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Re: [wsbpel] Issue 221 - discussion carried over from Issue 123
Hi, all,
[discussion carried over from Issue 123]
Ok ... let me summarize again on where to throw missingReply fault (to
make sure everyone is on the same page)
I believe that there are 2 variant of suggestions on the table:
Variant [A]: (which I am prefering)
It has two checkpoints:
checkpoint [A1]: S2 (at the end of normal execution) => S2
checkpoint [A2]: S2 (at the end of fault handler) => S1
[** A typical fault propagation flow would be:
At the end of normal execution, a missingReply is thrown by S2 to
S2.
Then, a fault handler (including the default one) will be
triggered, if the missingReply fault is NOT handled (aka default), the
fault will be propagated to S1. ** ]
Variant [B]:
It has only one checkpoint.
checkpoint [B1]: S2 (at the end of normal execution or fault handler)
=> S1
(You guys can think of [B1] is the morphed combination of [A1]+[A2]
plus modification.)
Concerns I have on Variant [B] are:
It seems to me that S2 does not receive any fault signal in the
case of at the end of normal execution. What is the state of S2?
Is S2 considered normally completed and up for compensation? Or, is it
faulted?
If S2 is considered normally completed, that is very strange. A
scope with an open-unfinished receive and a missing reply should not be
considered completed by itself.
If S2 is considered faulted, a fault signal should be sent to
S2 first (which may in turn propagate to S1, as depicted above).
Say, if we go for this variant [b]:
<onEvent> scenario: S2 belongs to an <onEvent>
construct. Now, one of <onEvent> S2 instances throws this
missingReply fault directly to S1. What can S1 do? Perform the
compensation on S2? But, remember we can only do group compensation of
all S2 instances? Can someone justify why previous successfully
completed S2 need to be affected?
Also, you guys can also think of scopes as components in a business
process. Component S1 makes use of component S2. If component S2 forgot
to send a reply (or make any other system-level mistake e.g. forgot to
close some system resources), it should be S2 considered at fault or
faulted. If S2 can clean up its own mess, S2 should handle it first. If
S2 cannot handle, then it will be S1's turn to handle it.
I hope I point out concerns that I have on Variant [B] more precisely
this time.
I also hope Variant [A] seems a better solution to you guys now.
Thanks!
Regards,
Alex Yiu
Alex Yiu wrote:
Hi all,
I am OK with moving this part of discussion to Issue 221 also, because
I also want to pass Issue 123 resolution in a timely fashion and the
decision on where to throw missingReplying is taking some discussion
time than expected.
Thanks!
Regards,
Alex Yiu
Danny van der Rijn wrote:
I agree with Yuzo and Chris. I still think, though, that we should
save this discussion for 221. 123 only deals with messageExchange,
while 221 holds for any of the 3 values.
Chris Keller wrote:
I support
Yuzo’s position
completely. A missing reply at the completion of S2 should be thrown
to
S1. The fact that we are saying that the missingReply is thrown is
when
the partnerLink or messageExchange goes out of scope says it all. How
does it go out of scope, it goes out of scope because the scope
completed.
After completion we can’t come back into ourselves to catch a fault.
All
that said I’d rather not have this delay 123. I’m ok with leaving
the text as it stands in the 123 proposal and solving the question of
which
scope catches the fault in issue 221.
- Chris
From: Alex
Yiu [mailto:]
Sent: Monday,
December
12, 2005
11:50 PM
To: Yuzo Fujishima
Cc: Danny van der
Rijn; Chris
Keller; 'ws bpel tc'; Alex Yiu
Subject: Re:
[wsbpel]
Issue 123 -
Proposal for vote
Hi Yuzo,
Thanks for following this discussion thread. Really appreicate it ...
:-)
[a]
The main motivations behind "S2-to-S2" for missingReply fault are:
==> Finer-grain reply / compensation
If the
fault is thrown to S1 instead of S2, S1 cannot do anything to fix the
missing reply situation. Because, one of resources (partnerLink or
messageExchange) used in the open IMA is already out of scope (S2) and
S1 has no access those out-of-scope resources anymore.
The
missingReply situation may be caused by the situation where some other
faults happened and the enclosed scope (S2) does not handle those fault
properly. (Please see more analysis of
"programming-error-vs-runtime-error" below.) ... In the
<onEvent> or parallel <forEach> situation, assuming there
are 5 instances created and 4 of them got executed successfully without
any missingReply situation. But, one of them ran into this missingReply
situation. If we throw the fault to S1 and if we want to every
compensate the work of S2, all 5 of them need to be compensated. We
cannot compensate just the faulty instance by itself. On the other
hand, if we throw to S2, we can do a partial compensation in its fault
handler.
[b]
About "Reason 1" : "programming-error-vs-runtime-error"
consideration:
I agree with you this is more likely a programming error, not a true
runtime
error. That is why Dieter introduced this exit-on-standard-fault
facility to
allow people to stop process in case of this programming error.
With or without using this exit-on-standard-fault facility, as a
complete
specification, we need to specify snapshot state of the process - i.e.,
what
scopes are faulted and not, when the "exit" takes place.
If you think of the <onEvent> example described above, S1 should
not be
faulted, when the "exit" happens. Similarly, the previous 4 instance
of "S2" should not be faulted either. Only the true missingReply
scope should be faulted.
