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RE: General Choreography and Bi-lateral Choreography

From: Furniss, Peter <Peter.Furniss@choreology.com>
Date: Mon, 17 Mar 2003 00:45:18 -0000
Message-ID: <221369570DEDF346AE42821041345E890E82A9@exchange1.corp.choreology.com>
To: "Ricky Ho" <riho@cisco.com>, <public-ws-chor@w3.org>
I agree with the view that concentrating on the bilateral case will be
helpful.
 
One test against the utility of "general" choreography is to consider
the case where some of the communication is not web-services
(effectively, not usefully represented in wsdl).  Take the use case
Ricky started with , of buyer - seller -shipper.  If the seller-shipper
communcations were by phone/fax, or legacy edifact, the buyer-seller and
buyer-shipper conversations could well be exactly the same.  So the
Choreography description as known and used by the buyer need not (and
indeed should not) be dependent on how the other two talk to each other.
 
Even the linkage between the conversations involving a single party is
rather limited. Much of it is kind of "typed reference" to another
service - if the buyer tells the seller the address of the buyers ws
that will be used by the shipper, then, if we have good bilateral
choreography descriptions, that passed address (uri) might be typed by
the appropriate description. And there is likely some data field passed
as well, so the buyer knows which incoming shipment corresponds to which
order. But again, that's fairly weak linkage.
 
one further comment inline

-----Original Message-----
From: Ricky Ho [mailto:riho@cisco.com] 
Sent: 16 March 2003 08:20
To: Burdett, David; public-ws-chor@w3.org
Subject: General Choreography and Bi-lateral Choreography


David,

I agree with everything you say.  However, I want to elaborate my view
on multi-party choreography.

Most real life B2B scenario are multi-party interaction.  But
"multi-party interaction" is NOT equivalent to "multi-party
choreography".  For example, a company (or "domain of control") can have
an orchestration that span multiple "bi-lateral" choreographies.  In
other words, "multi-party interaction" can be realized by multiple
"bi-lateral" choreographies linked together by orchestration.  The only
thing lost is the sequence dependencies between message exchanges within
choreographies is NOT captured externally.

I honestly don't think bi-lateral choreography is sufficient to capture
arbitrary public protocol sequence.  For example, the well-known 2-phase
commit protocol cannot be specified using bi-lateral choreography.  So I
agree with you that in a generic sense "choreography" should NOT be
restricted to 2 parties.
 
 
prf: on the 2-phase commit comment: I'm not sure if you mean
*bi-lateral" isn't sufficient, or "choreography" isn't sufficient. As I
said before, I think treating 2-pc as an example "application protocol"
- i.e. the subject for a choreography description - can be very
instructive.
 
2-pc can certainly be defined, precisely and fully, involving only two
parties. This was done in the OSI Commitment, Concurrency Recovery (CCR)
spec [ in fact, because standards politics meant it wasn't allowed to
have normative multi-party text, but also was required to have normative
semantic definition], and is also (I hope) complete in the BTP
specification of the "outcome" protocol (the superior:inferior bit).
There are implications for what is going on with other parts of the
transaction tree, but the fundamental protocol is in fact two-party. (my
diagram with the 3 dumbells and the box, on about my 4th slide was meant
to show this, but I fear may not have explained it well] (I mention CCR
and BTP because I know them - there may be other specs of similar
nature)
 
What you do need, beyond the simple message exchange rules, are
statements of the implied semantics of the messages. This is most
certainly part of the contract definition between the parties, and we
strongly believe any useful choreography specification needs to have
ways of stating the contractual implications (at least the
software-contractual implications) of the messages.
 
Peter

------------------------------------------
Peter Furniss
Chief Scientist, Choreology Ltd

   Cohesions 1.0 (TM)
   Business transaction management software for application coordination

web: http://www.choreology.com <http://www.choreology.com/> 
email:  peter.furniss@choreology.com
phone:  +44 20 7670 1679
direct: +44 20 7670 1783
mobile: +44 7951 536168
13 Austin Friars, London EC2N 2JX 

 
Received on Sunday, 16 March 2003 19:45:29 GMT

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