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RE: Abstract messages [Was: Multi-Party Binding Scenario]

From: Fletcher, Tony <Tony.Fletcher@choreology.com>
Date: Wed, 2 Apr 2003 12:43:29 +0100
Message-ID: <221369570DEDF346AE42821041345E8912A61E@exchange1.corp.choreology.com>
To: <jdart@tibco.com>, <public-ws-chor@w3.org>

Thank you to Jon and the others in this group for doing this work and
compiling this summary and returning it to the main group.

As we are all aware I am sure, there are many different groups that are
doing message design - UN/CEFACT Forum trade groups, OASIS UBL, OAG,
OTA, RosettaNet and UCC to name but a few (and there are many others,
particularly more industry specific ones).  Therefore I think we should
take it as given that this WS-Choreography group should not get into
message design (a requirement on what we should not do, if you like).

Although XML is very popular at the moment, there are other syntaxes
such as the various flavours of EDI and ASN.1 plus the various
constrained forms such as XHTML and WML - and I am sure there will be
others in the future.  To me this argues for defining a choreography
dependent only on message type / purpose rather than detailed content as
far as possible (may be a 'best practice') and expressing any dependence
on data in terms of the abstract data content (/meaning).  Then have a
separate binding to actual messages, specific WSDL, transport details
and so on.  This should allow the generation of re-usable 'prototype'
choreographies that can be particularised to specific circumstances
(parties requirements) with a specific binding. 

So my personal inclination at present (not a considered company
position!) is to say:

1.   Yes

2.   Yes

3.   No (not for the Choreography itself, but have a binding to WSDL and
XML message schema)

Best Regards     Tony
A M Fletcher
 
Cohesions 1.0 (TM)
 
Business transaction management software for application coordination
 
Choreology Ltd., 13 Austin Friars, London EC2N 2JX     UK
Tel: +44 (0) 20 76701787   Fax: +44 (0) 20 7670 1785  Mobile: +44 (0)
7801 948219
tony.fletcher@choreology.com     (Home: amfletcher@iee.org)


-----Original Message-----
From: public-ws-chor-request@w3.org
[mailto:public-ws-chor-request@w3.org] On Behalf Of Jon Dart
Sent: 01 April 2003 19:48
To: public-ws-chor@w3.org
Subject: RE: Abstract messages [Was: Multi-Party Binding Scenario]



As David Burdett suggested, we've taken some discussion of the WSDL
dependency issue discussed in the "Abstract messages" thread off-line.
Participants in this off-line discussion included David, myself, Assaf
Arkin, William Eidson, Ricky Ho, and Patil Sanjaykumar.

I'd like to summarize this discussion, which concluded with a conference
call yesterday.

The original issue was whether it is a requirement or not to decouple
the public (aka external) view of the choreography from WSDL.

The motiviation for decoupling the choreography from WSDL was to be able
to support general choreography definitions that can work with a variety
of message formats (representing, for example, standards that are
specific to certain industries). There was some discussion of how real a
use case this is, but David's input is that at least for Commerce One,
it is a common case (Assaf concurs).

This raises a difficulty in cases where the choreography flow depends on
the message contents. In those cases, you need to be able to extract the
relevant parts of the message in order to make a test. The logic behind
the test could be opaque (i.e. not specified in the external
definition), but if it is not opaque, then it does need to reference
message contents somehow and therefore (it seems) must assume something
about the schema.

David's original proposed solution was to separate the input and output
schemas of the messages in the choreography from the choreography
definition itself, and also to have the requirement that decision points
based on message contents could be abstractly specified, and later bound
to a particular schema. E.g. suppose we are testing the contents of an
address, but it might be in a "USAddress" element or "UKAddress"
element. In that case we may want to not specify the test (e.g. XPath
expression) in the choreography, but instead put it in an external
binding.

Bill pointed out that WSDL definitions can vary in degree of
abstractness. We could say it is necesary to specify a schema for the
messages, but that schema could specify very little about the message
structure, could define variant structures, and could leave places where
extensions can be made. This being the case, it is possible that a
single schema could express a range of possible message types. Also this
allows tests to be be made on those portions of the message that are
specified, without requiring that the message schema be made totally
specific in the choreography.

Finally, the issue was discussed, whether it is ever necessary to
specify message definitions in such an abstract form that they may not
even be XML, i.e. do we want to eliminate not only dependence on WSDL,
but also dependence on XML Schema? Or, on the contrary, can we assume
that at least at the point where the choreography is evaluating the
message contents, that they are XML?

IMO we could at this point just say that there's a requirement to
support use of the same choreography definition with varying message
schemas, and let it go at that. This is after all the requirements
phase.

However, at a lower level of detail (and closer to implementation) we
have these questions:

1. Do we need, or want, to support external binding of the message
schemas to the choreography definition, so that you can distinguish
between the abstract operation you are performing as written 
in the choregraphy, and the actual service with which you communicate?
(Thanks to Assaf for helping phrase this clearly. N.b. BPEL4WS does 
this, through the concepts of partners, roles, and serviceLinkTypes).

2. Do we need to allow external binding of decision points (based on
message content) to specific decision logic, or it is sufficient to
encode decisions in the choreography, given that they can be based on a
partially abstract schema?

3. (Most fundamental) Should the choreography definition explicitly 
assume that metadata exists in the form of WSDL and XML Schema?

David has an action item to further examine WSDL and XML Schema (esp.
the latter) to see if its abstraction capabilities could be made use of
to allow a solution to the message-independence requirement different
from the one he originally proposed (in particular: Bill's alternative).

IMHO: 1. may be desirable just as a good design principle as it furthers
re-use; 2. external binding is probably not necessary but we might vote
to allow it as an optional feature (I don't think it should be
required); 3. My preference would be to use WSDL + XML Schema, if these
are sufficiently expressive to meet the use cases (David's further input
needed here).

--Jon
Received on Wednesday, 2 April 2003 06:43:38 GMT

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