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

From: Assaf Arkin <arkin@intalio.com>
Date: Mon, 30 Jun 2003 14:34:44 -0700
Message-ID: <3F00ACF4.6020102@intalio.com>
To: jdart@tibco.com
CC: Steve <steve@enigmatec.net>, "Monica J. Martin" <monica.martin@sun.com>, Burdett David <david.burdett@commerceone.com>, Nickolas Kavantzas <nickolas.kavantzas@oracle.com>, Jean-Jacques Dubray <jjd@eigner.com>, "'Yaron Y. Goland'" <ygoland@bea.com>, "'WS Chor Public'" <public-ws-chor@w3.org>

Jon Dart wrote:

>
> The implication is that you have to treat reliable messaging as a 
> "black box" at the choreography layer. I.e., if it exists, its 
> supporting messages are not part of the message exchange captured at 
> the choreography layer. It is a QOS, nothing more.

+1

You can model reliable messaging down to the level of individual 
messages, and you can do it in many ways, e.g. send a message enough 
times with a good chance it will eventually reach its destination. So 
that's one option a good language would let you use, and in some cases 
it may actually be more efficient to bypass RM/coordination and just 
craft an appropriate choreography.

But for cases where you do -- always or sometimes -- rely on 
RM/coordination, I agree with Jon that we're better off treating it as a 
black box.

arkin

>
> There is some implication that you may have to handle timeouts in the 
> absense of RM.
>
> --Jon
>
> Steve wrote:
>
>>
>> I tried getting to the bottom of the remarks about RM below by going 
>> back through my email. Alas I cannot see the original email that 
>> raised this. Perhaps it was a private communication ;-(
>>
>> Whilst it is true that pi-c assumes that RM is in place I do believe 
>> that you can still model external observable behaviour without RM and 
>> using pi-c principles to guide you. An absence of a communication (a 
>> handshake) can be modeled as an implicit interaction both sides of 
>> the original interaction. This provides a way to model timeouts which 
>> can be used to handle issues about non-receipt of interactions or 
>> partial interactions.
>>
>> At the end of  the day all visible behaviour be it explicit 
>> interaction or implicit interaction (timeouts, exceptions etc) can be 
>> said to be an interaction. Thus we can model the external observable 
>> behaviour in most of these cases as a choice in pi from the 
>> perspective of one party. For example any process P that is in a 
>> state to receive a specific communication on channel "a" of type "x" 
>> can be re-written as:
>>
>>     P = ax.P'                - P receives x on channel x and 
>> continues as P' to do something
>>
>>     P = ax.P' + t<timeout>.P'' + e<exception>.P'''
>>             - as above unless a timeout occurs in which case it 
>> continues to do P'' or P''' if an exception is detected.
>>
>>     One can even use restriction so that the last two terms are not 
>> propagated outside of the scope of P.
>>
>> So I think it doesn't mean that because RM is normally required we 
>> cannot derive benefit from a pi-c approach.
>>
>> Comments ....
>>
>> Cheers
>>
>> Steve T
>>
>>
>> On Monday, June 30, 2003, at 04:05  pm, Monica J. Martin wrote:
>>
>>>
>>>>
>>>> Burdett: Thanks for the explanation - it makes complete sense. The 
>>>> essence of the idea id I understand it correctly is that Pi-c 
>>>> *relies* on the reliable delivery of messages which translates, as 
>>>> you describe, into the requirement that Pi-c would *have* to be 
>>>> used with a Reliable Messaging (RM) protocol. If RM is not used, 
>>>> then you have to introduce some way of compensating when the 
>>>> inevitable problems occur.
>>>>  However even if you do use a RM protocol, the RM protocol can 
>>>> still fail and leave the two roles in an inconsistent state where 
>>>> one side thinks the message was delivered and therefore is 
>>>> continuing while the other does not and is therefore halted. I 
>>>> won't go into the reasons why here since it has been discussed on 
>>>> various RM working groups before. Bottom line you "can't" 
>>>> completely guarantee that both sides know that a message has been 
>>>> delivered AND that therefore the one side won't start while the 
>>>> other is halted.
>>>
>>>
>>>
>>> mm1: However, this does not negate the value of using reliable 
>>> messaging because the key is to lower risk and increase 
>>> confidence/certainty in the results, correct?
>>>
>>>>  So I think the key question we have to answer is:
>>>> 1. Do we want to restrict our choreography definition language to 
>>>> be used *only* in conjunction with RM, so that we can support Pi-c, or
>>>> 2. Do we want remove that restriction and let each side to carry on 
>>>> processing independently and therefore not use Pi-c
>>>>
>>> mm1: I believe we may have to consider an approach that allows for 
>>> both.  And, we have to weigh whether (1) or (2) is given priority. I 
>>> would suggest we discuss this on tomorrow's call as it impacts many 
>>> assumptions, and we need to prioritize which will be the 'norm' in 
>>> the marketplace.  I believe the thrust for reliable messaging is 
>>> clear, but do realize you have to provide the lesser (if you 
>>> consider RM the greater) case to enable legacy migration and 
>>> real-world business constraints in some circumstances.
>>>
>>>> *My* answer would be to go for the second option as:
>>>> 1. RM is never 100% guaranteed and therefore
>>>> 2. You have to allow for the specification of the exception 
>>>> handling that occurs when processes get out sync anyway
>>>> 3. Forcing processes to wait while the RM protocol takes place 
>>>> could result in extended execution times. For example if you are 
>>>> using SMTP to deliver the messages.
>>>> 4. Carrying out process in parallel sometimes and handling 
>>>> inconsistencies when they occur is a natural way of doing many 
>>>> different types of processing.
>>>>
>>> mm1: See comment above. Reliable messaging could be an option.  This 
>>> depends on our focus.
>>>
>>>
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>>
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>>
>
>


-- 
"Those who can, do; those who can't, make screenshots"

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Assaf Arkin                                          arkin@intalio.com
Intalio Inc.                                           www.intalio.com
The Business Process Management Company                 (650) 577 4700


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Received on Monday, 30 June 2003 17:36:17 GMT

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