The “trouble”
with the WSFL style is that it uses links “indiscriminately” even
within the definition of each task J
From: Alex Yiu [mailto:alex.yiu@ORACLE.COM]
Sent: Thursday, April 07, 2005
1:29 PM
To: Satish Thatte
Cc: ygoland@bea.com;
wsbpel@lists.oasis-open.org; Trickovic, Ivana; Alex Yiu
Subject: Re: [wsbpel] Issue 6.2 -
Proposal For Vote
Just want to add few brief comments:
(A) During the last F2F, I thought
Satish only suggested to limit early completion to
<completionCondition> in <flow>, maybe in parallel
<forEach>. I guess I got the wrong impression. Restricting early
completion to parallel <forEach> only is a way too restrictive usage from
viewpoint. And, restricting to <forEach> only make the issue 6 discussion
totally dependent on Issue 147. And, if Issue 147 does not get passed,
the discussion of Issue 6 becomes meaningless.
(B) I tend to think it is not that
common to have early completion logic to apply to a flow where control links
are used. Because, when early completion condition is used, different branches
within a flow are usually relatively independent tasks to achieve the same goal
(i.e. completion condition). (Of course, I would be happy to hear convincing
counter examples from others).
(C) Ok. Let's say we want to cover
the edge case of control-links + early-completion. Assume we accept Satish's
supplementary interpretation of activity termination. Then, we shall be able to
allow a flow with early-completion condition with control links without
enforcing scope-based restriction. Then, it goes back to the same old question,
as Satish rephrased it:
"should we protect the naïve
process designer from unintended pitfalls by enforcing a scope boundary for
branches that may be prematurely terminated by early completion?"
Do we trust our users that they can write a flow with control links + early
completion and still get the compensation handler right? That is a
conscious choice that we need to make. As of now, I am neutral to this
question.
(D) BTW, if we pass Issue 147
parallel for-each, I tend to think a scope-based restriction should be applied
again to child activity in a parallel-each for to hold the iterator value
properly for compensation ... (Of course, that should be the discussion
of 147 ... and of course ... Yaron will yell at me again after he read this
part of email ... )
Regards,
Alex Yiu
Satish Thatte wrote:
One concern with making all children scopes is that this is kinda
useless when your control flow is governed mostly by links, in the WSFL
style. This also creates additional concerns relating to compensation
order and the constraints imposed by the resolution of issue 10 on link
structure crossing scope boundaries.
This is one reason why I had suggested limiting the early completion
proposal to the proposed parallel foreach construct, which, being a
specialized form of flow, can have more peculiar semantics, although I
am sure Yaron will not thank me for this suggestion :-)
Satish
-----Original Message-----
From: Yaron Y. Goland [mailto:ygoland@bea.com]
Sent: Tuesday, April 05, 2005 3:14 PM
To: Alex Yiu
Cc: wsbpel@lists.oasis-open.org; Trickovic, Ivana
Subject: Re: [wsbpel] Issue 6.2 - Proposal For Vote
Requiring that all the children of the flows be scopes is just bizarre.
If such contortions are needed to make this proposal work then I would
humbly suggest that this proposal is not ready for approval.
Yaron
Alex Yiu wrote:
Hi all,
Ivana and I were still discussing an open technical issue about this
6.2
proposal. That is: whether branches of <flow> should be restricted to
a
scope-based only when <completionCondition> / earlyCompletion is used.
However, since there is a Apr 1st deadline for proposal submission, it
may be
better to submit the proposal on what was drafted so far without
waiting to
resolve this open issue. And, let other people comment on this open
issue.
Here is my detailed viewpoints and analysis we need a scoped-only
restriction
when early completion is used:
========================================================
Scope-based-only branches restriction for early completion is needed,
because:
* early completion mechanics does not change fundamentally
regardless
whether we are using <complete> activity or
<completionCondition>
* we need a scope to encapsulate activities of branch as _"units
of work"_
to minimize the need of defining <completionHandler>
* a partial termination of a scope is NOT well-defined in BPEL,
while
termination of the whole scope is already well-defined in BPEL
spec.
