Handling Irregular Data Hierarchies

One of the benefits of using XML is that it allows you to model irregular data hierarchies, including data with the following characteristics.

• Collections of heterogeneous elements
• Structures with many optional elements
• Structures where the order is not apparent
• Recursive structures
• Structures with complex containment requirements

The pole.xml example in this section contains heterogeneous collections of elements, with a complex set of containment requirements. A <section> element can contain an arbitrary collection of <title> elements, <p> elements, <list> elements, and so on. Many elements are indeed optional: A <section> does not have to contain <p> or <list> elements, or other <section> elements. It is important to preserve the order of most elements in the output — that is, the first <section> comes before the second <section>. The structure is recursive because a <section> element can contain other <section> elements. The <emph> element is allowed anywhere. For example, you might add it to the <title> element, as follows:

<title>To the Pole... <emph>and Back!</emph></title>

The mechanism for handling data-driven transformations is similar to event-handling in traditional programming languages. Template rules or fragments are analogous to event handlers, which are called whenever the specified events occur. An element matched in the source document triggers a call to the specified template rule or fragment. A template rule or fragment can be called either by its name or according to the matched element type.

The next topic provides a Dissection of the pole.xml Example.