Re: [wsbpel] Issue 6 - Rough draft of proposal for vote

[Axel Martens <amarten@us.ibm.com>]

Dear Yaron, Alex, Ivana, Dieter, Vinky
and others

I have listed to your discussion, and
I am really afraid of the
tendency to introduce a new structure
(syntax and/or semantics)
for each use case separately. We really
should come up with an
generic and small solution that is hopefully
easily usable as well.

Because most of you do like the completionCondition
approach,
let's stick to this approach, but make
it as simple as possible. I
like Ivana's "isCompleted('A')",
because brings in a touch of a
final joinCondition, although no links
are present. But, I want to
generalize it, to get rid of ignorable
faults etc.

(1) Because we need the status of each
activity within the flow,
its is natural to me, to introduce predicates
that deliver this
status:
  - Each activity of the flow is
either running or ended. I
  propose a predicate "isEnded('A')"
which yields true, iff the
  activity is ended (both, normally
or faulty), and false otherwise.
  - Each scope within the flow
might catch faults. To distinguish
  the two way of a scope's ending,
I propose the two predicates
  "isCompleted('A')"
and "isFaulted('A')", which yields the final
  status of the scope. (Remark:
the predicate "isEnded('A')" is
  applicable to scopes as well.)
I guess, each middleware implementation
has to keep track of those
status information. Hence, it must be
easy to implement the
predicates.

(2) The completionCondition should be
a boolean predicate built
out of those predicates only. Otherwise,
part of the
completionCondition might not be defined
at evaluation time (e.g.
variables have not been written yet).

(2a) To make one simple case very simple,
I would agree on an
additional macro "numberOfEndedActivities(N)".
If we have the
activities A, B and C, the completionCondition
"numberOfEndedActivities(2)"
would be equivalent to the condition
"(isEnded('A') and isEnded('B'))
or (isEnded('A') and isEnded('C')) or (isEnded('B') and isEnded('C'))"

(3) The completion condition is evaluated
each time on of the 
predicates changes its truth value.
The flow is ended, iff
    (a) the completionCondition
could be evaluated true or
    (b) all activities of
the flow are ended.
In the first case, still running activities
are terminated. In the
second case, the fault "completionConditionFailure"
is thrown if
the completionCondition was evaluated
to false.

(4) Employing the already existing power
of fault handling, nested
scopes, etc. - all special cases of
severe and ignorable faults
could expressed without additional changes
to BPEL.

Regards, Axel.
----------------------------------------
Axel Martens

Post Doc Researcher
Component Systems Group
IBM TJ Watson Research Center 
Hawthorne, NY (USA)
Phone: (914) 784-7480
E-mail: amarten@us.ibm.com