Hi Björn,
Thanks for your reply.
Let me see if I understand correctly. If I have an actor with a counter,
then this code would be problematic:
class MyActor extends PersistentActor {
var counter = 0
override def receiveCommand: Receive = {
case Increment =>
counter += 1 // <<< shouldn't change state here
val m = Count(counter)
someActorRef ! m
persistAsync(m) { _ =>
sender ! Ack(counter)
}
}
}
But what if I don't want to wait for the actual persistence event to fire
to process the message? Or worse, what if I *have* to change the state from
*receive* and not the *persistAsync* handler?
As a more concrete example, I am wondering what would happen if I use the
AtLeastOnceDelivery trait. If I want to persist the deliveryId (or some
other data derived from it) I have to call *deliver* directly from *receive*.
However, that changes the trait's internal state without waiting for the
*persistAsync* handler. On top of that, I have to somehow synchronize "my"
state with the trait's when performing a snapshot.
So how would that all work together?
Thanks,
Tal
On Wednesday, March 4, 2015 at 4:18:53 PM UTC+2, Björn Antonsson wrote:
>
> Hi Tal,
>
> If you only update the state to be snapshotted in the handlers that you
> give to persistAsync, and send an immutable copy of the state to the
> snapshot method, then you should be fine. The async nature of persistAsync
> is that you continue processing messages while the persisting of the event
> happens, but you are still guaranteed that only one handler at a time will
> be executed in the actor, and that no other message processing is happening
> in the actor while the handler is being run.
>
> Immutability of the state to snapshot is the key here, since the
> "timestamp" and the state will be picked up at consistent point, and sent
> of to the snapshot store, and then normal processing of messages will
> continue inside the actor. If the state is mutated while in flight to the
> snapshot store, then all bets are off.
>
> B/
>
> On 3 March 2015 at 16:38:39, Tal Pressman ([email protected] <javascript:>)
> wrote:
>
> Hi,
>
> Sorry in advance if this has already been answered somewhere, I couldn't
> find it...
>
> I have a PersistentActor, and I use persistAsync to persist its events.
> Now I want to add snapshots into the mix. So from what I understand from
> the documentation, having a snapshot with a timestamp T means I wouldn't
> get any events that were persisted prior to T. But when are these
> timestamps generated? Is it possible that I will miss events? Is it
> possible that an event that was already taken into account in the snapshot
> be processed again?
>
> Thanks,
> Tal
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