Page 324 - Open Soource Technologies 304.indd

Web Technologies-I

Notes • Understand how to transform XML with XSLT

• Explain the web services

Introduction

XML presents the Extensible Markup Language at a reasonably technical level for anyone
interested in learning more about structured documents.
XML is a markup language for documents containing structured information.
Structured information contains both contents (words, pictures, etc.) and some indication of
what role that content plays. Almost all documents have some structures.
A markup language is a mechanism to identify structures in a document. The XML specification
defines a standard way to add markup to documents.

13.1 Basics of XML

13.1.1 Document

The number of applications currently being developed that are based on, or make use of, XML
documents is truly amazing (particularly when you consider that XML is not yet a year old)!
For our purposes, the word “document” refers not only to traditional documents, like this one,
but also to the myriad of other XML “data formats”. These include vector graphics, e-commerce
transactions, mathematical equations, object meta-data, server APIs, and a thousand other kinds
of structured information.
13.1.2 Is XML just like HTML?

No. In HTML, both the tag semantics and the tag set are fixed. An <h1> is always a first level
heading and the tag <ati.product.code> is meaningless. The W3C, in conjunction with browser
vendors and the WWW community, is constantly working to extend the definition of HTML to
allow new tags to keep pace with changing technology and to bring variations in presentation
(stylesheets) to the Web. However, these changes are always rigidly confined by what the
browser vendors have implemented and by the fact that backward compatibility is paramount.
And for people who want to disseminate information widely, features supported by only the
latest releases of Netscape and Internet Explorer are not useful.
XML specifies neither semantics nor a tag set. In fact XML is really a meta-language for describing
markup languages. In other words, XML provides a facility to define tags and the structural
relationships between them. Since there is no predefined tag set, there cannot be any preconceived
semantics. All of the semantics of an XML document will either be defined by the applications
that process them or by stylesheets.

13.1.3 Is XML just Like SGML?
No. Well, yes, sort of. XML is defined as an application profile of SGML. SGML which stand for
Standard Generalized Markup Language, defined by ISO, 8879. SGML has been the standard,
vendor-independent way to maintain repositories of structured documentation for more than
a decade, but it is not well suited to serving documents over the web. Defining XML as an
application profile of SGML means that any fully conformant SGML system will be able to
read XML documents. However, using and understanding XML documents does not require a
system that is capable of understanding the full generality of SGML. XML is, roughly speaking,
a restricted form of SGML.

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