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

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

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.yiu@oracle.com>

09/10/2004 12:10 AM

To	Alex Yiu <alex.yiu@oracle.com>
cc	"Trickovic, Ivana" <ivana.trickovic@sap.com>, Axel 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