I also agree with you that people should not rely on catching this
missingReply
fault to do any complicated business logic. At most, people should just
log the
fault and attempt doing the partial compensation, as described above.
(This
applies to most of standard fault catchers).
I am just promoting scope-modularity. Definitely, not promoting any
weired
coding style. If you want, I think we can add a caution statement along
the
intention of the passed resolution of Issue 192 (Dieter's
exitOnStandardFault
proposal) to spec to state that: "If people do not set
exitOnStandardFault
to true, it is a good practice to restrict logic of any standard fault
fault
handler to some minimalistic repair work (e.g. error logging and
partial
compensation)."
Actually, the same wisdom applies to most of conventional programming
language.
E.g. in Java, if one got an IOException, SQLException, or Security
permission
related Exception, there are only few things that a Java application
programmer
can do / should do (similar to above).
[c]
About "Reason 2: When to judge if a reply is missing":
Yuzo wrote:
we should expect a fault
handler to send a reply.
Yes, I totally agree with you
here. :-)
Our intended behavior are actually very similar. :-) ... I guess the
related text in my previous email writeup is muddled. Sorry for the
not-organized enough text.
Let me add some clarification and refinement here:
==================================
There are two
checkpoints to decide whether a bpws:missingReply fault needs to be
thrown in a
scope (S2):
After the normal
execution of the scope (S2) is finished but before the scope is
considered completed and the fault handlers of the scope (S2) are
uninstalled, if no fault has been thrown to the scope (S2) during the
normal execution.
After the execution of
a fault handler of the scope (S2) is finished, if a fault has been
thrown to scope (S2) during the normal execution of the scope (S2), and
that fault triggers the fault handler of the scope (S2), and no fault
has been thrown by execution of the fault handler
The bpws:missingReply fault
thrown by missingReply
checkpoint logic in
the case #1 will be caught by a fault handler at the
scope (S2)
which matches bpws:missingReply fault, if there is one.
If the triggered fault handler (either
by bpws:missingReply fault or other kinds of fault) does
not
generate proper replies to
any open IMAs, a missingReply fault MUST be thrown. This is the case
#2 checkpoint
logic.
Important
Notes:
In case #2,
there is NO an infinite loop situation, because any faults triggered or
thrown by a fault handler (e.g. <rethrow> activity) will NOT be
caught by any fault handler of the same scope (S2) and they can only be
caught by its ancestor scopes (e.g. S1).
If a fault is thrown
either during the normal execution of the scope (S2), checkpoint logic
of case #1 will not be executed. That implies, if such a fault happens
and the scope (S2) does not have a matching fault handler, this fault
will be propagated to the parent scope (S1) and it will overshadow
overall missingReply logic.
If a fault is thrown
or rethrown during the execution of a fault handler of the scope (S2),
checkpoint logic of case #2 will not be executed. That implies, if such
a fault happens, this fault will overshadow missingReply logic of case
#2.
Here is a fragment of process definition that illustrates the
relationship of scope
S1 and S2 described above:
------------------
<scope
name="S1">
...
<scope
name="S2">
...
<partnerLinks>
<partnerLink name="pl1" ... />
</partnerLinks>
...
<sequence>
<receive partnerLink="pl1" ... />
<!-- receive without reply -->
</sequence>
</scope>
...
</scope>
------------------
Note: The bpws:missingReply fault is thrown by scope S2 to scope S2
itself.
==================================
[I highlighted the clarification changes in GREEN.]
To summarsize, we agree more than disagree. :-) ....
Case #2 is where the fault is thrown by S2 to S1.
I am merely adding one more differentiated checkpoint (case #1) to
allow the
fault thrown by S2 to S2 to achieve the semantics of scope-modularity
and
finer-grain repair and compensation. (IMHO, that is very important for
<onEvent> cases).
I hope I have explain my intended behavior better this time. :-)
More thoughts on this topic ... ? :-)
Thanks!
Regards,
Alex Yiu
Yuzo Fujishima wrote:
Alex,
Alex Yiu wrote:
Hi Danny and Chris,
Please see inline.
...
*Q1*: /have the process'
faultHandlers been
uninstalled?/
Answer: As I mentioned in the my previous email: (please see the
example)
http://lists.oasis-open.org/archives/wsbpel/200512/msg00050.html
I prefer the direction of which: the fault is thrown by scope S2 to
scope S2.
I am not a big fan of this "S2 to S2" semantics for
the following reasons:
Reason 1: Credibility of the use case.
I am not convinced that there are (many) cases where
scope S2 fails to reply without throwing a fault (except
missingReply). If S2 does, it should be a programming
error rather than a runtime error.
More likely are, in my opinion, the cases where
scope S2 fails to reply due to some fault.
For such cases,
S2 should define a fault handler to catch the fault and
send a reply there.
Not catching the cause fault and instead waiting for the
missingReply to occur doesn't seem to be a good programming
practice. The language should not promote that.
Reason 2: When to judge if a reply is missing.
A missingReply should be thrown only after the whole execution
of the scope S2, including the fault handlers, completed
without sending a reply. (As explained in the example for Reason 1,
we should expect a fault handler to send a reply.)
Then the natural destination of the missingReply fault should be
the parent of S2, i.e., S1.
Yuzo Fujishima
NEC Corporation
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