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   XML doc valid against more than one DTD / Schema

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  • From: "Charles McCay" <charlie@ramseysystems.co.uk>
  • To: <xml-dev@xml.org>
  • Date: Fri, 7 Apr 2000 16:17:53 +0100

I am working with a complex DTD for a full medical record, and want to
define a simpler subset that could be used just for discharge letters.  This
would include restricting some of the enumerated attributes, and providing
FIXED values for others, and removing some of the optional elements.

What I would like to be able to do is to indicate that a document is valid
against both the restricted DTD and the full one, so that the receiving
system can validate it using whichever one they have.

Such simplified documents could be used as a stepping stone towards
implementing a richer data exchange using the full DTD, reducing the cost of
getting started  while having forward compatibility.

Also if DTD/schema editors could easily validate that their cut-down
document design defined a true subset of the documents valid under a larger
and more complex document design, then the risk of having multiple "similar
but not quite the same" documents would be reduced.
Conditional blocks in the full schema will not work, because that assumes
that the full schema can be amended whenever a new subset is wanted.

It is far easier to persuade a supplier to implement a standards based
interface where
they can do it in an incremental fashion, and so short schema that only
cover part of the
full information model are a good deal more attractive that having to deal
with everything at
once.  However once they have done the work to support the full model, they
want the payback
of not having to modify their work to support different subsets.

Looking at the W3C spec of schemas there seems to be some hope.  It appears
possible to validate a document against a schema other than the one
associated with the default namespace of the instance document, but there is
not a mechanism for saying that a document is valid under two namespaces.
It appears that restrictions will be available to define the simplified
version, but what this will not deliver is an easy to read short document
(schema) for the simplified document definition.  This will still be
referencing the full document, if anything adding the confusion of the

Any suggestions or corrections more thank welcome

Charlie McCay, charlie@RamseySystems.co.uk
Ramsey Systems Ltd, 23D Dogpole, Shrewsbury, Shropshire SY1 1ES
tel 01743 232278 / fax 01743 232295

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