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RE: Requirements: Decision Points Requirement Proposals

From: Jean-Jacques Dubray <jjd@eigner.com>
Date: Tue, 1 Jul 2003 16:59:13 -0400
To: "'Cummins, Fred A'" <fred.cummins@eds.com>, "'Jean-Jacques Dubray'" <jjd@eigner.com>, "'Martin Chapman'" <martin.chapman@oracle.com>, "'Yaron Y. Goland'" <ygoland@bea.com>, "'WS Chor Public'" <public-ws-chor@w3.org>
Message-ID: <01ca01c34013$b34e1120$176e050a@JJD>
Fred:

 

You are correct, the state of the choreography is immaterial and as you
pointed out the only state that is accessible is the public state of
each participant.

 

What would be wrong if we were to choose XPath predicates to express
conditions that specify the different paths a choreography can take? I
am all for hiding the XPath behind some logical construct like it was
suggested for BPSS that has the notion of a "logical document" behind
which either different physical documents or the same physical document
with 2 different XPath predicates.

 

Could you be more specific on how you would see this kind of logic
expressed?

 

Thanks, 

 

JJ- 

 

 

-----Original Message-----
From: Cummins, Fred A [mailto:fred.cummins@eds.com] 
Sent: Dienstag, 1. Juli 2003 16:07
To: Jean-Jacques Dubray; 'Martin Chapman'; 'Yaron Y. Goland'; 'WS Chor
Public'
Subject: RE: Requirements: Decision Points Requirement Proposals

 

JJ,

 

I think we should avoid talking about the "state of the choreography,"
and focus on the public state

of the participants.  By "public state" I mean the state that will be
expressed in the choreography

and that the other participant(s) are expected to know about and
understand.

 

The public state of a participant changes when it sends or receives a
message.  The participant must

also keep track of the perceived state of the other participant(s) which
is generally reflected in the last

message received from a participant.  When a participant sends a
message, the participant will typically 

change its state to waiting for a response.  The responses it should be
anticipating are constrained by the 

choreography specification.  When a participant receives a message, its
state should 

reflect that it is processing the message.  If it receives another
message, the choreography should

define what should happen, either explicitly or by default--typically it
is an error but the action could be

specified.

 

When a message is received, the recipient is no longer waiting for a
response, but will be processing the message

to determine its next action.  From a choreography perspective, its next
action is constrained by 

its state and the perceived new state of the sender.  The perceived
state of the sender is determined by the

content/semantics of the message.  How this is determined is an
implementation issue.  The choreography

only needs to define the possible alternatives and the possible
responses.  

 

The perceived state of the message sender requires interpretation of the
message.  This could be specified in

the choreography, but then the choreography must become a programming
language in order to process

various formats and potentially interpret the message in the current
context of the exchange.  I do not believe

this is appropriate. The choreograpy should only deal with the result of
the message interpretation.  In other

words, the message is processed by the recipient application, and the
result is the basis for determining 

compliance with the choreography specification as well as the acceptable
subsequent actions. 

 

Thus the choreography says, "Based on your current public state, if you
interpet this message as A, then you can 

respond with M or N, if you interpret it as B, then you must respond
with P or Q."  The choreography constrains

the exchange, but it does not interpret the message.  Similarly, the
choreograpy would define, "if you send a message

type M when you perceive the recipient to be in state G, then you should
expect to get back either X or Y and other responses are not valid."

 

Fred

 

-----Original Message-----
From: Jean-Jacques Dubray [mailto:jjd@eigner.com]
Sent: Tuesday, July 01, 2003 2:13 PM
To: 'Martin Chapman'; 'Yaron Y. Goland'; 'WS Chor Public'
Subject: RE: Requirements: Decision Points Requirement Proposals

Yaron:

 

I don't understand your rationale here, the only way the state of a
choreography can advance is by exchanging a message between two parties.
The only way two parties can agree on the state of the choreography is
by looking at the content of the message(s) (telepathy not being an
option here ;-). If there was a piece of information known by only one
party that would advance the state of the choreography, this party would
have to send a message to tell this information to the other party in
order for their state to be aligned. 

