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Subject: Re: [sca-bpel] Issue 3 - An amended proposal - Correlation Disagreementbetween SCA and BPEL proposal



Danny,

If I read you correctly, you seem to be taking a view that the SCA conversation mechanism can never be in conflict with the
BPEL correlation mechanism.

Logically, the SCA conversation identifies a process instance ( a particular instance of the component implementing the
service).

So too, logically, the BPEL process data identifies a process instance.


Surely it is possible for these two mechanisms to be in conflict?  ie SCA identifies process A and BPEL correlation identifies
process B.

Are you arguing that this cannot/will not happen?  Or are you saying that if there is ever conflict, the BPEL mechanism always
wins out?



Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431  
Email:  mike_edwards@uk.ibm.com



Danny van der Rijn <dannyv@tibco.com>

29/11/2007 23:46

To
Alex Yiu <alex.yiu@oracle.com>
cc
Michael Rowley <mrowley@bea.com>, Dieter Koenig1 <dieterkoenig@de.ibm.com>, Mike Edwards/UK/IBM@IBMGB, Najeeb Andrabi <nandrabi@tibco.com>, sca-bpel@lists.oasis-open.org
Subject
Re: [sca-bpel] Issue 3 - An amended proposal - Correlation Disagreement between SCA and BPEL proposal





Apologies for wading into this conversation late, and with such a long note, but...

I am somewhat troubled by the classification of the correlation data as some kind of ID that identifies the process instance.  The point of BPEL correlations is to get away from that style of usage, and, instead, use business data in the message payload.  The client should have no knowledge of how the services are constructed, whether the services are bound to one process, multiple processes, one engine, multiple engines, etc.  WSDL messages carry business data *only* and don't concern themselves with implementation details like process-ids, etc. whether in the header, or not.  See
http://www.oasis-open.org/committees/download.php/3249/Original%20Design%20Goals%20for%20the%20BPEL4WS%20Specification.doc, especially

Properties and Correlation

A running business process is an instance of a particular process model.  Thus, each running business process requires at least one unique identifier to be able to pass an incoming message to the correct instance of a process model.  Since running business processes are first class business artifacts, they are in practice identified in business terms.  Different participants of a particular process may want to identify the process in their own terms and they may even want to change the identifier used in the course of their interaction with the process.  We therefore came to the conclusion that defining artificial instance identifiers for processes is not the best solution.  Instead, we added a flexible mechanism to BPEL4WS that supports multiple user-defined identifiers for a process instance: Correlation via message properties. In particular, these message properties are embedded in the application messages (i.e., WSDL abstract messages) exchanged by the process and are not defined as header fields of the communication protocol.  This results in a binding independent correlation scheme.

Overall Goal 6: BPEL4WS should support an identification mechanism for process instances that allows the definition of instance identifiers at the application message level.  Instance identifiers should be partner defined and may change over time.



Of course, I understand that many implementations will violate this intent, and for good reasons, all.  However, I don't think that the combination of SCA and WS-BPEL is enough to warrant standardizing such violation.

OK, so what does all of this theoretical blather mean with respect to the current proposal (which I'm not sure I can actually piece together, by the way)?

There are only 2 dead-letter cases that I think we can standardize:

1) The protocol times out
2) Possibly in cases where a bpel:CorrelationViolation is thrown internally

My analyses:

1) If the protocol times out, well, then it's too late to send an sca exception, isn't it :-)  A wise implementation may choose to preempt the timeout and issue an exception on its own.  A case that I think that the proposed use of an sca exception is reasonable.

2) If a message is determined to be destined for a particular process (i.e. transport-level correlation via SCA, WS-Conversation, etc) but when the message is delivered to the process, the process rejects it for that process (and a bpel:CorrelationViolation is thrown).  Some engines may hang on to that message and try to deliver it to another IMA in that or other process instances, current or future.  Some engines may not.  In an engine that doesn't hang on to it, it's irrelevant whether a matching IMA will ever exist - even if it does, the engine will never try to deliver it.  In this case, the throwing of an sca exception as proposed, makes sense to me.

If we choose these 2 axes to consider, then they should probably be separate exceptions.

Evaluating this thought process against the ABC's of the thread:

(a) Process instance does not exist. e.g. the UUID-like "conversation-id" does not match any active process instance.

I posit that this is not a valid, spanning case to be talking about.  Correlation is not meant (solely) for instances, but for business data that is independent of process.

(b) Process instance exists (e.g. the "conversation-id" matches with an active process instance). However, there will NEVER be any potential matching IMA (e.g. the message is targetting operation "cancelPO" and the process has no IMA that match the operation at all. Or there is only potentially IMA and it is already enabled with a wrong CS value).

