Inside an XSLT Processor

1. Inside an XSLT Processor Michael Kay, ICL 19 May 2000

2. About me: • ICL Fellow, systems architect • Database background • Developer of SAXON • Author of XSLT Programmer’s Referencepublished by Wrox Press • Recently joined XSL WG as invited expert

3. About this talk: • The XSLT Processing Model • Structure of an XSLT Processor • Performance • current limitations • possible ways forward • Ideas on future development of the language

4. The XSLT Processing Modelfirst approximation Style sheet Source Result Document Document Transformation Process

5. The XSLT Processing Modelin more detail Style sheet Parsing Serialization Stylesheet Tree Source Result Document Document SourceTree ResultTree TransformationProcess

6. An XSLT Template Rule Pattern <xsl:template match="appendix/para[1]"> <h4> <xsl:number level="single"/> <xsl:value-of select="@title"/> </h4> <p> <xsl:apply-templates/> </p> </xsl:template> ResultElement XPathExpression Instruction

7. Architecture of an XSLT processor XML Parser Stylesheet Tree Builder XPathcompiler XSLTcompiler Compiled Stylesheet XSLT interpreter XPath interpreter Source Result XMLParser TreeBuilder OutputManager XML serializer HTML serializer Text serializer SourceTree

8. At compile time: • Parse and validate the stylesheet • Parse and validate all XPath expressions • and attribute value templates • Build rule base for matching patterns • Resolve references to named variables, functions, and templates • Flatten the import tree • Optimize XPath expressions

9. Where does the time go? Serialize Build Source Output Tree Process Templates Compile Stylesheet

10. Is Performance a Problem? • Client side: usually not • XSLT processing is generally faster than download speed • Server side: sometimes • CPU usage when handling very high throughput • Memory problems when handling very large documents

11. Some performance tips • Keep documents small: split them first • Process once, at publishing time • or use caching • Do several simple transforms in series • Avoid complex patterns in template rules • Use keys • Use external functions • Avoid "//item"

12. Performance progress Simpleoptimization Stylesheet compilation Java code optimization Lazy evaluation Simple XPath optimization Tail recursion 20 sec/Mb Incremental parsing Pipelining Use of schema Pattern matching Full XPath optimization Compile to bytecodes Advancedoptimization 5 sec/Mb 1 sec/Mb Today

13. Interesting research areas • Database integration: transforming a document without loading into memory • Applying regular expression theory • Execution as a sequence of serial passes • Using schema knowledge at compile time • Eager node numbering

14. Potential language features • Serial transformation language? • Multi-pass stylesheets • Higher-level "relational" constructs: grouping, joins, logical quantifiers • Richer data types • Assignment statement ????

15. Summary • XSLT language is now stable • XSLT processor technology is starting to be well understood • First crop of products are capable of significant performance • Now the research needs to start on the next phase of optimization techniques