Akka: Message ordering after Akka restart - akka

Question

The following code example (which you can copy and run) shows a MyParentActor that creates a MyChildActor.
The MyChildActor throws an exception for its first message which causes it to be restarted.
However, what I want to achieve is for "Message 1" to still be processed before "Message 2" on restart of the MyChildActor.
Instead, what is happening is that Message 1 is added to the tail of the mailbox queue, and so Message 2 is processed first.
How do I achieve ordering of the original messages on restart of an actor, without having to create my own mailbox etc?
object TestApp extends App {
var count = 0
val actorSystem = ActorSystem()
val parentActor = actorSystem.actorOf(Props(classOf[MyParentActor]))
parentActor ! "Message 1"
parentActor ! "Message 2"
class MyParentActor extends Actor with ActorLogging{
var childActor: ActorRef = null
#throws[Exception](classOf[Exception])
override def preStart(): Unit = {
childActor = context.actorOf(Props(classOf[MyChildActor]))
}
override def receive = {
case message: Any => {
childActor ! message
}
}
override def supervisorStrategy: SupervisorStrategy = {
OneForOneStrategy() {
case _: CustomException => Restart
case _: Exception => Restart
}
}
}
class MyChildActor extends Actor with ActorLogging{
override def preRestart(reason: Throwable, message: Option[Any]): Unit = {
message match {
case Some(e) => self ! e
}
}
override def receive = {
case message: String => {
if (count == 0) {
count += 1
throw new CustomException("Exception occurred")
}
log.info("Received message {}", message)
}
}
}
class CustomException(message: String) extends RuntimeException(message)
}

Answer 1

You could mark the failing message with a special envelope and stash everything up to the receiving of that message (see child actor implementation). Just define a behaviour where the actor stashes every message except for the specific envelope, processes it's payload and then unstashes all other messages and returns to it's normal behaviour.
This gives me:
INFO TestApp$MyChildActor - Received message Message 1
INFO TestApp$MyChildActor - Received message Message 2
object TestApp extends App {
var count = 0
val actorSystem = ActorSystem()
val parentActor = actorSystem.actorOf(Props(classOf[MyParentActor]))
parentActor ! "Message 1"
parentActor ! "Message 2"
class MyParentActor extends Actor with ActorLogging{
var childActor: ActorRef = null
#throws[Exception](classOf[Exception])
override def preStart(): Unit = {
childActor = context.actorOf(Props(classOf[MyChildActor]))
}
override def receive = {
case message: Any => {
childActor ! message
}
}
override def supervisorStrategy: SupervisorStrategy = {
OneForOneStrategy() {
case e: CustomException => Restart
case _: Exception => Restart
}
}
}
class MyChildActor extends Actor with Stash with ActorLogging{
override def preRestart(reason: Throwable, message: Option[Any]): Unit = {
message match {
case Some(e) =>
self ! Unstash(e)
}
}
override def postRestart(reason: Throwable): Unit = {
context.become(stashing)
preStart()
}
override def receive = {
case message: String => {
if (count == 0) {
count += 1
throw new CustomException("Exception occurred")
}
log.info("Received message {}", message)
}
}
private def stashing: Receive = {
case Unstash( payload ) =>
receive(payload)
unstashAll()
context.unbecome()
case m =>
stash()
}
}
case class Unstash( payload: Any )
class CustomException(message: String) extends RuntimeException(message)
}