entry ordering for blackboard-style communication?

Hi all,

I've been thinking of implementing a blackboard-style communication
system similar to:

http://graphics.stanford.edu/papers/eheap/

Basically, I'll have clients participating in the communication which
can read or write messages.  'Takes' won't occur very often, as
multiple 'consumer' clients may be interested in receiving an entry
that a 'producer' client writes.  When a consumer reads an entry,
though, what should be returned is the next matching entry (from any
client) that the client hasn't yet seen.

I'd like to build this on top of a JavaSpaces implementation if
possible.  First off, is there already a project like this that anyone
knows of?

The main problem I see is implementing the event ordering.  The
queueing patterns I've seen won't suffice for this, as they assume
that a consumer will want the next item in the queue, and I would need
the next 'matching' item in the queue.  I suppose I could take items
from the queue one at a time until I found a match, then replace the
non-matching entries, but this doesn't seem to be a very good solution
and would require a queue for each 'consumer' client.  Has anyone
worked on a similar problem?  I'm still pretty new to JavaSpaces and
blackboard-style communication in general, so I may be looking at this
all wrong.

Thanks,

-Sam

--
--------------------
J. Samuel Preston
Research Associate
Center for Advanced Engineering Environments
Old Dominion University Research Foundation

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