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Transforming XML The XSLT Language

This comprehensive overview of XSLT, the eXtensible Stylesheet Language Transformations, explores its usability, market success, and key features. It emphasizes the declarative and functional nature of XSLT as a tool for transforming XML data, and highlights its widespread implementation in various applications. The text discusses the pros and cons of XSLT, the significance of XML Schema, and the evolution from XSLT 1.0 to 2.0. Additionally, it examines the role of open-source implementations and the importance of performance factors in XSLT processing.

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Transforming XML The XSLT Language

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  1. Transforming XMLThe XSLT Language Michael H. Kay

  2. Topics XSLTas a language Usability and fitness for purpose Academicinterest Open Sourceand Investment Successin the market Successin the market Successin the market Technology&Architecture SAXON as a product SAXON as a product SoftwareEngineering

  3. Why did XML happen? • The Web needed something better than HTML • Data Interchange needed something better than CSV files • SGML was around, subsetting it was easier than reinventing • It had to be cheap, and it was • By luck, there was no competition

  4. XSLTExtensible Stylesheet Language - Transformations • A declarative language for transforming XML • Widely used in • publishing applications • messaging applications • anywhere else where XML is found • XSLT 1.0 (1999) was widely implemented • XSLT 2.0 (2007) is popular with users, but there are few products

  5. What kind of language is XSLT? • Declarative, functional • Rule-based • Uses XML syntax • Data model is “abstract XML” tree • Type system based on XML Schema • A two-language system: XSLT+XPath

  6. Template Rules <xsl:template match=“bibliography”> <h1><a name=“bibl”>Bibliography</a></h1> <dl> <xsl:apply-templates/> </dl> </xsl:template> <xsl:template match=“bibl-entry”> <dt> <xsl:value-of select=“@ref”/> </dt> <dd> <xsl:apply-templates/> </dd> </xsl:template>

  7. Style sheet Parsing Serialization Stylesheet Tree Source Result Document Document SourceTree ResultTree TransformationProcess The XSLT Processing Model

  8. Pros and ConsDeclarative, Functional PRO • Optimizable • Safe • Robust • Productive CON • Script kiddies hate it • Slow • Recursion is mind-numbing

  9. Pros and ConsRule-based PRO • Great for text and semi-structured data • Potential for change CON • Script kiddies hate it • Makes static analysis hard

  10. Pros and ConsUses XML Syntax PRO • Templates: “fill in the blanks” • Common infrastructure (editors, parsers) • Extensibility CON • Verbose • Ugly

  11. Pros and ConsXML Tree Data Model PRO • Abstracts away from the lexical XML detail • But it’s still distinctively XML CON • Too abstract for some • In-memory assumption • Inadequate for complex algorithms

  12. The XPath Axes (1) parent preceding-sibling following-sibling child self

  13. The XPath Axes (2) ancestor preceding following descendant

  14. Pros and ConsUse of XML Schema PRO • Everyone uses XML Schema • Type safety • Optimization • Better diagnostics CON • Everyone hates XML Schema • Strong typing is for wimps

  15. Success Factors for XSLT • People needed it • There wasn’t much competition • Good open-source implementations appeared early • High level of spec conformance • Adequate performance • Browser support • Endorsement/credibility

  16. Architectureof an XSLT Processor Stylesheet Parser Builder Compiler CompiledStylesheet ResultDocument SourceDocument XPath Outputter Serializer Parser Builder

  17. Factors driving Performance • Tree model: searching and matching • Pipelining, streaming • Static code optimization • Use of schema • Code generation • Basic good programming • Engineering Methodology

  18. XSLT and XQuery XQuery 1.0 overlaps in capability: • Much smaller language, less power • Clean design, easier to learn • Backed by database vendors • who have money • Backed by academics • who want money • More oriented to data than documents • which is where the money is

  19. New in XSLT 2.0 • Grouping • Regular Expressions • Schema-awareness • Functions • Multiple output • Date/time handling • ... and much more

  20. Where are we today? • XSLT 2.0 came out in Jan 2007 • 2½ implementations (Saxon, Altova) • Highly popular with users • but not with non-users • The big vendors have failed to produce products • not for want of trying

  21. Saxon (XSLT and XQuery) 10 years 300 emails/month 500downloadsper day £xxxK revenue 1 developer 300K test cases 180K LOC 10 bugs/month

  22. Engineering Model • “Alpine mountaineering” • Agile, high-speed, high-risk • Small teams, fast decision making • Importance of tooling • IDEs • test automation • Ship frequently • low half-life for bugs • Support community

  23. Business Model • Open Source has driven out the profit • Only the low-cost operators are able to make money • IBM, Oracle, Microsoft, Intel are failing to deliver product • Is this good for users?

  24. Summary: XSLT • XML concentrates on information interchange • This creates a need for transformation • Many processors available • Excellent conformance to standards • Good performance and reliability • Often open source and/or free • Role of XSLT • Styling XML for presentation • Transforming XML for application interworking • Middle-tier business logic

  25. Wider lessons • Open source changes everything • Ultra-low-cost vendors have a significant advantage • But this is reducing investment and reducing quality (on some measures) • The future is uncertain...

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