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Subject: Re: [wsbpel] Issue 6 - Rough draft of proposal for vote
Just for completeness (sorry if this has been mentioned somewhere already):
for "N out of M", is there a notion of "end activities" in a flow? M would
then be the number of such end activities. This would be interesting for a
static analysis in order to avoid specification where N > M where one would
wait forever.
Examples:
flow name=flow1 <!-- a flow with two "end activities" -->
link name=from-a-to-c
link name=from-b-to-c
any-activity name=a
source name=from-a-to-c
any-activity name=b
source name=from-b-to-c
any-activity name=c
target name=from-a-to-c
target name=from-b-to-c
any-activity name=d
flow name=flow2 <!-- a flow with three "end activities" -->
link name=from-a-to-b
link name=from-a-to-c
any-activity name=a
source name=from-a-to-b
source name=from-a-to-c
any-activity name=b
target name=from-a-to-b
any-activity name=c
target name=from-a-to-c
any-activity name=d
(Embedded image moved to file: pic12343.jpg)
Kind Regards
DK
Alex Yiu
<alex.yiu@oracle.
com> To
"Trickovic, Ivana"
13.09.2004 08:00 <ivana.trickovic@sap.com>
cc
Axel Martens <amarten@us.ibm.com>,
Frank
Leymann/Germany/Contr/IBM@IBMDE,
Dieter Roller/Germany/IBM@IBMDE,
Dieter Koenig1/Germany/IBM@IBMDE,
"'edwink@collaxa.com'"
<edwink@collaxa.com>,
"'satisht@microsoft.com'"
<satisht@microsoft.com>,
"'wsbpel@lists.oasis-open.org'"
<wsbpel@lists.oasis-open.org>, Alex
Yiu <alex.yiu@oracle.com>
Subject
Re: [wsbpel] Issue 6 - Rough draft
of proposal for vote
Hi,
Ivana, thanks for sending out this proposal.
I believe a number of us (including me) like the <completionConditon>
construct from your proposal high level speaking.
After reading the proposal, I guess we need to work on a number of details.
I would like to ask a few questions / mention a few points:
Minor syntax and semantics questions:
Do we want to allow expression for branch attribute? instead of
just a constant integer?
We may need to some minor syntax adjustment for expression
attribute because we have passed Isseu 13 already.
If answers to the above question are yes, then the condition
syntax may become a choice (xsd:choice) of the following two:
<condition
expressionLanguage="anyURI"?>boolean-expression</condition>
<branchCondition
expressionLanguage="anyURI"?>integer-expression</condition>
About completionConditionFailure: it looks like a nice idea so far
...
About the new "isCompleted()" function: I am not against this
function. But, I tend to think there are usually BPEL variables being
modifed as the execution result of a flow activity. (See the
train-or-plane-and-hotel example). Is it more typical to have
completionCondition to be evaluated on those variables?
Also, after the completion of the whole flow, it is not uncommon for
people to have logic to find out which branch is finished (especially
when "or" is involved in the condition). The "isComplete()" function
would be available only within the completionCondition within the
flow. People may get confused and attempt to use the "isComplete()"
function outside the flow. If the completionCondition relies on
variables, the same logic is applicable outside of the flow.
About the IgnoreFault semantics: I recall this ignoreFault idea back
to March F2F. With ignoreFault semantics, it essentially introduces
two brand new semantics.
It turns <flow> into a semi-<scope> like construct
It adds this new "ignoreFault" semantics: which catches certain
fault and the catching action will not cause any scope to be
marked as faulted.
I tend to think this "ignoreFault" functionality has quite a heavy
weight impact to <flow> and fault handling picture in BPEL. If there
is an alternative way to achieve similar business logic, I would go
for the alternative way.
Quoted from Ivana's email:
"Not all activities must complete in order the enclosing flow
activity to complete. The way to identify that an enclosed activity
did not complete is to propagate faults."
I agree with the first sentence. But, I am not sure that propagating
fault is the only way to identify an enclosed activity did not
complete. I would rather use a scope as an activity enclosed by flow.
If we wish to suppress any minor fault which happens during the
execution of the scope, then we can just add a fault handler. If the
fault that happen, the scope will be marked as "faulted".
The completeConditon can identify the "incomplete" state of the scope
by two ways:
completeCondition can be evaluated based on BPEL variables,
which are set to "incomplete" state by pre-flow initialization
or related fault-handler logic.
branch-based condition will interprete a faulted scope as
"incomplete" branch and will not count them into the required N
branches.