Again, this is process-identity based, and is therefore not suitably wide-enough scope to consider.

(c) A potential matching receive may be enabled in future, but not enabled in a "timely fashion".

This is my (1) above.

Thanks and condolences for reading this far,
Danny

Alex Yiu wrote:


Hi, Michael,

[1]

I think I am sold on why you want to view them differently. :-)

The reason that I agree with you may be slightly different than your own wordings. According to you, the main reason is who is causing this problem - the message sender? or process itself? The distinction may not be that clear cut in that aspect. [e.g. case (b) can be caused a mistake in process definition as well]

IMHO, the key differences what factors are involved in determining whether we hit one of these problematic cases.

For the cases of (a) and (b), the SCA+BPEL infrastructure can detect deterministically by introspecting the state of the process instance and the process definition (alone).

On the other hand, case (c) are problems situation that the infrastructure cannot detect deterministically by introspecting the state of the process instance and the process definition alone. The infrastructure can safely classify the situation as problems based on other pre-arranged heuristic or protocol (e.g. timeout values).

(Side Note: for case (c): the BPEL process definition itself may be correct. It may fail to enable an IMA due to another external error the process cannot control.)

I believe the consensus so far is: case (c) is not something we try to cover in SCA-BPEL spec. We will talk about case (a) and (b) here.

I think I will re-visit the wordings of the proposal by using the Michael's "never" wording plus variants of the underlined wording above.

[2]

May a SCA standard fault be reused and thrown in other situation not covered by the spec?
E.g. May "sca:DeadLetterMessageError" fault be thrown in other situations, other than case (a) and (b)?

Should the spec be silent on this topic? Or, we should explicitly allow or disallow such a usage?


Thanks!



Regards,
Alex

Michael Rowley wrote:

 
In my opinion, the important difference between (a), (b) and (c) is that, for (a) and (b) it is likely that the client did something wrong.  However, I expect that for timeout (case c), the error either exists in the infrastructure (something is down) or, quite likely, the BPEL process was designed incorrectly such that it has a potential deadlock.
 
This is why I think there should be two different faults.  I think they would be treated differently, at to the extent that _any_ faults are treated differently from any other.
 
Michael
 




From: Alex Yiu [mailto:alex.yiu@oracle.com]
Sent:
Friday, November 16, 2007 3:26 PM
To:
Dieter Koenig1
Cc:
Michael Rowley; Mike Edwards; Najeeb Andrabi;
sca-bpel@lists.oasis-open.org; ALEX.YIU@oracle.com
Subject:
Re: [sca-bpel] Issue 3 - An amended proposal - Correlation Disagreement between SCA and BPEL proposal

 

Hi, Mike and Dieter,

Thanks for the follow up discussion emails.

It looks like there are 3 cases listed on the table in the previous emails sent by Mike and Dieter.
(a) Process instance does not exist. e.g. the UUID-like "conversation-id" does not match any active process instance.
(b) Process instance exists (e.g. the "conversation-id" matches with an active process instance). However, there will NEVER be any potential matching IMA (e.g. the message is targetting operation "cancelPO" and the process has no IMA that match the operation at all. Or there is only potentially IMA and it is already enabled with a wrong CS value).
(c) A potential matching receive may be enabled in future, but not enabled in a "timely fashion".

If I read Michael's email correctly, his two cases are (a)+(b) together as one case and (c) as the other case. On the other hand, Dieter's two cases are (a) and (b).

IMHO, (a) and (b) are definitely close enough to be treated together, as both of them do not involve "timely fashion" concept.

Few additional points to consider:

        When a UUID-like conversation is used (Oracle's product has similar feature in the form of WS-Addressing for a while - pre-dated SCA), most users would not use BPEL-CS as an additional correlation mechanism anymore. That implies frequency of case (b) is actually very small compared with (a) and (c).  
        Even though the underlying causes for cases (a), (b) and (c) are different, the behavior observable by the message sender is exactly the same in these 3 cases. Do we want to define another fault for user to catch differently? That is a usability concern.
o        My thought is: case (c) is not applied, if there is no "timeout" value specified. For example, if someone use plain simple SOAP without any special protocol related headers, case (c) would not be applied.
        If we decide to have a different fault for case (c),
o        Do we want to define this separate fault in SCA spec?
o        In that case, the "sca:DeadLetterMessageFault" will be mainly for case (a) in real life usage.
        In BPEL spec, there was a formal discussion on whether user-code can throw a standard fault defined by BPEL spec (e.g. "bpel:selectionFailure"). I guess similar questions are applied here: whether "sca:DeadLetterMessageFault" can be re-used outside the scenario described by the SCA spec.

