RE: [xml-dev] Backward and forward compatible schemas ... RelaxNG --> Y

[Rick Jelliffe <rjelliffe@allette.com.au>]

On Thu, 2007-08-23 at 22:33 -0700, Dave Orchard wrote:
> The blanket statement that XML schema 1.0 does not support forwards and backwards compatibility is incorrect.  Using an "Extension"
> or "Sentinel" element, one can get around XSD 1.0 UPA constraints at the cost of extra markup in instances.  This is shown in the
> W3C TAG Versioning Finding(editors draft) XML and Schema doc, section 7.4 http://www.w3.org/2001/tag/doc/versioning-xml#versionid3
> and in the section 4 "Version Strategy" of an XML.com article explaining forwards compatibility using XSD 1.0
> (http://www.xml.com/pub/a/2004/10/27/extend.html?page=4)
 
I think Dave is replacing one blanket statement with another. To say "it
supports it in a particular case" is no rebuttal to people saying "it
doesn't support it in every case, by which I mean 'in my case'."  

What people complain about with XML Schemas 1.0 is having to adjust the
base schema when they want to just be able to adjust the derived
schemas. (That you have to add wildcards or <other> elements willy nilly
is as old as the hills, and no advance on parameter entities or
<redefine>.) 

However, the whole basis of XML Schemas is namespaces and modularity,
and that means adopting standard or industry vocabularies which almost
universally have not been written to be open. People are really loath to
rewrite standard or industry vocabularies.

That being said, the new features in XSD 1.1 for openness look a step in
the right direction for capabilities, except for adding to the monolith
and thereby being a step in the wrong direction complexity-wise.

Cheers
Rick