*_How termination mechanism works in BPEL: _*
Here is my understanding of how the termination mechanism in BPEL
works: When a
scope is sent with a termination signal, the targeted scope will: (1)
cascade
the termination signal inner child scopes (2) stop all the inner
activities
within the targeted scope (3) execute the termination handler. Then,
done.
As of now, the termination signal MUST alway go through a scope to
other
NON-scope activity. And, the termination Handler of the corresponding
scope MUST
got activated.
Hence, termination of the whole scope is well defined. However, a
partial
termination of a scope is NOT well-defined. That is we have not
defined:
* what should happen if some branches got terminated, while some
branches
are completed.
* what should happen if the termination signal to sent to
NON-scope
activities without going through a <scope>.
*_Example #1_*
Consider the following, a flow with two sequence as branches, which is
NOT
scope-based.
<scope name="A">
<flow>
<completionCondition> ... </completionCondition>
<sequence name="B1">
...
<scope name="B1SS1"> ... </scope>
<receive name="B1R2" ... />
...
</sequence>
<sequence name="B2">
...
<scope name="B2SS1"> ... </scope>
<receive name="B2R2" ... />
...
</sequence>
</flow>
</scope>
Say, sequence "B2" finishes. The completionCondition is fulfilled.
Sequence
"B1" is being terminated when it is waiting a message at the receive
"B1R2".
Now, we got the following questions _unanswered, if we don't want to
invent a
brand variation of termination mechanism_:
* Will the terminationHandler of scope "A" be activated?
* Should the compensation handler of scope "B1SS1" be invoked?
Which
"handler" logic is responsible to determine that?
Ok. Let's change it into scope-based branches.
<scope name="A">
<flow>
<completionCondition> ... </completionCondition>
<scope name="B1">
<terminationHandler> ... </terminationHandler>
<sequence>
...
<scope name="B1SS1"> ... </scope>
<receive name="B1R2" ... />
...
</sequence>
</scope>
<scope name="B2">
<terminationHandler> ... </terminationHandler>
<sequence>
...
<scope name="B2SS1"> ... </scope>
<receive name="B2R2" ... />
...
</sequence>
</scope>
</flow>
</scope>
The above questions got _answered clearly, with reusing the current
termination
mechanism easily_:
* Will the terminationHandler of scope "A" be activated?
o No.
* Should the compensation handler of scope "B1SS1" be invoked?
Which
"handler" logic is responsible to determine that?
o It is up to the terminationHandler to decide whether to
invoke the
compensation handler of scope "B1SS1". By default, the
terminationHandler of scope "B1" will invoke that
compensation handler.
Hence, we minimize the need of "completionHandler". I hope I have
refreshed your
memory on why we come up with this scope-based branches. (Regardless
we use
<complete> activity or not).
*_Example #2_*
One important to remember: <flow> only controls the parallelism of
execution. It
does not interfere the compensation and termination structure of a
process.
Without scope-based-branch only restriction, the net effect of a
<flow> is just
a parallellized execution of activities inside the flow in an
undeterministic
order. So, in one extreme case of this undeterministic order, one
inner branch
got completed first and then the other inner branch got executed.
Hence, one
extreme case of <flow> is equivalent to the following nested sequence
pattern in
the context of compensation and termination. (I replace <flow> in
exmple #1 with
an outter <sequence> ... You can also think of one control-link is
used from
sequence "EX2B1" to "EX2B2").
--------------------------------
<scope name="EX2A">
<sequence>
<sequence name="EX2B1">
...
<scope name="EX2B1SS1"> ... </scope>
<receive name="EX2B1R2" ... />
...
</sequence>
<sequence name="EX2B2">
...
<scope name="EX2B2SS1"> ... </scope>
<receive name="EX2B2R2" ... />
...
</sequence>
</sequence>
</scope>
--------------------------------
Say, the execution is waiting at receive "EX2B2R2". Now due to early
completion,
we need to do this "partial" termination magic. We don't have a
well-defined way
to terminate sequence "EX2B2" without affecting sequence "EX2B1".
