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RE: General Choreography and Bi-lateral Choreography

From: Assaf Arkin <arkin@intalio.com>
Date: Sun, 16 Mar 2003 18:39:53 -0800
To: "Furniss, Peter" <Peter.Furniss@choreology.com>, "Ricky Ho" <riho@cisco.com>, <public-ws-chor@w3.org>
Message-ID: <IGEJLEPAJBPHKACOOKHNCEJIDFAA.arkin@intalio.com>
Message
  -----Original Message-----
  From: public-ws-chor-request@w3.org
[mailto:public-ws-chor-request@w3.org]On Behalf Of Furniss, Peter
  Sent: Sunday, March 16, 2003 4:45 PM
  To: Ricky Ho; public-ws-chor@w3.org
  Subject: RE: General Choreography and Bi-lateral Choreography


  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.

  Most ERP vendors will tell you this is a non-issue. While they support
phone, fax and other forms of non-IP communication, that information is fed
into the system manually and therefore there is a representation of the
information exchange irrelevant of the protocol used. In fact, in one of the
scenarios we have been dealing with part of the information is exchanged by
fax. But form that point on it either vanishes into thin air and neither
buyer nor seller can use it, or it is fed into a system which can track it,
in which case it becomes part of the choreography and a message described by
WSDL. The fact that a lot of information is carried over non-IP protocols
seems to orthogonal to the fact that any computer system dealing with that
information can in fact describe it as a message exchange.

  They will also tell you it's a non-issue because most of the problems they
are trying to solve are not of the buyer-seller-shipper partner variety, but
more of the buyer-seller-shipper components/gateway interaction, so the
choreography is defined in terms of buyer proxy talking to seller proxy,
where one can be represented by the ERP and another by an EDI gateway. I
know of at least one big manufacturer who views their partner relationships
in exactly that way.

  There's kind of a mental shift here. In the CORBA/DCOM/EDI world, if it
wasn't travelling over the wire in that one and only protocol then it could
not be described. If you used IIOP but it was travelling over EDI it was
invisible to you, and vice versa.

  On the other hand with Web services, if you can proxy it, transform it,
scan it, or type it in, you can describe it. You don't need to talk to your
partner using HTTP, you can have someone feed the information to a WS proxy
HTML form, and you can define that WS proxy as representing the interaction
with that partner. Which says a lot about the revolutionary approach of Web
services*

  arkin

  * Needless to say if you were ever thinking of enabling road warriors to
conduct business online having just a PDA and base Internet link, or even a
Web-enabled phone, that's the way to do it.

  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
    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 21:40:51 GMT

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