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Choreography and Orchestration

From: Ricky Ho <riho@cisco.com>
Date: Sat, 15 Mar 2003 08:10:14 -0800
Message-Id: <4.3.2.7.2.20030315075057.02898318@franklin.cisco.com>
To: public-ws-chor@w3.org
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 13:54:17 GMT

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