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While XML provides means for structured content, it presents a different (but not
necessarily opposing) view on content than relational database systems. XML doesn’t
provide a relational model. It allows unlimited nested levels, which could not be
handled by a database system. On the other hand, it misses features found in an
RDBMS, such as stringent field types, constraints, keys, and so on. Of course, there are
similarities in the two concepts and there is indeed development going on to create a
SQL-like query language for XML documents.Anyway, the success of XML shouldn’t
make you forget the usefulness of the traditional RDBMS; they provide many
important processing features that could hardly be modeled in XML, and they’re
optimized for speed from the ground up.
The overall and killer advantage of XML is the separation of logical structure from
layout. By having your documents in XML, you can transform them into any
representation you want: HTML, PostScript, PDF, RTF, plain text, audio, Braille—from
one single source. And as XML (plain text) documents can be parsed with your
favorite scripting language, it’s easy to change hyperlinks dynamically, change element
contents, or associate structures with a database.
And if you’re still not convinced, review all those Document Type Definitions that
are being developed or are already in use. XML itself is mostly an “under the hood”
technology—the meat is the applications that use XML.
What Is XML Used For?
As a structured information markup language, XML is of course used in content
management systems, archiving solutions, and corporate document repositories. But
plenty of other XML applications and subprotocols exist. Due to the open nature of
the standard, DTDs have been developed at a fast pace.
DocBook
The DocBookX DTD is a very popular set of tags for describing books, articles, and
other prose documents, particularly technical documentation. It was originally
developed in 1991 by the publisher O’Reilly as an SGML DTD for in-house use. It
soon became popular with authors and spread to other publishing houses, a change
embraced by O’Reilly, which handed over further development to the Davenport
Group. In mid-1998, OASIS (Organization for the Advance of Structured Information
Standards) officially took over the maintenance of DocBook.When XML became
increasingly popular, an unofficial XML version (3.1) was created by Norman Walsh;
work is currently underway to transform this to an official release—DocBook 5 will
most probably come in SGML and XML flavors.
When we started writing this book, it was clear that we wanted to use an open
format such as XML.The DocBook DTD was consequently chosen because it offered
all the features we would ever need. All the elements typically used in technical
writing are present and, to tell you the truth, even very esoteric ones are included—or
have you ever seen a MouseButton element (from the quick reference: The conventional
name of a mouse button) in your word processor?
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