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Re: Choreography State Definition (was: RE: More requirement

From: Nickolas Kavantzas <nickolas.kavantzas@oracle.com>
Date: Tue, 24 Jun 2003 09:41:55 -0700
Message-ID: <3EF87F53.D5859EB0@oracle.com>
To: "Burdett, David" <david.burdett@commerceone.com>
CC: Jean-Jacques Dubray <jjd@eigner.com>, "'Yaron Y. Goland'" <ygoland@bea.com>, "'WS Chor Public'" <public-ws-chor@w3.org>

The global knowledge of state is clearly an issue and I think we should record it as such.

Said that, I believe that the allignment of any kind of observable state (including the observable state of a
pi-c channel) is a very important requirement.

How to accomplish this at the language level and how a possible implementation will work in a distributed setting
is something we should discuss in our task force (no. 3).

"Burdett, David" wrote:

> JJ
>
> I agree with you that a) a choreography is a state machine, and b) the
> exchange of messages changes the state.
>
> However, I don't beleive it is possible for the parties involved in a
> choreography to know the *full* state of a choreography at any point in time
> because of a) the finite time it takes for a message to be sent, and b)
> sometimes a message may not be delivered.
>
> For example if a Buyer sends an order to a Supplier, then the sequence of
> states would be something like:
> 1. Initial states: Buyer: idle, Supplier: idle
> 2. Buyer sends the order. States: Buyer, Order Sent; Supplier, Idle
> 3. Supplier receives the order. States: Buyer, Order Sent; Supplier, Order
> Received
> 4. Buyer sends an acknowledgement. States: Buyer, Order Sent; Supplier,
> Order Ack Sent
> 5. Buyer receives the ack. States: Buyer, Order Ack Received; Supplier,
> Order Ack Sent
>
> Even in the last case, the Supplier doesn't really know the state of the
> Buyer as the Supplier does not know if the ack was received. Similarly, if
> one of the messages doesn't get through, e.g. the original order message,
> then they Buyer has no way of knowing what the state of the Supplier is with
> the choreography as illustrated above.
>
> I tend think that state belongs to an individual role rather than the
> choreography as a whole and so would propose the folowing "Choreography
> State Definition" ...
>
> "The state of a choreography instance at a point in time is defined as the
> combination of the states of the individual roles that are participating in
> that choreography instance at that time".
>
> Since communication between roles can't be instantaneous, it also means that
> the actual state of a complete choreography is actually unknowable whilst
> the choreography is in progress. Only when the choreography is halted (i.e.
> the states can't change) and queries made of each role to discover their
> state can the full state of the choreography be discovered. This is also one
> reason why I think that enabling "state queries" is important as you will
> often need to know the full state of the complete choreography to determine
> what to do if you want to recover after a failure to execute a choreography
> successfully.
>
> Thoughts?
>
> David
>
> -----Original Message-----
> From: Jean-Jacques Dubray [mailto:jjd@eigner.com]
> Sent: Wednesday, June 18, 2003 5:40 AM
> To: 'Yaron Y. Goland'
> Cc: 'WS Chor Public'
> Subject: RE: More requirement
>
> If two parties believe that the choreography is in a different state,
> one can have many surprises about the outcome of this choreography. A
> choreography is a state machine (in the mathematical sense) and as such
> is always in a given state. The language must be unambiguous about the
> different state. It is merely the exchange of a message that changes the
> state of the choreography. However, it is a very peculiar state machine,
> since it has no run-time associated with it that keeps ITS state.  Its
> state is rather "spread" over n parties, which are all represented by
> their individual state machines participating in the choreography. As
> you can imagine if these state machines gets out of synch only the most
> catastrophic results can be expected.
>
> Cheers,
>
> Jean-Jacques Dubray____________________
> Chief Architect
> Eigner  Precision Lifecycle Management
> 200 Fifth Avenue
> Waltham, MA 02451
> 781-472-6317
> jjd@eigner.com
> www.eigner.com
>
>
>
> >>-----Original Message-----
> >>From: Yaron Y. Goland [mailto:ygoland@bea.com]
> >>Sent: Dienstag, 17. Juni 2003 13:05
> >>To: Jean-Jacques Dubray; 'WS Chor Public'
> >>Subject: RE: More requirement
> >>
> >>I have to admit that the term 'choreography state synchronization
> >>mechanism'
> >>makes me really nervous only because it is so broad that it could
> >>potentially mean anything. But let's adopt your proposed new language
> >>anyway
> >>and when we do the full requirements review we can do a pass to look
> for
> >>phrases such as that one and make sure they are used in a consistent
