Transforming XMLThe XSLT Language

Michael H. Kay

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TopicsXSLT

as a language

Successin the market

SAXONas a product

Usability andfitness forpurpose

Academicinterest

Technology&

Architecture

SoftwareEngineering

Successin the market

SAXONas a product

Successin the market

Open Sourceand

Investment

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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

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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

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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

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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>

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The XSLT Processing Model

SourceDocument

ResultDocument

TransformationProcess

SourceTree

ResultTree

StylesheetTree

Stylesheet

ParsingSerialization

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Pros and Cons

Declarative, Functional

PRO

• Optimizable

• Safe

• Robust

• Productive

CON

• Script kiddies hate it

• Slow

• Recursion is mind-numbing

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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

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Pros and Cons

Uses XML Syntax

PRO

• Templates: “fill in the blanks”

• Common infrastructure (editors, parsers)

• Extensibility

CON

• Verbose

• Ugly

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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

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The XPath Axes (1)

parent

following-sibling

preceding-sibling

child

self

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The XPath Axes (2)

ancestor

following

preceding

descendant

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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

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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

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Architectureof an XSLT Processor

Serializer

Stylesheet

Builder

Parser

Source

Document

Builder

Parser

Compiler

CompiledStylesheet

Outp

utter

Result

Document

XPath

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Factors driving Performance

• Tree model: searching and matching

• Pipelining, streaming

• Static code optimization

• Use of schema

• Code generation

• Basic good programming

• Engineering Methodology

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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

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New in XSLT 2.0

• Grouping

• Regular Expressions

• Schema-awareness

• Functions

• Multiple output

• Date/time handling

• ... and much more

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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

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Saxon (XSLT and XQuery)

1developer

10years

180KLOC

500downloads

per day

£xxxKrevenue

300Ktest cases

10bugs/month

300emails/month

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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

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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?

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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

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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...