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

[Doug Davis <dug@us.ibm.com>]

Lei,
  Yes that is true.  So there
are two thing here: 1) the proposal doesn't say why the RMS is closing
the sequence nor does it say what it will use the final ACK state for.
 This is very important.  It purposely does not get into that
area because, as others have stated, things like linking of sequence should
probably be done by some higher level processing, which is out of scope
(as of now anyway).  And  2) this problem exists for other areas
of the spec, not just this proposal.  Let's say the an RMS reaches
the MaxMessageNumber, as defined by the RMD.  The way to resolve this
is to create a new sequence to continue - well how can we guarantee the
ordering will be preserved across the sequences?  Same problem.  So
this proposal does not introduce a new problem, the problem is already
there.  However, that being said, and even though several people have
said that doing things like preserving the order across sequences is something
for a higher-level processing, I do personally believe that the spec should
provide an aide to that processing.  But I view that as a separate
issue and not part of this one.
thanks
-Doug

"Lei Jin" <ljin@bea.com>

08/30/2005 08:15 PM

To	<ws-rx@lists.oasis-open.org>
cc
Subject	RE: [ws-rx] i0019 - a formal proposal - take 2

Let's assume we are using an in-order
delivery assurance.  I am sending 10 messages from AS to AD, and after
some time, I decide to send a <close> for which I receive a final
ack with (1 - 5).
 
If I understand correctly, one
of the motivations for having a final ack is so that you know the accurate
state of all the received messages, so that you can decide what to resend
later in a new sequence.  So let's say we start up a new reliable
sequence and resends messages 6 - 10.  However, here is a problem.
 How do I preserve the in-order delivery assurance?  How do I
guarantee that message 6 will be delivered after message 5.  Note
that the final ack only says (1 - 5) is received, not delivered.  It's
perfectly reasonable if message 5 is not delivered by the time another
sequence is set up and message 6 arrives.  In that case, are we going
to have to worry about delivery assurances across multiple sequences?
 
Lei
 
 
 -----Original Message-----
From: Doug Davis [mailto:dug@us.ibm.com] 
Sent: Tuesday, August 30, 2005 12:08 PM
To: ws-rx@lists.oasis-open.org
Subject: RE: [ws-rx] i0019 - a formal proposal - take 2


Yet more comments. :-) 
-Doug

"Stefan Batres" <stefanba@microsoft.com>

08/30/2005 03:35 PM

To	Doug Davis/Raleigh/IBM@IBMUS, <ws-rx@lists.oasis-open.org>
cc
Subject	RE: [ws-rx] i0019 - a formal proposal - take 2

Doug, 
  
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? 

<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.
</dug>
 
Finally, I agree with you that considering a gap-filling mechanism would be a good thing for this TC to do.
 
 
--Stefan