RE: State Alignment and Standard Signals

Gary:

 

The "paper world" is really hard to emulate electronically. It is far
easier to forge an XML text file than a piece of paper. It is also very
easy to blame something like an Enigmatec infrastructure for not having
delivered an XML document to my ERP system than the postmaster for not
delivering a registered piece of mail. 

 

So here is a scenario that could happen in business (either by mistake
or wrongfully):

- You send me an invoice and I don't "respond". Then I call you and you
claim you never received it. How would you strategy work in that case?

- You can also have (very common scenarios - see my article:
http://www.ebpml.org/state.htm article) the case where the response can
only come, days or weeks after the request (e.g. an RFQ). It is a very
good practice to indicate that you received the RFQ and you were able to
process it, it does not mean that you are in the position to respond
just yet. Imagine how disastrous it could be for both parties if the RFQ
fell through the cracks and you waited on a timeout on the response to
detect it.

 

So at the end of the day, we are either in the business of Distributed
computing or in the business of B2B, but applying DC tricks to B2B will
lead to a vastly suboptimal solution. When you apply DC internally, you
tend to detect all these situations and find an ad-hoc solution. When
you problem is to get a 1000 or 10000 companies to interoperate over
some message interchanges definition, you cannot afford to have little
tricks figured two by two. You need a holistic solution.

 

I hope you understand a little more the value a little signal can have.

 

CQFD.

 

Jean-Jacques

 

________________________________

From: public-ws-chor-request@w3.org
[mailto:public-ws-chor-request@w3.org] On Behalf Of Gary Brown
Sent: Wednesday, July 14, 2004 11:41 PM
To: david.burdett@commerceone.com; tony.fletcher@choreology.com;
public-ws-chor@w3.org
Cc: Robin.Milner@cl.cam.ac.uk; kohei@dcs.qmul.ac.uk;
yoshida@doc.ic.ac.uk
Subject: Re: State Alignment and Standard Signals

 

Hi David

 

As Monica pointed out in a previous email, "we are implementing the
technical interactions, as the 
business aspects are outside of WS-CDL".

 

I would suggest that the legal status of a message
(signal/acknowledgement) is a business level consideration. For example,
if participant A sends a request to participant B, and the CDL defines a
sequence that indicates that a response is then sent from B to A
following the receipt of this request, then that implies participant B
has received and processed the message.

 

<sequence>

    <interaction A->B />

    <interaction B->A />

</sequence>

 

As Participant B has sent a follow-up communication that could only have
resulted as a consequence of receiving the request, it would not be able
to argue that it hadn't received the request.

 

I am not sure what value defining a specific message type would add.

 

Regards

Gary

 

 

----- Original Message ----- 

	From: david.burdett@commerceone.com 

	To: tony.fletcher@choreology.com ; public-ws-chor@w3.org 

	Cc: Robin.Milner@cl.cam.ac.uk ; kohei@dcs.qmul.ac.uk ;
yoshida@doc.ic.ac.uk 

	Sent: Thursday, July 15, 2004 1:24 AM

	Subject: State Alignment and Standard Signals

	 

	Tony

	 

	I thought it might be worthwhile putting on record the comment I
made on the call on Tuesday in that I think that there are two different
"state alignment" problems to be solved: "real" state alignment, and
standard signals.

	 

	REAL STATE ALIGNMENT

	The first is the problem you discuss below that there is a
general requirement at various points in a choreography that each
participant has a common shared understanding of the state of the other
participants in terms of the messages that have (or have not) been
received ... I hope I am paraphrasing/simplifying your requirement as
described below.

	 

	STANDARD SIGNALS

	The second is the problem that Anders Tell described in his
email [1] that I responded to in my email [2]. Anders talks about a need
that a recipient of a message "legally" accepts that he has received the
message. In this case, the message is more like a signal that informs
the sender of the original message what their "state" is with respect to
legal acceptance of the message. There are also other signal messages
that can occur, for example to indicate that a message has been
received, i.e. a simple Ack, or has been "Accepted for Processing" as
standards like BPSS suggest.

