RE: Versioning of XML Schema and namespaces

> -----Original Message-----
> From: Eliot Kimber [mailto:ekimber@innodata-isogen.com] 
> Sent: Wednesday, May 04, 2005 8:10 AM
> To: John.Hockaday@ga.gov.au; xmlschema-dev@w3.org
> Cc: :;
> Subject: Re: Versioning of XML Schema and namespaces
> 
> John.Hockaday@ga.gov.au wrote:
> > I expect that document instances using W3C XML Schemas will use a 
> > "namespace" declaration to identify which XML Schema should be used to 
> > validate that document instance.  The problem that I see with the 
> > namespace it that a URI is the unique identifier.  There is no PUBLIC 
> > identifier.  As we have all probably experienced with old bookmarks, 
> > the content at URLs change a lot. If an XML Schema's version is not 
> > part of the URI and a new version of that XML Schema is made then it 
> > is likely that this will *not* be reflected in the URI and hence the 
> > namespace.
> 
> Thus, I think the appropriate approach in your case is to require the 
> use of schemaLocation= with absolute URIs that include version 
> information--that gives you the same control you had before.

Remember, however, the xsi:schemaLocation [1] is just a hint...schema processors are not required to honor it.  In a similar vein, the presence of <!DOCTYPE> in an instance does not mean that processors must perform DTD validation, although most processors do so by default.

For what it's worth, the reason the XML Schema WG made xsi:schemaLocation a hint was for security reasons.  Depending on circumstances, it can be very dangerous to trust the sender/author to tell you what DTD/schema to use to validate against.  After all, one of the main reasons to perform validation is because you don't trust the sender/author...so why would you trust them when they tell you what DTD/schema to use?

pvb

[1] http://www.w3.org/TR/xmlschema-1/#xsi_schemaLocation