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Re: A schema for each mep?

From: Amelia A. Lewis <alewis@tibco.com>
Date: Mon, 10 Nov 2003 10:56:28 -0500
To: FABLET Youenn <youenn.fablet@crf.canon.fr>
Cc: www-ws-desc@w3.org
Message-id: <20031110105628.7e5f1eab.alewis@tibco.com>

A couple of comments.

It appears that the main motivation for this is to set default values.

May I recommend that folks track down what James Clark had to say about
schemas setting default values?  In a nutshell, it's a bad idea, because
it means that schema-validating and non-schema-validating parsers
produce different results.  It means that the schema is, in a sense,
part of the instance.  RNG doesn't support it, for those reasons.

Second, it is not possible to give elements of the same name different
default values in W3C XML Schema (Youenn's 'quick question', below).

Finally, there are real namespace problems here.  In order to define an
element "wsdl:input", the schema namespace has to be that of WSDL. 
Which breaks just *lots* of things, although scoping could, potentially,
resolve some of it (except that it's instance-level scoping, and we
should not be requiring folks to reference a schema from an instance of
WSDL).  There is no mechanism (notoriously) for W3C XML Schema to vary
the content model of an element based on the value of an attribute,
which is effectively what this requires.

The schema, thus, could not be used for automatic validation, but would
require special code in order to be programmatically invoked for each
operation in each interface (presumably, the code would read the value
of the pattern attribute, and load a schema based on that name).  Does
it really add significant value to do this?  It seems to me that we are
only specifying default values, and trading programmed-in default values
for programmed-in custom schema loading extensions seems to me no great
win.

Amy!
On Mon, 10 Nov 2003 14:21:54 +0100
FABLET Youenn <youenn.fablet@crf.canon.fr> wrote:

> At last f2f we discussed about rules to retrieve default values for 
> message-related constructs.
> It seems interesting to have a mep schema for at least two reasons:
>     - allow the validation of the content of each wsdl:operation
>     instance- allow defaulting of values (such as messageRef or
>     direction)
> The idea would be that any author of a wsdl mep spec would also write
> a mep schema.
> 
> Attached are examples of what could resemble such schemas (these are
> not complete schemas, for instance the open content model is not taken
> into account). There are two examples, one to accomodate the current
> notation for contents in the wsdl:operation construct and one to
> accomodate the notation as proposed by Jean-Jacques. In each file
> (example1.xml and example2.xml), a <syntax> element is related with a
> <schema> element. Each schema should do some validation of the
> <syntax> example and give default values when possible.
> 
> What do you think ? Is this worth continuing exploring this subject?
>     Youenn
> 
> Quick question: in the case of meps with more than one input message,
> we might have several <wsdl:input> in a <wsdl:operation> construct. Is
> it possible with XML Schema to give a different type for each of these
> 
> <wsdl:input>? If I recall correctly I would say no, but I am not very 
> sure (If it is not possible, it might lead to problems for defining 
> mep's schemas, with XML Schema at least).
> 


-- 
Amelia A. Lewis
Architect, TIBCO/Extensibility, Inc.
alewis@tibco.com
Received on Monday, 10 November 2003 10:56:21 GMT

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