Learning XML

5.6. XML Schema: An Alternative to DTDs

Some people have complained that DTDs use an old and inflexible syntax and aren't expressive enough for some needs. Others find it strange that documents follow one syntax and DTDs another. The content models and attribute list declarations are difficult to read and understand, and it's frustrating that patterns for data in elements and attributes can't be specified.

For these reasons, there are a number of proposed alternatives to the venerable DTD. XML Schema (sometimes referred to as XSchema) is one example, which we introduce here. Though it is still just a candidate recommendation of the XML Schema Working Group at the W3C, the essentials represented here shouldn't change much. Unlike DTD syntax, XML Schema syntax is well-formed XML, making it possible to use your favorite XML tools to edit it. It also provides much more control over datatypes and patterns, making it a more attractive language for enforcing strict data entry requirements.

Consider this example, a census form. The census-taker, going door to door, enters information in a little electronic tablet. A schema helps keep her data organized by enforcing datatypes, in case she writes something in the wrong field. Here's how an instance of the document type might look in XML.

<census date="1999-04-29"> <censustaker>738</censustaker> <address> <number>510</number> <street>Yellowbrick Road</street> <city>Munchkinville</city> <province>Negbo</province> </address> <occupants> ...