Hi,
Thanks for your response. Let me (try to) clarify my questions.
I guess I oversimplified my example so it, so I'll try again. I have an
actor (persistent with at-least-once-delivery), and I want it to process
messages "immediately" upon arrival. That is, I don't want to wait for the
persistence handlers to fire before updating my state, delivering messages
downstream, etc.. I would also like to be able to recover from crashes to a
consistent state.
So now let's look at an example actor (assuming recovery behaves exactly
the same, but without the persistence calls):
class MyActor extends PersistentActor with AtLeastOnceDelivery {
var counter = 0
override def receiveCommand: Receive = {
case Increment =>
counter += 1
deliver(somewhere, id => Count(counter, id))
persistAsync(Increment) { _ =>
sender ! Persisted
}
case a@Ack(id) =>
counter -= 1
confirmDelivery(id)
persistAsync(a) { _ =>
// nothing to do here
}
case Snapshot =>
saveSnapshot(Snapshot(counter, getDeliverySnapshot))
}
}
Here, I would expect the counter to be the same as the number of
unconfirmed messages as long as there are no crashes.
Could I miss messages here? Would crashes always restore my actor to a
consistent state as well?
I didn't observe any problems, it was just something I thought about during
the design phase, and since it relates to thread timings and crashes it
would be kind of hard to create the problematic scenarios ahead-of-time.
Thanks,
Tal
On Monday, March 9, 2015 at 3:55:39 PM UTC+2, Patrik Nordwall wrote:
>
> I don't see what this has to do with snapshots, and what the problem could
> be. This is not the kind of mutability that Björn warned about. He was
> thinking about mutability of the instance passed to saveSnapshot.
>
> You can do that, but then you risk that the message may not be delivered
> (re-delivered) if things crash before the event has been saved.
>
> Snapshots and persistent events are not re-ordered, so it is supposed to
> just work, and that should not change because of persistAsync. Have you
> seen any problems?
>
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