Dear all:
I'd like to participate in your discussion
with a proposal that
comes with minimal changes to the current
BPEL specification but
is able to handle the mentioned use
cases, and even more.
First, I'd like to introduce myself.
I am a post-doc researcher at
IBM Watson, working in the group of
Paco Curbera. In my PhD thesis
I have dealt with Web service based
business processes and Petri
nets. Hence, I know the discussion on
the mismatch between BPEL
and the workflow pattern à la van der
Aalst quite well.
I completely agree on searching for
a general solution to the
request of issue 6 rather than fixing
distinguished cases. The
problem (or restriction) of BPEL is
that you can't enable or
disable an activity easily w.r.t. to
the complex context, like you
can do modeling with Petri nets or related
methods. The deferred
evaluation of the join condition (all
incoming links has to be
signaled first) is one of the mayor
reasons for it. Hence, our
group proposes to allow an immediate
evaluation of the join
condition, too. This feature was already
covered by IBM's Flow
Definition Language (FDL) and Web Service
Flow Language (WSFL).
The next paragraph explains the proposed
change to syntax and
semantics. Afterwards I show three examples
to demonstrate the
expressive power of this changes.
Syntax & semantics:
The changes to syntax are minimal. Each
activity gets a new
standard attribute "joinEvaluation"
- of course, this attribute
has only influence on activities that
are target of at least two
links. The attribute can have the values
"deferred" (the default
value) and "immediate".
As a kind of syntactical sugar, each
flow or the whole process
might redefine the default value for
all its child activities
through the additional attribute "defaultJoinEvaluation".
But, we
do not insist on this change.
In case, the attribute was set to "deferred",
the join behavior
stays untouched. If the attribute was
set to "immediate", the join
condition is evaluated each time one
of the incoming links has
changed its value. Some examples:
"true" and "yet
undefined" yields "yet undefined"
"false" and "yet
undefined" yields "false"
"true" or "yet
undefined" yields "true"
"false" or "yet
undefined" yields "yet undefined"
This evaluation may yield "yet
undefined", than nothing happens.
If the join condition could be evaluated
to "true" or "false", the
activity is executed or skipped, and
further changes to the link
values are ignored.
Because links are not allowed to cross
the borders of a while loop
and each time a while loop is entered,
its internal link value
should be cleared, there can't be any
confusion.
Examples:
This changes together with the already
existing concepts of scope
and fault handling allow an strait forward
mapping of the missing
pattern.
One-out-of-Many
First, we take a look on a selection
of one result out of three,
where the not chosen alternatives are
allowed to finish normally.
The following figure shows the example
of contacting three
airlines and booking the first available
(for reasons of clarity,
the bpel-code is not fully specified).
Because of the immediate evaluation
of the join condition, the
activity "InvokeBooking" is
started directly after the termination
of one preceding activity. Whereas the
flow is exited only if all
its child activities are terminated.
In case, the customer should
be informed as soon as possible, the
reply activity should be
moved into the flow in sequence to "InvokeBooking".
N-out-of-M
The second example shows two new aspects.
First, the join requires
at least two of the three referees to
have finished the review.
This is done by a modified join condition.
Second, after two
results are available, still running
activities of the flow are
explicitly terminated. This is modeled
by help of a scope around
the flow with an empty fault handler
that catches the
"forcedTemination" fault.
These mechanisms already exit. Hence,
no further changes to the
language itself are necessary. Of course,
a user should be
supported, e.g. by generating such structures
automatically on
demand, but this is a task of tooling
rather than language design.
More complex relations
Finally, the join condition can specify
precisely which
combination of link values is needed
to proceed, instead of just
the number. The last figure shows an
example employing this
feature: For booking a trip, it is necessary
to find an available
hotel and at least on kind of transportation.
This example mixes all concepts. In
case, no hotel or no
transportation is available, an information
for the customer is
generated ("assign") and the
flow is terminated ("throw").
Otherwise, the trip is booked ("InvokeBooking")
and still running
activities are allowed to terminate
on their own before the
customer is informed.