On the topic: "who specifies timeout and what timeout value"
I am not proposing we should define a detailed timeout mechanism in SCA-* spec, because it may depend on the actual underlying protocol. If a protocol with a business-conversation timeout mechanism exists, my gut feeling is that the sender should specify the value. The actual time out will vary depending on the nature of business conversation.


Thanks!


Regards,
Alex Yiu


Dieter Koenig1 wrote:

Mike, my preference goes into a similar direction.
 
(1) Indicating that no existing process instance can be correlated with a
request - note that this may not always be because of a faulty correlation
set; it may also happen if a branch of the process navigates to an <exit>
activity such that a <receive> on another branch is effectively cancelled -
this would not be a problem caused by the client.
 
(2) Indicating that an existing process instance that has been correlated
with a request can never consume the request - of course only if this
situation can really be detected - I agree that any good timeout value can
be wrong in another scenario.
 
Kind Regards
DK
 
 
 
                                                                                                                                 
  From:       "Michael Rowley" <mrowley@bea.com>                                                                                
                                                                                                                                 
  To:         "Alex Yiu" <alex.yiu@oracle.com>                                                                                  
                                                                                                                                 
  Cc:         Dieter Koenig1/Germany/IBM@IBMDE, "Mike Edwards" <mike_edwards@uk.ibm.com>, "Najeeb Andrabi" <nandrabi@tibco.com>,
              <sca-bpel@lists.oasis-open.org>                                                                                    
                                                                                                                                 
  Date:       16.11.2007 16:22                                                                                                  
                                                                                                                                 
  Subject:    RE: [sca-bpel] Issue 3 - An amended proposal - Correlation Disagreement between SCA and BPEL proposal              
                                                                                                                                 
 
 
 
 
 
 
My first inclination would be to have two different faults.  One fault that
says we know, logically, that you have sent a faulty correlation set.  A
different fault would be used to report that your seemingly valid
correlation set hasn’t been matched by some designated timeout period
(although I’m not sure which side should define that timeout).
 
If others feel strongly that these two error conditions should be combined
into one fault, I would be OK with that, but my preference would be to keep
them separate.  I’m also not sure that I would bother standardizing on the
timeout fault, since it may be appropriate for it to be different for
different bindings.  Essentially, this is an argument to keep it as
“never”.
 
Michael
 
 
From: Alex Yiu [mailto:alex.yiu@oracle.com]
Sent: Wednesday, November 14, 2007 4:21 PM
To: Michael Rowley
Cc: Dieter Koenig1; Mike Edwards; Najeeb Andrabi;
sca-bpel@lists.oasis-open.org; ALEX.YIU@oracle.com
Subject: Re: [sca-bpel] Issue 3 - An amended proposal - Correlation
Disagreement between SCA and BPEL proposal
 
 
Hi Michael,
 
I understand your concern.
 
Wording versions:
Version 1: "will never be matched"
Version 2: "will not be matched"
Version 3: "can not be matched"
 
I think the current version 3 has the least interpretation space for the
engine to queue up messages, because it loses the time dimension semantic.
 
Differences on whether to use the term "never":
Consider the case that: (one of the bullet in the proposed non-normative
text)
        matching IMAs blocked by other activities within a sequence
Say, a matching IMA is blocked by another preceding receive. Or, a matching
IMA is blocked by an ill-defined preceding loop.
 
If we pick the term "never" in version 1, then using this standard fault to
notify the message sender won't be covered by the if-condition. Because,
the engine cannot logically conclude the external message for the blocking
receive will never arrive or the engine cannot logically conclude the
ill-defined loop will never finish.
 
My intention here is: the engine may notify the sender of this error
condition, if a matching IMA cannot be found in a timely manner.
 
E.g. if a section of a process typically takes only few minutes or hours,
after days or weeks of waiting for the blocking receive / loop, and, if the
engine decides to notify the sender of this error condition by this new
fault, it should be covered by the spec.
 
So, I prefer wordings of version 2 or a new version wording (modified based
on version 3):
Version 4: "can not be matched ... in a timely manner"
 
Of course, the exact details of "timely manner" will depend on the
underlying protocol (transport and coordination) and etc.
 
For example, if:
        the operation is request-response
        HTTP is the transport protocol (say the HTTP timeout is: 360
seconds)
        an IMA cannot be matched within the transport timeout period
        but a potential matching IMA is pending but not activated yet.
If we use the term "never", then this new standard fault does seem to be
applied and the existing transport level time out fault will be applied
instead. If we use wordings of version 2 or version 4, then this standard
fault can be applied and covered by the spec.
 
