Re: using namespaces to version

[Tony Coates <Tony.Coates@reuters.com>]

On 03/05/2001 21:02:50 Jonathan Borden wrote:

>> What is the mechanism for declaring which version of the resource (schema,
say)
>> is the correct one for a given instance document?
>>

[ ... snip ...]

>Using the general rule that backwards compatible changes to a schema might
>stay within the same namespace, the current schema for a particular
>namespace would be identified by the purpose:
>http://www.rddl.org/purposes#schema-validation

I don't quite see how this can work.  I mean, from what I understand, the
purpose is embedded in the RDDL document, so there is still no way of knowing
which actual version of the schema is required.  The problem here is that of
what is meant by "backwards compatible".  Many XML authors think of it in terms
of "instance documents which are valid under (some?) previous versions of the
schema remain valid instances under the new schema".  That's fine, but for an
application developer, a more useful definition of "backwards compatible" would
be "instance documents which are valid under the new schema are also valid under
the version of the schema against which the application was written".  As you
can see, the direction of "backwards" is opposite in these two cases, yet both
are perfectly valid views.  It is really because of this conflict that I
consider it so important to be able to resolve the precise versions of the
schemas used by a document, and namespaces with explicit versions do seem to be
the only practicable solution at present.

Clearly this is not satisfying from a theoretical standpoint.  It's easy to
criticise with the benefit of hindsight, so I don't intend this as a carping
criticism at all, but my experience is that one of the major architectural
implications of writing applications that handle XML is that you have build an
architecture that can cope with changes to versions of schemas.  There really
isn't a clear W3C story (as far as I can see) on how versioning should be dealt
with.  It might have been nice if it could have been dealt with at the same time
as namespaces, though I clearly understand the desire at that point to keep
namespaces as simple as possible.  I don't know what the way forward is with
versioning, but we are going to keep revisiting this problem until we do agree
on a generic solution.

     Cheers,
          Tony.
========
Anthony B. Coates
Leader of XML Architecture & Design
Chief Technology Office
Reuters Plc, London.
tony.coates@reuters.com
========


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