W3C home > Mailing lists > Public > public-ws-chor@w3.org > April 2003

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

From: Assaf Arkin <arkin@intalio.com>
Date: Wed, 02 Apr 2003 11:55:42 -0800
Message-ID: <3E8B403E.2030307@intalio.com>
To: William Eidson <weidson@tibco.com>
CC: Fletcher Tony <Tony.Fletcher@choreology.com>, public-ws-chor@w3.org



William Eidson wrote:

>  Tony -
>  I think you hit the nail on the head when you described EDI and ASN.1 as
>syntaxes.  I strongly believe that we should tie ourselves to WSDL/Schema,
>but that does not mean that we are tied to the XML syntax in the actual
>messages exchanged.
>  Both schema and WSDL both operate at the xml information level not the xml
>syntax level, so, for example, a WSDL interface can easily be used to
>describe an EDI or ASN.1 message.  What is missing right now are standard
>bindings to EDI/ASN.1/name-your-favorite-encoding+transport, however, there
>are plenty of companies already providing and using EDI<->XML converters.  A
>processor is, of course, able internally to represent the data however it
>chooses, provided it can logically present it as XML when needed.
>  I think the cost of adding a level of indirection above WSDL/Schema would
>be enormous in terms of extra complexity and user-confusion.  Instead, we
>should work to leverage the extensibility and openness already in WSDL and
>the richness and expressiveness already in schema.
>  Thanks,
>  - Bill
>>-----Original Message-----
>>From: public-ws-chor-request@w3.org
>>[mailto:public-ws-chor-request@w3.org]On Behalf Of Fletcher, Tony
>>Sent: Wednesday, April 02, 2003 3:43 AM
>>To: jdart@tibco.com; public-ws-chor@w3.org
>>Subject: RE: Abstract messages [Was: Multi-Party Binding Scenario]
>>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
>>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
>>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).

"Those who can, do; those who can't, make screenshots"

Assaf Arkin                                          arkin@intalio.com
Intalio Inc.                                           www.intalio.com
The Business Process Management Company                 (650) 577 4700

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Received on Wednesday, 2 April 2003 14:56:56 UTC

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