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

From: Jean-Jacques Dubray <jjd@eigner.com>
Date: Wed, 25 Jun 2003 00:39:47 -0400
Message-ID: <012601c33ad3$cfda3340$136e050a@jjd2>
To: "Burdett, David" <david.burdett@commerceone.com>, "'Yaron Y. Goland'" <ygoland@bea.com>
Cc: "'WS Chor Public'" <public-ws-chor@w3.org>

David:

you are absolutely right, unless we can use a "shared" memory space, it is
impossible to guarantee an absolute state synchronization mechanism at a
reasonable cost (we don't want to ack the acks !! and so on).

The question is then is it necessary to have more than RM ? and would it be
lest cost to the users to use a choreography protocol on top of RM than
using nothing?

I would tend to think that a choreography protocol is needed because:
1) once two party exchange important messages with a large latency between
them, you need "signals" to make sure that things are going as planed, as I
said, you can't ever be 100% sure of what the state is. However, you want
either a NAK to come back to tell you that something went wrong, or a short
timeout on the ACK (not the subsequent messages) so that you don't have to
wait on the time out of the response to discover that something went wrong.
It is exactly like factory automation system, you just sent a new setpoint
to your production equipment, you either get an ack/nak or it timeouts. In
each case, you know what to do. Imagine if you had sent the setpoint, and
the only feedback you get is looking at what is coming out !!

2) you want to offer a fairly generic mechanism to do that to avoid
choreography designers to re-invent the wheel. They can always do it by the
way, but a great service would be to declaratively specify whether and how a
protocol is used, as opposed to specific messages embedded in the
choreography definition itself.

JJ-


----- Original Message -----
From: "Burdett, David" <david.burdett@commerceone.com>
To: "Jean-Jacques Dubray" <jjd@eigner.com>; "'Yaron Y. Goland'"
<ygoland@bea.com>
Cc: "'WS Chor Public'" <public-ws-chor@w3.org>
Sent: Tuesday, June 24, 2003 11:24 AM
Subject: Choreography State Definition (was: RE: More requirement


> 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 12:34:51 GMT

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