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Re: [xml-dev] RE: Is schemaLocation just a hint in schema import?

From: Dan Vint <dvint@dvint.com>
Date: Mon, 16 Oct 2006 17:31:41 -0700
Message-Id: <>
To: "C. M. Sperberg-McQueen" <cmsmcq@acm.org>,Dan Vint <dvint@dvint.com>
Cc: "C. M. Sperberg-McQueen" <cmsmcq@acm.org>, "Michael Kay" <mike@saxonica.com>, "'Antoli, Leo'" <Leo.Antoli@Misys.com>,<xml-dev@lists.xml.org>, <xmlschema-dev@w3.org>


At 02:25 PM 10/16/2006, C. M. Sperberg-McQueen wrote:
>On 15 Oct 2006, at 13:08 , Dan Vint wrote:
> > This seems another instance where the schema folks changed the
> > path of the XML standard. In a DTD, it is the last definition
> > read that becomes the definition - seems like the import rules
> > should treat schemas in the same way.
>Either your memory is playing tricks on you, or mine is playing
>tricks on me.  I believe the rule in SGML and XML DTDs is
>normally either that multiple declarations are illegal (you
>mustn't declare the same element twice, for example) or the first
>one wins (entities, attributes).

<drv>Well you got the idea. Multiple declarations are not an error 
and first (not last) wins. I knew I should have looked up the exact 
details. ;-)

>Sec. 4.2 of the XML spec says
>     If the same entity is declared more than once, the first
>     declaration encountered is binding; at user option, an XML
>     processor MAY issue a warning if entities are declared
>     multiple times.
>Sec. 3.3 says
>     When more than one definition is provided for the same
>     attribute of a given element type, the first declaration is
>     binding and later declarations are ignored.
>But the parallel you are drawing seems off to me: an import
>functions in part like a declaration (it declares that references
>to components in the imported namespace should be legal in the
>schema document with the xsd:import element), and in part like an
>xsd:include (in the sense that the user will frequently want the
>processor to go find components for the imported namespace,
>possibly using the hint given as to location).
>To the extent that xsd:import acts like a declaration, the
>question of first-wins vs. last-wins makes no sense.  Both
>declarations do exactly the same thing - they make references to
>components in that namespace legal.  So there's no question of a
>If the two carry different schemaLocation hints, then in a
>situation where we are following the location hints, the correct
>analogy seems to me to be with entity declarations, where XML
>Schema 1.0 follows the same first-wins rule as XML.
>You may perhaps be thinking of the last-wins rule of XSLT 1.0.
>Sec. 5.5 of XSLT 1.0 says

<drv>No it was XML and the first defined as you indicated above.

>     It is an error if this leaves more than one matching template
>     rule. An XSLT processor may signal the error; if it does not
>     signal the error, it must recover by choosing, from amongst
>     the matching template rules that are left, the one that
>     occurs last in the stylesheet.
> > The only place that seems like it needs special handling is
> > with redefine in the situation that I just got caught in.
> > I have Schema A that is imported into Schema B, I then redefine
> > Schema A and import Schema B. This also seems to be a
> > implementation dependent problem and the solution that was
> > proposed but I haven't tested was to remove the schema location
> > on the import of Schema A into schema B. I find it strange that
> > some how the physical file location might control part of the
> > triggering of this rule. Seems to me that the namespace should
> > be the equivalent of specifying a file location, but maybe it
> > isn't.
>This seems to be an instance similar to the one described in
>Bugzilla issue 2577
>(http://www.w3.org/Bugs/Public/show_bug.cgi?id=2577), about what
>happens when the same schema document is both included and
>redefined (normally, as in your description, via different
>Your processor may be taking the simple and often unproblematic
>approach of handling a schema location hint on an import by
>retrieving the schema document indicated and adding the relevant
>components to the schema being assembled -- i.e., treating the
>import a lot like an 'include'.  If so, then you can see why it's
>running into problems and why.  You import B, and when it sees
>the import with a hint pointing to schema document A, it goes and
>reads A and adds the components defined in A to the schema it's
>collecting.  Then it sees your redefine, and it builds components
>from schema document A, modifying them as specified in the
>redefine.  For each thing you redefine in A, it's now got two
>copies, one the original and one the modified copy.  Not a legal
> > How about a push to simplify XML schema instead of changing the
> > syntax and features of the core XML spec?
>The Working Group is working as hard as we can to make XML Schema
>1.1 clearer than 1.0, to fix bugs, and to add useful
>functionality.  Simplification in the form of eliminating
>features has proven to be a very hard sell -- almost everyone
>agrees that there are a lot of features no one would miss, but
>the lists people give don't converge.  Just this morning I had a
>conversation in which someone suggested that the easiest way to
>simplify the description of schema composition would be to
>eliminate xsd:redefine entirely.  They are probably right, as far
>as it goes, but if you are using redefine you might feel that
>that would be going about simplification the wrong way.

<drv>No I understand the situation, I was part of the group that met 
at Oracle on this topic. Had redefined never been created I would 
have never used it though ;-)

<drv>The reason I bring up the redefine as it seems to be handled 
differently, and  it was suggested that removing the file reference 
would then make the error go away. To me wether the schemaLocation is 
there or not, the namespace should trigger the same problem. How can 
I import the namespace and not have a definition for what it contains 
- my SGML senses say this is just plain wrong. But then I didn't 
agree with the notion that a location is a hint either - why couldn't 
XML Catalogs handle any mapping that you wanted to change?

<drv>Anyway, The redefinition process I wanted to do is as described. 
It was alternately defined as a implementation dependent detail, or 
could be handled by removing the schemaLocation. I believe removing 
the schemaLocation would then remove the content validation I wanted 
for reuse of those structures. Now if you said that changing the 
schemaLocation on the Import should point to the redefining schema, 
then I could go along with that, but to remove it completely seems 
wrong as well.


>best regards,
>--C. M. Sperberg-McQueen
>   World Wide Web Consortium

Danny Vint

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voice: 510-522-4703

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Received on Tuesday, 17 October 2006 00:32:36 UTC

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