Re: [xml-dev] [Summary] Backward and forward compatible schemas ... Rela

["Stephen Green" <stephengreenubl@gmail.com>]

Very excited to see this. Is there an all global way to do this
or a way to create a reusable library schema set preserving
such forward and backward compatibility - something which
might be desirable for typical cases needing such versioning?

I guess any use of imports might make it less straightforward
but does XML Schema 1.1 help with such like?

Many thanks

-- 
Stephen Green

Partner
SystML, http://www.systml.co.uk
Tel: +44 (0) 117 9541606

http://www.biblegateway.com/passage/?search=matthew+22:37 .. and voice



On 24/08/07, Dave Orchard <orchard@pacificspirit.com> wrote:
> > -----Original Message-----
> > From: Costello, Roger L. [mailto:costello@mitre.org]
> > Sent: Friday, August 24, 2007 8:32 AM
> > To: xml-dev@lists.xml.org
> > Subject: [xml-dev] [Summary] Backward and forward compatible
> > schemas ... Relax NG --> Yes ... XML Schema -->Yes
> >
> >
> > Thanks everyone for your excellent comments!
> >
> > Special thanks to David Orchard.  David, I read your article.
> >  I now understand how to implement backward and forward
> > compatible schemas using XML Schema.  The technique you
> > illustrate is very powerful.
>
> Not a problem!  And with Schema 1.1, it is much easier to do backward and forward compatible schemas..
>
> > Notice that successive schema versions "added" new elements.
> > Is there a way for successive versions to remove elements and
> > still remain backward and forward compatible?
> >
>
> It depends what you mean by "remove".
>
> If you mean remove and not accept, then the options depend upon the cardinality in V1.  If you mean remove the definition but still
> accept, then in general the language can still be fully compatible.  An example of remove and still allow would be to replace an
> element with a wildcard that allows that element.  As we've previously said, this is possible only in some specific cases in XSD 1.0
> - such as an element in a non-targetnamespace being replaced by an any with namespace="##other" - and more generally allowable in
> XSD 1.1.  The theory is that you've reduced the "defined text set" size but not the "accept text" set, which is forwards and
> backwards compatible change.  See a formal model of compatibility at
> http://www.xml.com/pub/a/2006/12/20/a-theory-of-compatible-versions.html and the TAG versioning finding terminology
> (http://www.w3.org/2001/tag/doc/versioning) for more explanation of the terms and set theory.
>
> Under remove and not accept, elements with maxOccurs greater than minOccurs can have maxOccurs reduced to the minOccurs and the
> newer language is forwards compatible because an "newer" producer will then not produce the content up to the old maxOccurs.  In the
> easy case, this means removing optional content is forwards compatible.  The newer language is not fully backwards compatible
> because an "older" producer may produce the content with cardinality greater than the new "maxOccurs".  If the "older" producer only
> produces content within the new maxOccurs, then the producer has effectively subsetted the language such that the language it is
> using is backwards compatible.  This is one advantage of "partial understanding".
>
> It is not possible to reduce the maxOccurs less than the previous minOccurs (ie removing mandatory elements) and be forwards
> compatible because older consumers will not accept the newer documents because there aren't enough of the element or backwards
> compatible because newer consumers will not accept the older documents because there are too many of the element.
>
> In a more formal model, it is forwards incompatible because Defined Text set of the new language is smaller than the older languages
> Defined Text Set (not enough) and it is backwards incompatible because the Defined Text set of the old language is larger than the
> Accept Text Set (too many) of the newer language.
>
> Cheers,
> Dave
>
>
>
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