Re: [wsn] Open issues concerning ordering and interleaving.

[Samuel Meder <meder@mcs.anl.gov>]

On Thu, 2005-01-13 at 11:55 -0500, David Hull wrote:
> 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.  

Are you saying we should make that guarantee in BaseNotification? I
don't think we should. You can for example easily have a scenario where
a event occurs on a topic but the policy is such that notification
messages are only sent at fixed intervals.

/Sam

> 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?
-- 
Sam Meder <meder@mcs.anl.gov>
The Globus Alliance - University of Chicago
630-252-1752