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Subject: Re: [ebxml-cppa] RE: [ebxml-msg] Comments on ebMS_v1.04c

   Date: Mon, 15 Oct 2001 15:34:48 -0500
   From: David Fischer <david@drummondgroup.com>

   Reliable Messaging has to do with retries/acks, not with duplicate elimination.

While we can define the term "reliable messaging" to mean whatever we
want it to mean, I think it would be rather weird to use the name
"reliable messaging" for a facility that sometimes delivers
duplicates.  It seems to me that "reliable messaging" should mean, at
the very least, that a message is delivered "once and only once".

   RM could actually work w/o duplicate elimination -- but who would want to do

I don't think there's any such thing as "reliable message without
duplicate elimination".  But retry/ack without duplicate elimination
is meaningful.  In the computer science literature it's known as "at
least once" semantics.  It's useful in cases where

 -- not getting the message at all is BAD
 -- getting the message more than once is OK

If the service being provided is both inexpensive and idempotent, "at
least once" does a good job.

Why not just always use "once and only once"?  Because duplicate
elimination costs something, and there's no reason to pay that
cost if you don't need it.

For example, party A wants to keep party B up-to-date about how many
washing machines party A has in his inventory.  So whenever party A's
inventory level of washing machines changes from X to Y, he sends
party B a message saying "I now have Y washing machines".  If the
message gets through more than once, nothing is harmed.  It simply
becomes a matter of performance: if B's cost to process such a message
is low compared with the cost of duplicate elimination in the message
layer, then "at least once" performs better overall than "once and
only once".

-- Dan

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