RELAX NG

XML Foundations [./]
Fall 2013 — INFO 242 (CCN 41613)

Erik WildeUC Berkeley School of Information, UC Berkeley School of Information
2013-12-04

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E. Wilde: RELAX NG

(2) Abstract

Schema languages as a general concept in XML are used to (1) prescribe the allowed document structure, and/or (2) validate a document against a description of what is allowed in a document and what isn't. DTDs and XSDs are particularly important schema languages because DTDs are part of XML itself, and XSD was the first major improvement of the rather limited capabilities of DTDs. Recently, however, XSD is increasingly criticized for its complexity, and the RELAX NG schema language is gaining popularity instead. RELAX NG is a grammar-based schema language (like DTD and XSD), but adds a human-friendly syntax, the ability to use datatypes, and it removes the ability to allow validation to change a document.

E. Wilde: RELAX NG

(3) Design by Committee

XSD was a political decision
- several schema languages were competing to replace DTDs
- DCD, DDML, SOX, XML Data, and XDR were inputs to XSD
- XSD became the first unreadable W3C specification
- implementing XSD correctly is hard (large number of specialized rules)
Researchers were looking for a more elegant solution
- the underlying formalism should be well-defined and well-studied
- the schema language should be easy to learn and use
- lessons learned from DTDs should be included
RELAX NG is the merger of two similar approaches
- Tree Regular Expressions (TREX)
- Regular Language description for XML (RELAX)

E. Wilde: RELAX NG

(4) RELAX NG +/-

RELAX NG and XSD are direct competitors
Advantages of RELAX NG
- RELAX NG Compact Syntax [RELAX NG Compact Syntax (1)]
- the document element is well-defined
- SGML's & is supported (all is extremely limited)
- non-deterministic content models
Disadvantages of RELAX NG
- no datatype support (datatype libraries can be included)
- no modeling facilities in the spirit of XSD's type derivation
- less popular than XSD
- no support for XSD's numeric occurrence constraints (minOccurs/maxOccurs)

Principles

Outline (Principles)

Principles [2]
Example [3]

Principles E. Wilde: RELAX NG

(7) Validation

Validation should not change the document
- there are no default values
Only schema↔instance tests are supported
- there is no type hierarchy as in XSD (schema↔schema)
- there are no identity constraints (instance↔instance)
Grammars should not be restricted
- DTDs and XSD no not allow non-determinism
- RELAX NG allows non-deterministic content models
```
chess = white, (black, white)*, black?
```

Principles E. Wilde: RELAX NG

(8) Grammars

RELAX NG grammars have a start symbol
- DTDs and XSD do not have start symbols
Attributes are defined as part of the content model
- a more homogeneous view of the XML document tree
- this allows alternatives of elements and attributes
Grammars are a set of named rules
- rules define how an element is composed
- local definitions (nested specifications of content models) are possible

Example

Outline (Example)

Principles [2]
Example [3]

Example E. Wilde: RELAX NG

(10) DTD and XSD

<!ELEMENT document (heading, chapter) >
<!ELEMENT heading  (#PCDATA) >
<!ELEMENT chapter  (heading, para+) >
<!ATTLIST chapter  id ID #REQUIRED >
<!ELEMENT para     (#PCDATA) >

document.dtd

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
 <xs:element name="document">
  <xs:complexType>
   <xs:sequence>
    <xs:element ref="heading"/>
    <xs:element ref="chapter"/>
   </xs:sequence>
  </xs:complexType>
 </xs:element>
 <xs:element name="heading" type="xs:string"/>
 <xs:element name="chapter">
  <xs:complexType>
   <xs:sequence>
    <xs:element ref="heading"/>
    <xs:element name="para" type="xs:string" maxOccurs="unbounded"/>
   </xs:sequence>
   <xs:attribute name="id" type="xs:ID"/>
  </xs:complexType>
 </xs:element>
</xs:schema>

document.xsd

Example E. Wilde: RELAX NG

(11) RELAX NG

<grammar xmlns="http://relaxng.org/ns/structure/1.0">
 <start><ref name="document"/></start>
 <define name="document">
  <element name="document">
   <ref name="heading"/>
   <ref name="chapter"/>
  </element>
 </define>
 <define name="heading">
  <element name="heading"><text/></element>
 </define>
 <define name="chapter">
  <element name="chapter">
   <attribute name="id"><text/></attribute>
   <ref name="heading"/>
   <oneOrMore>
    <element name="para"><text/></element>
   </oneOrMore>
  </element>
 </define>
</grammar>

document.rng

Example E. Wilde: RELAX NG

(12) RELAX NG Compact Syntax

start    = document

document = element document { heading, chapter }

heading  = element heading { text }

chapter  = element chapter {
    attribute id { text },
    heading,
    element para { text }+
  }

document.rnc

RELAX NG

XML Foundations [./]
Fall 2013 — INFO 242 (CCN 41613)

Erik WildeUC Berkeley School of Information, UC Berkeley School of Information
2013-12-04

Contents

(2) Abstract

(3) Design by Committee

(4) RELAX NG +/-

(5) RELAX NG Syntaxes

Principles

Outline (Principles)

(7) Validation

(8) Grammars

Example

Outline (Example)

(10) DTD and XSD

(11) RELAX NG

(12) RELAX NG Compact Syntax

RELAX NG

XML Foundations [./]Fall 2013 — INFO 242 (CCN 41613)

Erik WildeUC Berkeley School of Information, UC Berkeley School of Information2013-12-04

Contents

(2) Abstract

(3) Design by Committee

(4) RELAX NG +/-

(5) RELAX NG Syntaxes

Principles

Outline (Principles)

(7) Validation

(8) Grammars

Example

Outline (Example)

(10) DTD and XSD

(11) RELAX NG

(12) RELAX NG Compact Syntax

XML Foundations [./]
Fall 2013 — INFO 242 (CCN 41613)

Erik WildeUC Berkeley School of Information, UC Berkeley School of Information
2013-12-04