Further thoughts and comments?
 
Thanks!
 
 
Regards,
Alex Yiu
 
 
 
Michael Rowley wrote:
The reason I had included the word "never" was precisely for this
reason.  I didn't want it to read that there is no IMA active right now.
I wanted to say that the system determined that there never would be
one.  There certainly are times when that can occur, such as when you
are using a combination of engine-managed correlation and explicit
correlation sets.
 
If we don't say "never", then I'm not sure how we would word this to
allow for queuing up messages until an IMA becomes active.
 
Michael
 
-----Original Message-----
From: Dieter Koenig1 [mailto:dieterkoenig@de.ibm.com]
Sent: Tuesday, November 06, 2007 4:31 AM
To: Mike Edwards
Cc: alex.yiu@oracle.com; Najeeb Andrabi; sca-bpel@lists.oasis-open.org
Subject: Re: [sca-bpel] Issue 3 - An amended proposal - Correlation
Disagreement between SCA and BPEL proposal
 
Alex, Mike E, Mike R, Najeeb,
 
I assume we agree that we want to allow a BPEL implementation to store a
message if it can be consumed by a process instance at a later point in
time. Do we want to make this explicit in the normative language? (Of
course, it cannot be decided in many cases, and additional
considerations
apply - e.g., it makes a lot of sense for one-way messages and it makes
less sense for request-response operations called via synchronous
bindings
[<- btw, this is another scenario for issue BPEL-12]).
 
For those cases where we finally decide to return an
"sca:DeadLetterMessageError" fault to the sender, I have additional
questions:
 - in case of request-response operations, would this fault manifest
itself
as an SCA ServiceRuntimeException?
 - in case of one-way operations, will SCA (the assembly spec?) define
the
coordination protocol messages?
 
Kind Regards
DK
 
 
 
 
 
  From:       Mike Edwards <mike_edwards@uk.ibm.com>
 
 
 
  To:         sca-bpel@lists.oasis-open.org
 
 
 
  Cc:         Najeeb Andrabi <nandrabi@tibco.com>, alex.yiu@oracle.com
 
 
 
  Date:       06.11.2007 09:46
 
 
 
  Subject:    Re: [sca-bpel] Issue 3 - An amended proposal - Correlation
Disagreement between SCA and BPEL
              proposal
 
 
 
 
 
 
 
 
 
Alex,
 
I agree with your point about "never", but I don't like your additional
words in parentheses at all.  The extra qualifiication just complicates
things needlessly
- let us leave it to the SCA & BPEL infrastructure to work out whether
matching can be done.  I changed "will" to "can" as well, to get rid of
this flavour of
"future-ness" in the wording which is not appropriate, since this
statement
is about a state and what to do when this state is detected.
 
How about:
 
"If the SCA or BPEL infrastructure is able to determine that a message,
that has been sent to an endpoint address of a business process, can not
be
matched with a corresponding inbound message activity (i.e. receive,
onMessage or onEvent), then:"
 
Yours,  Mike.
 
Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431
Email:  mike_edwards@uk.ibm.com
 
 
 
 Alex Yiu
 
 <alex.yiu@oracle.
 
 com>
 
 
To
                          Michael Rowley <mrowley@bea.com>
 
 05/11/2007 19:23
cc
                          Najeeb Andrabi <nandrabi@tibco.com>,
 
                          sca-bpel@lists.oasis-open.org,
 
                          ALEX.YIU@oracle.com
 
 
Subject
                          Re: [sca-bpel] Issue 3 - An amended proposal -
 
                          Correlation Disagreement between SCA and BPEL
 
                          proposal
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Hi, Michael,
 
Thanks for the quick feedback.
I agree with you that the "if" condition in the normative text can be
tightened up. I like your changes in general.
 
I am wondering whether the word "never" may be a bit too strong. I am
thinking to use the word with a weaker tone with extra qualification
description.
 
How about:
"If the SCA or BPEL infrastructure is able to determine that a message,
that has been sent to an endpoint address of a business process, will
not
be matched with a corresponding inbound message activity (i.e. receive,
onMessage or onEvent) (possibly based on a message protocol or some
infrastructure heuristic), then:"
 
 
If the extra qualification description "(possibly based on a message
protocol or some infrastructure heuristic)" opens new cans of worms, I
am
comfortable to drop it.
 
 
Thanks!
 
