RE: CDL Challenge

You say "I don't see the logic behind not supporting a 'state alignment'
communication protocol". When you use the word "supporting" do you
mean "binding to" or do you mean "enabling the definition of" such a
protocol. If it is the former then I certainly would agree with you. 
The latter,
which I don't think you mean, would not be a good idea.

Examining this notion of "binding to" and thinking further ahead...
If a CDL description describes the observable interaction between 
multiple
parties then we need to have some way of stating the assumptions we 
make.
<JJ>We agree that ideal would be "binding to". In the lack thereof, I
would suggest to fall back on defining. I curious whether someone has an
opinion on if we can use WSDL MEP (part 2) extension mechanisms to
specify such a protocol? I spoke briefly with Jean-Jacques Moreau who
gave me some tips about how that works, since it did not appear clearly
on the spec itself. I am not sure I can specify something like an MEP
with concrete messages like signals.</JJ>


For example, when we describe such interaction in the pi-calculus there
are some assumptions that are made such as synchronous communication
and guaranteed delivery. That is we do not take account of failed 
communication
within the calculus itself. It can of course be modeled (see Banana 
Calculus).
But then again the pi-calculus is very well documented and the 
assumptions
are made clear. With CDL this is less so.
<JJ>Hum... are you faced with the classical "theory and practice"
dilemma? </JJ>


I think all of this could well live in some sort of "policy" statement 
that describes
security, coordination protocols, messaging protocols and so on. What 
we would
need to be able to to do though is "bind-to" specific places in the CDL 
description
and not just present a top-level policy statement.
<JJ>Yes, this sounds like the right place to do, again, which state
alignment protocol do we bind to?</JJ>

JJ-

On 29 Jun 2004, at 17:24, Jean-Jacques Dubray wrote:

