I'm not really sure that open-world/closed-world is the right way to
characterize this question. I see that as being more about whether or not
one can assume one has access to all instances of a type (for example, all
employees) or only to selected instances.

It's true that derivation-by-extension poses problems for substitutability.
But there is an OO analogy: adding new methods/fields in a subclass does not
invalidate callers who only use the methods/fields defined in the
superclass. Applications that use the paths a/b or a/c will continue to work
when a subtype is defined that also permits a/d.

Regards,

Michael Kay
http://www.saxonica.com/
http://twitter.com/michaelhkay 
 

> -----Original Message-----
> From: Olivier Rossel [mailto:olivier.rossel@g...] 
> Sent: 05 October 2009 10:04
> To: xml-dev@l...
> Subject: [xml-dev] Schemas and the open world assumption.
> 
> Hello everyone.
> 
> Sorry to be 5 years late, but it is just today that I 
> question myself about derivation by extension and closed 
> world assumption.
> 
> Please do not hesitate to comment the following points:
> 
> If I define "a" as being a sequence of b,c:
> <a>
>  <b/>
>  <c/>
> </a>
> 
> and i extend "a" into "aa" that extends that sequence with d:
> <aa>
>  <b/>
>  <c/>
>  <d/>
> </aa>
> 
> then any "aa" will not validate against the definition of "a".
> right?
> this sounds like a MAJOR difference with OO paradigm (where 
> any "aa" is also a "a").
> 
> that is what i call the closed world assumption in xml validation.
> 
> considering i need a more open world approach, i plan to 
> relax my schema by defining "a" in this way:
> <a>
>  <b/>
>  <c/>
>  <xsd:any>
> </a>
> 
> then i feel like i could extend my "a" definition  without 
> breaking the "subclass" philosophy.
> 
> can anyone comment that point of view?
> i am especially interested in possible pitfalls i could have 
> missed in using the "any" statement.
> i am also interested in best practices when defining modular 
> expandable models.
> 
> any help is very welcome.
> 
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