<vaninwagen>
Concerning your question about the general pattern of when an actor or a group of actors is finished doing their computation:
<vaninwagen>
However you setup your actors doing the computation, due to the asynchronous nature of actors, they need another actor to signal when stuff is done
<vaninwagen>
Promises give you kind of a rendezvous point in between, modelled with another actor, but you can also model the reporting of success/failure in a simpler way, by passing a reference to the actor to be notified
<vaninwagen>
I am unsure how to abstract from this pattern, all solutions i can think of are tightly bound to the concrete problem