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June 28, 1999

XML Makes Object Models More Useful

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Take, for instance, code generation. If a user-interface tier can be successfully created by applying a style sheet to an XML document, then why not try creating the business tier and the data-services tier as well. In other words, if an XSL style sheet can be used to generate HTML code, then a different XSL style sheet can be written to generate different code, such as Visual Basic, Java, and SQL.

A data schema can be created by writing an XSL style sheet that will transform an XML model into the appropriate "Create Table" SQL statements. The beauty of this approach is that now that we're driving the generation of screens and database tables off the same XML source document, we're guaranteed there will be a column in the database for each element on the screen.

For the business layer we can, at a minimum, generate all of our object definitions in whatever programming language we like: Visual Basic, Java, or C++. The more elaborate the model, the more code that can be generated.

We've actually used this approach at KPMG to generate an application prototype entirely out of a Rational Rose UML model. At the push of a button, a Rational Rose model is converted to XML and styled first into HTML, again into Visual Basic, and finally into SQL statements. The result is a fully functional (albeit not pretty) application prototype that can actually accept data into a Web-based form and save it to a database.

Theory To Practice
The neat thing about many of the concepts described above is that they can be implemented with skills you have in-house. Creating an XML representation of an object model can be accomplished within a modeling tool such as Rational Rose by using the scripting capabilities of the tool to extract the model data and reassemble it in XML format. In the near future, this step may not even be necessary, as UML tool vendors are working toward their own XML-based standard representation.

XML vocabularies will end up looking a lot like the object attributes you have already defined. Writing XSL style sheets is a skill that needs to be learned but can be picked up fairly quickly, especially by those already familiar with the pattern-matching facilities of environments such as Perl.

XML is not only the answer to the question of what comes after HTML. The very properties that make it so appropriate for transmitting structured data on the Web can be applied in the architectural arena as well. The result is an architectural approach that enhances the best architectural practices by making application development more efficient and more responsive to changing customer and business needs. That may end up being the most exciting thing yet about XML.

Bruce Klein is a manager in KPMG's electronic-commerce practice in New York. He can be reached at bklein@kpmg.com. David Vun Kannon, who works on XML initiatives at KPMG, contributed to this article.

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