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Subject: RE: [ws-rx] i0019 - a formal proposal - take 2

Jaques, you said:

The general use case is the one where gaps exist and persist and for a variety of reason, were not / could not be filled at the time the sequence is no longer to be used (for whatever reason) and needs to be disposed of.


In following the many proposals being made here it continued to strike me that what you are looking for is a way to fill gaps so it is nice to see that confirmed. Couldn’t filling gaps in a sequence be done in a much simpler manner than the current proposal?


From: Jacques Durand [mailto:JDurand@us.fujitsu.com]
Sent: Wednesday, August 31, 2005 1:24 PM
To: Stefan Batres; Doug Davis; ws-rx@lists.oasis-open.org
Subject: RE: [ws-rx] i0019 - a formal proposal - take 2


2 comments Inline <JD>


From: Stefan Batres [mailto:stefanba@microsoft.com]
Sent: Tuesday, August 30, 2005 10:48 PM
To: Doug Davis; ws-rx@lists.oasis-open.org
Subject: RE: [ws-rx] i0019 - a formal proposal - take 2




You mention a specific situation: An RMD experiences a failure that prevents it from receiving application messages. I agree in so far as saying that in such a failure case this proposal could be helpful in that it helps the RMS to engage in recovery of some sort (either inform applications that a specific message was not sent or open a new sequence, assuming ordering is not important). But this is not the only failure case that applications will want to deal with (with or without help from the protocol).

Consider the case where connectivity is lost for long enough for both sequences to expire or consider the case where the destination suffers a loss of session state. In such failure modes this solution is not helpful - yet applications will need a recovery strategy of some sort. It might be that it is application specific, or it might be that a general failure recovery specification is created and ratified at some point. The important idea is that the only way to deal with all failure modes is at higher level. This proposal leverages the protocol to optimize recovery in specific circumstances that should be relatively rare. RM implementations should not be required to support failure mode recovery mechanisms that either don't apply to them or that they choose to implement in a uniform way at a higher level.


<JD> I do not see better recovery as the main driver behind resolving i019 - though enhanced recovery can certainly be a byproduct of it, yet in no different way than say the recovery made possible by the mechanisms behind AtLeastOnce DA  ("... or else an error will be raised on at least one endpoint"). Such error-raising is serving a purpose, whatever usage is made of these "errors",  (and indeed in many cases they require application-level  handling as you said - sometimes also just application awareness may have great value). But just because of this, we want errors to be raised as accurately as possible. I believe the proposal for i019 allows for achieving greater awareness of delivery failure on RMS / AS side at no greater cost, and that applies not just to I019 but to i028 as well, where the sequence is not faulted. The general use case is the one where gaps exist and persist and for a variety of reason, were not / could not be filled at the time the sequence is no longer to be used (for whatever reason) and needs to be disposed of.







From: Doug Davis [mailto:dug@us.ibm.com]
Sent: Tuesday, August 30, 2005 1:08 PM
To: ws-rx@lists.oasis-open.org
Subject: RE: [ws-rx] i0019 - a formal proposal - take 2


Yet more comments. :-)

"Stefan Batres" <stefanba@microsoft.com>

08/30/2005 03:35 PM


Doug Davis/Raleigh/IBM@IBMUS, <ws-rx@lists.oasis-open.org>




RE: [ws-rx] i0019 - a formal proposal - take 2




Some more comments and thoughts on your proposal:
<dug>... When or why an RMS uses CloseSequence is up to it to decide.
All we know is that it wants to shut things down and get an accurate ACK from the RMD.</dug>
I still have not heard of a plausible reason why an RMS "wants to shut things down" and the current spec presents a problem. Comparing the spec as it stands today vs. the spec + this proposal:

  • TODAY: RMS wants to end the sequence so it sends a LastMessage and must wait for a complete set of acks; this might require retransmitting messages. Once a full set of acks is received RMS sends TerminateSequence.


