XML in a Nutshell, 3rd Edition

Extended Links

Whereas a simple link describes a single unidirectional connection between one XML element and one remote resource, an extended link describes a collection of resources and a collection of paths between those resources. Each path connects exactly two resources. Any individual resource may be connected to one of the other resources, two of the other resources, zero of the other resources, all of the other resources, or any subset of the other resources in the collection. It may even be connected back to itself. In computer science terms, an extended link is a directed, labeled graph in which the paths are arcs, the documents are vertices, and the URIs are labels.

Simple links are very easy to understand by analogy with HTML links. However, there’s no obvious analogy for extended links. What they look like, how applications treat them, what user interfaces present them to people, is all up in the air. No simple visual metaphors like “click on the blue underlined text to jump to a new page” have been invented for extended links, and no browsers support them. How they’ll be used and what user interfaces will be designed for them remains to be seen.

In XML, an extended link is represented by an extended link element; that is, an element of arbitrary type that has an xlink:type attribute with the value extended. For example, this is an extended link element that refers to the novel The Wonderful Wizard of Oz:

<novel xlink:type="extended"> <title>The Wonderful Wizard of Oz</title> ...