> and
> >>well defined manner throughout the requirements.
> >>
> >>              Yaron
> >>
> >>> -----Original Message-----
> >>> From: Jean-Jacques Dubray [mailto:jjd@eigner.com]
> >>> Sent: Wednesday, June 11, 2003 12:44 PM
> >>> To: 'Yaron Y. Goland'; 'Jean-Jacques Dubray'; 'WS Chor Public'
> >>> Subject: RE: More requirement
> >>>
> >>>
> >>> Yaron:
> >>>
> >>> >>
> >>> >>BPSS has chosen one of a number of possible MEPs and each MEP has
> its
> >>> own
> >>> >>benefits and drawbacks that I don't believe this group needs to
> >>> address.
> >>> >>In
> >>> >>fact I expect that each industry will pick the MEPs that best meet
> >>> their
> >>> >>functional and legal requirements. Therefore I would propose that
> our
> >>> job
> >>> >>is
> >>> >>to enable the creation of such MEPs rather than specifying exactly
> >>> what
> >>> >>they
> >>> >>are.
> >>> [JJ] +1
> >>> >>
> >>> >>As such I would propose rewriting Jean-Jacques' proposed
> requirement
> >>> as:
> >>> >>
> >>> >>The WS-Chor message sequence description language MUST take into
> >>> >>consideration the need to manage signals where a signal is defined
> as
> >>> an
> >>> >>application level processing error that is expressed as a message
> >>> visible
> >>> >>by
> >>> >>other partners in the choreography.
> >>> [JJ] I am not sure I would translate it this way. Signals are not
> just
> >>> application level processing error messages. You may also have
> "message
> >>> format errors" that could be trapped above the system of record.
> >>>
> >>> I would suggest:
> >>>
> >>> >>The WS-Chor message sequence description language MUST take into
> >>> >>consideration the need to manage signals where a signal is defined
> as
> >>> <a choreography state synchronization mechanism> that is expressed
> as a
> >>> <standard> message visible
> >>> >>by
> >>> >>other partners in the choreography.
> >>>
> >>>
> >>> >>
> >>> >>> -----Original Message-----
> >>> >>> From: public-ws-chor-request@w3.org
> >>> >>> [mailto:public-ws-chor-request@w3.org]On Behalf Of Jean-Jacques
> >>> Dubray
> >>> >>> Sent: Thursday, June 05, 2003 2:51 AM
> >>> >>> To: 'WS Chor Public'
> >>> >>> Subject: More requirement
> >>> >>>
> >>> >>>
> >>> >>>
> >>> >>> I would like to add another requirement:
> >>> >>>
> >>> >>> The WSC-Languange MUST provide specific Message Exchange Pattern
> >>> >>> templates that establish a reliable state of the WSC-instance
> when
> >>> >>> needed.
> >>> >>>
> >>> >>> This requirement is essential since RM itself is not enough to
> >>> guaranty
> >>> >>> that the state of the choreography is identically represented in
> >>> each
> >>> >>> party. For instance a party sends a message with an incorrect
> >>> format. If
> >>> >>> we have RM only, then the state of the collaboration says that
> the
> >>> >>> message got there, so the choreography should proceed as normal.
> >>> >>> However, if this is a request, the responding party cannot send
> the
> >>> >>> response since the message was structurally incorrect.
> >>> >>>
> >>> >>> Unless the WSC-definition specifies that at this point the
> >>> responding
> >>> >>> party can send a "INVALID MESSAGE" response, we get into a
> deadlock
> >>> >>> (requesting party waiting for response, responding party unable
> to
> >>> >>> respond).
> >>> >>>
> >>> >>> A similar deadlock can happen when the message is structurally
> >>> valid,
> >>> >>> but cannot be processed by the corresponding system of record
> (that
> >>> is
> >>> >>> in charge of producing the response).
> >>> >>>
> >>> >>> Providing MEP-templates would greatly simply the work of the
> >>> designers
> >>> >>> by establishing clear patterns, with standard error messages
> that
> >>> can be
> >>> >>> used over and over by anybody.
> >>> >>>
> >>> >>> This approach also offloads the business logic of the
> application/or
> >>> >>> process engine to deal with "protocol" levels. Can you imagine
> the
> >>> >>> simplification for the Orchestration/Process Definition-instance
> if
> >>> >>> these concepts are implied rather than explicitly handled by the
> >>> process
> >>> >>> instance?
> >>> >>>
> >>> >>> See also this article:
> >>> >>> http://www.looselycoupled.com/stories/2003/message-infr0528.html
> >>> >>>
> >>> >>> Of course most people would have recognized the BPSS business
> >>> >>> transaction protocol, that itself has its root in prior work at
> RN
> >>> and
> >>> >>> UN/CEFACT. I think that generalizing this idea would be helpful.
> >>> >>>
> >>> >>> Cheers,
> >>> >>>
> >>> >>> Jean-Jacques Dubray____________________
> >>> >>> Chief Architect
> >>> >>> Eigner  Precision Lifecycle Management
> >>> >>> 200 Fifth Avenue
> >>> >>> Waltham, MA 02451
> >>> >>> 781-472-6317
> >>> >>> jjd@eigner.com
> >>> >>> www.eigner.com
> >>> >>>
> >>> >>>
> >>> >>>
> >>> >>>
> >>> >>>
> >>>
> >>>
Received on Tuesday, 24 June 2003 12:42:28 GMT

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