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RE: Internal processes and/or external choreographies (was RE: Ev ents and States ...

From: Steve Ross-Talbot <steve@enigmatec.net>
Date: Fri, 11 Apr 2003 09:51:21 +0100
To: "'Cummins, Fred A'" <fred.cummins@eds.com>, "'Burdett, David'" <david.burdett@commerceone.com>, "'Assaf Arkin'" <arkin@intalio.com>
Cc: <jdart@tibco.com>, <public-ws-chor@w3.org>
Message-ID: <001701c30007$863f91f0$f400a8c0@ENIGMAW07>

I'd like to echo Fred's comments. I think external is the way to go and
if we can provide "compatibility" as a minimum and "verifiablity" as a
nice to have we would have done a great job.

The notion of "observable behaviour" and the associated "bi-simulation"
are in essence how we should approach the issue of verfiability. We want
to ensure that "contracually" the external behaviour defined (allowable
message patterns over given states) is the observably equivalent to any
implementation. It doesn't matter if the implemetation is done over
BPML, Java, BPEL or even by people. If we can observe it as equivalent
then we can say it bi-simulates the externally defined behaviour and so
meets the contract.

One issue I am less sure on is the issue of time. If observable
behaviour is allowable message patterns over given states, what role
does time play in all of this? Does it have a role? Thoughts .....

Cheers

Steve T

-----Original Message-----
From: public-ws-chor-request@w3.org
[mailto:public-ws-chor-request@w3.org] On Behalf Of Cummins, Fred A
Sent: 11 April 2003 03:43
To: Burdett, David; Cummins, Fred A; Assaf Arkin
Cc: 'jdart@tibco.com'; public-ws-chor@w3.org
Subject: RE: Internal processes and/or external choreographies (was RE:
Ev ents and States ...



David,

My view is that the scope should be external only, but for now that
should be the same as external first.  There may be many implementations
of a choreography with diverse processes as appropriate to the
particular processes.  Consequently, there must be a clear separation
between external and internal, and the same choreography should be
suitable for designing the various internal implementations that marry
to it.

I will be useful for tools to be able to validate the compatibility of
an internal process with a choreography (but not necessary). In any
event, the choreography should be specified such that this compatibility
can be determined, hopefully in a straightforward manner.

Essentially, a participant must send a message when in an 
external state that calls for it to send a message, and the message must
be such that it will cause the recipient to transition to the state
desired by the sender based on the choreography.  A participant has no
control over the other participant(s) except through sending an
appropriate message--it cannot control what the other participant sends,
when it is sent, or if it is sent at all.

I suggest that the proper actions of a participant can be defined by its
public state machine.  Typically, each state will have two or more exit
transitions that depend on what is sent or received. The selection of
the exit transitions is made by the message sender (unless there is an
error or time-out which may be viewed as a third message/transition
type).

So an internal process can be validated against the participant's public
state machine specification.

