Akka Java here. I just read the Akka docs on the “ask” pattern using futures and am not understanding how several things work, with respect to scenarios when both supervisor strategies (deciders) and failure callbacks are part of the picture.
Parent asking a Child
Say I have two actors, Fizz and Buzz, where Fizz is the parent/creator of Buzz. Because Fizz is Buzz’s parent, it has a SupervisorStrategy for Buzz that handles its failures:
// Groovy pseudo-code
class Fizz extends UntypedActor {
ActorRef buzz
// Contstructor omitted for brevity, but Buzz is the child of
// Fizz.
@Override
void onReceive(Object message) {
if(message instanceof FizzRequest) {
FizzRequest fReq = message as FizzRequest
// Exceptions thrown here (inside of Buzz) will be
// handled by Fizz’s supervisor strategy.
Future<BuzzData> bDataFut = Patterns.ask(buzz,
fReq.buzzRequest, 500)
bDataFut.onComplete(new GetBuzzDataCallback())
// etc.
} else {
unhandled(message)
}
}
@Override
SupervisorStrategy supervisorStrategy() {
new FizzSupervisorStrategy()
}
}
class Buzz extends UntypedActor {
// …etc.
}
class FizzDecider extends Function<Throwable,Directive> {
@Override
Directive apply(Throwable throwable) {
if(throwable instanceof BuzzIsAngryException) {
return SupervisorStrategy.restart()
}
SupervisorStrategy.stop()
}
}
class FizzSupervisorStrategy extends OneForOneStrategy {
FizzSupervisorStrategy() {
super(true, new FizzDecider())
}
}
class GetBuzzDataCallback extends OnComplete<BuzzData> {
@Override
void onComplete(Throwable failure, BuzzData bData) {
if(failure != null) {
// If Buzz is the child of Fizz, does this code execute, or
// just the FizzDecider above? Or both? I’m so confused!
} else {
// Handle success. Likely use an ‘Inbox’ to send
// ‘bData’ back to Fizz.
}
}
}
Sometimes, Fizz needs to ask Buzz for some data. When this happens, there can be one of three results:
Buzzreturns successfully and provides theGetBuzzDataCallbackwithbData; orBuzzthrows aBuzzIsAngryException; orBuzzthrows some other kind of exception/error
I’m wondering what happens with the latter two cases:
- Who is notified of the exception, in what order, and how? In other words, is the
GetBuzzDataCallbacksent the exception as itsThrowable failureargument? Or, is theFizzFailureDeciderinvoked? Or both (it seems a bit redundant and complicating if both the callback and the decider/supervisor strategy are passed the error)?
Parent asking a non-Child
Same scenario as above, except now Fizz is not the parent/creator of Buzz. In this case, can I just assume that the GetBuzzDataCallback is sent the exception (as its Throwable failure argument)?
I guess at the root of my question(s) is this: when a supervisor strategy and a future callback are involved, who gets notified when a child exception is thrown, and in what order? As I mentioned above, to me it would be extra confusing if both receive the failure/exception because then you might have a supervisor strategy trying to restart a Buzz while the callback is trying to do something else (possibly conflicting) with the exception.
Please note: Although certainly not a requirement, I would greatly appreciate it if any code snippets provided be in Java and not Scala (Scala looks like hieroglyphics to me)!