Similarly, we
don't have a well-defined way to terminate sequence "EX2B2" without
affecting
"EX2B2SS1"
Now think again, if we have scope-based-only branches, all activities
are
separated in units-of-work. Now those branches have independent
compensation and
termination structure now.
Last not least, if we map this early completion idea to transaction
world, it
does not seem like a good idea to allow partial transaction-work
rollback
(mapped to termination & compensation) without any units-of-work
boundary marker
or checkpoints (mapped to <scope>).
========================================================
Thanks!!!
Thank you so much for reading this long email!
Regards,
Alex Yiu
Trickovic, Ivana wrote:
Motivation
===========
The current semantics of the flow activity is that it completes when
all
its (directly) nested activities have completed, either successfully
or
unsuccessfully. However, there are scenarios where it is necessary to
have ability to complete the flow activity before all its nested
activities complete in order to speed up the process execution. For
example, a process waits in parallel for 3 reviews of a paper. If 2
positive reviews are received the process may continue with the
execution without waiting for the last, third response.
The completion condition of may have the following flavors:
* Wait for N out of M nested activities to complete
* Wait until after boolean condition C evaluates to true
The completion condition is interesting for a flow activity enclosing
identical nested activities and for the parallel for-each activity
(still under discussion).
Proposal
=========
Syntax:
<flow standard-attributes>
standard-elements
<links>?
<link name="ncname">+
</links>
<completionCondition>?
activity+
</flow>
<completionCondition>
<branches expressionLanguage="URI"?
countCompletedBranchesOnly="yes|no"?>
an-integer-expression
</branches>?
<booleanExpression expressionLanguage="URI"?>
a-boolean-expression
</booleanExpression>?
</completionCondition>
Semantics:
(1) The completionCondition element is an optional element of the flow
activity. Default behavior of the flow activity is that it waits for
all
its nested activities to complete.
(2) There are two kinds of completion condition:
A> <booleanExpression>: A boolean condition operating upon process
variables. It is evaluated at the end of execution of each nested
activity.
B> <branches>: An integer value expression which is used to define
condition of flavor N out of M. It is evaluated at the end of
execution
of each nested activity. This condition has "at least N out of M"
semantics. (The exact N out of M condition semantics involve resolving
racing condition among nested activities.)
(3) Both conditions (<branches> and <booleanExpression>) may be
specified at the same time. They will be evaluated at the end of
execution of each nested activity. If at least one condition evaluates
to true the <flow> activity completes successfully terminating all
remaining running nested activities. If both conditions are specified,
the <branches> will be evaluated first. If the boolean condition is
specified the evaluation of the condition is done in a serialized
fashion with respect to the nested activities directly enclosed in the
flow activity.
(4) If the integer value evaluated from the <branches> expression is
larger than the number of nested activities in the <flow>, then
bpws:invalidBranchCondition fault MUST be thrown. Note that the number
of branches may be known only during runtime in some cases. Static
analysis should be encouraged to detect this erroneous situation at
design time when possible. (E.g. when the branches expression is a
constant.)
(5) <branches> expression has an optional attribute
"countCompletedBranchesOnly". Its default value is "no". If
countCompletedBranchesOnly is "no", it means the BPEL processor will
count branches which have completed (either successfully or
unsuccessfully). If countCompletedBranchesOnly is "yes", it means the
BPEL processor will count branches which have completed successfully
only.
(6) If flow activity specifies a completionCondition element the
completion condition is evaluated each time a nested activity
completes.
If the completion condition evaluates to true the flow activity
completes successfully. All still running nested activities will be
terminated.
(7) Standard BPEL termination semantics applies to running nested
activities when the completion condition is met. The termination of
running nested activities follows the termination semantics defined in
the specification (see section 13.4.4 Semantics of Activity
Termination).
(8) If all nested activities of the flow activity have been completed
but the completion condition evaluates to false the
"bpws:completionConditionFailure" MUST be thrown by the flow activity.
(end)
----------------
Ivana
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