This will allow us to suppress minor fault with or without the
<completionCondition> construct. Please see the following example. It
is already legal and working today. It shows how to suppress
"foo:barFault" without the <completeCondition> construct
<flow>
<scope name="A">
<faultHandler>
<catch faultName="foo:barFault" ... > ... </catch>
</faultHandler>
<sequence>
... <invoke name="doA" ... /> ...
</sequence>
</scope>
<scope name="B"> ... <!-- similar to A --> ... </scope>
</flow>
If the invoke of "doA" triggers "foo:barFault", the scope "A" will be
marked as faulted. On the other hand, scope "B" will be allowed to
continue and complete. After scope "B" is completed, the compensation
handler of scope "B" will be installed. And, the overall execution of
flow is still considered a normal completion.
I hope I have illustrated we can achieve similar logic without
introducing a new "ignoreFault" construct and concept to BPEL.
What "completionCondition" does should be just providing a way to
pro-actively cancel/terminate other flows without waiting to them to
complete or to be faulted.
About "Cancel-other-flows": (Let me consolidate what I am leaning
towards so far.) We need to formal define the notion of a "
cancel-other-flows" mechanism. This mechanism is specified and
attached to <flow> construct (and future parallel forEach construct).
When this mechanism is triggered by one of parallel flows, it will
cancel/terminate other parallel flows which are still running, after
the triggering flow is completed. If the other flow activties are
scope activities, the scope will be marked as cancelled.
This mechanism will be triggered upon the completionCondition is
evaluated to be true.
[Note: (a) since this mechanism is triggered by without using any
fault directly. This will again decrease the need of "ignoreFault"
construct (b) This mechanism will affect the wordings of Issue 135 to
an extent.]
[Question: again, should we enforce scope-only activity when
completionCondition is used? I tend to say yes.]
Thank you all for following and reading this email thread!!!
Regards,
Alex Yiu
Trickovic, Ivana wrote:
Here is a proposal for the completion condition - it addresses some
issues not tackled so far (at least not in case of
<completeCondition>). It is a version of the proposal for completion
condition for the bundle element. This proposal includes comments
from many people including Frank Leymann, Dieter Koenig, Dieter
Roller and Satish Thatte. It does not mean that they completely
support the proposal - they may have issues with any part of the
proposal.
Completion condition
=====================
The current semantics of the flow activity is that it waits for all
concurrent activities to complete. The completion also means that an
enclosed activity/scope may end abnormally or be skipped (e.g. the
join condition of the activity evaluated to false). If a fault is
thrown within an enclosed activity/scope and one of the local fault
handlers catch the fault (and does not rethrow the fault), the
enclosed activity/scope will be deemed to be completed (although
ended abnormally). If a fault is not caught by any local fault
handler (or is rethrown) the flow activity will terminate all active
concurrent activities and corresponding fault handler may be
initiated.
A completion condition for the flow activity is needed for scenarios
where not all concurrent activities of a flow activity must complete
in order the flow activity to complete. Note: We are not talking
about "successful completion" of enclosed concurrent activities
because that would not be consistent with the semantics of the
current flow activity.
The completion condition may have different flavors, such as:
(1) N out of M
(2) The two most important requests completed
(3) A Boolean condition operating upon process variables
Not all activities must complete in order the enclosing flow activity
to complete. The way to identify that an enclosed activity did not
complete is to propagate faults. We may distinguish between severe
faults and those that can be ignored. Severe faults cause the
enclosing flow activity (or more precisely, enclosing scope) to
terminate the flow activity, including all active concurrent
activities, and corresponding fault handler may be initiated. Other
faults may be ignored - the flow activity is "informed" that a
concurrent activity did not complete but still allows other active
concurrent activities to continue with execution.
Ignore semantics
=================
Faults thrown within enclosed concurrent activities/scopes and not
handled by local fault handlers are rethrown. Enclosing <flow>
element decides which of these rethrown faults can be ignored. This
new "ignore" semantics should be part of the completion condition and
should apply to all enclosed activities. This new semantics does not
introduce a new fault handling mechanism. It is needed for
identifying how many of the enclosed activities failed.
Proposed syntax
================
<flow standard-attributes>
standard-elements
<completionCondition/>?
<links>?
<link name="ncname">+
</links>
activity+
</flow>
<completionCondition>
<conditions branch="xsd:integer"?
expression=xpath?/>?
