RE: Abstract Bindable Choreography

Martin

I agree that you can represent these flows using UML, but I think that if
you were to do that you would miss some of the potential benefits that are
easier to get using XML.

I also understand and accept that we should not include diagrammtic
representations within the scope of what we do, on the other hand if you
want to communicate a choreography to someone then literally, one diagram is
worth a thousand words!

So why do we need a formal representation of choreographies in XML?

Here's my reasoning ...

1. The only reason for standardizing anything is to make interoperability
easier to achieve as the information being described needs to be shared.

2. Choreography definitions (e.g. the sequence of exchanging business
documents between partners) is an example of information that MUST be shared
if interoperability is to occur therefore there is benefit in having a
standard representation for it.

3. Now we *could* agree that UML be that standard representation as an
interchange format so that the information can be exchanged for example
between the parties involved in eCommerce on the other hand we could use
XML, so which is better?

4. To answer the question of which is better, let's think what the
representations could be used for. I can think of two main reasons:
  a) So they can be used at design-time to make sure that you build a
solution that meets the constraints implied by the choreography
  b) So they can be used at run-time to make sure that a choreography is
being correctly followed.

5. At design time you need to know:
  a) The role you are going to play in the choreography (e.g. are you a
buyer or a seller)
  b) The sequence in which you MUST exchange messages in order to comply
with the rules of your role
If a formal representation of the choreography exists, then a business
process design tool could use it in some automated to check that a design is
correct.

6. Ensuring a choreography is being correctly followed at run time is also
important especially when you consider how complex a choreography can get
(e.g. consider the international procurement use case I discussed on the
last call). Here's the reason why:
  a) You can't control what the other roles in a choreography do. Therefore
if they send a message out of the expected sequence, then your solution
won't work. Therefore at run-time you have to check that a choreography is
being followed correctly and raise errors if it is not.
  b) If you want to check that a choreography is being followed correctly
then you need to be able to identify in  a message, the choreography that is
being followed as well as the specific message within the choreography that
is being sent - a SOAP header would be ideal for this
So really what you are doing is running a state machine that validates that
the sequence of messages run in a choreography is correct. The alternative
is to design all the exception handling into your business process. This
makes the busines process design more complex.

6. So which should you use, UML or XML? Now UML does have an XML
represenation, but it is proprietary (I think) to Rational and focuses on
describing the structure of any UML document rather than the structure of a
choreography. On the other hand using XML to define a choreography would
provide a development environment neutral definition which is specifically
designed for the purpose. It would be easier to feed into a state machine
that was validating that a choreography was being correctly followed at
run-time.

Thoughts?

David

-----Original Message-----
From: Martin Chapman [mailto:martin.chapman@oracle.com]
Sent: Friday, April 04, 2003 10:12 AM
To: 'Burdett, David'; 'WS Choreography (E-mail)'
Subject: RE: Abstract Bindable Choreography


David,

I have a strong feeling that you can get what you want by exstiing
technologies such as UML. In the past I have used use cases and activity
diagrams to express reusable interactions between parties. Diagramtic
notations are explicitly out of scope of our charter, and I'm not sure
if there is any benefit in a specific xml language  to express the same
thing.

Martin.

> -----Original Message-----
> From: public-ws-chor-request@w3.org 
> [mailto:public-ws-chor-request@w3.org] On Behalf Of Burdett, David
> Sent: Thursday, April 03, 2003 11:09 AM
> To: WS Choreography (E-mail)
> Subject: Abstract Bindable Choreography
> 
> 
> There has been some discussion around the idea of an abstract 
> bindable choreography so I thought I would provide an example 
> in the form of a diagram (PDF) which shows the flow 
> associated with the placement of an order and an XML 
> representation of the same in a declarative style. I strongly 
> suggest you look at the diagram first.
> 
> Comments welcome ;-)
> 
> David
>  <<PlaceOrderChoreography.pdf>> 
>  <<PlaceOrderChoreography.xml>> 
> 
> 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; Web: http://www.commerceone.com