Can actors read messages under a certain condition? - scala

Question

I have this situation:
ActorA sends ActorB start/stop messages every 30-40 seconds
ActorA sends ActorB strings to print (always)
ActorB must print the strings he receive, but only if ActorA sent just a start message
Now i wonder if i can do the following things:
Can ActorB read messages only under a certain condition (if a boolean is set as true) without losing the messages he receives while that boolean is set as false?
Can ActorB read a start/stop message from ActorA before the other string messages? I'd like to have this situation: ActorA sends a start message to ActorB, ActorB start printing the strings he received before the start messages (and that is still receiving) and then stop as soon as it receives a stop messages?
I don't know if I explained it well.
EDIT: Thank you, the answers are great, but I still have some doubts.
Does the become mantain the order of the messages? I mean, if i have "Start-M1-M2-Stop-M3-M4-M5-Start-M6-M7-Stop", will the printing order be "M1-M2" and then "M3-M4-M5-M6-M7" or could M6 be read before M3, M4 and M5 (if M6 is received just after the become)?
Can I give a higher priority to start/stop messages? If ActorB receives "M1-M2-M3", and then it receives a stop message while it is printing "M1", i want that ActorB saves again M2 and M3.

Answer 1

You can exactly solve your problem with the Stash trait and the become/unbecome functionality of Akka. The idea is the following:
When you receive a Stop message then you switch to a behaviour where you stash all messages which are not Start. When you receive a Start message, then you switch to a behaviour where you print all received messages and additionally you unstash all messages which have arrived in the meantime.
case object Start
case object Stop
case object TriggerStateChange
case object SendMessage
class ActorB extends Actor with Stash {
override def receive: Receive = {
case Start =>
context.become(printingBehavior, false)
unstashAll()
case x => stash()
}
def printingBehavior: Receive = {
case msg: String => println(msg)
case Stop => context.unbecome()
}
}
class ActorA(val actorB: ActorRef) extends Actor {
var counter = 0
var started = false
override def preStart: Unit = {
import context.dispatcher
this.context.system.scheduler.schedule(0 seconds, 5 seconds, self, TriggerStateChange)
this.context.system.scheduler.schedule(0 seconds, 1 seconds, self, SendMessage)
}
override def receive: Actor.Receive = {
case SendMessage =>
actorB ! "Message: " + counter
counter += 1
case TriggerStateChange =>
actorB ! (if (started) {
started = false
Stop
} else {
started = true
Start
})
}
}
object Akka {
def main(args: Array[String]) = {
val system = ActorSystem.create("TestActorSystem")
val actorB = system.actorOf(Props(classOf[ActorB]), "ActorB")
val actorA = system.actorOf(Props(classOf[ActorA], actorB), "ActorA")
system.awaitTermination()
}
}

Answer 2

Check the Become/Unbecome functionality. It lets you change the behavior of the actor.
If I understood correctly you want your ActorB to have two different states. In the first state it should cache the messages it receives. In the second state, it should print all the cached messages and start printing all the new ones.
Something like this:
case class Start
case class Stop
case class Message(val: String)
class ActorB extends Actor {
var cache = Queue()
def initial: Receive = {
case Message(msg) => cache += msg
case Start =>
for (m <- cache) self ! Message(m)
context.become(printing)
}
def printing: Receive = {
case Message(msg) => println(msg)
case Stop => context.become(initial) //or stop the actor
}
def receive = initial
}

Answer 3

Have Actor B alternate between two states (two different behaviours). In the initial 'pending' state, it waits for a 'start' message, while stashing any other messages.
On receipt of a 'start' message, unstash all the stored messages and become 'active', waiting on a 'stop' message and writing out the other messages received (which will include the unstashed ones). On receiveing a 'stop' message, revert to the 'pending' behaviour.

Answer 4

Some of my thoughts
Yes if the boolean flag is got from some system resource like db or a config file, but I don't think it should be dependent on any external message, given that the actor receive messages from multiple other actors. If ActorB is only used by ActorA, the two can be merged to one
Similar as 1, how to handle the messages from multiple actors? If there is only one actorA, the two actors can be merged. If there are multiple, the flag can be set in database, actorA change the flag in db to "Start" or "Stop". and Actor B will print or not based on the flag.
An actor should be doing something very independently on other actor's state. The start and stop is actually some state of ActorA instead of ActorB

Answer 5

You already have a lot of good answers, but somehow I feel compelled to offer something more brief, as what you need is not necessarily a state machine:
class ActorB extends Actor {
def receive: Receive = caching(Nil)
def caching(cached: List[String]): Receive = {
case msg: String =>
context.become(caching(msg :: cached))
case Start =>
cached.reverse.foreach(println)
context.become(caching(Nil))
}
}