 

Incidentally, we are not talking about "routing" messages here but
rather "choreography state". If "this message contains this value" then
"the choreography will continue like this" else "continue like that".
Once a message comes in the choreography definition could not care less
where it is routed. This will typically be a private decision. Of course
the line may blur when we talk about multi-party choreographies.

 

JJ-

 

 

 

-----Original Message-----
From: public-ws-chor-request@w3.org
[mailto:public-ws-chor-request@w3.org] On Behalf Of Martin Chapman
Sent: Montag, 30. Juni 2003 17:33
To: Yaron Y. Goland; WS Chor Public
Subject: RE: Requirements: Decision Points Requirement Proposals

 

BPEL duplicates the work of java, c#, c++, ...... but the authors had
their reasons. Therefore I don't   find that to be  a compelling
argument. 

Simple parseable if-then-else statements seem a necessity IMHO.

 

Martin.

 

-----Original Message-----
From: public-ws-chor-request@w3.org
[mailto:public-ws-chor-request@w3.org]On Behalf Of Yaron Y. Goland
Sent: Monday, June 30, 2003 2:19 PM
To: WS Chor Public
Subject: RE: Requirements: Decision Points Requirement Proposals

I agree with #1 wholeheartedly. In fact I would like to see our
priorities be:

    #1 - Be able to define and validate at runtime a choreography
without reference to any outside facility such as BPEL

    #2 - After we have successfully shipped #1 out the door then we
would worry about how to directly integrate our work with BPEL

 

As for #2, I respectfully disagree. 

 

I believe that including the ability to make machine readable state
change decisions (e.g. decision logic) based on the contents of a
message will have a number of detrimental consequences:

 

    #1 - In real world choreographies it is exceedingly rare that the
decision regarding how to deal with a message is made exclusively based
on the contents of the message itself. Even in the most simplistic
scenario, a message router, there is almost universally a recourse to a
routing table which is fully dynamic. So message control logic expressed
exclusively based on message content (which is the only platform and
implementation independent way of doing it) will always be incomplete
and thus of little utility.

 

    #2 - The only way to define a useful message control mechanism to
allow for routing on message contents will require the introduction of a
full programming language since anything but the most trivial scenario
will require sophisticated data handling capabilities. This quickly
takes us down the course of inventing our own programming language and
thus duplicating the work BPEL is doing.

 

As such I believe that our solution should not specify a machine
readable mechanism for specifying how a routing decision is made in the
choreography graph. Rather the logic should be specified via text. It
will then be the job for a latter group to perhaps specify a mechanism
by which one could point at code (such as BPEL) that could encode the
actual decision logic.

 

    Just my two cents,

 

            Yaron

-----Original Message-----
From: Fletcher, Tony [mailto:Tony.Fletcher@choreology.com]
Sent: Monday, June 16, 2003 4:45 AM
To: Burdett, David; Yaron Y. Goland; WS Chor Public
Subject: RE: Requirements: Decision Points Requirement Proposals

Dear Yaron, David and others,

 

Out of this debate I would like re-assurance that we are agreeing to
support the following two 'propositions' :

 

1)  The WS-Choreography language should be usable on its own, as well as
usable in unison with BPEL4WS.

 

Note :use alone and use with BPEL4WS are two separate goals.  Personally
I would be happy if we tried to support them both and I think it makes
sense to try to do so.

 

2)   The next state of a choreography can be determined by some value in
a field of an incoming message.

 

Please refer to the attached slide for a simple example.  Not that how
the supplier determines the accept or reject value for the result field
is hidden.  However, at the buyer side the next step in the Choreography
is determined by the value of this field.  It seems to me that
Choreographies will not be comprehensible (to humans - machines can be
made to accept anything!) with out this sort of facility.

 

Note:  This should be possible both when the WS-Choreography is used
alone, and when used together with BPEL4WS to expand on some (or all) of
the roles.  This is so that the WS-Choreography language can be used on
its own (at least initially) to design choreographies - and as a means
of agreeing a design amongst interested parties.

 

One potential solution to these requirements is to copy some of the
syntax and semantics from BPEL4WS into the WS-Choreography language, but
there may also be other approaches.

