Re: entry ordering for blackboard-style communication?

Sam Preston wrote:
> 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?
>

Not that I'm aware 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.
>

Hmmm, okay so I can think of several different ways of doing this but
first let's get a better idea of the constraints you have (I could go
skim the paper but this is probably faster):

(1)     How timely does "delivery" of these events need to be from a
client's perspective?

(2)     How many different fields do you think you might be matching on?

(3)     How much variation are you expecting in the values of those fields?

(4)     Does a consumer have interest in particular producers or just some
types of event in general or both?

Dan.

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