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This is an excellent question, and the problem isn't confined to publicly
available schemas. Even with your own schemas, you might have some
applications that want to impose tighter constraints than others, for
example some applications might be prepared to deal with incomplete
documents in which some information is not yet available, while others
require the information to be complete.
The schema specs seem to have a split personality on this. On the one hand,
they allow mechanisms such as xs:redefine which are based on the premise
that different versions of a schema can coexist. On the other hand, the fact
that processors are expected to be able to find schema definitions given
only the target namespaces carries a strong presumption that a given
namespace can only have one schema.
I don't know how to resolve this dilemma. It's not going to be possible, for
example, to write a schema-aware transformation that converts an input
document conforming to a loose version of schema to an output document
conforming to a tighter schema for the same namespace. If you are only using
schemas for validation, then you might be able to get by defining the
tighter version of the schema using xs:redefine, but I think it's a very
difficult solution to manage.
Michael Kay
http://www.saxonica.com/
> -----Original Message-----
> From: Fraser Crichton [mailto:fraser.crichton@solnetsolutions.co.nz]
> Sent: 03 April 2005 23:56
> To: xml-dev@lists.xml.org
> Subject: [xml-dev] How can I constrain publicly available schemas?
>
> Hi,
>
> I'm working on an application where I need to use subsets of publicly
> available schemas.
>
> For instance, to model contacts information I'm using xCIL
> however xCIL
> has a very large range of formats, for instance one of the immediate
> children the CustomerDetails element can be either
> OrganisationInfo or
> PersonInfo. My system, however, only wants to process PersonInfo
> elements and never OrganisationInfo so, do I -
>
> a) create a more specific schema using the xCIL namespace (!) (if so,
> should I then validate it twice - once against xCIL and then
> against my
> more specific schema)
>
> b) XML.com has this article - Designing a New Schema with XML Design
> Patterns - http://www.xml.com/pub/a/2003/06/04/patterns.html - which
> would seem to suggest selectively reusing types and global
> elements from
> other namespaces, are there any problems with this approach?
>
> c) is the schema just a contract and I should use something
> else again
> for validation?
>
> Is this a dumb question or has anybody else come across this
> issue and
> if so how have they approached it?
>
> Cheers,
>
> Fraser
>
>
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