 

 

Best Regards     Tony

A M Fletcher

 

Cohesions 1.0 (TM)

 

Business transaction management software for application coordination

 

Choreology Ltd., 13 Austin Friars, London EC2N 2JX     UK

Tel: +44 (0) 20 76701787   Fax: +44 (0) 20 7670 1785  Mobile: +44 (0)
7801 948219

tony.fletcher@choreology.com     (Home: amfletcher@iee.org)

-----Original Message-----
From: public-ws-chor-request@w3.org
[mailto:public-ws-chor-request@w3.org] On Behalf Of Burdett, David
Sent: 07 June 2003 06:31
To: Yaron Y. Goland; WS Chor Public
Subject: RE: Requirements: Decision Points Requirement Proposals

Yaron 

Several detailed comments (with alternative suggested wording) are
included inline below. I would also add another few requirements ...

"The WS-Chor choreography definition MUST provide mechanisms by which
new choreography definitions can be composed out of other choreography
definitions". The use case for this is that you might have a
choreography that defines how to place an order, you also might have
another choreography that defines how to send an invoice. If you then
want to define another choreography that defines how you place an order
that is followed by one to send an invoice, then a composition
capability would allow the original choreographies to be reused. Another
consideration on this is that there a many different functionally
equivalent ways of placing an order. Similarly there are several
functionally equivalent ways of sending an in invoice, so it would
really be useful to be able to compose a choreography that said
something like "Do one of n ways of placing an order, followed by doing
one of n ways of placing an invoice".

"The WS-Chor choreography definition MUST provide mechanisms by which
the execution of one choreography definition is dependent on the
execution of the instance of some other choreography definition". The
use case for this is where you want to execute a choreography that
determines the current state of processing of some earlier choreography.
The "query" choreography can only validly be executed if there is some
earlier instance of the a choreography that can be referenced.

The following couple of requirements are ones that have been discussed
much earlier on the list however I am not sure that we really want to do
them, at least not initially, but I do think they are worth discussing
...

"The WS Choreography specification MUST provide standardized, reusable
choreography definitions that allow one role to determine another roles
state of processing of a choreography instance, no matter what
choreography definition was being followed."

"The WS Choreography specification MUST provide standardized, reusable
choreography definitions that allow one role to request another role to
restart the processing of a "stalled" choreography instance, no matter
what choreography definition was being followed." This could simply be a
request to resend some earlier message that got lost.

The rationale for both of these is that querying the status of a
choreography and re-starting a choreography will be common requirements
for many (but not all) choreographies and therefore having a standard
way of doing these functions will make choreographies easier to design
and develop.

As stated earlier, more comments inline below. 

David 

-----Original Message----- 
From: Yaron Y. Goland [mailto:ygoland@bea.com] 
Sent: Wednesday, June 04, 2003 4:28 PM 
To: WS Chor Public 
Subject: Requirements: Decision Points Requirement Proposals 

 

I propose the following requirements be added to the requirements
document: 

The WS-Chor choreography description format MUST provide mechanisms to 
enable a choreography to specify that a process in a particular role
MUST 
send zero, one or more messages from a statically defined set of
messages in 
parallel, serial or any combination of the two. 
<DB> A couple of comments: 
1. I think a role that MUST send zero messages doesn't work as if the
role MUST send zero messages, then why is it in the choreography. 

2. Why do you use the term "description format" instead of the simpler
"definition' because, aren't the properties you seek a characteristic of
the definition rather than the format of the definition.

3. The first sentence is circular as it says ... "The WS-Chor
choreography description format MUST enable a choreography ..." without
specifying what a choreography is. 

4. I think you mean when you say a "statically defined set of messages"
that the actual messages definitions that can be sent are finite in
number and from a proscribed list. There has been a lot of discussion on
the idea of variability in the detailed message content which means that
limiting a choreography to specific message formats will inhibit
choreography reuse. Instead I thinkt that we should refer to "Message
Types" or "Message Families" rather than "messages".

5. This requirement is also very similar to the next so my alternative
is described below 
</DB> 

The WS-Chor choreography description format MUST be able to describe 
decision points where a process in a particular role MAY send zero, one
or 
more messages from a statically defined set of messages in parallel,
serial 
or any combination of the two. 
<DB>So how about the following requirement that combines the previous
two and takes into account the comments I made ...

