Re: [xsl] XML Schema Design for Effective Utilization by XSLT and XPath

[Ronan Klyne <ronan.klyne@xxxxxxxxxxx>]

Costello, Roger L. wrote:
> Hi Folks,
>
> Below are some thoughts on how to design an XML Schema so that it can
> be effectively utilized by XSLT and XPath.  I would appreciate your
> comments, particularly on any errors, and on poor/incorrect wording.
> Thanks!  /Roger
>
> -------------------------------------------------------------
> XML SCHEMA DESIGN FOR EFFECTIVE UTILIZATION BY XSLT AND XPATH
>
> XSLT and XPath can leverage the information in an XML Schema when
> processing input documents.  
>
> However, leveraging the information in an XML Schema is possible only
> if the schema is designed in a certain fashion.  Only globally declared
> elements and elements with named types can be leveraged.  Elements that
> are locally declared and have anonymous types cannot be leveraged.
> (Note: these comments also apply to attributes)
>
> For example, note the declaration for Publisher in this schema:
>
> <?xml version="1.0"?>
> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema";
>            elementFormDefault="qualified">
>
>     <xs:element name="Book">
>         <xs:complexType>
>             <xs:sequence>
>                 <xs:element name="Title" type="xs:string"/>
>                 <xs:element name="Author" type="xs:string"/>
>                 <xs:element name="Date" type="xs:gYear"/>
>                 <xs:element name="ISBN" type="xs:string"/>
>                 <xs:element name="Publisher">
>                     <xs:simpleType>
>                         <xs:restriction base="xs:string">
>                             <xs:enumeration value="O'Reilly Media,
> Inc." />
>                             <xs:enumeration value="Simon &amp;
> Schuster" />
>                             <xs:enumeration value="Three River Press"
> />
>                             <xs:enumeration value="W. W. Norton &amp;
> Company, Inc." />
>                             <xs:enumeration value="Harvard Business
> School Press" />
>                             <xs:enumeration value="Random House Trade
> Paperbacks" />
>                             <xs:enumeration value="Wrox Press" />
>                             <xs:enumeration value="Prentice Hall" />
>                             <xs:enumeration value="McMillan Publishing"
> />
>                         </xs:restriction>
>                     </xs:simpleType>
>                 </xs:element>
>             </xs:sequence>
>         </xs:complexType>
>     </xs:element>
>
> </xs:schema>
>
> Publisher is declared using an inline element declaration and has an
> anonymous type.  XSLT and XPath will not be able to leverage this
> element declaration.  
>
> All the other elements can be leveraged: the Book element is globally
> declared, and Author, Title, Date, and ISBN all have named types.
>
> There are two ways to redesign the schema so that Publisher can be
> leveraged by XSLT and XPath.
>
> Design 1: make Publisher global. 
>
> <?xml version="1.0"?>
> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema";
>            elementFormDefault="qualified">
>
>     <xs:element name="Book">
>         <xs:complexType>
>             <xs:sequence>
>                 <xs:element name="Title" type="xs:string"/>
>                 <xs:element name="Author" type="xs:string"/>
>                 <xs:element name="Date" type="xs:gYear"/>
>                 <xs:element name="ISBN" type="xs:string"/>
>                 <xs:element ref="Publisher"/>
>             </xs:sequence>
>         </xs:complexType>
>     </xs:element>
>     <xs:element name="Publisher">
>         <xs:simpleType>
>             <xs:restriction base="xs:string">
>                 <xs:enumeration value="O'Reilly Media, Inc." />
>                 <xs:enumeration value="Simon &amp; Schuster" />
>                 <xs:enumeration value="Three River Press" />
>                 <xs:enumeration value="W. W. Norton &amp; Company,
> Inc." />
>                 <xs:enumeration value="Harvard Business School Press"
> />
>                 <xs:enumeration value="Random House Trade Paperbacks"
> />
>                 <xs:enumeration value="Wrox Press" />
>                 <xs:enumeration value="Prentice Hall" />
>                 <xs:enumeration value="McMillan Publishing" />
>             </xs:restriction>
>         </xs:simpleType>
>     </xs:element>
>
> </xs:schema>
>
> Here is an example of an XPath statement that leverages the Publisher
> element declaration:
>
> if ($p instance of schema-element(Publisher)) then
>       concat('The publisher is: ', data($p))
> else
>       error()
>
> Read as: "if the value of the 'p' variable validates against the
> Publisher element declaration in the XML Schema then concatentate the
> string 'The publisher is: ' with the variable's value, otherwise
> generate an error."
>
> Design 2: give Publisher a named type.
>
> <?xml version="1.0"?>
> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema";
>            elementFormDefault="qualified">
>
>     <xs:element name="Book">
>         <xs:complexType>
>             <xs:sequence>
>                 <xs:element name="Title" type="xs:string"/>
>                 <xs:element name="Author" type="xs:string"/>
>                 <xs:element name="Date" type="xs:gYear"/>
>                 <xs:element name="ISBN" type="xs:string"/>
>                 <xs:element name="Publisher" type="publishers"/>
>             </xs:sequence>
>         </xs:complexType>
>     </xs:element>
>
>     <xs:simpleType name="publishers">
>         <xs:restriction base="xs:string">
>             <xs:enumeration value="O'Reilly Media, Inc." />
>             <xs:enumeration value="Simon &amp; Schuster" />
>             <xs:enumeration value="Three River Press" />
>             <xs:enumeration value="W. W. Norton &amp; Company, Inc." />
>             <xs:enumeration value="Harvard Business School Press" />
>             <xs:enumeration value="Random House Trade Paperbacks" />
>             <xs:enumeration value="Wrox Press" />
>             <xs:enumeration value="Prentice Hall" />
>             <xs:enumeration value="McMillan Publishing" />
>         </xs:restriction>
>     </xs:simpleType>
>
> </xs:schema>
>
> Here is an example of an XPath statement that leverages the Publisher
> element:
>
> if ($p instance of element(Publisher, publishers)) then
>       concat('The publisher is: ', data($p))
> else
>       error()
>
> Read as: "if the value of the 'p' variable is an element and has the
> type 'publishers' then concatentate the string 'The publisher is: '
> with the variable's value, otherwise generate an error."
>
> Lesson Learned: when designing an XML Schema, to facilitate its use by
> XSLT and XPath, avoid anonymous, local element declarations.

Hi Roger,

A nice piece. I have one issue with the wording; 'leverage' is a noun, 
not a verb - did you mean 'lever'?... In this context, I think 
'utilise(s)' might be a better word to use.

	# r

--
Ronan Klyne
Business Collaborator Developer
Tel: +44 01189 028518
ronan.klyne@xxxxxxxxxxx
www.groupbc.com