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RE: Requirements: Decision Points Requirement Proposals

From: Burdett, David <david.burdett@commerceone.com>
Date: Fri, 6 Jun 2003 22:30:35 -0700
Message-ID: <C1E0143CD365A445A4417083BF6F42CC08391979@C1plenaexm07.commerceone.com>
To: "Yaron Y. Goland" <ygoland@bea.com>, WS Chor Public <public-ws-chor@w3.org>
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 Saturday, 7 June 2003 01:29:41 GMT

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