scala - Flaky onSuccess of Future.sequence

Question

I wrote this method:

import scala.concurrent._
import ExecutionContext.Implicits.global
import scala.util.{ Success, Failure }

object FuturesSequence extends App {
  val f1 = future {
    1
  }

  val f2 = future {
    2
  }

  val lf = List(f1, f2)

  val seq = Future.sequence(lf)

  seq.onSuccess {
    case l => println(l)
  }
}

I was expecting Future.sequence to gather a List[Future] into a Future[List] and then wait for every futures (f1 and f2 in my case) to complete before calling onSuccess on the Future[List] seq in my case.

But after many runs of this code, it prints "List(1, 2)" only once in a while and I can't figure out why it does not work as expected.

Answer 1

Try this for once,

import scala.concurrent._
import java.util.concurrent.Executors
import scala.util.{ Success, Failure }

object FuturesSequence extends App {
  implicit val exec = ExecutionContext.fromExecutor(Executors.newCachedThreadPool)
  val f1 = future {
    1
  }

  val f2 = future {
    2
  }

  val lf = List(f1, f2)

  val seq = Future.sequence(lf)

  seq.onSuccess {
    case l => println(l)
  }
}

This will always print List(1,2). The reason is simple, the exec above is an ExecutionContext of threads (not daemon threads) where as in your example the ExecutionContext was the default one implicitly taken from ExecutionContext.Implicits.global which contains daemon threads.

Hence being daemon, the process doesn't wait for seq future to be completed and terminates. if at all seq does get completed then it prints. But that doesn't happen always