Summary
A minimal change to BPEL's syntax and
an easy to implement change
to its operational semantics enrich
the expressive power
enormously, such that more useful modeling
pattern could be mapped
into this language. To increase the
convenience of usage, tools
may provide predefined macros or generate
necessary structures
automatically on demand.
Regards, Axel.
----------------------------------------
Axel Martens
Post Doc Researcher
Component Systems Group
IBM TJ Watson Research Center
Hawthorne, NY (USA)
Phone: (914) 784-7480
E-mail:
"Trickovic, Ivana"
<>
09/01/2004 10:32 AM
To
"''"
<>
cc
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<>, "''" <>,
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Subject
RE: [wsbpel] Issue 6 - Rough
draft of proposal for vote
Alex,
Regarding the proposed semantics:
Would you elaborate how the fault (bpws:earlyCompletionFault) will be signaled
to other incomplete flow activities? Those activities are enclosed within
the flow activity. If I understand the fault handling model correctly there
is no possibility to communicate faults to nested activities/scopes except
canceling the enclosing activity (in this case, the flow activity).
Regards,
Ivana
-----Original Message-----
From: Alex Yiu [mailto:]
Sent: Mittwoch, 1. September 2004 04:28
To: Satish Thatte
Cc: ; ; Alex Yiu
Subject: Re: [wsbpel] Issue 6 - Rough draft of proposal for vote
Hi, Yaron and others,
I agree with the concern of Edwin and Satish. We should define a general
mechansim to express early completion condition. Then, define the "orJoin"
attribute or other syntax as a macro/special syntax short cut.
If you guys don't mind, I can supplement some alternative ideas to this
issue. I am in the thinking-out-loud mode again. That means, I need to
verify whether this idea makes sense to other people at Oracle. [
But, as Ron said, this thinking-out-loud mode has worked quite well for
me in the past. ;-) ]
We should allow a <completeCondition> subelement attached to parallel-flow-starter
activity (e.g. <flow> or future potential parallel <forEach>).
Syntax of completeCondition:
It has two forms:
(1) <completeCondition>a_boolean_expr</completeCondition>
(2) <completeCondition minFlow="a_number" />
... or ...
<completeCondition countMinFlow="yes">a_number_expr</completeCondition>
The first form is the most generic form of completion condition.
The second form provides a convienent shorthand to achieves the N out of
M semantics.
The suggested orJoin="yes" is equivalent to:
<flow>
<completeCondition minFlow="1" />
...
</flow>
Semantics of completeCondition:
The complete condition will be evaluated at the completion of each flow.
If the condition is true, a standard fault bpws:earlyCompletionFault will
be signaled to other incomplete flow activities. Each incomplete flow activity
can potentially be compensated if each flow is a scope activity. (Potentially,
we can add static analysis to enforce that when a completionCondition is
used in <flow>, <scope> must be used as activities within the
flow.)
The completeCondition itself is executed in the same scope context of the
<flow> activity. [ And, the completeCondition should be executed
serially, to avoid racing condition among flows (?)]
I hope this rough draft of ideas serves as a good base of discussion for
this issue.
Thanks!
Regards,
Alex Yiu
Satish Thatte wrote:
I echo Edwin's second question.
Yaron, your proposal clearly doesn't address the intent of 6. It
seems too narrow to just address this corner case of completion of one
among many (corner case from my perspective).
Satish
From: Edwin Khodabakchian [mailto:]
Sent: Friday, August 27, 2004 5:38 PM
To:
Subject: RE: [wsbpel] Issue 6 - Rough draft of proposal for vote
Hi Yaron,
2 questions:
1) are we sure that termination is the desired
behavior for the non completed activity?
2) what happens if the join rule is: "the
first search result which is non empty or includes at least 3 entries with
more than 85% accuracy"? The point here is that a lot of join patterns
might have logic in them. I know that in the past we talked about having
something like <break> for flow. (This is something that a lot of
our customers are asking us and might address the same us case is a more
flexible way specially if the <break> can be configure to tell the
engine to compensate or terminate).
Best,
Edwin
