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