RE: Message reliability

It was agreed to accept Roger's definition in today's discussion, so the
text has been modified to reflect the decision. 

Editors of the architecture document, please incorporate the text into the
document.

Hao


 2.3.1.13 Message reliability
2.3.1.13.1 Definition
Message reliability is the degree of certainty that a message will be
delivered
and that sender and recipient will both have the same understanding of
the delivery status.

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2.3.1.13.3 Explanation


The goal of message reliability is to both reduce the error frequency for
messaging and to provide sufficient information about the status of a
message delivery. Such information enables a participating agent to make
a compensating decision when errors or less than desired results occur.
High level correlation such "two-phase commit" is needed if more than two
agents are involved. Note that in a distributed system, it is theoretically
not possible to guarantee correct notification of delivery; however, in
practice, simple techniques can greatly increase the overall confidence
in the message delivery.

It is important to note that a guarantee of the delivery of
messages alone does not improve the overall reliability of a Web service due
to the "end-to-end argument."[1]  It may, however, improve the performance
of
messaging, and therefore, the overall performance of a Web service.

Message reliability may be realized with a combination of message receipt
acknowledgement and correlation. In the event that a message has not been 
properly received and acted upon, the sender may attempt a resend, or some
other compensating action at the application level.

[1]http://www.reed.com/Papers/EndtoEnd.html