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Subject: about ordering

Title: about ordering

I propose the following (partial) answer for Rel 57, see below,
and also after this, see my concern on the way we seem to tie the "ordering" vs "no ordering" decision
to the seq number value (section 9.5 in the meeting minutes)



Rel 57:

Proposal: Message ordering has actually two variants (o1) and (o2) that address different ordering requirts. Roughly:

The spec seems to endorse either o1 (section 2.4) or o2 (section 3.3.2). Instead, it should clearly define these two variants.

Definition 1: (message ordering, o1 above):
When "message ordering" is required, an  RMP implementation that is required to do "message ordering" , MUST NOT make any two received messages available to the application in a different order than their sending order.

Definition 2: (consecutive message ordering, o2 above):
When "consecutive message ordering" is required, an RMP implementation MUST enforce "message ordering", and in addition MUST NOT make a received message available to the application if its was sent after another message that is not yet available to the application.

Depending on which variant is used, o1 or o2, either one or the other of the current options (b)and (a) respectively
 in Rel 57 will apply.
The decision when to decide to "not wait anymore", remains to be decided (TimeToLive of the first unordered message? The min of TimeToLives of all unordered messages? Or a max unordered window size parameter, related to Rel 56.)

-------------------------------- meeting minutes 9.5:

"...The text is currently unclear on whether sequenceNO other than 0 can be used without ordering. The general consensus is no."

Issue: restricting the use of sequence numbers to the message ordering feature alone - and enforcing one message
per groupId otherwise, with seq nbr equal to 0 - will preclude implementations from using of a whole set of fast and space-efficient algorithms for duplicate elimination.

Proposal: Do not preclude using sequence numbers for other purpose than message ordering, at least until some conclusive results have been obtained on testing duplicate elimination algorithms. The spec should only require

that (groupId + seq.nbr) be always unique.
If the concern is about how to instruct an RMP what to do when it sees a sequence number in a message (should order? Should not?), that is another problem that could be solved separately.

If we resort to such tricks as using a different groupId for each message that does not require ordering, this amounts to enforcing duplicate check algorithms that are based on unique messageId strings. This will lead to more costly duplicate check (less scalable and less performant) and to the use of two different algorithms, if we still want to use sequence number-based duplicate check for ordered sequence that also need exactly once delivery.

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