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RE: Definition of Terms

From: Edwin Khodabakchian <edwink@collaxa.com>
Date: Wed, 19 Mar 2003 12:43:15 -0800
To: "'Assaf Arkin'" <arkin@intalio.com>, "'Burdett, David'" <david.burdett@commerceone.com>, "'WS Choreography \(E-mail\)'" <public-ws-chor@w3.org>
Message-ID: <002401c2ee58$2c634070$690aa8c0@collaxa.net>
Assaf,
 
I think that we would benefit not reusing the same term for what we do
with a choreography and orchestration. Meaning avoid reusing the term
execute. This is an area where there is already a *lot* of confusion on
the market and I think that adds to it.
 
It seems to me that choreography is about creating protocols that
services comply with and orchestration flows that gets executed by an
engine. So I would keep execute for orchestration and find another term
for choreography.
 
I also think that JJ's suggestion to use "Choreography" and
"Choreography Instance", "Orchestration and Orchestration Instance" is a
good suggestion because it reduced the number of new terms and potential
sources of confusion.
 
Edwin
 

-----Original Message-----
From: public-ws-chor-request@w3.org
[mailto:public-ws-chor-request@w3.org] On Behalf Of Assaf Arkin
Sent: Wednesday, March 19, 2003 12:13 PM
To: Burdett, David; WS Choreography (E-mail)
Subject: RE: Definition of Terms


Absolutely.
 
arkin

-----Original Message-----
From: public-ws-chor-request@w3.org
[mailto:public-ws-chor-request@w3.org]On Behalf Of Burdett, David
Sent: Wednesday, March 19, 2003 6:34 AM
To: Assaf Arkin; Burdett, David; WS Choreography (E-mail)
Subject: RE: Definition of Terms



Assaf 

I quite like the definitions: 
- The process defined by an orchestration is executed by a single 
role/service type 
- The process defines by a choreography is executed by a combination of
all 
roles/service types 

... but would add ... 
-  A process defined by an orchestration is constrained by the
choreographies in which the process takes part.  

David 

-----Original Message----- 
From: Assaf Arkin [mailto:arkin@intalio.com] 
Sent: Tuesday, March 18, 2003 1:41 PM 
To: Burdett, David; WS Choreography (E-mail) 
Subject: RE: Definition of Terms 



> > 4. Can't be executed directly. Because there can never be a 
> > single point of 
> > control that governs the behavior of all the domains of control 
> > involved in 
> > the choreography. 
> 
> Again I definitely agree with the intent but has an issue with 
> the wording. 
> I could argue that when a buyer and seller interact they 'execute' the

> choreography. So perhaps a refinement would be 'a choreography can be 
> executed only when all participants perform their designated
activities'. 
> <DB>I suppose the point I am trying to say is that you can't feed a 
> choreography definition into some process management software and 
> expect it 
> to be executed directly. Instead, you need to feed the *same*
choreography 
> definition into the process managers running at all the roles involved
in 
> executing an instance of the choreography so that they can check they
are 
> following the choreography in a correct way for the role that they are

> taking.</DB> 

How about: 

- The process defined by an orchestration is executed by a single 
role/service type 
- The process defines by a choreography is executed by a combination of
all 
roles/service types 


> So there's a definition of what service X does for its part (XSDL +
WSDL + 
> interface) and a definition of what service Y does for its part 
> (ditto) and 
> a composition of both parts (model) which is the definition of the 
> choreography. 
> <DB>I think this needs exploring further. I don't think that defining
a 
> choreography in terms of WSDL works if you want a reusable 
> choreography that 
> can be used for B2B (see my other email on Uses of the WS Choreography

> spec). On the other hand it does work very well, if you have a one-off

> choreography agreed between two parties. There are two different uses
here 
> which have different requirements. I also think there needs to be 
> two parts 
> to a chorepgraphy definition: a) an abstract choreography 
> definition that is 
> indpendent of the precise message format and service format, and b)a 
> "Choreography Implementation Binding" which binds the abstract 
> choreography 
> definition to specific message formats and service instances to an 
> implementation.</DB> 

If I understand the other terminology list correctly then a message
would be 
something like a WSDL abstract message definition, and a representation 
would be the protocol binding/encoding. So if we express the
choreography in 
terms of WSDL operations (as WSCI and BPEL4WS do), it would meet the 
abstraction requirement. 

I know that there are issues with WSDL if you want more abstraction in
the 
message definition. But I don't think this is a problem that occurs only

when you use choreography, and I'm not sure if this group should try to 
solve it, though we should highlight problems/issues/proposals. I am
fine 
working with WSDL here and then working in the WSD to improve what needs
to 
be improved. I'm not too excited about the possibility of redoing WSDL 
outside of the WSD WG. 



> Here I would disagree only because I consider the orchestration to be 
> separate from the implementation, sometimes overlapping (e.g 
> BPEL, BPML) and 
> sometimes being a non-complete subset. By definition any choreography 
> language (WSCI, BPSS, yet-to-be-imagined) can define an orchestration
that 
> fulfills the service's role in the choreography, but such an
orchestration 
> may be a simple template that requires much more details to construct
an 
> orchestration that will also serve as an implementation. 
> 
> In OO terms I would say that an orchestration is like a Java class. It
may 
> be the class you use to create objects from (implementation process 
> instances), but it can also be an abstract class that only matches the

> interface but must be extended before any objects can be created. 
> 
> The analogy is not complete. Process models are type systems that 
> can define 
> behavior while OO are type systems that can only describe points of 
> interaction (much like WSDL operation). So one has to look at how
mobile 
> process calculus has been used to define behavior of objects beyond
the 
> limitation of OO languages (benefiting from >10 years of experience). 
> <DB>I think you are hinting at some layering in the spec when you 
> talk about 
> templates. We need greater clarity on what these layers are.</DB> 

The way I see it, to draw a very loose analogy, is like interface,
abstract 
class and class. (Or if you like you can substitute abstract class with
some 
templating mechanism). A choreography is like a definition of interfaces

that mesh together (think one interface per role for a second). An 
orchestration is like a class. 

An orchestration can be as full as the implementation (a concrete
class), or 
it can be less than an implementation (abstract or template) to the
point 
where it's so abstract that it parallels the interface. In fact, if you
look 
at Java, an interface is an abstract class that has no non-abstract
methods. 

So one orchestration can extend another. I may have an orchestration: 

orch A 
  do X 
  ... 
  do Z 

and another orchestration: 

orch B extends A 
  do X 
  do Y 
  do Z 

orch A may say what my constraints are for participating in the 
choreography, and orch B may include some other things I do in private
and 
may well be my implementation. Being able to say that orch B extends
orch A 
is useful because if I can understand how A fits with the choreography I
can 
also understand how B can fit. I may also have another orchestration
called 
C (a different implementation altogether): 

orch C extends A 
  do X 
  do U 
  do V 
  do Z 
  do W 

but notice that both B and C extend A so I may use either of these 
implementation orchestrations for the choreography requiring me to use
A. 

A behavioral type system based on mobile processes deals with exactly
that 
kind of type checking: not just the input/output, but also the
sequencing 
rules. (This example is simplified, but it can deal very well with more 
complex things like parallel flows, branching, exceptions, cycles, etc) 

arkin 

> 
> arkin 
> 
> > 
> > Also see comments inline ... 
> > 
> > David 
> > 
Received on Wednesday, 19 March 2003 15:43:31 GMT

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