Re: [xml-dev] XML Database Decision Tree?

[Dan Weinreb <dlw@exceloncorp.com>]

   Date: Wed, 24 Oct 2001 10:36:12 +0200
   From: Nicolas LEHUEN <nicolas.lehuen@ubicco.com>

   This raises the following question : are there schema mutation tools in
   native XML databases ? I.e. you provide a new target schema and the XSLT
   stylesheet to apply to move one collection of document from an old schema to
   the new one, and the database processes all relevant documents in a safe
   way, e.g. by locking the collection or any other means that would prevent
   clients to get a mix of old and new documents.

Short answer: Yes.  eXcelon's "Stylus Studio" tool has a component
called the "XML mapper", which provides assistance in defining a
mapping from one DTD to another, and generates a stylesheet that
implements the defined mapping.

Caveats: Some possible mappings (even some possible DTD's) aren't
expressible using the XML mapper's graphical notation.  Stylus Studio
allows you to edit the generated stylesheet by hand, and does its best
to recover the graphical representation of the modified mapping, but
that recovery isn't always possible.  Considering stylesheets in
isolation as representations of mappings, there is no assurance that
all possible mappings are representable in XSLT, and the properties of
a particular stylesheet with respect to source and target DTD's are
not easily (if at all) provable.  I don't know of any standard
notation or theory for reasoning about DTD's and relations or mappings
between DTD's, in general.  Assuming that you can find such a notation
for the particular case at hand, and can represent the desired mapping
as a stylesheet (and somehow prove or test or otherwise convince
yourself that it does in fact represent the desired mapping), then you
can use the XSLT processor in XIS to generate a new file from the old.
Also, the current implementation may have effective size limits on the
XML document being transformed; future releases will increase or
eliminate these limits.

(This info is from Paul Rabin, the engineering manager in charge of
eXcelon's native XML database product, officially known as Extensible
Information Server or XIS.)

(Also, XML documents in XIS aren't required to have prescriptive
schemas.  So if your position was that you didn't want a prescriptive
schema in the first place, but just wanted to allow any well-formed
XML, XIS can work that way too.  But I guess you were referring to
cases where you do want a prescriptive schema and then you want to
change it.)

-- Dan