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RE: Choreography and Orchestration

From: Burdett, David <david.burdett@commerceone.com>
Date: Sat, 15 Mar 2003 12:24:36 -0800
Message-ID: <C1E0143CD365A445A4417083BF6F42CC053D1807@C1plenaexm07.commerceone.com>
To: "'edwink@collaxa.com'" <edwink@collaxa.com>, "'Ricky Ho'" <riho@cisco.com>, public-ws-chor@w3.org
I totally agree with Ricky on separating choreography and orchestration but
disagree with Edwin on restricting collaborations to just two. 
 
Details below.
 
CHOREOGRAPHY AND ORCHESTRATION
I agree that there is a clear distinction between:
1. The *choreography* that occurs between businesses or what I would
generalize to "domains of control"
2. The *orchestration* of (business) processes within a business (or within
a "domain of control").
 
The Choreography HAS to be agreed between the parties if interoperability is
to occur. The Orchestration, on the other hand, does not. Better internal
business processes are often a way in which a business realizes competitive
advantage.
 
WHY DOMAIN OF CONTROL
I am suggesting using the term "domain of control" (or something similar)
because you have the same issues arising, for example, when a business
process needs to interact with any system over which it has no control even
if the system is within the same business.
 
For example consider a procurement system interacting with an ERP system.
The procurement system might be implemented using an orchestration and
therefore can be changed flexibly, but the ERP has its own internally fixed
and unchangeable rules on the sequence in which messages are exchanged with
it. This means that ERP system is really imposing a constraint on what the
procurement system, in *exactly* the same was as an external business might
impose a constraint on what the procurement system could do. So really there
is a "choreography" between the procurement and and the ERP system since the
procurement system MUST conform to it and cannot control it - even though
they are both being run by the same business.
 
NUMBER OF PARTIES IN A COLLABORATION
Although you can restrict collaborations to just two, there are several
common use cases when three parties or more parties can be involved, such as
the one at [1] from the WS Architecture list which describes an exchange
between a buyer, a seller and a third party, independent shipper. There is
clearly one transaction going on and it is hard and definitely unnatural to
split the three-party choreography down into a set of two party
interactions, even if it is possible to do. The group might want to consider
this use case when defining the scope of the activity.
 
So I would suggest that we allow more than two parties to be involved in a
choreography. My personal view is that it is not too difficult to do either.

 
Regards
 
David Burdett
Director, Product Management, Web Services 
Commerce One 
4440 Rosewood Drive, Pleasanton, CA 94588, USA 
Tel/VMail: +1 (925) 520 4422; Cell: +1 (925) 216 7704 
mailto:david.burdett@commerceone.com;
<mailto:david.burdett@commerceone.com;>  Web: http://www.commerceone.com
<http://www.commerceone.com/>  

 
[1] http://lists.w3.org/Archives/Public/www-ws-arch/2002Oct/0369.html
<http://lists.w3.org/Archives/Public/www-ws-arch/2002Oct/0369.html> 

-----Original Message-----
From: Edwin Khodabakchian [mailto:edwink@collaxa.com]
Sent: Saturday, March 15, 2003 11:52 AM
To: 'Ricky Ho'; public-ws-chor@w3.org
Subject: RE: Choreography and Orchestration


Ricky,
 
*Great* work: Simple and clear. This is one of the pieces I have seen
regarding Choreography Versus Orchestration. The balance between definition
and use case/example is very helpful.
 
Constraining choreography to bi-lateral interactions would be a good
property of the architecture in my opinion.
 
Edwin
 

-----Original Message-----
From: public-ws-chor-request@w3.org [mailto:public-ws-chor-request@w3.org]
On Behalf Of Ricky Ho
Sent: Saturday, March 15, 2003 8:10 AM
To: public-ws-chor@w3.org
Subject: Choreography and Orchestration


I try to put up my own definition of "Choreography" and "Orchestration" and
use a simple buyer/seller use case to illustrate what I mean.
I'm particularly interested to see how the "Choreography" portion of this
simple example get represented by WSCI and BPSS.

1       Definitions

1.1     Choreography
=================
CHOREOGRAPHY defines the public part of a bi-lateral interaction between two
communicating parties.  It formalize a contractual agreement between these
parties.

CHOREOGRAPHY defines TWO communicating parties in terms of ROLES, which will
be bound to the actual business entity when the choreography instance
starts.  The binding doesn't change throughout the lifecycle of the
CHOREOGRAPHY INSTANCE.

CHOREOGRAPHY defines a set of "SHARED STATES" between the TWO communicating
parties.

CHOREOGRAPHY defines the TRANSITIONS of SHARED STATES in terms of MEP, where
one ROLE sends a message to another ROLE.  In other words, the purpose of
MEP is to align the SHARED STATES between the two ROLES.

CHOREOGRAPHY does NOT reflect the perspective of a single party.  It can be
taken by any parties who wants to play a role within it.

