schema. According to the guide:

An ontology differs from an XML schema in that it is a knowledge representation, not a message format.

This is an important point. XML Schemas and message-based uses of XML focus on specific pieces of data and specific uses of data, such as sending a message and processing its results. Nothing within basic XML or within the XML Schema allows one to derive information outside of the context of the specific use. For instance, the tool I used to maintain a weblog might support SOAP requirements that allow me to publish a new posting, but nothing associated with the SOAP request allows me, or anyone, to learn more about that specific posting, or even the weblog, other than what's included within the transaction. OWL differs from XML Schema (as well as SOAP and many of the other uses of XML) in that it allows you to record data about an object outside of any specific transaction associated with that data. It allows you to record knowledge. OWL (and by its association, RDF) focuses on data rather than process.

The guide provides an overview of three different types of OWL:

OWL Lite

Supports simple classifications, allowing only cardinalities (member count) of 0 or 1 and only minimal constraints. An example would be a taxonomy.


Supports more complex ontologies, but with some guarantees, such as processing finishing in finite time, restricting elements to be one type, and so on. According to the guide, it's called DL, meaning "description logics."

OWL Full

Full support for maximum freedom of RDF, with no computational guarantees and the possibility of indefinite processing time.

These specific designations have more to do with what certain tools can and will support, which of course influences the design and implementation of a specific ontology. Looking at PostCon's RDFS definition, PostCon could be ported to an ontology, with any restrictions and constraints added to it fitting comfortably within those allowed by OWL DL.

The rest of the guide then covers the basic components of OWL. Later, as we review each of these, I'll demonstrate the concepts by porting PostCon over to OWL, to supplement the example of the Wine ontology used within the OWL document.

At this point, you may want to take a look at a tutorial on OWL, "Ontology Development 101: A Guide to Creating Your First Ontology," found at http://www.ksl.stanford.edu/people/dlm/papers/ontology-tutorial-noy-mcguinness-abstract.html.

12.4.2 OWL Reference 1.0

The OWL Reference document provides the formal specification of the language. It is equivalent to the XML specification for RDF/XML covered in Chapter 3 and Chapter 4. However, unlike the RDF document, the OWL documents are a work in progress and far from complete.

The section of most interest in the reference is the one covering the language structure. Unlike RDF, the OWL vocabulary is quite large. Like RDF, though, it makes use of elements from RDFS (and from RDF).

The Reference document can be found at http://www.w3.org/TR/owl-ref/. The prefix for the OWL namespace used in the OWL documents is owl, a convention I'll follow in this chapter. OWL is based on RDF and RDFS, which means it must first and foremost validate as proper RDF/XML.

The Reference document breaks the structure of OWL down into separate components, most of which are already familiar to you from previous chapters, such as the concepts of classes, properties, and enumerations (collections). Section 12.5 provides an overview of these items. However, OWL also has several concepts unique to it, such as the Boolean combination of class expressions and property restrictions, which add the additional layer of reasoning you would expect for a language defining a business domain. These warrant a closer examination, which occurs in Section 12.6.

You'll sometimes find reference to owl:Thing in the document, but it doesn't show in any formal definition of elements, though it is listed as a term. This element represents the class of all classes and is an artifact of the DAML+OIL effort from which OWL is derived.

12.4.3 OWL Abstract Syntax and Semantics

The Abstract Syntax and Semantics document provides a breakdown of the model theoretical axioms and rules guiding the implementation and interpretation of OWL. It provides a semantic definition of what is a "fact" within OWL, as well as a high-level overview of how OWL differs from DAML+OIL and how OWL Lite differs from the full-featured OWL.

The WebOnt is considering making a minor modification to the name of this document, changing it to Semantics and Abstract Syntax.

A major difference between OWL and OWL Lite is the inclusion of what the document calls OWL descriptions. The formal definition of a description is:

< description> ::= <classID>
                | <restriction>
                | unionOf( {<description>} )
                | intersectionOf( {<description>} )
                | complementOf( <description> )
                | oneOf({<individualID>} )

Primarily, an OWL description is one of a class identifier, a property restriction, or a complex class association. These descriptions enhance the reasoning inherent within OWL ontology?reasoning that goes beyond that allowed in RDFS.

Regarding the separation of OWL and OWL Lite in this chapter, for the most part Section 12.5 applies to both OWL and OWL Lite, though the data typing discussed in the section is beyond OWL Lite. Additionally, the property restrictions covered in "Bits of Knowledge: More Complex OWL Constructs" apply to both. However, the discussion about complex classes in this section applies purely to the fully featured OWL, as these make up most of the options from the OWL description just provided.

Another section in the Abstract document, "RDFS-Compatible Model-Theoretic Semantics," provides a semantic description of the relationship between OWL and RDFS. Though much of the section is given over to theorems and proofs, the first part provides a basic overview describing the compatibility between the two. In particular, it describes the differences between OWL Full and OWL DL.

This chapter focuses on OWL Lite and OWL Full. You'll want to check out the Abstract document for more information on OWL DL. The latest document can be found at http://www.w3.org/TR/owl-semantics/.

12.4.4 Feature Synopsis for OWL Lite and OWL

The Feature Synopsis document provides a summary of features for OWL and OWL Lite. It makes a good "in a nutshell" review of the material and is definitely worth a read as you learn more about OWL.

Access the recent version of the Synopsis document at http://www.w3.org/TR/owl-features/..