How to save data from actors on system shutdown - scala

Question

For example I have following actors: Player and GameRoom.
GameRoom holds players with scores. When user left(terminates), we save player score in database:
class Player extends Actor {
...
}
object GameRoom {
case object Join
}
class GameRoom(database:ActorRef) extends Actor {
type Score = Int
var players: Map[ActorRef, Score] = Map.empty
def receive: Receive = {
case GameRoom.Join =>
context.watch(sender())
players = players + (sender() -> 100)
case Terminated(player) =>
players = players - player
database ! SavePlayerScore(...)
}
}
But what if I want to kill jvm process (SIGTERM)? In that case i have no way to save all users score to database on shutdown.
Any hints how to implement needed behaviour?

Answer 1

You can install a shutdown hook that will terminate your ActorSystem (using ActorSystem#terminate()) on application shutdown.
That will trigger an ordered termination of the Actors hierarchy, and in turn result in your GameRoom to receive the Terminated for the players.
Here is a small code snippet to install that shutdown hook:
Runtime.getRuntime.addShutdownHook(
new Thread("shutdown-hook") {
override def run() {
// This obviously needs to
try{
Await.ready(actorSystem.terminate(), Duration(2, TimeUnit.MINUTES))
}catch{
case _ : InterruptedException => // Termination was interrupted
case _ : Throwable => // Exception thrown by actor system termination
}}
})}
One important thing to notice here is: once the code in the Shutdown Hook is completed, the JVM will shutdown (killing all threads even if they are not done), so if you have any other cleanup to do, add it to the shutdown hook.
EDIT 1: The JVM will terminate even if the Await.ready threw an Exception. This means that, some of your state might have not been saved or something. You might want to handle those exception then and there because, again, once the run() method is complete, the JVM will die.