<ignoreFaults>?
<fault name="qname"? value="ncname"?/>*
<ignoreAll/>?
</ignoreFaults>
</completionCondition>
Attribute branch is used to specify a condition of flavor "wait for N
out of M activities to complete", or more precisely value N.
Attribute expression is used to specify a Boolean condition operating
upon process variables or a condition of flavor "the two most
important requests completed".
Both conditions (branch and expression) may be specified at the same
time. They will be checked when one instance of the scope activity
reaches the end. If at least one condition evaluates to true all
active instances will be terminated.
Element <ignoreFaults> specifies faults that may be ignored. Element
fault is used to specify a fault which may be ignored (fault name and
fault data may be specified). Element <ignoreAll> would mean that all
faults thrown/rethrown by any concurrent activity/scope may be
ignored. If this element is specified <fault> element must be
omitted.
Completion condition failure
=============================
A new standard fault, e.g. completionConditionFailure, should be
introduced to notify that the completion condition of a flow activity
evaluated to false (note: all concurrent activities have been
completed). The fault is thrown in the scope enclosing the flow
element.
Completion condition and links
==============================
There should be no difference between a flow activity with a
completion condition and a flow activity without completion
condition. For example, if the completion condition fails all links
leaving the flow activity should have value "false" (or be reverted
to a negative status).
There are just a few additional rules:
(1) Let's assume enclosed activity A is the source of a link. If the
completion condition evaluates to true and activity A is not
completed it will be terminated and the value of the link will be set
to "false".
(2) Let's assume enclosed activity A is the source of a link and the
activity failed but the fault is "ignored" by the enclosing flow
activity. The value of the link will be set to "false".
New function for completion condition of flavor "the two most
important requests completed"
============================================================================================
For completion conditions of flavor "the two most important requests
completed" standard attribute "name" must be specified for all
enclosing activities in order to be able to distinguish them. In
addition a new function, e.g. isCompleted('activityName') must be
introduced. The semantics of the function is: if activity completed
successfully the function returns value true.
Example
<completionCondition expression=
"isCompleted('A') AND isCompleted('B')"
</completionConditions>
Regards,
Ivana
-----Original Message-----
From: Alex Yiu [mailto:alex.yiu@oracle.com]
Sent: Samstag, 11. September 2004 00:49
To: Axel Martens
Cc: 'edwink@collaxa.com'; Trickovic, Ivana; 'satisht@microsoft.com';
'wsbpel@lists.oasis-open.org'; Alex Yiu
Subject: Re: [wsbpel] Issue 6 - Rough draft of proposal for vote
Hi, Axel,
Thanks for liking parts of my proposal so far.
Actually, the idea of my proposal is borrowed from a part of
presentation from Ivana and Dieter Roller in March F2F. I just
generalize it to apply to <flow> also.
About using links or not:
I don't have a huge opposition to Axel's proposal which enriches the
semantics of joinCondition evaluation. However, it is not that
difficult for people to make mistakes in joinCondition. Look at the
2-out-of-3 example. The join condition already gets so complicated.
If people make a mistake in their joinCondition, the whole flow can
get struck for no good reasons. (The completeCondition approach will
less likely to have the whole flow struck. Because, it does not
introduce a new parallel activity in the flow). But, if this is what
Petri-net-oriented and control-link-oriented audience really want, I
can accept it in a sense, as long as this is not the only way to
achieve similar business logic.
About static vs dynamic parallelism:
Static parallelism is basically <flow> construct in BPEL. Dynamic
parallelism is "parallel forEach" or "bundle". Issue 4 and 147 are
related issues.
http://www.choreology.com/external/WS_BPEL_issues_list.html#Issue4
http://www.choreology.com/external/WS_BPEL_issues_list.html#Issue147
About "Terminating still running tasks versus proceeding in control
flow while those tasks are
still running":
I am not 100% that I understand your question. Let me try to answer
it anyway. The completeCondition is about triggering termination of
still-running tasks. The "proceeding in control flow" (if I
understand you correctly) will be handled by an outter flow. [I
guess it is related to (hotel and (train or plane)) flow??]
About changes of "forcedTermination" fault handler:
If you got a chance to see some recent emails on Issue 135, Satish
has recently suggested to remove the notion "forcedTermination"
fault and replace its fault handler with cancelHandler. Frank,
Yaron, Edwin and I all agree with this direction. Because,
overloading fault with the concept of "forcedTermination" is a
"false economy" and it creates quite a bit of unnecessary confusion.
