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   Re: XML Schema?

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  • From: Rick JELLIFFE <ricko@geotempo.com>
  • To: XML-DEV@xml.org
  • Date: Thu, 29 Jun 2000 21:08:39 +0800

Development wrote:
> Request for clarification,
> I would like to build on this question: are they (schemas) a replacement
> for DTD's?
> Or
> are they validated using a DTD? - What is the future of DTDs - with respect
> to XML.
For  back-end data interchange, especially from a DBMS, you can expect
XML Schemas to replace DTDs. They provide better datatyping and they are
designed to fit in conveniently with RDBMS and OO languages such as

For data that is closer to human users or file-systems, XML Schemas
cannot entirely replace DTDs yet:

 1) there is no way to define entities, in particular entities for
special characters or entities to allow a document to be split into
several shared pieces;

 2) type is directly keyed from the element name (or an xsi:type
attribute), which does not allow the user-friendly markup idiom of where
the element name gives the basic type of the element and an attribute
declares the subtype; instead, if one wants subtyping, one has to make
an explicit new element named for that--one must have <textInput> rather
than <input type="text">...

  3) there may be some uses of parameter entities which do not have
explicit equivalents in XML schemas: in particular, INCLUDE marked
sections (i.e. schemas paramterized by a header in the document);  

  4) if the constraints in your document are not regular and not keys,
or if you have a lot of type selection based on the occurrence of values
in attributes or from external vocabularies, then perhaps XML Schema
will offer little in advance of DTDs for validation.

  5) if you have a severe sensitivity to data size, DTDs may be
preferred as a distribution schema. For XML Schemas which do not make
use of the tag/type distinction (i.e. for schemas where an element name
always selects the same content model), then XML Schemas can be
transformed (e.g. by XSLT) into DTDs for distribution, but at the cost
of losing the ability to validate datatyping.

  6) XML Schemas is namespace based, so if you do not use namespaces you
cannot use XML Schemas (last time I checked...hmmm don't quote me, I
have to check the status quo and my shocking memory confuses me easily)

So the big question is whether XML Schemas provides enough extra bangs
per buck from its data orientation to overcome the deficiencies listed
above, where it does not replace DTD functionality.  

And, of course, there are some kinds of data for which neither XML
Schemas nor DTDs seem useful: XSL-FO has a lot of complex inheritable
declarations (if X has an attribute Y, the children of X can also have Y

I am sure the XML Schema WG at W3C will be very interested in finding
out how well it fits user's requirements; I hope people will take a good
hard look at the upcoming CR draft.  I think readers of this list will
know, from the carefully prepared 210+ long issues list so dillegently
prepared by Michael Sperberg-McQueen, that the XML Schema WG is trying
very hard to have public review systematically addressed, since this is
such an important issue.

Rick Jelliffe
(not writing on behalf of XML Schema WG)

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