>
> A state alignment protocol is in my views a feature of communication
> protocol (i.e. it is part of the communication stack), just like RM is
> another.
>
> A coordination protocol is using various communications protocols to
> achieve its goals. A distributed transaction is a coordinated
protocol.
> A distributed transaction is not a communication protocol, it requires
> some features to be supported by the underlying communications
> protocols. In a distributed transaction, the parties involved don't 
> just
> "communicate" they perform a "unit of work". We should really separate
> semantically communication from coordination (of course a
communication
> with guaranteed state alignment can be viewed as a "unit of work" but
> please let's not confuse everything).
>
> So
> a choreography of message exchanges can pertain
> - to "communication" only
> - to "units of work" only
>
> "Communication" failures & successes may change the outcome of the
unit
> of work. Some units of work could not be implemented by underlying
> communication mechanisms and is required to understand some of the
> states associated to the communications features.
>
> I think it would be a mistake to force people to specify both at the
> same level. It would lead to very poor re-use and abysmal
> interoperability (with a large number of proprietary ways to add
> features to existing communication protocols).
>
> Features that can be implemented at the communication level: reliable
> messaging, non-repudiation, state alignment, ...
>
> If we lived in an ideal standards world, there will be an OASIS TC
> working on WS-SA and WS-NR specification and WS-CDL would just have to
> specify a binding to these communication protocol features (and 
> states).
> Unfortunately this is not the case, the standards world is far from
> ideal. Since "state" is such a core concept of WS-CDL, I don't see the
> logic behind not supporting a "state alignment" communication
protocol.
>
> JJ-
>
> -----Original Message-----
> From: Steve Ross-Talbot [mailto:steve@enigmatec.net]
> Sent: Tuesday, June 29, 2004 9:01 AM
> To: Jean-Jacques Dubray
> Cc: distobj@acm.org; david.burdett@commerceone.com;
> public-ws-chor@w3.org
> Subject: Re: CDL Challenge
>
> In particular can we look at my last comments about defns.
>
> As a community at large it would be good to agree on some
> and as a W3C working group it would be good to do likewise.
>
> Cheers
>
> Steve T
>
> On 29 Jun 2004, at 16:43, Jean-Jacques Dubray wrote:
>
>>
>>
>>
>> Here's an explanation of how CDL could "support" state alignment.
>>
>> Firstly CDL supports the idea that each of the roles involved in a
>> choroegraphy are a) stateful, i.e. they are aware of their own state,
>> <JJ>This is great, new and forward thinking</JJ>
>>
>> and b) their state is changed by either: the role sending or
receiving
>
>> a
>> message, or some other event internal to the role, e.g a timeout.
>> <JJ>This is incorrect, state can only change after a message can be /
>> has been correctly processed not just received. This is only true for
>> the protocol states not the choreography states.</JJ>
>>
> <SRT>
> JJ you are correct and I think this is what David means. When a role
> receives
> it is somewhat implicit that the application playing that role has
> received and
> understand the message sent to it.
> </SRT>
>> This means that in CDL you could define a very small choreography
that
>> described the state alignment protocol you describe below. This would
>> require that you define:
>> 1. The messages - the business message, the receipt signal message
and
>> the acknowledgement signal message.
>> 2. The states at the sender and receiver which arose from sending
>> and/or
>> receiving those messages.
>> <JJ>The only issue is that protocol exceptions are part of the
>> choreography definition so you cannot have a complete independence
>> between the choreography and protocol. You need to say, if I get a
NAK
>> here, the choreography continues this way, otherwise, it continues
> this
>> way, ...</JJ>
> <SRT>
> If the CDL description model NAK or ACK explicitly then you are 
> correct.
> If it is implied by binding in some protocol for message delivery then
> it
> is simply a badly modeled CDL description.
> At a high level CDL is a language for describing a business level
> protocol
> that is multi-party. I think this is something that Nick has always
> stated.
> </SRT>
>
>>
>> You could then "Perform" the "State Alignment Protocol", passing the
>> details of the business message a paramter, as part of a real
protocol
>> such as a purchasing protocol with a Send PO, PO response etc, where
>> each perform would mean that the state alignment protocol would be
>> followed. As part of the perform, state information would be returned
>> that provided details of the outcome of the perform, e.g. whether or
>> not
>> the receipt signal was received before a timeout.
>>
>> So basically, CDL allows a state alignment protocol to be defined but
>> does not require that one is used.
>> <JJ>I disagree</JJ>
> <SRT>
> Can we go back a couple of steps here. What exactly do we mean by a
> state alignment
> protocol vs some communication protocol vs some coordination protocol.
> Time for
> robust defns.
> </SRT>
>> The reason is that you will not always want or need state alignment
>> depending non what you want to do.
>> <JJ>I agree, further there is not just "one" protocol</JJ>
>> For example, if you are doing a query, e.g. stock availability, and
it
>> does not work then you can just do the query again.
>>
>> Hope this helps.
>> <JJ>Thanks</JJ>
>>
>> -----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 24, 2004 7:32 AM
>> To: Mark Baker
>> Cc: WS-Choreography List
>> Subject: RE: CDL Challenge
>>
>>
>>
>> Mark:
>>
>> I apologize I don't have such an extensive historical perspective.
>>
>> Is this why REST talks about State without talking about State
>> Alignment?
>>
>> I am wondering how State Alignment works over the web with web
>> technologies I have the feeling that this might not be implemented
>> properly by application developers all the time. I cannot tell you
the
>> peace of mind it gives me when I receive an email with a subject like
>> "Your order..." (I don't even look at it...).
>>
>> Anyways, reading your response lead me to believe that I might want
to
>> explain one more time state alignment in BPSS (which is a business
>> document exchange choreography standard).
>>
>> 1) RM tells you only that a message got to its receiver safely (and
in
>> the right sequence if necessary)
>>
>> 2) However, it is not because I got a message that I will be able to
>> understand its content, it is not because I can understand it that I
>> will act on it.
>>
>> 3) Therefore BPSS has 2 signals:
>> 	a) a receipt signal that says that the message I received
>> matches the agreement that we have (this message was the one I was
>> expected as defined in the collaboration, and it had the right
message
>> format if specified).
>> 	b) an acknowledgement signal that is returned when the message
>> was successfully processed by the receiving application, system, ...
>> whatever (you don't want to expose this kind of detail to the other
>> party in general)
>>
>> I content that state alignment requires at least the acceptance
>> acknowledgement. The receipt ack is rather used for non repudiation
> and
>> is not part of the state alignment question per se but helps provide
>> feedback about what might have gone wrong. If you get a negative
>> receipt, you know you may not have sent the right thing based on the
>> agreement you had with this party.
>>
>> The acceptance ack is often called a non-substantive response. It
does
>> not mean yes or no to a request, it simply means that the receiver of
>> the request was able to process the request (it did not get lost
>> internally).
>>
>> Is the BPSS state alignment protocol perfect? No, I can give you an
>> example where it fails. Should we make it more robust, absolutely.
>>
>> I am concerned that since WS-CDL (or REST for that matter) speaks
> about
>> state and state alignment but does not offer anyways to guaranty
state
>> alignment, this remains an issue. If the states are RM states (sent /
>> received) I would content that's completely useless, this is because
> RM
>> gives that for free, no need to make them explicit at the
choreography
>> level. If the states have business semantics associated to them
(Order
>> Processed) I am wondering how this information can be "signaled" back
>> to
>> guarantee state alignment.
>>
>> JJ-
>>
>>
>> -----Original Message-----
>> From: Mark Baker [mailto:distobj@acm.org]
>> Sent: Thursday, June 24, 2004 5:45 AM
>> To: Jean-Jacques Dubray
>> Cc: WS-Choreography List
>> Subject: Re: CDL Challenge
>>
>> On Wed, Jun 23, 2004 at 10:20:56PM -0700, Jean-Jacques Dubray wrote:
>>> If you or someone from the WS-CDL team have some time, I would
really
>> be
>>> interested to understand how WS-CDL can claim state alignment
without
>> a
>>> state alignment protocol. I have details ebXML BPSS state alignment
>>> protocol: http://www.ebpml.org/state.htm
>>
>> "The web services gurus are at least 2 light years away from
>> understanding the problem. ebXML solved it in 2001 and RosettaNet
>> before
>> it in 1999."
>>
>> and Internet gurus solved it in 1970; some of the first application
>> protocols were state alignment (aka state transfer) protocols.
>>
>> Mark.
>> -- 
>> Mark Baker.   Ottawa, Ontario, CANADA.        http://www.markbaker.ca
>>
>>   Seeking work on large scale application/data integration projects
>>   and/or the enabling infrastructure for same.

Received on Monday, 5 July 2004 14:58:12 UTC