xml-dev - RE: Default DTD URI

[Kay Michael <Michael.Kay@icl.com>]

> The problem is like this: the server (accepting commands in XML) has a
> well - published DTD. So all documents sent by client must 
> conform to this.
> If the XML document contains ELEMENT or ATTLIST declarations, then the
> server will have to do extra job of checking whether that is 
> canonically equivalent to predefined DTD. 

I think this is a real problem, and I have in the past recommended against
using DTDs for validation for this reason. A validating parser checks that
the document conforms to whatever rules the sender wants it to conform to,
not that it conforms to the rules required by the recipient. Since DTDs are
only capable of expressing a small subset of the application-level validity
rules anyway, I've found it easier in practice to do all the validation at
application level. Perhaps I didn't try hard enough: there are parsers that
allow you to build a DOM, modify the doctype declaration to reference your
own DTD, and then validate against that. But I wanted to be
parser-independent, and SAX1 didn't even allow the application to discover
whether the parser was validating or not.

I'm hoping that XML Schemas will improve this situation, but it depends  on
how they are supported in products.

Mike Kay 

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