 
Regards,
Alex Yiu
 
 
Michael Rowley wrote:
Alex,
 
Thanks for the background material.  I also the proposed normative text,
except for the reference to "a dead letter message", without having a
definition for that in our spec.  I also thought that "message receiver"
might be too vague.  Perhaps the first sentence, which you had as:
 
"If the SCA and BPEL infrastructure of the message receiver is able to
detect a dead letter message:"
 
could instead read:
 
"If the SCA or BPEL infrastructure is able to determine that a message
that
has been sent to an endpoint address of a business process will never
match
a corresponding inbound message activity (i.e. receive, onMessage or
onEvent), then:
 
Michael
 
 
 
 
 
From: Alex Yiu [mailto:alex.yiu@oracle.com]
Sent: Thursday, November 01, 2007 12:34 PM
To: Najeeb Andrabi
Cc: sca-bpel@lists.oasis-open.org; ALEX.YIU@oracle.com
Subject: Re: [sca-bpel] Issue 3 - An amended proposal - Correlation
Disagreement between SCA and BPEL proposal
 
 
Hi all,
 
Here is an amended proposal for this Issue 3.
 
I believe the spirit of the proposal is the same. But, wordings and
details
are different. This amended proposal avoids/addresses a few issues that
the
original proposal has:
*         "bpel:correlationViolation" is for BPEL's CorrelationSet
lifecycle violation - not CS mismatch. I am not sure we want to overload
it. All existing "bpel:*" fault thrown are for internally consumption
only.
Not directly visible through BPEL partnerLink.
*         There is no formal notion of "wrapping" in WS fault.
*         We need to address the difference in one-way or
request-response
MEP.
*         The flow chart has "wait until all the IMA in all existing
processes have been activated". That seems to be not so feasible /
well-fit.
The proposal has two parts: background (non-normative) text and
normative
text.
 
If the background text is too long for some audience, I am happy to trim
and para-phrase it. I send this longer version for the benefit for the
TC
discussion.
 
About the normative text, I hope it is short yet concise.
One thing to highlight is: it may be infeasible to specify a universal
dead
letter detection algorithm, which has real values to users. Hence, I am
using SHOULD and MAY here, instead of MUST.
 
------------------------------------
[background text begins here ...]
When an inbound message comes into the SCA and BPEL infrastructure, such
a
message is normally consumed by a matching inbound message activity
(IMA)(e.g. a <receive> activity). However, due to process model error or
runtime message data error, there is no matching IMA at all or a
matching
IMA is not enabled within the expected time limit of the
(system/business
level) protocol between the message sender and receiver. This kind of
messages, which do not have a matching IMA, are termed as "dead message
messages"
 
Examples of process model error are:
*         matching IMAs are skipped by faults
*         matching IMAs blocked by other activities within a sequence or
an
impossible-to-fulfill control link transition condition.
*         IMAs cannot receive message due to incorrect usage of message
correlation mechanism, including BPEL correlation set and SCA
conversational interface
Examples of runtime message data error are similar to above, as the
above
error are not inside the process definition itself but caused by
incorrect
data values.
 
There might not be a universal way to determine a message is truly a
"dead
letter message" without any additional protocol between message senders
and
receivers. Consider the following example, an message is dispatched to a
BPEL process instance by SCA conversational mechanism. At the moment
when
the message is matched with the BPEL process instance, there might be no
<receive> activity enabled for the matching partnerLink and operation at
all, or there is a <receive> activity enabled for the matching
partnerLink
and operation but with a mismatched correlation set. Some users might
think
this is certainly a dead letter message situation. However, a matching
IMA
may be enabled minutes, hours, or days later, as the matching IMA might
be
blocked in the process model.
 
On the other hand, there might be some cases that the BPEL
infrastructure
can determine there will never be a matching IMA enable in future. And,
some advanced features in BPEL infrastructure (e.g. process instance
repair
or process definition repair) might make the detection of "dead letter
message" cases more difficult. However, with some additional
system-level
protocol coordination between the message sender and receiver, it might
make detection easier.
------------------------------------
 
------------------------------------
[normative text begins here ...]
If the SCA and BPEL infrastructure of the message receiver is able to
detect a dead letter message:
*         If the message is sent through a request-response operation,
"sca:DeadLetterMessageError" fault SHOULD be thrown to the message
sender
*         If the message is sent through a one-way operation and
additional
system-level protocol is  used between the message sender and receiver,
the
dead letter message error situation MAY be notified to the message
sender,
according to the protocol used.
------------------------------------
 
 
Looking forward to further comments and fine-tuning.
 
Thanks!
 
 
 
Regards,
Alex Yiu
 
 
 
 
 
 
 
 
 
 
 
 
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