	 

	SOLVING THE PROBLEMS

	To solve the problem you are suggesting, then we need to
continue discussing the state alignment approaches you describe below.

	 

	To solve Anders problem and also the issues I think Jean-Jacques
was raising, we could define some "standard" Message Content Types that
have specific semantics, message flow patterns and behaviors associated
with them and then recommend use of these standard types when
choreography designers have a need to use them. Note that these would
specify the types and not the representation of those messages in XML
which could potentially be done in different ways.

	 

	Thoughts?

	 

	David

	 

	[1]
http://lists.w3.org/Archives/Public/public-ws-chor/2004Jul/0009.html

	[2]
http://lists.w3.org/Archives/Public/public-ws-chor/2004Jul/0012.html

		-----Original Message-----
		From: public-ws-chor-request@w3.org
[mailto:public-ws-chor-request@w3.org]On Behalf Of Tony Fletcher
		Sent: Tuesday, July 13, 2004 3:56 AM
		To: public-ws-chor@w3.org
		Cc: Robin Milner; 'Kohei Honda'; Nobuko Yoshida
		Subject: Tony's nightmare - wake me up please

		Dear Colleagues,

		 

		I hope this is a nightmare and someone can wake me up
and re-assure me that everything is all right really!  I know that Nick
(who could answer this concern as one of our main language designers) is
on holiday for awhile now, so I have copied this message to your
resident experts as well, though Steve, Gary or somebody else may well
be able to answer simply.  I started writing this email a few days ago
before Gary Brown's message on 'Perform' came out.  Having now had a
look at that I wonder if my concern is a more extreme form of his.

		 

		Following discussions with Peter last week I have a
major concern about how we are designing the choreography description
language.

		 

		I am no mathematician, but my understanding is that
pi-calculus describes each 'node' (a partner according to our current
definitions) as a process and the allowed message sequences (I think
mathematicians call these 'traces') are generated by mathematically
'composing' each node process with the others and letting the maths tell
you what the various allowed message exchanges are (etc.)

		 

		Now I am more familiar with state machines, and I will
therefore continue the discussion based on these, but I think the same
principles apply to process algebras.

		 

		Consider a relationship between two nodes that we wish
to describe.  The usual way to describe this fully suing state machines
is to develop a state machine for each end and see how they interact.
Suppose one state machine has N states and the other M (N and M will be
close to the same integer value but not necessarily equal).  So we have
had to use N + M (approx 2*N) states to describe this relationship
precisely.  However, there are altogether N*M (approx N squared) states
that the system can be in.  Now depending on the whether each of our
state machines are relatively simple and only have entries on the
diagonal cells (or nearly so) or a very complicated with entries in most
cells the number of valid, reachable states may be near to 2N or to N
squared).  The answer is usually a lot more than 2N even if less than
the worst case.

		 

		Now it is possible, in principle (I think) to have a
single state machine that describes the permitted message sequences for
the relationship but this is usually only attempted for very simple
cases due to the state 'explosion' described above.  This single state
machine has to represent all the valid states which is between N+M and
N*M and usually well above N+M for any 'interesting' protocol.

		 

		In the current version of CDL we do not seem to be
describing the process at each node then letting these interact with the
others, but describing the interactions directly.  My concern is that
this approach will suffer from state space explosion when one tries to
include every conceivable message sequence that could happen, and that
actually the current CDL may be fine for describing message sequence
charts in XML (which is useful but not I thought what we were attempting
to achieve) which are fine for illustrating message flows, but do not,
in general, cover every possible case the protocol designer needs to
allow for, but will become unwieldy / impossible to use for a complete
description.

		 

		I hope the point is clear and that someone can answer
it.

		 

		Best Regards     Tony

		A M Fletcher

		 

		Cohesions  (TM)

		 

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