Falk HartmannResearch Associate, SAP AG

An Architecture for an XML-Template Engine enabling Safe Authoring

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2/25Agenda

Motivating Example

Definitions

Goals

Requirements

Proposed Architecture

Solution Elements

Related Work

Conclusion & Open Issues

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Servlet Container

Template(JSP)

Template(JSP)

Data Tier

Motivating Example

Safe Authoring:Ensure, that the instantiated template conforms to

the target language by inspecting the template!

BrowserJSP EnginePage

(XHTML)

User

Author

Page(XHTML)

Browser

Data

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JSP

Definitions

Binding Language

<%= table.date() %>

Slot Markup Language

<%= %>

Term Language

table.date()

= +

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5/25Goals

Safe Authoring

All errors detectable upfront are indeed detected upfront

Structural errors like incorrectly nested elements, missing attributes

Separation of Concerns

Templates are a mean for separation of concerns

Concerns depend on use case, e.g., between web designer and developer

Adequate Error Handling

Errors are clearly and understandably signaled to the user

Both for errors detectable upfront and at runtime

Broad applicability

Restriction on target language: XML

No restrictions on the instantiation data (esp. its addressing)

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6/25Requirement 1: Preservation

Goals: Safe Authoring

A template engine and language must preserve all constraints that are valid in the target language also in the template language.

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7/25Requirement 2: Coverage

Goals: Broad Applicability, Separation of Concerns

The template language should cover the target language by allowing to produce every target language document from a template, no matter which parts of the target language document are created from the instantiation data.

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8/25Requirement 3: Inferibility

Goals: Safe Authoring, Broad Applicability

It must be possible to automatically infer the schema the template must comply to from the schema defined for the target language.

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9/25Requirement 4: Control

Goals: Separation of Concerns

The binding language must support control statements for conditional and repeated inclusion of template fragments.

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10/25Requirement 5: Type safety

Goals: Adequate Error Handling

In order to assert, that the instantiated template complies to the target language schema the types of the instantiation data must be correct.

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11/25Requirement 6: Independence

Goals: Broad Applicability

The architecture should be independent of the data source and the way data in it is addressed.

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12/25Proposed Architecture

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13/25Solution Element 1: Binding Language Design

Requirements: Coverage, Control, Independence

General: Independence makes reasoning about instantiation data impossible

No switch statements which allows multiple branches

Coverage and control imply the necessity for conditional and repeated content

XML Specific: Leverage namespaces as concept for slot markup language

No extra markup, existing tools can be reused

XTL = XML Template Language

Prototypical approach: XTL is embedded into target language

Elements: xtl:text, xtl:attribute, xtl:if, xtl:for-each

Decision against xtl:element

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14/25…Prototypical vs. Transformational Approach

Prototypical Approach:Target language syntax is "top-level" syntaxBinding language syntax is embedded

JSP: <html>…<%= name %>…</html>

+target language syntax checkable (with limitations improvable)

+easier to learn

Transformational Approach:

Binding language syntax is "top-level" syntaxTarget language fragments are embedded

XSLT: <xslt:transform>…<html>…</html>…</xslt:transform>

+binding language syntax checkable

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15/25Solution Element 2: Grammar Transformer

Requirements: Preservation, Inferibility

General: Separation of constraints: upfront checkable and instantiation data

XML Specific: Target language grammar (input): XML Schema Template language grammar (output): not expressible in XML Schema

→ enhancement of XML Schema with OCL constraints (CXSD) Schema without constraints should not declare valid templates invalid Instantiation data constraints: XML Schema Types

Enabling xtl:attribute for all elements with attributes Enabling xtl:if/xtl:for-each around elements with appr. multiplicity Forcing ID to be replaced by xtl:attribute in xtl:for-each

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Problem:

- Attribute might be substituted by xtl:attribute

- Change of content model and attribute model

Current Approach:

<element name="external-graphic"> <complexType> <attribute name="src" type="string" use="required"/> </complexType></element>

Problem:

- Attribute might be substituted by xtl:attribute

- Change of content model and attribute model

Current Approach:

<element name="external-graphic"> <complexType> <sequence> <element ref="xtl:attribute"/> </sequence> <attribute name="src" type="string" use="optional"/> </complexType></element>

<element name="external-graphic"> <annotation> <appinfo source="…"> <cxsd:inv> let count : Integer = self->select(xtl:attribute[@name='src'])->size() in if @src->isEmpty() then count = 1 else count = 0 endif </cxsd:inv> </appinfo> </annotation> <complexType> <sequence> <element ref="xtl:attribute"/> </sequence> <attribute name="src" type="string" use="optional"/> </complexType></element>

… Enabling xtl:attribute

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17/25… Enabling xtl:if/xtl:for-each

Problem:- Modeling as attribute vs. as element

- Definition of xtl:if/xtl:for-each with wildcard (lax processing)

- Problem: asserting local validity

Current Approach:

<element name="label" type="string" minOccurs="0"/><element ref="xtl:if"> <annotation> <appinfo source="http://research.sap.com/cxsd/1.0"> <cxsd:element name="label" type="string"/> </appinfo> </annotation></element>

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18/25… Forcing ID to be replaced

Problem:

-Attributes and elements with type 'ID' must be instantiation data inside xtl:for-each

-Treatment of IDREF unsolved (conversion to string?)

Current Approach:

<cxsd:inv> let count : Integer = self->select(ancestor::xtl:for-each->size()) in count > 0 implies count(@id-attribute) = 0</cxsd:inv>

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19/25Solution Element 3: Template Validator

Requirements: Preservation

General: Grammar checking tool

Depending on template language grammar used

XML Specific: XML Schema with OCL constraints

OCL navigation expressions replaced by XPath

Implementation via XMLBeans and Dresden OCL Toolkit

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20/25Solution Element 4: Template Engine

Requirements: Control, Independence

General: Enable independence using plug-in architecture

XML Specific: Efficient (memory, time) implementation not straight-forward

SAX implementation hard due to necessary look-ahead

StAX allowed an efficient implementation based on a two-stream machine

Linear Memory and Time Complexity

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21/25Solution Element 5: Term Evaluator

Requirements: Independence

General: Term evaluation outsourced from template engine

XML Specific: Implementation of XPath plug-in makes XTL comparable to XSL-T

XTL cannot be translated directly to XSL-T as this would require dynamic evaluation of XPath (however, with this feature or with a two-stage XSL-T it is possible)

Implementation of OCL plug-in plannedOCL plug-in would enable efficient generation of XML documents from UML

models (XMI, XML Schema using the corresponding profile)

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22/25Solution Element 6: Inst. Data Validator

Requirements: Type Safety

General: Enables exact error messages in case of mismatching instantiation data

types

XML Specific: Only simple types

Straight-forward, check using XMLBeans

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23/25Related Work

• T. J. Parr: Enforcing strict model-view separation in template engines. WWW’04

• S. Maneth, A. Berlea, T. Perst, and H. Seidl: XML type checking with macro tree transducers. PODS’05

• M. Murata, D. Lee and M. Mani:Taxonomy of XML Schema Languages using Formal Language Theory.Extreme Markup Languages 2001

• V. Wallentine, S. Zhou:Validating XML Document Content with the Object Constraint Language.Documents in Computing and Information Science 2002

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24/25Conclusions & Open Issues

Conclusions:

- Safe authoring can be supported with this architecture

- XML Schema wildcards lack expressive power

- XML Schema transformations are hard to implement

Open Issues:

- Search existing XML Schema transformation engines

- Alternative schema languages

- Define white space handling

- Treatment of identity constraints

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25/25Questions

Thank you very much!

Questions…?