  • TODAY + THIS PROPOSAL: RMS wants to end the sequence so it sends Close, waits for a CloseResponse, possibly retransmitting the Close. Once a CloseResponse is received RMS sends TerminateSequence.

The problem with the TODAY scenario, as I've heard it in this forum, is that the RMS might have to wait unacceptably long between sending LastMessage and getting a full ack range. But if getting some messages or acks across proves difficult; why would the RMS expect that getting Close across would be any easier?

<JD> Some messages may not have made it to RMD for various reasons that do not necessarily apply to the Close op. You may also have the option of resending the Close op in a way (say over 24h) that you could not afford to do on large scale via a policy that has to apply to all regular messages, due to network bandwidth or due to the time-bound value of these messages (message may loose value if untimely - yet RMS and AS  want to be sure which ones were lost) . So even a delayed closing still have value for accuracy of acknowledgements.

<dug> 1 - I don't believe your text is accurate in that Close is supposed to be used in cases where the sequence needs to end due to something going wrong.  You've described a case where the sequence is functioning just fine - and while Close can be used in those cases as well, it provides no additional value.  2- Sending a Close and sending application data can have quite a different set of features executed so I don't think its hard to imagine cases where RM messages can get processed just fine but application messages run into problems.  I believe Chris mentioned on some call the notion of two different persistent stores - one for RM data and one for app-data.  Its possible that the app-data one is running into problems.  3 - Using the CloseSequence operation is option - if you feel that, as an RMS implementor, you'll never see its usefulness then you're free to never implement/send it.  However, I'd hate remove this option for those of us who do see value in it.  </dug>

<dug>The case that I keep thinking about is one where the RMD is actually a cluster of machines and when a sequence gets created it has an affinity to a certain server in the cluster - meaning it processes all of the messages for that sequence. If that server starts to have problems, and for some reason it just can't seem to process any new app messages then the RMS can close down the sequence and start up a new one. Hopefully, the new sequence will be directed to a different server in the cluster. </dug>
There are two problems with this scenario and the proposed solution.
1.      If an RMD has sequence-to-machine affinity that should be strictly the RMDs decision and the RMDs problem. The RMS is autonomous; this proposal puts expectations on the RMS' behavior based on particularities of the RMD implementation. To be clear, I'll note that affinity can be achieved in two ways:
                                                        i.            By performing stateful routing at the RMD; basically the RMD has to remember every active sequence and what machine it has affinity to. In this case it would be simple to change the RMD's routing table when a machine fails.
                                                       ii.            By generating different EPR's for each machine. For affinity to function this way two things are necessary:
1.      Some sort of endpoint resolution mechanism would have to be devised for the RMS to learn the EPR that it should target.
2.      A mechanism for migrating that EPR.

Clearly 1) and 2) are outside the scope of the TC and, in my view, this proposal might be defining 2) in an informal way that is specific to WS-RM.

2.      If the RMS somehow guesses that there is a problem on the EPR to which it is sending its messages and somehow decides that Closing the sequence and starting a new one is the right course of action, ordering guarantees are compromised.

<dug> I probably didn't state the problem very well.  I didn't intend to claim that the RMS knew about this affinity, but instead it knew that something was wrong with the current sequence and in order to try to fix the situation it decided to try another sequence.  The affinity bit was thrown in there to explain why starting a new sequence _might_ fix the problem.

I should also point out that while a lot of these discussions have focused on InOrder+ExactlyOnce DA, this feature is still useful in other DAs.  For example, if the DA is just ExactlyOnce - having an accurate accounting of the ACKs allows a subsequent sequence to send just the gaps from the first, so getting an accurate list of the gaps becomes critical.  And this of course leads us to the discussion of how to determine the DA in use - which I think might be part of issues 6, 9, 24 and 27.
Finally, I agree with you that considering a gap-filling mechanism would be a good thing for this TC to do.


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