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   RE: How far current XSLT processors will be useful in real applications?

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  • From: "Ingo Macherius" <macherius@darmstadt.gmd.de>
  • To: "Simon Wilson" <Simon.Wilson@crealogix.com>, <xml-dev@xml.org>
  • Date: Tue, 28 Mar 2000 16:09:04 +0200

> -----Original Message-----
> From: owner-xml-dev@xml.org [mailto:owner-xml-dev@xml.org] On Behalf Of
> Simon Wilson

> You could take this further and say that, due to the fact that
> next to nobody processes XML directly, but rather uses standardized
> (DOM/SAX) for processing the XML content, it should be possible to
> XML and access ANY hierarchical data via the DOM API.

Danger, part 1. XML is not about closely coupled interfaces. Do I smell
CORBA in your suggestion ? Integrating systems via direct, synchronous
method calls on objects is a thing XML should overcome, and not perpetuate.
Like http, XML draws its power from the exchange of self contained,
stateless documents.

> An OODB engine might, for example, provide a DOM interface for accessing
its object as
> if they were  XML structures. This would be preferable to extracting an
> representation from the DBMS (expensive), feeding it to SAX/DOM, and then
> your processing.

Danger, part 2. While reading virtual XML documents is a pretty tempting
idea, the nightmare begins when you try to synchronize update operations.
However, the DOM contains methods that *write* into your document. How to
update a virtual XML document of autonomous, distributed sources ?
> In the  end you would have a vendor independent API for the extraction and
querying of
> hierarchical data. Kind of a web-standardized ODBC for hierarchical stuff
> everything looking like XML/DOM, and seeing that hierarchies can be used
> model both relational data and objects, you would have a pretty powerful
> general purpose data access interface.

Danger, part 3. Objects have the tendency not to be hierarchical for even
trivial n:m relations like "student takes courses and courses are taken by
students". This would result in objects graphs rather then hierarchical
DOMs. The results are no longer realizable in XML in a standard way. You
need to overload XML with marshalling/serialization semantics for object
graphs. This takes away basic properties of XML, such as its tree structure
and freedom of DTD design.

> This idea seems so good that someone must be implementing it already...

I bet. Ever looked at RDF, W3C's model for object graphs ?

> Out with ODBC/OLEDB and in with "VXML/DOM" or whatever you might
> want to call it!

Agreed, but don't call the result XML just because XML syntax is used. We no
longer have hierarchical data structures. Beside RDF, you may also want to
have a look at SOAP and it's object serialization algorithm


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