How can I retrieve the first-completed Actor in a group of Actors in Scala? - scala

Question

I have a moderate number of long-running Actors and I wish to write a synchronous function that returns the first one of these that completes. I can do it with a spin-wait on futures (e.g.,:
while (! fs.exists(f => f.isSet) ) {
Thread.sleep(100)
}
val completeds = fs.filter(f => f.isSet)
completeds.head()
), but that seems very "un-Actor-y"
The scala.actors.Futures class has two methods awaitAll() and awaitEither() that seem awfully close; if there were an awaitAny() I'd jump on it. Am I missing a simple way to do this or is there a common pattern that is applicable?

Answer 1

A more "actorish" way of waiting for completion is creating an actor in charge of handling completed result (lets call it ResultHandler)
Instead of replying, workers send their answer to ResultHandler in fire-and-forget manner. The latter will continue processing the result while other workers complete their job.

Answer 2

The key for me was the discovery that every (?) Scala object is, implicitly, an Actor, so you can use Actor.react{ } to block. Here is my source code:
import scala.actors._
import scala.actors.Actor._
//Top-level class that wants to return the first-completed result from some long-running actors
class ConcurrentQuerier() {
//Synchronous function; perhaps fulfilling some legacy interface
def synchronousQuery : String = {
//Instantiate and start the monitoring Actor
val progressReporter = new ProgressReporter(self) //All (?) objects are Actors
progressReporter.start()
//Instantiate the long-running Actors, giving each a handle to the monitor
val lrfs = List (
new LongRunningFunction(0, 2000, progressReporter), new LongRunningFunction(1, 2500, progressReporter), new LongRunningFunction(3, 1500, progressReporter),
new LongRunningFunction(4, 1495, progressReporter), new LongRunningFunction(5, 1500, progressReporter), new LongRunningFunction(6, 5000, progressReporter) )
//Start 'em
lrfs.map{ lrf =>
lrf.start()
}
println("All actors started...")
val start = System.currentTimeMillis()
/*
This blocks until it receives a String in the Inbox.
Who sends the string? A: the progressReporter, which is monitoring the LongRunningFunctions
*/
val s = receive {
case s:String => s
}
println("Received " + s + " after " + (System.currentTimeMillis() - start) + " ms")
s
}
}
/*
An Actor that reacts to a message that is a tuple ("COMPLETED", someResult) and sends the
result to this Actor's owner. Not strictly necessary (the LongRunningFunctions could post
directly to the owner's mailbox), but I like the idea that monitoring is important enough
to deserve its own object
*/
class ProgressReporter(val owner : Actor) extends Actor {
def act() = {
println("progressReporter awaiting news...")
react {
case ("COMPLETED", s) =>
println("progressReporter received a completed signal " + s);
owner ! s
case s =>
println("Unexpected message: " + s ); act()
}
}
}
/*
Some long running function
*/
class LongRunningFunction(val id : Int, val timeout : Int, val supervisor : Actor) extends Actor {
def act() = {
//Do the long-running query
val s = longRunningQuery()
println(id.toString + " finished, sending results")
//Send the results back to the monitoring Actor (the progressReporter)
supervisor ! ("COMPLETED", s)
}
def longRunningQuery() : String = {
println("Starting Agent " + id + " with timeout " + timeout)
Thread.sleep(timeout)
"Query result from agent " + id
}
}
val cq = new ConcurrentQuerier()
//I don't think the Actor semantics guarantee that the result is absolutely, positively the first to have posted the "COMPLETED" message
println("Among the first to finish was : " + cq.synchronousQuery)
Typical results look like:
scala ActorsNoSpin.scala
progressReporter awaiting news...
All actors started...
Starting Agent 1 with timeout 2500
Starting Agent 5 with timeout 1500
Starting Agent 3 with timeout 1500
Starting Agent 4 with timeout 1495
Starting Agent 6 with timeout 5000
Starting Agent 0 with timeout 2000
4 finished, sending results
progressReporter received a completed signal Query result from agent 4
Received Query result from agent 4 after 1499 ms
Among the first to finish was : Query result from agent 4
5 finished, sending results
3 finished, sending results
0 finished, sending results
1 finished, sending results
6 finished, sending results