Messaging requirements: some ideas

[Alastair Green <alastair.green@choreology.com>]

Some points on messaging requirements that arise from the models work:

-1) We assume that party and counterparty have already bootstrapped a
carrier protocol selection, and that the process for achieving that
agreement is out of our scope.

0) We need a compound address where the first part is a fully valid
carrier protocol address, and the second part is an opaque suffix. The
requirement on implementers is that the first part is good enough to
allow contact with the Cohesion Manager (listener for cohesion protocol
messages) even if the suffix is duff.

1) Some carrier protocols support headers/enveloping/context
augmentation, and others do not. We need to be able to map (Header
(Header (Header (Payload))) to both types, which probably means some
generic "put it in an envelope inside the payload" facility for unaware
protocols.

2) We need to define a standard way of "boxcarring" messages, and how
this relates to context augmentation. For example: a one-shot service
would probably receive a message

    CONTEXT + app_request

and a reply would be sent

    app_response + ENROLL + VOTE

3) Very frequently the protocol messages will go in a request-response
or message-ack pattern. The messages may need to be numbered under some
circumstances (some messages can be duplicated, they may need to be
request-response paired in some cases).

4) I believe that the current protocol proposal, which uses a standard
2PC/presumed abort model, does not require reliable or guaranteed
message delivery, nor does it require exactly-once operation delivery
(although incidentally it will permit both to be created without need
for queuing mechanisms, as a by-product of the ability to create
transactional RPCs).

Alastair
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