Yes Michael, you are right. I based my example code on the documentation
you referenced, assuming it was correct.
What would be the correct way to do, the same as when sender is involved or
is there something special here?
// is this the safe way to proceed?
val s = self
Future { expensiveCalculation() } onComplete { f => s ! f.value.get }
// or is this better?
Future { expensiveCalculation() } pipeTo self()
Could someone help me understand the following statement from the newly
opened issue
https://github.com/akka/akka/issues/17805#issuecomment-114389213: "(after
all, even if you share the result of self() the message will end up in
DeadLetters anyway)". This doesn't match my experience, as actors are
restarted, message sent to a copy of self() end up in the actor's mailbox
and not in deadLetters.
Thanks, Fred
On Monday, June 22, 2015 at 2:35:41 PM UTC-7, Michael Frank wrote:
>
> the current documentation appears to be incorrect, which i believe was
> the source of confusion. on line 20 of the code block of the section
> "Actors and shared mutable state" on page
> http://doc.akka.io/docs/akka/2.3.11/general/jmm.html#jmm-shared-state it
> specifically states:
>
> 18 // Completely safe, "self" is OK to close over
> 19 // and it's an ActorRef, which is thread-safe
> 20 Future { expensiveCalculation() } onComplete { f => self !
> f.value.get }
>
> i opened an issue for it: https://github.com/akka/akka/issues/17805
>
> -Michael
>
> On 06/20/15 01:45, Roland Kuhn wrote:
>
> Hi Frederic,
>
> you violate the first rule of Actors: never call *any* of their methods
> from outside of their context of execution. This is true for all of them,
> including context() and self(). While the result of calling self() can then
> safely be shared afterwards (an ActorRef is thread-safe), the result of
> context() cannot (because ActorContext is not thread-safe).
>
> What happens in your sample is that the Actor is destroyed due to the
> Exception (that triggers a restart), so anything referencing the old Actor
> instance will see a dead entity with nulls in it (in order to minimize
> memory leaks in case someone does something wrong).
>
> Regards,
>
> Roland
>
> 19 jun 2015 kl. 02:02 skrev Frederic <[email protected] <javascript:>>:
>
> There's more... as context is also null in the Future's callback,
> anything using context won't work, see below for an updated example using
> the Akka's scheduler and dispatcher:
>
> import akka.actor._
> import scala.concurrent._
> import scala.concurrent.duration._
>
> case class Done(s: String)
> case object Greet
> case class Complete(p: Promise[Unit])
>
> class Greeter extends Actor {
> import context.dispatcher
> def receive = {
> case Done(s) => println(s"Done($s) (system still running)!")
> case Complete(p) => p.success(())
> case Greet =>
> val p = Promise[Unit]()
> val saveMyself = self
> val sched = context.system.scheduler
> val disp = context.dispatcher
> p.future onComplete {
> case _ =>
> println(s"self is $self")
> println(s"self is expexted to be $saveMyself")
> // cannot use self to send a message, need to close on self
> self ! Done("Send using self")
> saveMyself ! Done("Send using saveMyself")
> // are we supposed to avoid using context here?
> println(s"context is $context") // outputs: context is null
> // is this meant to work?
> // context.system.scheduler.scheduleOnce(1.millisecond,
> saveMyself, Done)
> // will fail because we can't get the scheduler as context is
> null
> try { context.system.scheduler.scheduleOnce(1.millisecond,
> saveMyself, Done("scheduler 1")) } catch { case e => println(e) }
> // The following will still fail because implicit dispatcher is
> context.dispatcher and context is null
> try { sched.scheduleOnce(1.millisecond, saveMyself,
> Done("scheduler 2")) } catch { case e => println(e) }
> // that will work...
> sched.scheduleOnce(1.millisecond, saveMyself, Done("scheduler
> 3"))(disp, saveMyself)
> }
> self ! Complete(p)
> throw new Exception("Messing with onComplete callback, self will be
> dead letters")
> }
> }
>
> object HelloAkkaScala extends App {
> val system = ActorSystem("helloakka")
> val greeter = system.actorOf(Props[Greeter], "greeter")
> greeter ! Greet
> }
>
> Program output :
>
> Running HelloAkkaScala
> self is Actor[akka://helloakka/deadLetters]
> [ERROR] [06/18/2015 16:47:50.464]
> [helloakka-akka.actor.default-dispatcher-3] [akka://helloakka/user/greeter]
> Messing with onComplete callback, self will be dead letters
> java.lang.Exception: Messing with onComplete callback, self will be dead
> letters
> at Greeter$anonfun$receive$1.applyOrElse(HelloAkkaScala.scala:40)
> at akka.actor.Actor$class.aroundReceive(Actor.scala:467)
> at Greeter.aroundReceive(HelloAkkaScala.scala:9)
> at akka.actor.ActorCell.receiveMessage(ActorCell.scala:516)
> at akka.actor.ActorCell.invoke(ActorCell.scala:487)
> at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:238)
> at akka.dispatch.Mailbox.run(Mailbox.scala:220)
> at
> akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:397)
> at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
> at
> scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
> at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
> at
> scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
>
> self is expexted to be Actor[akka://helloakka/user/greeter#-1327535750]
> context is null
> java.lang.NullPointerException
> java.lang.NullPointerException
> Done(Send using saveMyself) (system still running)!
