The data sets exchanged could in fact be quite small. Imagine
shopping for a new PC on the Web. If you could capture your system
requirements as a small document using an XML DTD, you could send
that spec to a hundred different vendors to quote you a system. If
you extend that model to include almost anything you can shop
for -- from cars to hot tubs -- XML provides an elegant base
layer of communication among cooperating vendors on the Internet.
Almost any data that is now captured and stored can be more easily
shared using XML. For many systems, the XML DTDs will define a data
transfer protocol and nothing more. The data may never actually be
stored using the XML-defined markup; it may exist in an
XML-compatible form only long enough to pass on the wire between two
systems.
In conjunction with XML-based data exchange, the Extensible
Stylesheet Language, or XSL, will be increasingly used to describe
the appearance and definition of the data represented by these XML
DTDs. Much like Cascading Style Sheets and their ability to transform
HTML documents, XSL will support the creation of style sheets for any
XML DTD. CSS can be used with XML documents as well, but it is not as
programmatically rich as XSL. While CSS stops with style sheets, XSL
is a style language. XSL certainly addresses the need for data
display, and it also provides rich tools that allow data represented
with one DTD to be transformed into another DTD in a controlled and
deterministic fashion. A complete discussion of XSL is beyond the
scope of this book; consult an appropriate O'Reilly reference
for complete details.
The potential for XML goes well beyond that of traditional markup and
presentation tools. What we now see and use in the XML world is only
scratching the surface of the potential for this technology.