No Standard way to reference XML Schema? Was Re: (Many) XML Schema Questions

Henry S. Thompson ht at
Thu Dec 30 13:26:33 GMT 1999

[Thread wrt instance->schema connections, lack of rigidity thereof]

With apologies for the 'turgid prose' of the draft in this area, let
me try to explain why flexibility IN THE REC in this area is a Good

Schemas are a powerful and useful mechanism, with a wide range of
possible deployment scenarios.  Different schemas may usefully be
employed with respect to the same instance document for different
purposes, all legitimate.  'xsi:schemaLocation' is a means by which a
document author can signal A location for A schema with respect to
which s/he warrents the instance at hand is schema-valid.  It will
often be appropriate for schema-aware processors to exploit this
information.  But it may not always be possible (the processor may be
offline) or appropriate (the processor may have other schema-based
processing in view) to do so.  We have tried in the current draft to
indicate that 'xsi:schemaLocation' is the preferred, inter-operable
means by which instances signal schemas to processors, WITHOUT making
this connection make-or-break mandatory.

A moment's thought about experience with XML's instance->DTD linkage
will perhaps suggest some benefits of this approach:  as it stands, if 
I wish to validate an XML instance which references no external DTD, I 
have to edit it to incorporate a suitable DOCTYPE declaration.  Even
if the document has a DOCTYPE, if the URL it references is unavailable 
or out-of-date, I again must have recourse to a text editor to fix
this.  We've tried to do better for XML Schema.  Another experience
we've tried to learn from is the instance->stylesheet one, with
similar lessons we believe.

Hope this helps,

  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht at

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