> [INFO] [06/18/2015 16:47:50.480]
> [helloakka-akka.actor.default-dispatcher-4] [akka://helloakka/deadLetters]
> Message [Done] from Actor[akka://helloakka/deadLetters] to
> Actor[akka://helloakka/deadLetters] was not delivered. [1] dead letters
> encountered. This logging can be turned off or adjusted with configuration
> settings 'akka.log-dead-letters' and
> 'akka.log-dead-letters-during-shutdown'.
> Done(scheduler 3) (system still running)!
>
>
> On Thursday, June 18, 2015 at 2:37:35 PM UTC-7, Frederic wrote:
>>
>> Hello,
>>
>> I found a quite specific case involving Actors, Future callbacks and
>> exceptions where self becomes deadLetters.
>>
>> I have the feeling I'm using self the way it is meant to be used, as
>> described here:
>> http://doc.akka.io/docs/akka/2.3.11/general/jmm.html#jmm-shared-state
>>
>> Am I doing something wrong or is it a bug?
>>
>> Please see code and output below to reproduce/illustrate the issue.
>>
>> Thanks, Fred
>>
>> import akka.actor._
>> import scala.concurrent._
>>
>> case object Done
>> case object Greet
>> case class Complete(p: Promise[Unit])
>>
>> class Greeter extends Actor {
>> import context.dispatcher
>> def receive = {
>> case Done => println("Done (system still running)!")
>> case Complete(p) => p.success(())
>> case Greet =>
>> val p = Promise[Unit]()
>> val saveMyself = self
>> p.future onComplete {
>> case _ =>
>> println(s"self is $self")
>> println(s"self is expexted to be $saveMyself")
>> // cannot use self to send a message, bug?
>> self ! Done
>> saveMyself ! Done
>> }
>> self ! Complete(p)
>> throw new Exception("Messing with onComplete callback, self will be
>> dead letters")
>> }
>> }
>>
>> object HelloAkkaScala extends App {
>> val system = ActorSystem("helloakka")
>> val greeter = system.actorOf(Props[Greeter], "greeter")
>> greeter ! Greet
>> }
>>
>> Program output :
>>
>> Running HelloAkkaScala
>> self is Actor[akka://helloakka/deadLetters]
>> [ERROR] [06/18/2015 14:25:41.510]
>> [helloakka-akka.actor.default-dispatcher-4] [akka://helloakka/user/greeter]
>> Messing with onComplete callback, self will be dead letters
>> java.lang.Exception: Messing with onComplete callback, self will be dead
>> letters
>> at Greeter$anonfun$receive$1.applyOrElse(HelloAkkaScala.scala:27)
>> at akka.actor.Actor$class.aroundReceive(Actor.scala:467)
>> at Greeter.aroundReceive(HelloAkkaScala.scala:8)
>> at akka.actor.ActorCell.receiveMessage(ActorCell.scala:516)
>> at akka.actor.ActorCell.invoke(ActorCell.scala:487)
>> at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:238)
>> at akka.dispatch.Mailbox.run(Mailbox.scala:220)
>> at
>> akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:397)
>> at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
>> at
>> scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
>> at
>> scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
>> at
>> scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
>>
>> self is expexted to be Actor[akka://helloakka/user/greeter#990239115]
>> Done (system still running)!
>> [INFO] [06/18/2015 14:25:41.516]
>> [helloakka-akka.actor.default-dispatcher-3] [akka://helloakka/deadLetters]
>> Message [Done$] from Actor[akka://helloakka/deadLetters] to Actor[
>> akka://helloakka/deadLetters] was not delivered. [1] dead letters
>> encountered. This logging can be turned off or adjusted with configuration
>> settings 'akka.log-dead-letters' and
>> 'akka.log-dead-letters-during-shutdown'.
>>
>
> --
> >>>>>>>>>> Read the docs: http://akka.io/docs/
> >>>>>>>>>> Check the FAQ:
> http://doc.akka.io/docs/akka/current/additional/faq.html
> >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user
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>
>
>
>
> *Dr. Roland Kuhn*
> *Akka Tech Lead*
> Typesafe <http://typesafe.com/> – Reactive apps on the JVM.
> twitter: @rolandkuhn
> <http://twitter.com/#%21/rolandkuhn>
>
> --
> >>>>>>>>>> Read the docs: http://akka.io/docs/
> >>>>>>>>>> Check the FAQ:
> http://doc.akka.io/docs/akka/current/additional/faq.html
> >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user
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--
>>>>>>>>>> Read the docs: http://akka.io/docs/
>>>>>>>>>> Check the FAQ:
>>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html
>>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user
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