XML DATA TRANSFORMATION

• XML was first introduced as a metalanguage for data

description.

• A metalanguage represents a well-defined interface that

evolves and is transformed into derived languages. XML is

simply the foundation interface for a number of specific

markup languages, each of which is based on its own

vocabulary and schema.

• The schema syntactically differentiates XML languages from

each other. XML is key for data exchange and interoperability,

and the schema is essential for providing XML documents with

a typed and well-defined structure. Unfortunately, in the

imperfect world in which we live, schemas often express the

same semantics through different syntaxes.

• An XML transformation is simply the XML workaround for this

relatively common

situation. An XML transformation is a user-defined algorithm that

attempts to express

the semantics of a given document using another equivalent syntax. A

transformation is much like a type cast in programming. You can

always try to coerce the type, but in

doing so you could face and accept compromises like syntax

adaptations and,

sometimes, loss of data and logic.

• In XML, the transformation process is seen as the application of a

style sheet to the

source document. The style sheet is a declarative and user-defined

document that is

referred to as extensible. The term Extensible Stylesheet Language

(XSL) indicates a

metalanguage designed for expressing style sheets for XML

documents. An XSL file

contains the set of rules that will be used to transform a document into

another,

• XSL files were originally conceived as the XML counterpart of HTML's

cascading style

sheets (CSS). In this context, XSL files were simply extensible and user-

definable tools

to render an XML markup in HTML for display purposes. The growing

complexity of

style sheets, as well as the advent of XML schemas, changed the

perspective of XSL

and led to XSL Transformations (XSLT).

• The goal of XSL has evolved over time. Today, XSL is a blanket

term for a number of derived technologies that altogether

better qualify and implement the original idea of styling XML

documents. The various components that fall under the

umbrella of XSL are the actual software entities that you use in

your code:

• XSLT Rule-based language for transforming XML documents

into any other

text-based format. XSLT provides for XML-to-XML

transformation, which

mostly means schema transformation. An XSLT program is a

generic set of

transformation rules whose output can be any text-based

language,

including HTML, Rich Text Format (RTF), and Wireless Markup

Language

• XPath Query language that XSLT programs use to select specific

parts of an

XML document. The result of XPath expressions is then parsed

and

elaborated by the XSLT processor

• XSL Formatting Objects (XSL-FO) Advanced styling features

expressed by

• an XML vocabulary that define the semantics of a set of

formatting

• elements

XSLT TEMPLATE PROGRAMMING

XSLT is a process that combines two XML documents—the XML source

file and the style sheet—to produce a third document. The resultant

document can be an XML document, an HTML page, or any text-based

file the style sheet has been instructed to generate.

The source document must meet only one requirement: it must be a

well-formed XML

document. The style sheet must be a valid XML document that contains

the transformation logic expressed using the elements in the XSLT

vocabulary. An XSLT

style sheet can be seen as a sequence of templates.

AN OVERVIEW OF THE XSLT PROCESS

• The core part of the transformation process is the application

of templates to XML source elements. Other ancillary steps

might include the expansion of elements to text, the execution

of some script code, and the selection of a subset of nodes

using Xpath queries. The layout of a generic XSLT script is

shown here:

• <xsl:stylesheet

• xmlns:xsl="http://www.w3.org/1999/xsl/transform">

• <xsl:template match="/">

• ⋮

• </xsl:template>

• <xsl:template match="...">

• ⋮

• </xsl:template>

• ⋮

• </xsl:stylesheet>

• The final output of each template must form a syntactically valid fragment in

the target

• language—be it XML, HTML, RTF, or some other language. You are not

required to

• indicate the target language explicitly, although the XSLT vocabulary

provides a tailormade

• instruction to declare what the expected output will be. The main

requirement for

• the XSLT style sheet is that its overall text be well-formed XML. In addition,

it must

• make syntactically correct use of all the XSLT instructions it needs. The

syntax of each

APPLYING AN XSLT TEMPLATE TO SOURCE MARKUP TEXT.

XSLT INSTRUCTIONS

• The XSLT vocabulary consists of special tags that represent particular

operations you

• can perform on the source markup text or passed arguments.

Although the overall

• syntax is that of a rigorous XML dialect, you can easily recognize the

main constructs of a high-level programming language.

• The following subsections summarize the main XSLT instructions you

are likely to run

• across in your XSLT experience. The XSLT instructions are divided

into four categories:

• templates, data manipulation, control flow, and layout.

THE STYLE SHEET IS FIRST NORMALIZED TO AN XPATH NAVIGATOR AND THEN COMPILED

MANAGING THE PROCESSOR'S STATE

• The style sheet compiler populates three internal data

structures with the data read from the source.

PERFORMING TRANSFORMATIONS

FURTHER READING:

• http://www.w3.org/TR/xslt