Re: JavaSpace.notify() "not reliable"

Mark Brouwer wrote:
> Gregg Wonderly wrote:
>
>> Except for notify not working across firewalls, etc. I've just
>> written off the conveniences of listeners. They won't work in all
>
> Which I consider a pity as 'event driven programming' as you say is very
>  convenient and in case of the inverted event model (polling in the
> smart proxy) you really make use of the mobile code aspect opposed of
> people polling themselves (writing a decent polling mechanism is often
> more work as one believes it is in the first place).
>
>> situations without a polling based implementation (as Mark has done
>> in Seven/"The JSC"). The only working mode is to register the
>> listener (so that you can get immediate notifications on local
>> networks where latency will be expected to be minimal), then
>> make the query to see if there are any there (which covers both the
>> data race
>> in the server and the non-working call back on listeners case). Finally,
>> using a background timer based operation to rerun the query
>> periodically. I've actually pushed this into a recovery strategy for
>> failed notify() registrations as well.
>
> Due to the ease of implementing distributed events as part of your
> service with Seven (both Jini distributed events as the inverted JSC
> event model) I've used distribute events a lot (next to the ordinary
> query methods for bootstrapping etc.) but I always ended up with an
> 'alive' notification scheme. As part of the remote events to be received
> one should expect a NOOP remote event top indicating the server is alive
> and kicking if it has no actual remote event to send and also for the
> first remote event to validate whether callback in case of the Jini
> Distributed Event Model does work, if not one could decide to switch to
> the 'inverted' event model.
>

Hmmmmm as yet I'm not clear - what are these NOOP events intended to convey?

Is it a liveness test or simply an indication that probably no events
have been dropped or something else?

> I wonder whether such a 'I'm alive' notifications have ever been
> considered as part of the event protocol for JavaSpaces and Lookup
> Server. I think it is not too late to add these to the event protocols
> by making them optional and to enabled them through the usage of some
> invocation constraint. So far I always coded the 'I'm alive'
> notification scheme myself but this could even be shifted to the
> framework part, although that is totally irrelevant for the discussion
> whether such a scheme would be worth 'standardizing'.
> --
> Mark
>
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