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RE: Versioning of XML Schema and namespaces

From: Hans Teijgeler <hans.teijgeler@quicknet.nl>
Date: Thu, 5 May 2005 00:25:00 +0200
Message-Id: <200505042225.j44MP1Go027731@vmx40.multikabel.net>
To: "'Eliot Kimber'" <ekimber@innodata-isogen.com>, <John.Hockaday@ga.gov.au>, <xmlschema-dev@w3.org>
Cc: <":www-xml-schema-comments"@w3.org>

Hi John,

My two cents of wisdom. In ISO 15926-7 we have defined nine interlinked XML
Schemas, the top one, for the data ISO 15926-2 data model, starting with:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema 
	xmlns:xs="http://www.w3.org/2001/XMLSchema" 
	xmlns="http://www.tc184-sc4.org/iso15926-7/datamodel/2005-1.xsd" 
	
targetNamespace="http://www.tc184-sc4.org/iso15926-7/datamodel/2005-1.xsd" 
	elementFormDefault="qualified" attributeFormDefault="unqualified">
	...

This is imported in the next level schema, for Templates:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema 
	xmlns="http://www.tc184-sc4.org/iso15926-7/templates/2005-1.xsd" 
	xmlns:p2="http://www.tc184-sc4.org/iso15926-7/datamodel/2005-1.xsd" 
	xmlns:xs="http://www.w3.org/2001/XMLSchema" 
	
targetNamespace="http://www.tc184-sc4.org/iso15926-7/templates/2005-1.xsd" 
	elementFormDefault="qualified" attributeFormDefault="unqualified">
	<xs:import
namespace="http://www.tc184-sc4.org/iso15926-7/datamodel/2005-1.xsd" 
	
schemaLocation="http://www.tc184-sc4.org/iso15926-7/datamodel/2005-1.xsd"/>
	...

and this is imported again in the next level, for Object Information Models:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema 
	xmlns="http://www.tc184-sc4.org/iso15926-7/oim/2005-1.xsd" 
	xmlns:p2="http://www.tc184-sc4.org/iso15926-7/datamodel/2005-1.xsd" 
	
xmlns:templ="http://www.tc184-sc4.org/iso15926-7/templates/2005-1.xsd" 
	xmlns:xs="http://www.w3.org/2001/XMLSchema" 
	targetNamespace="http://www.tc184-sc4.org/iso15926-7/oim/2005-1.xsd"

	elementFormDefault="qualified" attributeFormDefault="unqualified">
	<xs:import
namespace="http://www.tc184-sc4.org/iso15926-7/datamodel/2005-1.xsd" 
	
schemaLocation="http://www.tc184-sc4.org/iso15926-7/datamodel/2005-1.xsd"/>
	<xs:import
namespace="http://www.tc184-sc4.org/iso15926-7/templates/2005-1.xsd" 
	
schemaLocation="http://www.tc184-sc4.org/iso15926-7/templates/2005-1.xsd"/>

etc, etc.

The targetNamespace contains (here) the suffix 2005-1, meaning that it is
the first version of the year 2005. The schemas are (in due time) located on
a publicly accessible web server.

I hope this helps somewhat.

Regards,
Hans
 

-----Original Message-----
From: xmlschema-dev-request@w3.org [mailto:xmlschema-dev-request@w3.org] On
Behalf Of Eliot Kimber
Sent: woensdag 4 mei 2005 17:11
To: John.Hockaday@ga.gov.au; xmlschema-dev@w3.org
Cc: ":www-xml-schema-comments"@w3.org
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.

I think you're confusing two similar but distinct functions here: 
namespaces and schema locations. DTD declarations with external identifiers
are equivalent to schemaLocation specifications, not namespace
declarations--SGML (and XML without namespaces) had no equivalent of
namespaces [except for HyTime architectures].

For DTDs, which are a syntactic part of the document that references them,
the PUBLIC identifier is nothing more than an alias for the external DTD
subset's storage location (i.e., it's filename). Thus it is completely
appropriate that it include a version identifier since if the external
declaration subset is changed it's a new object and should be identified as
such.

By contrast, a document's namespace *does not* directly identify a schema.
It identifies (or rather, can be exclusively associated with) an
(abstract) "application" that might have any number of schemas associated
with it. That is, for a given application, with a single associated
namespace, there might be different schemas *at the same time*, reflecting
different profiles or uses of the application, or there might be different
schema versions over time reflecting changes over time to the details of the
namespace. But the namespace itself is unchanged because the namespace
identifies the application independent of it's various implementations over
time. [For example, the XSLT namespace is invariant across versions of the
XSLT spec because, as an abstract application, XSLT is XSLT regardless of
the currently-defined details of it.]

In DTD-based documents that use external declaration subsets you always have
to have an external identifier for the subset, so you always had something
you could resolve or use in a catalog.

For non-DTD-based documents, there are two possible cases (assuming
namespaced documents--the no-namespace case is degenerate and not worth
considering because it allows no good general solution):

1. The document uses the schemaLocation= hint to say which specific schema
it wants you to use.

2. The document specifies only a namespace.

In the first case, the schema location can either be local, relative URI or
it can be an absolute URI. In this case of the absolute URI, the URI
functions essentially as a PUBLIC ID does: it essentially demands local
mapping to a local resource via some sort of catalog method (for the simple
practical reason that most processing environments aren't always net
connected or because the schema is not in fact served on a
publicly-available server). If the absolute URI includes some sort of
version value, then you have *exactly the same* functionality and semantics
as with PUBLIC IDs for external DTD subsets.

In the second case, the implication is that the system must determine which
version of the schema to use, which typically would be done using a catalog
and probably implies that in most cases you want the latest or more general
version of the schema. But, a processor could use other heuristics to decide
which version to use, for example, looking in the document for other clues
or using some outside information, such as metadata held in a document
management system.

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.

It's important to remember that there is (and never was) any particular
magic to PUBLIC identifiers--they are just magic strings that require
indirection to be resolved. In that respect that are indistinguishable for
URIs that also require indirection to be resolved to real resources.

Cheers,

Eliot

--
W. Eliot Kimber
Professional Services
Innodata Isogen
9390 Research Blvd, #410
Austin, TX 78759
(512) 372-8155

ekimber@innodata-isogen.com
www.innodata-isogen.com
Received on Wednesday, 4 May 2005 22:25:31 GMT

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