XML: Separating Data and Its Structure from Presentation 

• Why XML - the problem with HTML
  • No way to easily identify data in documents
  • Difficult to identify meta data
• General purpose of XML
  • XML gives us a way to create and maintain structured documents in plain text that can be rendered in a variety of different ways.
  • A primary objective of XML is to separate content from presentation.
  • XML describes data through the use of markup tags - Example
  • Works well for exchanging data between applications
  • XML documents can be presented in different ways through the use of extensible Stylesheet Language transformation (XSL)
• Basics of XML
  • Dialect of SGML
  • Standards defined by World Wide Web Consortium (W3C)
  • Plain-text markup language
  • Extensible
  • Complements, not replaces, HTML
  • Not just for the Web
• XML Documents
  • XML elements - Example
  • Well-formed - properly nested, end tags used
  • Validated - matches the data schema defined by DTD or Schema
• DTD (Document Type Definition)
  • Defines the legal building blocks (elements) of an XML document
  • Acts as the data schema
  • Can be included inline with XML document or as separate document
  • Standard DTDs are evolving
• Schema
  • Adds typing, default values, constraints on values to DTD
• XSL
  • A Transformation language (XSLT) and a formatting language (XSL-FO)
  • XSLT most developed
  • XSLT used to transform XML document into another form:
    • Different XML
    • HTML
    • PDF
    • Etc.
  • Example: XML --->XSL ---> Result
  • Declarative language – analogous to SQL
  • Based on matching templates and selecting data

Diagram of XML Processing

Sample DTD from RosettaNet

Sample XML (to really see the XML, view the source)

Sample XSL

• XML Parser
  • Microsoft parser is a COM component that comes with IE5
  • Netscape does not include a parser, but other parsers are available
  • List of parsers
• Namespaces
  • Method for qualifying element names
  • Allows for mixing of “vocabularies” (DTDs)
  • URI: Universal Resource Identifier (more info, still more info)
    • URL - defines specific location
    • URN - assumes persistence through either location or through agent that will find location
• Document Object Module
  • Provides a standard programming interface to an XML document
  • Creates a tree data structure of document in memory
  • Nodes (elements) can be accessed, added, deleted, or modified