persisting external resources

[raft]

just thinking aloud.. maybe someone can suggest a simpler system..

our prevelant system (call it game) will use a kind of external engine
for some complicated calculations. the calculations are expensive, so
we want to cache the results if possible. calculations arent
instantaneous but spread over time. results are retrieved via a
callback interface. the time results arrive is important (changes
game's state)

 to make things simpler, a transparent proxy (implementing calculation
engine's interface) will be placed in front of calculation engine and
will act as the cache. proxy will have its own prevayler to persist
previous results

1. game asks proxy to calculate something
2. proxy checks its cache if it already has the results. if so proceed to step 6
3. otherwise, proxy asks calculation engine for calculation and
registers its listener
4. when results came via listener, proxy persists them with a
prevayler transaction and forwards results to game's listener.
5. sequence ends
6. proxy retrieves results from cache, schedules some events to game's
event queue for the time of results' arrival. when that events are
executed, results are forwarded  to game's listener. (remember, time
of results are important)
7. sequence ends

cases:
a. in a regular run, all will be ok: 1,2,3,4,5
b. during recovery, if calculation isnt requested yet, again all will
be ok: 1,2,6,7
c. calculation is requested and a snapshot is taken after that but
before results arrive. here we have problems:

* game expects results but nobody is aware of situation: we can do
some hacking here, for example, implement readObject method in game,
so when it is deserialized from a snapshot, it can re-request
calculations. alternatively after cache recovery, it may re-request
result waiting calculations. may lead to some complications but say
that are ok for now.
* results arrival time is also a problem here. hours or even days may
be past after last snapshot, and we need to insert results at a proper
time at game's event queue. again may lead to some complications

regards,
r a f t

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