"The WS-Chor choreography definition MUST provide mechanisms that define
the sequence in which one or more messages types are exchanged between
two or more roles either in parallel, serially or any combination of the
two, together with the conditions that cause those messages to be
sent."</DB>

The WS-Chor choreography description format MUST be able to describe who
is 
to receive a message by referencing their role. 
<DB>I would add the sender to this definition to give ... "The WS
Choreography definition MUST be able to describe who the sender of a
message is and who the receiver should be by referencing their role."
The rationale for this is that what you do with a message may well
depend on the role of the sender ... assuming that the same message can
be sent by different roles.</DB>

The WS-Chor choreography description format MUST make it possible to
specify 
a role's binding to an actual web service instance either statically,
when a 
web service using that choreography is deployed, or dynamically at run
time. 

The WS-Chor choreography description format MUST provide mechanisms to
allow 
messages to be sent to a particular member of a set of web services in
the 
same role. 
[Ed Note: What I'm very inelegantly trying to capture is the idea that
if 
you are running an auction service and you just found out that one of
the 
bidders isn't qualified to bid you need a way to say things like "I'm
now 
going to send out an unsolicited 'get lost you dead beat' message to one
web 
service that is in the role of bidder." This could then trigger a whole
set 
of messages back and forth between the auction service and the dead beat

bidder, the choreography needs some way to capture the fact that you are

still talking to the same member of the role group.] 
<DB>This example introduces the idea of a role group, which I don't
*think* we need. If we take this use case, then you can actually
consider it as an internal "business process" problem, for example:

The auctioneer has a business process that consists of a set of separate
individual identical choreographies between the auctioneer and the
bidder where each choreography instance would take the following form
...

AUCTIONEER       BIDDER 
Bid Invite -------> 
Either ... 
Get Lost ---------> 
... or ... 
<--------------- Bid 
... etc ... 
The fact that there are several bidders involved is something that only
the auctioneer needs to be concerned of. 

This means that this is really a business process (e.g. BPEL ) problem
rather than a choreography problem especially as the auctioneer is in
complete control of what goes on. For example, the auctioneer could
treat all the interactions as being part of one choreography by using
the same identifier for the correlation of all the messages irrespective
of the bidder. 

Now there may be a use case where you really do the need the
variability, but I can't think of one. On the other hand, if we can
avoid this variability, then it will simplify the specification we need
to write significantly.

</DB> 

The WS-Chor choreography description format MUST NOT require that the
logic 
used by a sender in a decision point to decide how to act be exposed in
the 
choreography. 
<DB>There's a corollary, I think, that says something like ... "The
WS-Chor choreography definition MUST enable the results of decisions
made by one role that affect the behavior of another role to be
communicated to the other role." This is really about the transmission
of relevant state information between roles.</DB>

The WS-Chor choreography description format MUST enable the annotation
of 
all actions with human readable descriptions. 
<DB>I agree but would go further and replace the last phrase with "...
with clear semantic definitions." Something might be human readable but
that does not mean it explains the purpose well.</DB>

The WS-Chor choreography description format MUST provide an abstract 
mechanism where by the logic used to make a decision at a decision point
can 
be expressed through reference to a WSBPEL abstract or executable
process or 
similar machine readable logic. 
<DB>I don't have an alternative definition, but this pre-supposes a
binding to WSBPEL that we might (or might not) want to make unless and
until we collectively (i.e. WSBPEL and WSCHOR) work out what the goals
and relationships of each activity will be.</DB>

The WS-Chor choreography description format base specification MUST NOT 
specify bindings for the abstract mechanism used to reference machine 
readable logic, rather extension specifications on top of the base 
specification MUST be used. 
<DB>As a general comment, we could do with developing definitions of
various terms, e.g. "decision point", "base specification" which
although quite intuitive, could be open to miss-interpretation.</DB>

I would appreciate it if objections to these requirements were stated in
the 
form of alternate proposals. It's easy to say why something is wrong,
it's a 
lot harder to spend the time to specify what is right. 

                Yaron 
Received on Tuesday, 1 July 2003 17:00:49 GMT

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