1. Transforming XML The XSLT Language Michael H. Kay

2. 2 Topics XSLT as a language Success in the market SAXON as a product Usability and fitness for purpose Academic interest Technology & Architecture Software Engineering Success in the market SAXON as a product Success in the market Open Source and Investment

3. 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. 4 XSLT Extensible 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. 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. 6 Template Rules Bibliography

7. 7 The XSLT Processing Model Source Document Result Document Transformation Process Source Tree Result Tree Stylesheet Tree Style sheet Parsing Serialization

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

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

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

11. 11 Pros and Cons XML 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. 12 The XPath Axes (1) parent following-sibling preceding-sibling child self

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

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

15. 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. 16 Architecture of an XSLT Processor Serializer Stylesheet Builder Parser Source Document Builder Parser Compiler Compiled Stylesheet Outputter Result Document XPath

17. 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. 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. 19 New in XSLT 2.0 Grouping Regular Expressions Schema-awareness Functions Multiple output Date/time handling... and much more

20. 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. 21 Saxon (XSLT and XQuery) 1 developer 10 years 180K LOC 500 downloads per day £xxxK revenue 300K test cases 10 bugs/month 300 emails/month

22. 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. 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. 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. 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...