Open issues concerning ordering and interleaving.

[David Hull <dmh@tibco.com>]

For simplicity, assume there are only two topics in the world, topic A and topic B, and that a topic expression may specify any combination of them.  Events are generated in a given order, and we should at least guarantee that, for a given topic, the NP submits events for delivery in the order in which they were generated.  The issues here concern whether we should have policy utterances to describe stronger guarantees.

Suppose that topic A carries notifications A1, A2 and A3, and similarly topic A carries notifications B1, B2 and B3.  At the very least, we guarantee that A1 will arrive before A2, etc.  The first issue is what to do if there is some inherent ordering across these event streams.  E.g., each carries a timestamp from an external reference clock.  In this case, A1 might be marked as occurring before (or after) event B1, and the "correct" order of notifications might be [A1, B1, B2, A2, A3, B3].  Should we provide a way of flagging these situations, or should we leave that as application dependent.

The second issue comes up when there is no externally-defined ordering across topics.  Should the NP impose one?  If two separate subscriptions both include topic A and topic B, should they see the same interleaving of messages?  This might seem automatic, but I can imagine a distributed implementation in which different physical processes handle different subscriptions and so may have different views of interleaving.

For that matter, under what circumstances is is the original requirement that notifications should be sent in order of their underlying events meaningful at all?