Overview

The most characteristic navigation method for the World Wide Web is the hyperlink as supported by HTML. The XML specification, in contrast, does not define a navigation method. In Section 7.1 we first discuss XLink, a W3C recommendation that introduces powerful hyperlink techniques into the world of XML. A discussion of WebML completes this section. WebML is a modeling method that integrates a structural model, a navigational model, and a presentational model.

Concepts such as RDF and topic maps extend the hypertext metaphor to semantic navigation. RDF (see Section 3.3) was developed in order to describe Web resources, while topic maps had their first application in the area of knowledge bases and interactive encyclopedias. We will discuss topic maps in detail in Section 7.2.

Hypertext techniques are based on the signpost metaphor the end user is led through the Web, link by link. This is not always sufficient, as the existence of search engines proves. Services that lie beyond the horizon cannot be seen by the surfer. In Section 7.3 we discuss UDDI, an XML-based language that supports the registration and discovery of Web services through public repositories.

Peer-to-peer technology relies on a different concept. Systems such as Gnutella or Freenet do not rely on signposts or central repositories but work by word-of-mouth propaganda. To find certain information items, you simply ask your neighbors. They in turn ask their neighbors, and so on. We will discuss peer-to-peer technology in Section 7.4.

7.1 Hypermedia

Originally, the World Wide Web was designed as...