The CHOREOGRAPHY INSTANCE starts when the following occurs
*       One party sends the first message (which propose the initial SHARED
STATES) to another party.
*       This another party verifies that the initial SHARED STATES meets the
pre-requisite to start the CHOREOGRAPHY

1.2     Orchestration
=================
ORCHESTRATION defines the private part of the implementation of a particular
party who plays a ROLE in the CHOREOGRAPHY.  It formalize the execution
logic of that party throughout the message exchanges.

ORCHESTRATION realize a particular ROLE of a CHOREOGRAPHY.  Therefore,
ORCHESTRATION needs to be conformed with the CHOREOGRAPHY.

ORCHESTRATION can potentially span across multiple CHOREOGRAPHIES.
Therefore, CHOREOGRAPHY INSTANCES can form inter-dependent relationship at
the ORCHESTRATION level.

Note here that I try to restrict choreography to 2 parties and disallow
changes of role binding throughout the lifecycle of choreography instance.
The downside is now a multi-party interaction needs to be broken down into
multiple bi-lateral choreographies and their inter-dependencies is not
externalized at the choreography level.  It is up to the implementation
(which is the orchestration) to determine such inter-dependencies.  The
purpose of these restrictions is to simplify the choreography model which I
think still address 80% of the real life use cases.  I would like to see
where it breaks before remove this restriction.

2       Use Case Example

Lets look at a very simple example of the product purchase interaction
between a BUYER, a SELLER, and a COURIER.
*       The buyer send a PURCHASE ORDER message to the seller.
*       The seller check the credit history of the seller as well as the
product availability and decide either to accept or reject the purchase
order.
*       If the seller decide to reject the order, he send an ORDER REJECTION
message back to the buyer.  The interaction ends here.
*       If the seller decide to accept the order, he will arrange shipment
of the purchased product by selecting one of his preferred couriers.
*       The selected courier pickup the product from the seller and deliver
to the buyer's location.  The courier start a new interaction with the buyer
by sending a SHIPMENT NOTICATION message.
*       The buyer verifies the product is delivered in good shape and send
back a SHIPMENT RECEIVED message to the courier as well as FULFILLMENT
COMPLETE message to the seller.  Otherwise, the buyer sends back a SHIPMENT
REJECTED message to the courier as well as FULFILLMENT FAILED message to the
seller.

3       Illustration

3.1     Choreography
=================
There are four possible CHOREOGRAPHIES in this example
*       Product Purchase (Buyer and Seller)
*       Credit Checking (Seller and CreditCheck provider)
*       Arrange Delivery (Seller and Courier)
*       Shipment Delivery (Courier and Buyer)

For the purpose of this discussion, I'll focus in the first one.

3.1.1   Public / Shared States
=======================
Product Purchase choreography defines the following "PUBLIC STATES"
*       OrderNo
*       OrderStatus ("Submitted", "Accepted", "Rejected", "Delivered",
"Returned", "Terminated")
*       ShipmentNo

3.1.2   Message Exchanges
======================
Product Purchase Choreography defines the PUBLIC STATE TRANSITIONS in terms
of the following message exchanges ...

Start State = "submitted":  (OrderStatus="submitted")
Triggered by: when buyer send a "PurchaseOrder" to sender

State = "accepted":  (OrderStatus= "accepted", OrderNo, ShipmentNo)
From State "submitted"
Triggered by when seller send a "OrderAcceptance" message to buyer

State = "rejected":  (OrderStatus= "rejected")
From State "submitted"
Triggered by when seller send a "OrderRejection" message to buyer

End State = "delivered":  (OrderStatus= "delivered", OrderNo, ShipmentNo)
From State "accepted"
Triggered by when buyer send a "FulfillmentCompleted" message to seller

End State = "returned":  (OrderStatus= "returned", OrderNo, ShipmentNo)
From State "accepted"
Triggered by when buyer send a "FulfillmentFailed" message to seller

3.2     Orchestration
=================
Here is the Orchestration of the Seller within the Product Purchase
Choreography

Wait for receiving a PurchaseOrder message from buyer
Starts a new instance of "Credit Check" choreography by invoke the
CreditCheck web services.  After receiving the response, this "Credit Check"
choreography instance is terminated.
Invoke an internal web service to check the stock level of product
availability
If (credit is OK and product is available) {
    Invoke an UDDI search to lookup shipping companies.
    Select one courier based on company specific decision logic
    Starts a new instance of "Arrange Shipment" choreography by invoking the
ShipmentHandling web services.
    Send an "OrderAcceptance" message (which include the shipment No) to the
buyer
    Wait for receiving either "FulfillmentComplete" or "FulfillmentFailed"
message from the buyer and update the OrderStatus correspondingly.  The
choreography instance ends here.
    If the OrderStatus is "return", log into the customer care DB.
} else {
    Send an "OrderRejection" message to the buyer
}

As you can see, some activities within the orchestration is not visible by
the buyer and hence is the private part of the seller.
- Check the credit history
- Check the product availability
- Start another choreography with the courier


Comments and Thoughts ?

Best regards,
Ricky 
Received on Saturday, 15 March 2003 15:25:01 GMT

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