Steve Graham wrote:
Hi David:
David Hull <dmh@tibco.com> wrote on
01/13/2005
11:55:19 AM:
> 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.
Is there indeed such a guarantee? How can we
make this guarantee, independent of
certain assumptions of QOS in the transport?
Sorry, I should have said "be submitted for delivery" instead of
"arrive".
> 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.
Or, perhaps a simpler, monotonically increasing
sequence
number on the notificaiton
messages?
> 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.
My choice would be to leave this as application
dependent.
There may be some naturally
occurring sequence number (CBE, for example, has
naturally
occuring creation time attribute).
So in some cases the NP would be obligated to submit in sequence order,
and in some cases not? Would this be advertised?
>
> The second issue comes up when there is no externally-defined
ordering
across
> topics. Should the NP impose one?
IMO, no. Again, leave the application to define
this. CBE will have timestamp, I am
not keen on making the WSN specs more complicated.
Again, I think the only WSN issue is should the NP be able to advertise
that this sort of thing is going on, and if so, how.
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