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RE: CDL Challenge

From: Jean-Jacques Dubray <jeanjadu@Attachmate.com>
Date: Tue, 29 Jun 2004 09:24:05 -0700
Message-ID: <D15269CBED76D51185280008C73323FA02F23F8F@exch-bel6.attachmate.com>
To: "Steve Ross-Talbot" <steve@enigmatec.net>
Cc: <distobj@acm.org>, <david.burdett@commerceone.com>, <public-ws-chor@w3.org>

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 Tuesday, 29 June 2004 12:27:49 UTC

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