Interaction between reliable messaging and transport level

[mwsachs@us.ibm.com]

Although Reliable Messaging is intended to be neutral with regard to which
transport protocol is in use, "neutral" means that it can work with any of
them, not that we can disregard all of them.

I offer, for your consideration, the following thought about interaction
between transport ACKs and RM acks:

One expects that if a message is acknowledged at the transport level, it
will be delivered to the application. One can provide a committment to
deliver the message by placing it in persistent storage before sending the
ACK. Of course, it is up to the implementation design to ensure that the
message is actually delivered.  Now consider:

   If the reliable messaging function take place after the transport ACK is
   sent, the message may not be delivered if a problem arises in the RM
   group.  At best, the message will be seriously delayed because of the RM
   group latency and retries of other messages in the group (which I
   pointed out in my comments to the RM spec.).  One might also consider
   the possibility that the message which has been ACKed at the transport
   level then gets lost due to some other RM problem.

   If the reliable messaging processing takes place before the transport
   ACK is sent

   - We are now mixing together transport level and messaging level
   functions.

   - We are imposing significant delay before a synchronous transport-level
   ACK is sent, due to the normal RM group latency plus additional delays
   caused by retries of other messages in the group.

Regards,
Marty

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Martin W. Sachs
IBM T. J. Watson Research Center
P. O. B. 704
Yorktown Hts, NY 10598
914-784-7287;  IBM tie line 863-7287
Notes address:  Martin W Sachs/Watson/IBM
Internet address:  mwsachs @ us.ibm.com
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