I don't think we should go back to this route or even further
overload the "forcedTermination" fault handler semantics. Because,
fault handling is way too generic and the changes you mentioned will
create even more confusion. (e.g. Will this change of marking as
"completed" apply to all "forcedTermination" fault handler / cancel
handler of all scopes? )
I guess most of us agree that we should have a new mechanism to kill
/ do an early completion of a flow in a nice and clean way. However,
overloading a standard fault handler does not seem to be the best
one. I would rather introduce a construct which is very specific to
<flow> or parallel-forEach. For example, this <completeCondition>
construct or maybe a new activity called <completeFlow />. I will
send out more emails on joint thinking of both Issue 6 and Issue
135. Please stay tuned.
Thanks!
Regards,
Alex Yiu
Axel Martens wrote:
Hi,
I like the simplicity of the syntax of Alex's proposal.
Although,
for me it is quite easy to model in terms of links and join
conditions, and I want to keep my proposal alive (because of
the
minimal changes to BPEL's syntax and semantics), it looks like
a
nice macro to provide more convenience to the customers.
Alex, could you shortly explain to me, what do you mean by
static
and dynamic parallelism? How do you handle the two different
cases
after evaluating the completeCondition: Terminating still
running
tasks versus proceeding in control flow while those tasks are
still running (Sorry, I missed your previous emails)?
I agree to Alex's opinion, that we need a mechanism to kill
parallel flow nice and clean. This could be done either by a
new
mechanism which does not throw a fault or by changing the way
fault are handled. I have discussed already an example of the
second case with Ivana, and I like to tell you, what I have in
mind. Look at the following example:
<scope name="scopeFlow">
...
<flow>
<link name="linkA"/>
<link name="linkB"/>
<scope name="scopeA">
...
<invoke name="A" ...>
<source linkName="linkA" ...>
</invoke>
</scope>
<scope name="scopeB">
...
<invoke name="B" ...>
<source linkName="linkB" ...>
</invoke>
</scope>
<throw name="C" faultName="bpws:forcedCompletion"
joinCondition="linkA OR linkB"
joinEvaluation="immediate">
<target linkName="linkA" ...>
<target linkName="linkB" ...>
</throw>
</flow>
</scope>
First, I explain the situation: In the example above, activity
C
will be executed if one of the two activities A and B was
successfully completed. Activity C throws the fault
"forcedCompletion". Like each other fault, this forces the
scope
"scopeFlow" to terminate still running activities. Assume,
there
was a fault handler defined in this scope which catches the
fault
"forcedCompletion" (omitted in here), the process continues
after
scope "scopeFlow". A problem arises, if scope "scopeFlow"
should
be compensated. Because it was exited from a fault handler, no
compensation handler was installed.
Now, I explain my solution: In the example, I have chosen a
new
"standard" fault name: "forcedCompletion". The only necessary
change is to allow a fault handler that catches this fault to
install a compensation handler for the same scope, i.e. to
mark
the scope "scopeFlow" as "completed" instead of "exited".
The standard compensation mechanism will only undo those
("scoped") activities within the scope, which actually have
been
successfully completed, i.e. scopeA or scopeB or may be both.
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
Alex Yiu
<alex.yi
u@oracle
.com> To
Alex Yiu <alex.yiu@oracle.com>
cc
09/10/20 "Trickovic, Ivana"
04 12:10 <ivana.trickovic@sap.com>, Axel
AM Martens/Watson/IBM@IBMUS,
"'edwink@collaxa.com'"
<edwink@collaxa.com>,
"'satisht@microsoft.com'"
<satisht@microsoft.com>,
"'wsbpel@lists.oasis-open.org'"
<wsbpel@lists.oasis-open.org>, Alex
Yiu <alex.yiu@oracle.com>
Subject
Re: [wsbpel] Issue 6 - Rough draft of
proposal for vote
Hi,
Here are some examples how to use completeCondition to express
similar business logic in Axel's previous email:
(A) "Select one out of three" example:
<flow>
<completeCondition branch="1" />
<invoke name="CheckAirlineA" ... />
<invoke name="CheckAirlineB" ... />
<invoke name="CheckAirlineC" ... />
</flow>
(B) "Select two out of three" example:
<flow>
<completeCondition branch="2" />
<invoke name="AskRefereeA" ... />
<invoke name="AskRefereeB" ... />
<invoke name="AskRefereeC" ... />
</flow>
As you guys can see, the completeCondition declaration is very
straight forward and simple. No complicated links and
joinCondition usage.
