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   Re: FW: [xml-dev] Partyin' like it's 1999

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Bullard, Claude L (Len) wrote:
> From: Rick Jelliffe [mailto:ricko@allette.com.au]
> At ISO DSDL, we have been sitting around waiting for someone to make
> up a small key/reference/integrity schema language that would
> allow validation and graph-building without requiring full XML Schemas
> validation. The absense of such a thing (e.g. something that
> could be used to create an ID list next to a DOM as a SAX insert)
> suggest either that it is not a pressing need, or that people
> are feckless and lazy, or that XML Schemas and Schematron are
> adequate, or that IDs are enough, or that the people that need
> extended references are using DBMS which already provide
> integrity checks and this is a requirement coming from
> people who want to ramp up "native" XML DBMS.

I think the need is pressing, but that most people have simply not 
thought about generalizing this. Instead, they've added it to the 
application that processes the document, which undoubtedly checks other 
non-declarative constraints as well.

This would be a nice addition for native XML databases. As far as I 
know, none enforce referential integrity at the moment, although you can 
probably do it in a few of the databases via triggers.

I'll go out on a limb and say that I don't think that the language would 
be that hard to design. XML Schemas already covers most of it with their 
  unique, key, keyref stuff. All you would need to do is generalize that 
to cross schemas, which you could probably do by replacing the XPath 
expressions with [IRI#]XPointer expressions, where the IRI gave the 
namespace of the target schema if the pointer crosses documents.

(Question: Is it worth considering references in the case of schema-less 
documents? If so, the IRI obviously can't point to a schema, but then 
what does it point to? Perhaps just a well-defined IRI that say, 
"Another document needed here?")

While this would cover things at the schema level, instance processors 
which actually enforce integrity would need to know the location of the 
target document(s) in the case of references that crossed schemas. Since 
I have no grand ideas here, I'll take the cheap way out and say that 
this is implementation specific :)

-- Ron


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