Terminating a flailing akka actor - akka

Question

I have the following actor setup, using Akka actors (2.10)
A -spawn-> B -spawn-> C
A -sendWork-> B -sendWork-> C
C -sendResults-> A (repeatedly)
However, at some point A notices that it should change the workload sent to B/C because C is sending a large number of messages that turn out to be useless. However, in such situations C's inbox seems to be very full, and/or C may be blocked.
How can A tell B to shutdown C immediately? Losing the state and messages of both B and C is acceptable, so destroying them and spawning new ones is an option.

Answer 1

Given the actors are started the way you described, then using stop in the right way will do what you require. According to the docs, calling stop will both:
1) stop additional messages from going into the mailbox (sent to deadletter)
2) take the current contents of the mailbox and also ship that to deadletter (although this is based on mailbox impl, but the point is they won't be processed)
Now if the actor will need to completely finish the message it's currently processing before it's all the way stopped, so if it's "stuck", stopping (or anything for that matter) won't fix that, but I don't think that's the situation you are describing.
I pulled a little code sample together to demonstrate. Basically, A will send a message to B to start sending work to C. B will flood C with some work and C will send the results of that work back to A. When a certain number of responses have been received by A, it will trigger a stop of B and C by stopping B. When B is completely stopped, it will then restart the process over again, up to 2 total times because it stops itself. The code looks like this:
case object StartWork
case class DoWork(i:Int, a:ActorRef)
case class WorkResults(i:Int)
class ActorA extends Actor{
import context._
var responseCount = 0
var restarts = 0
def receive = startingWork
def startingWork:Receive = {
case sw # StartWork =>
val myb = actorOf(Props[ActorB])
myb ! sw
become(waitingForResponses(myb))
}
def waitingForResponses(myb:ActorRef):Receive = {
case WorkResults(i) =>
println(s"Got back work results: $i")
responseCount += 1
if (responseCount > 200){
println("Got too many responses, terminating children and starting again")
watch(myb)
stop(myb)
become(waitingForDeath)
}
}
def waitingForDeath:Receive = {
case Terminated(ref) =>
restarts += 1
if (restarts <= 2){
println("children terminated, starting work again")
responseCount = 0
become(startingWork)
self ! StartWork
}
else{
println("too many restarts, stopping self")
context.stop(self)
}
}
}
class ActorB extends Actor{
import concurrent.duration._
import context._
var sched:Option[Cancellable] = None
override def postStop = {
println("stopping b")
sched foreach (_.cancel)
}
def receive = starting
def starting:Receive = {
case sw # StartWork =>
val myc = context.actorOf(Props[ActorC])
sched = Some(context.system.scheduler.schedule(1 second, 1 second, self, "tick"))
become(sendingWork(myc, sender))
}
def sendingWork(myc:ActorRef, a:ActorRef):Receive = {
case "tick" =>
for(j <- 1 until 1000) myc ! DoWork(j, a)
}
}
class ActorC extends Actor{
override def postStop = {
println("stopping c")
}
def receive = {
case DoWork(i, a) =>
a ! WorkResults(i)
}
}
It's a little rough around the edges, but it should show the point that cascading the stop from B through to C will stop C from sending responses back to A even though it still had messages in the mailbox. I hope this is what you were looking for.