Extensibility limitation in XML schema

>Working Group,
>
> 
>
>Just in case you all are not aware of this issue already:
>
> 
>
>Problems with XML schema.  Unique Particle Attribution.
>
> 
>
>Say you have a hierarchy:
>
> 
>
>1st Generation
>
>---------------
>
>xa
>
>  xab
>
>  xad
>
>  xsd:any (optional - placeholder for future extension, 1st Generation
>apps will not use
>
>this element)
>
> 
>
>Now in 2nd Generation apps, it is desired to add an optional xae member
>of xa.
>
>Because of the Unique Particle Attribution rule, this optional element
>may
>
>not precede xsd:any, and so we must remove xsd:any as so:
>
> 
>
>2nd Generation
>
>--------------
>
>xa
>
>  xab
>
>  xad
>
>  xae (optional)
>
> 
>
>Now because there is no xsd:any we cannot add another element xaf in
>
>a 3rd generation app:
>
> 
>
> 
>
>3rd Generation
>
>--------------
>
>xa
>
>  xab
>
>  xad
>
>  xae (optional)
>
>  xaf (optional)
>
> 
>
>Because the 2nd Generation apps won't accept xaf (1st generation apps
>will).
>
>With these 3 generations:
>
> 
>
>1 may receive from 1, 2, and 3
>
>1 may send to 1, 2, and 3
>
> 
>
>2 may receive from 1 and 2
>
>2 may send to 1, 2, and 3
>
> 
>
>3 may receive from 1, 2, and 3
>
>3 may send to 1 and 3
>
> 
>
> 
>
> 
>
>Solution
>
>-----------
>
>XML Schema SHOULD but DOES NOT (thus a flaw in the design) allow
>
>optional elements to be placed before an xsd:any optional element.
>
> 
>
>Use xpath/DOM to marshal the XML to java.  This will ignore all the
>
>optional extensions.  Do your own marshalling to XML.
>
> 
>
> 
>
> 
>
>xpath package --> javax.xml.xpath
>
> 
>
>Regards,
>
> 
>
>Andrew Shropshire
>
>AT&T Government Solutions, Inc. 
>
>703-506-5708
>
> 
>
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