(C) "(Plane or Train) and Hotel" example: It would become two
flow constructed (nested).
<assign> ... <to variable="planeResult"></assign>
<!-- initialize "planeResult" with NOT-OK value -->
<assign> ... <to variable="trainResult"></assign>
<!-- initialize "trainResult" with NOT-OK value -->
<flow name="checkIterinary">
<flow name="PlaneOrPlane">
<completeCondition>
fn:planeOK(planeResult) or fn:trainOK(trainResult)
</completeCondition>
<invoke name="CheckPlane" outputVariable="planeResult"
... />
<invoke name="CheckTrain" outputVariable="trainResult"
... />
</flow>
<invoke name="checkHotel" />
</flow>
<switch name="bookingSwitch">
<case>
<condition>
(fn:planeOK(planeResult) or fn:trainOK(trainResult))
and fn:hotelOK(hotelResult)
</condition>
<invoke name="invokeBooking" ... />
</case>
<otherwise>
<invoke name="writeInformation" ... />
</otherwise>
</switch>
Please note that regardless whether we pick a link-oriented
approach or completeCondition approach:
The initialization of result variables are needed
because of potential cancellation of one of the invoke
between train and plane
The "fn:*()" represents the logic to determine whether a
traveling resource is available. They are used in either
the transitionCondition of links or the condition of
switch/case.
As you guys see, we don't need declare to six links. The
completeCondition and case-condition are much simpler and
easier to understand.
I attach a diagram to illustrate the above flows.
I guess we can still more time in terms of refiniment of
joinCondition evaluation. However, I don't think that should
be the only approach to achieve complete condition related
logic.
Thanks!!!
Regards,
Alex Yiu
Alex Yiu wrote:
Hi,
+1 to what Ivana said in general.
Few points to add:
Axel's proposed enhancement to control links evaluation
works to an extent for static parallelism (e.g.
<flow>). However, control links do not work well in
dynamic parallelism (e.g. "parallel forEach" or
"bundle"). I think the notion of completeCondition
(borrowed from Ivana and DK) is general enough and it
should be applied to both static and dynamic
parallelism. A general completeCondition mechanism will
be easier for BPEL users to learn.
Even in a pure static parallelism case,
completeCondition has much better code clarity. It is
more declarative and easier for BPEL users to
understand. It will eliminate significant amount of
joinCondition programming, which may be error prone. (I
will send another email later to show how
completeCondition can be used to express the same
semantics of Axel's example).
In one of my previous emails, I also tried to use a
"macro" way to illustrate how outstanding running flows
can be cancelled by throwing a fault within a scope. The
illustration has the same compensation handler
installation problem that Ivana has pointed out. The
"illustraction macro" does NOT carry a desirable and
intended semantics. We need to create / describe a new
mechanism to cancel parallel flow without throwing a
fault. (That was discussed briefly between Edwin and me
at Oracle).
Thanks!
Regards,
Alex Yiu
Trickovic, Ivana wrote:
Axel,
I find the idea interesting. In fact, I was discussing the
completion condition issue with Dieter Koenig during the last
f2f meeting and his suggestion was also to try to resolve this
issue using links. And we identified that several changes need
to be done, including removing restriction for jonCondition,
that "the join condition is evaluated as soon as all incoming
links of the activity are determined" - so definitely
"immediate" semantics needs to be introduced.
I have the following comments on your proposal.
1. In your proposal you are using a fault
(bpws:forcedTermination) to terminate all active parallel
branches. But this changes the outcome of the flow activity.
It will always end abnormally and compensation handler (if it
is defined) will never be installed. Although completion
condition has evaluated to true and needed activities have
completed the enclosing flow activity will end abnormally. Is
this really intended semantics?
2. Your proposal does not address some pain points. For
example, in case of "N out of M", N<M there many possible
"variations":
(A) One enclosed activity may experience problems but the
<flow> activity may succeed
(B) One of enclosed activities may experience a severe error,
which may have impact on the <flow> activity
The question is what to do with running activities? In the
latter case, reasonable behavior would be: if one enclosed
activity does not succeed other running activities should be
cancelled and the flow activity should try to recover. In the
former case, we should allow active parallel activities to
complete their work. This is not supported in your proposal.
Regards,
Ivana
[stuff deleted]
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