Download (direct link):
4Dieter Fensel, "Semantic Enabled Web Services," XML-Web Services ONE Conference, June 7, 2002.
Much effort is going into ontologies in Web services. DARPA Agent Markup Language Services (DAML-S) is an effort that is specifically addressing this area. Built on the foundation of Resource Description Framework (RDF), RDF Schema, and DAML+OIL, DAML-S provides an upper ontology for describing properties and capabilities of Web services in an unambiguous, computer-interpretable markup language.5 Simply put, DAML-S is an ontology for Web services. In addition, Semantic Web Enabled Web Services (SWWS) was developed in August 2002 to provide a comprehensive Web service description framework and discovery framework, and to provide scalable Web service mediation. Together, both of these technologies have the potential to increase automated usability of Web services.
As we build ontologies (models of how things work), we will be able to use this common language to describe Web services and the payloads they contain in much more detail. The rest of this book focuses on this vision.
In this chapter, we have given you a high-level introduction to Web services. In defining Web services, we gave business reasons and possible implementations of Web service technologies. We provided an overview of the basic technologies of Web services, we discussed orchestration and security in Web services, and we provided a vision of where we believe Web services will be tomorrow.
Web services have become the standardized method for interfacing with applications. Various software vendors of new and legacy systems are beginning to provide Web services for their application platforms, and this trend is leading to quick and inexpensive application integration across platforms and operating systems. Businesses are currently deploying internal Web services-related projects, creating powerful EAI processes, and the development of B2B Web services in extranet environments and global Internet environments is on the horizon. We are currently at the beginning of the evolution of Web services. As ontologies are developed to provide richer descriptive content, and as distributed technologies such as grid computing merge with Web services, the future is very bright.
5Sheila McIllraith, "Semantic Enabled Web Services," XML-Web Services ONE Conference, June 7, 2002.
Understanding the Resource Description Framework
"In short, the Semantic Web offers powerful new possibilities and a revolution in function. These capabilities will arrive sooner if we stop squabbling and realize that the rift between XML and RDF-based languages is now down to the minor technical details easily ironed out in the standards process or kludged by designing interoperable tools."
—James Hendler and Bijan Parsia,
"XML and the Semantic Web," XML-Journal
In this chapter, you will learn what the Resource Description Framework (RDF) is, why it has not yet been widely adopted and how that will change, how RDF is based on a simple model that is distinct from the RDF syntax, and how RDF Schema is layered on top of RDF to provide support for class modeling. We then examine some current applications of RDF, including noncontextual modeling and inference. We conclude the chapter by examining some of the current tools for editing and storing RDF. After reading this chapter, you should have a firm understanding of how RDF provides the logical underpinnings of the Semantic Web.
What Is RDF?
At the simplest level, the Resource Description Framework is an XML-based language to describe resources. While the definition of "resource" can be quite broad, let's begin with the common understanding of a resource as an electronic file available via the Web. Such a resource is accessed via a Uniform Resource Locator (URL). While XML documents attach meta data to parts of a document, one use of RDF is to create meta data about the document as a standalone entity. In other words, instead of marking up the internals of a document, RDF captures meta data about the "externals" of a document, like the author, the creation date, and type. A particularly good use of RDF is to
describe resources, which are "opaque" like images or audio files. Figure 5.1 displays an application, which uses RDF to describe an image resource.
The RDFPic application is a demonstration application developed by the W3C to embed RDF meta data inside JPEG images. The application can work in conjunction with the W3C's Jigsaw Web server to automatically extract the RDF meta data from images stored on the server. As you see in Figure 5.1, the application loads the image on the right side and allows data entry in a form on the left side. The tabbed panels on the left side allow you to load custom RDF schemas to describe the image. The two built-in schemas available for describing an image are the Dublin Core (www.dublincore.org) elements and a technical schema with meta data properties on the camera used. Besides embedding the meta data in the photo, you can export the RDF annotations to an external file, as shown in Listing 5.1.