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unambiguously define the data model. Before XML, many times canonical 

data models were established but never documented by their developers. If you 

were lucky, developers might have described the model in a text document. It 

was easy for such a document to get out of sync with the actual types used by 

the model.

  XML schemas are enforceable. You can validate an XML document to ensure 

that it conforms to the schema of the canonical data model. A text document 

cannot enforce use of the proper types. 

  XML is easier to convert to an alternate form, by using declarative means such 

as style sheets.

  XML is both platform and programming language neutral. Hence, you can 

have a variety of systems use the same canonical data model without requiring 

a specific programming language or platform. 

This is not meant to imply that XML is perfect, since there are some disadvan 

tages to using XML to represent a canonical model. For one, to use XML you

need to either learn the XML schema language (XSL) or use a good tool. Often,

transforming XML requires you to use XSL, which is not an easy language to

learn, especially since XSL is different from traditional programming languages.

There are also performance overheads with XML. However, for many enterprise

settings (and especially for integration purposes) the benefits of using XML out 

weigh its disadvantages, and the benefits of XML become more significant when

you factor in future maintenance costs.

Code Example 6.3 shows an XML document representing invoice informa 

tion. This might be the canonical model of an invoice used by the adventure

builder enterprise. Since this document is published internally, all new applica 

tions requiring this data type can make use of it.