Fred 

> -----Original Message-----
> From: Burdett, David [mailto:david.burdett@commerceone.com]
> Sent: Thursday, April 10, 2003 9:42 PM
> To: 'Cummins, Fred A'; Assaf Arkin; Burdett, David
> Cc: 'jdart@tibco.com'; public-ws-chor@w3.org
> Subject: Internal processes and/or external choreographies (was RE: 
> Events and States ...
> 
> 
> Thanks ... but one thing we haven't nailed down yet is the
> extent to which
> the scope of this group covers definition of languages to 
> define internal
> process definitions (as in WSCI and BPEL4WS) as well as external
> choreographies. I have been focusing on the latter but we 
> need to be clear
> what we are doing about the former.
> 
> For example we could have the following as requirements for
> internal process
> definitions ...
> 
> "An internal process definition MUST be capable of defining
> the sequence and
> rules by which software is executed within a 'Control Domain' 
> " ... Control
> domain has been defined/described/discussed earlier.
> 
> "An internal process definition MUST be capable of identifying the 
> relationships and dependencies it has on an external choreography 
> definition."
> 
> ... I am sure there are more, like internal process
> definitions being Turing
> complete ...
> 
> Any thoughts chairs?
> 
> David
> 
> -----Original Message-----
> From: Cummins, Fred A [mailto:fred.cummins@eds.com]
> Sent: Thursday, April 10, 2003 5:38 PM
> To: Assaf Arkin; Burdett, David
> Cc: 'jdart@tibco.com'; public-ws-chor@w3.org
> Subject: RE: Events and States (was: timeouts & states (was: Abstract 
> Bind able Choreography))
> 
> 
> David,
> 
> I'm not sure why I haven't received your message directly, but I like 
> your linkage of state machine to process functionality. This provides 
> a clean separation of the external choreography from the internal 
> process.  We can then focus on how messages are exchanged between 
> participants according to the state transitions of their public state 
> machines.
> 
> Fred
> 
> > -----Original Message-----
> > From: Assaf Arkin [mailto:arkin@intalio.com]
> > Sent: Thursday, April 10, 2003 7:17 PM
> > To: Burdett, David
> > Cc: 'jdart@tibco.com'; Cummins, Fred A; public-ws-chor@w3.org
> > Subject: Re: Events and States (was: timeouts & states
> (was: Abstract
> > Bind able Choreography))
> > 
> > 
> > +1
> > 
> > arkin
> > 
> > Burdett, David wrote:
> > 
> > > >>>Very good questions. But what do you want (or perhaps more
> > > importantly,
> > > need) it to do? As you say, a state machine is really a
> > mechanism. What
> > > is the functional requirement?<<<
> > >
> > > I would put the functional requirements for which state
> > machines are a
> > > possible answer as follows:
> > >
> > > "An implementation of a process that is following a
> > choreography MUST
> > > be able to verify that the choreography is being followed
> > correctly as
> > > specified in the choreography definition."
> > >
> > > I would then have two further more closely defined but related
> > > requirements of the products of this group ...
> > >
> > > "A choreography definition should be usable at Design Time
> > to validate
> > > that a process should be capable of carrying out a choreography
> > > correctly as specified."
> > >
> > > "A choreography definition shoule be usable at Run Time
> to validate
> > > that a process is executing a choreography correctly as
> specified".
> > >
> > > ... and finally one more ...
> > >
> > > "If a process detects that a choreography is not being followed
> > > correctly, then the process SHOULD be able to use the 
> choreography
> > > definition to identify exactly what went wrong."
> > >
> > > This last one means that you stand a better chance of
> being able to
> > > fix the problem when it occurs.
> > >
> > > Thoughts?
> > >
> > > David
> > >
> > >
> > > -----Original Message-----
> > > From: Jon Dart [mailto:jdart@tibco.com]
> > > Sent: Thursday, April 10, 2003 2:56 PM
> > > To: Cummins Fred A
> > > Cc: public-ws-chor@w3.org
> > > Subject: Re: Events and States (was: timeouts & states
> > (was: Abstract
> > > Bindable Choreography))
> > >
> > >
> > >
> > > Cummins, Fred A wrote:
> > > > This raises questions about the scope of a choreography.
> > When does
> > > > it end?  When a disconnect occurs?  When a particular business 
> > > > transaction is completed?  When a relationship is terminated? 
> > > > Maybe any of the above?
> > > >
> > > > Do the state machines provide the mechanism for nesting
> > of component
> > > > choreographies?
> > >
> > > Very good questions. But what do you want (or perhaps more
> > importantly,
> > > need) it to do? As you say, a state machine is really a
> > mechanism. What
> > > is the functional requirement?
> > >
> > > At minimum, I would guess it is the ability to transition
> > to a distinct
> > > state when a timeout occurs. This state could be the
> > termination of the
> > > choreography (implying no more processing will occur). Or
> > it could be an
> > > error state (implying there might be some warning given, or some 
> > > recovery effort made, e.g. a retry - this assumes you are
> > doing this at
> > > the application level and not in some lower-level
> reliable messaging
> > > protocol). Certainly I can think of real-world examples
> > where you'd need
> > > this functionality. This is something of a simplification
> of earlier
> > > proposals. If we need something more complex, I'd like to see some

> > > rationale behind it.
> > >
> > > --Jon
> > >
> > 
> > 
> > --
> > "Those who can, do; those who can't, make screenshots"
> > 
> > 
> ----------------------------------------------------------------------
> > Assaf Arkin                                          
> arkin@intalio.com
> > Intalio Inc.                                           
> www.intalio.com
> > The Business Process Management Company                 
> (650) 577 4700
> > 
> > 
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Received on Friday, 11 April 2003 04:51:28 UTC

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