Specifying XML Structures Using Schema OVERVIEW

Specifying XML Structures Using Schema
OVERVIEW
We've already seen that XML documents can be described using Document Type Definitions, DTDs. DTDs originated with SGML and show those origins all too visibly. XML documents are far more complex and varied than their SGML cousins because XML is used in far more ways than SGML. This creates a problem. While DTDs are perfectly suitable for SGML, where they have been used successfully for many years, they are inappropriate for the newer technology of XML. DTDs cannot be processed by XML-only applications. Developers need to learn two relatively complex languages to use DTDs and they cannot be validated using XML validators. XML has more data types than can be expressed in DTDs, and is generally far richer. Basically DTDs cannot be used to express XML documents.

To remedy this situation, W3C has created a language called XML Schema which can be used to define XML structures. A number of different schema languages exist. In this chapter I will be writing specifically about XML Schema because it is a Recommendation of W3C. I'll be using the terms XML Schema and schema interchangeably – my choice being based purely upon which reads better in a given context. If I wanted to be precise all of the time I would use XML Schema when referring to the language and Recommendation, and schema when referring to a particular document that uses the language.

As I write this, far more tools exist to handle DTDs than XML Schema. This situation is changing rapidly since everyone sees the advantages of using schemas. DTDs are really a technical dead-end, although understanding them will remain important since so many exist. It's likely that when you are using older documents, they'll continue to be described using DTDs. New documents should always be described using XML Schema.[1]

The most important omission in the DTD is the idea of a data type. SGML documents tend to contain mostly plain text. Almost all data in an SGML application can be treated as strings of characters in definitions and applications. XML documents require a far richer set of data types, including strings of characters, numbers, both whole and decimal, and complex types such as dates and times. XML Schema introduces data types which, in turn, leads to more tightly defined XML structures which can be used with current database technologies or in conventional applications written in general-purpose programming languages. Other new, and useful, features in the XML Schema Recommendation include:

a simple pattern matching grammar which might be used, for example, to define the structure of an order code,

defined ordering of subelements so that document structure can be tightly controlled,

selection between different elements so that documents can share a schema without having identical structure.

DTDs are described using their own, unique, syntax. Using them means having to learn, and apply, two sets of syntactic rules in one application. While DTDs are not the most complex documents imaginable, it is vital that developers define them correctly. Equally as important, parsing and manipulating DTDs within applications requires special libraries. XML Schema documents can be handled much more easily because they are fully compliant XML documents in their own right. What does this mean in practice? The tools that you use to develop, parse and manipulate your XML can also be used for your schemas. Developers need learn only one set of rules for schema and document, and both could be created using the same pieces of editing software.

Using XML Schema requires an understanding of namespaces. Schema definitions always use namespaces, so much so that namespaces are one of the cornerstones of schema technology. I've mentioned namespaces before; now is the time to examine them in detail and learn how to use them.

[1]Although pragmatic realities such as organizational politics, historical preferences or the tools you have available may force you to use DTDs.