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RE: Requirements: Decision Points Requirement Proposals

From: Yaron Y. Goland <ygoland@bea.com>
Date: Mon, 30 Jun 2003 14:18:51 -0700
To: "WS Chor Public" <public-ws-chor@w3.org>
Message-ID: <GMEOJGJFKALPDCNPFMDOGEOEDEAA.ygoland@bea.com>
MessageI 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 Monday, 30 June 2003 17:18:54 GMT

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