Sending back to sender, from supervisor, in case of failure

Question

I have an Actor, which acts as a Supervisor, but also needs to "return" data to the caller, wether this is an actor or not shouldn't matter.

I am asking my Supervisor, let's call him SV.

SV Processes the message I send to him, and sends back a response.

val system = ActorSystem("ActorSystem")
val sv = system.actorOf(Props[SV], name = "SV")

sv ? msg

And SV's recieve method looks like this:

def receive = {
    case msg => (someChild ? msg).pipeTo(sender)
    ...
}

This all works jolly fine. The problem is when the child throws an exception, this exception is caught by the supervisor strategy.

override def supervisorStrategy = OneForOneStrategy () {
    case e : Throwable => {
        val newResponse = someNewResponse
        sender ! newResponse
        ...
    }
}

sender is not longer a reference to whoever called SV in the first place, and I can't seem to work out how to send a message back to the askee, and get back to my original flow.

score 3 · Answer 1 · answered Dec 09 '15 at 07:30

3

One of the three Actor rules is: “An Actor can send a finite number of messages to other Actors it knows.” The last two words are critical here: if the supervisor has not been introduced to the original sender somehow and the failure (exception) itself does not contain the sender’s reference either, then the supervisor cannot possibly send a message to the sender. Either you catch all exceptions in the child actor, wrap them together with the sender in your own exception type and then rethrow, or the original message needs to travel via the supervisor to the child and back, so that the supervisor can see that a reply is outstanding when a failure occurs.

answered Dec 09 '15 at 07:30

Roland Kuhn

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I'm not sure I understand, "the original message needs to travel via the supervisor to the child and back", is this not what I'm doing when i `ask` the child, from the supervisor? It never returns directly to where I asked though, because an exception happens, so the SupervisorStrategy is hit instead. From here I now want to send a message back to the original sender (From the supervisor), with a different response, If I am still not clear enough in my question, I can update it with a flowchart if needed. – Kao Dec 09 '15 at 09:12
Ah, right, I didn’t parse the usage of `ask` correctly; yes, that is what I meant. Then you’ll still need to capture the original `sender` in the supervisor when formulating the supervisorStrategy, which means managing the number of outstanding requests (so as to not get confused). – Roland Kuhn Dec 09 '15 at 13:23

score 1 · Accepted Answer · answered Dec 09 '15 at 10:55

Are you using the supervisor strategy and exceptions to control the flow or your data? Consider using the type system (Option type or a Failure on the response Future) for "exception" cases in your child actors and handle the response flow without exceptions.

The supervisor strategy is for unhandled exceptions. When an unhandled exception occurs you lose the ability to respond with a message to the sender. Unless you wrap the sender in the unhandled exception like Roland Kuhn suggests.

Let the supervisor strategy instead handle how the actor system should respond to your unhandled exceptions by mapping them to a Directive.

Sending back to sender, from supervisor, in case of failure

2 Answers2