XQuery-Schema integration

Hi all

while trying to come up with some solution that would allow XPath 
queries to schemata stored in eXist (those queries should not return 
blank XML but a Java representation so that the result is easily 
useable), I discovered that this will probably lead to writing 
classes that represent schema. I could not find any existing 
libraries that would be capable of receiving an XPath expression and 
returning part of a schema (XMLBeans looked promissing, but their 
support for XPath is extremely limited, I would like to use eXist's 
query processor for orthogonality reasons). 
That said, my first attempt was to just use JAXB to run it over XML 
Schema. Then I realized that XQuery also has schema support, but that 
the datamodel used in XQuery is a bit different (but you can 
convert). This leads to a couple of suggestions:

- Use the XQuery type system to internally represent schemata. There 
are conversion rules for XML Schema in the XQuery specs.

- Write our own validator (this task is greatly simplified if the type 
system is that of XQuery and not XML Schema). Rolling our own seems 
like reinventing the wheel, but it would be able to do one-pass 
validation because it knew about the eXist schema store (and thus 
could ask for elements on the fly). Currently (using Sun's MSV) we 
have to do two-pass validation because you have to hand all the 
necessary schemata to the MSV beforehand. If anybody knows a 
validator we could use, PLEASE come forward!

- Make the result of the validation (i.e. the type of the element) 
persistent. This way you only need to revalidate what is changed when 
you update a document. The XQuery implementation also could take 
advantage of this in the long run (e.g. XPath steps could prune 
results early).

To implement this, the next steps would be:
- add the schema syntax to the XQuery implementation. Looking at 
XQuery.g I honestly have no idea how to do this :-(
- implement the conversion from XML Schema to XQuery typing and store 
schemata in XQuery format internally.
- write our own parser based on the XQuery schemata (this means 
implementing XQuery's "validate").

This is clearly not an option for beta-2. I don't know whether schema 
support would make it into release-1.0.
What do you think?

Sebastian

-- 
Sebastian Bossung
Software Systems Group (4-02)
Hamburg University of Technology
Harburger Schlossstrasse 20, 21071 Hamburg, Germany
http://www.sts.tu-harburg.de/~se.bossung


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