Skip to content
Satisfy Scala Futures quickly
Branch: master
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.


You can't stop the future but you can always move to the fast lane


libraryDependencies += "com.softwaremill.undelay" %% "undelay" % "0.2.0"


long division

Scala Futures provide a nice interface referencing a deferred value. This deferral will not make your slow operation run any faster. A slow operation scheduled in a Future will result in a future that will be slow to satisfy.

import scala.concurrent.Future
import scala.concurrent.duration._

val longDivision = Future {
  1 / 1

longDivision.onComplete {
  case res => println(s"long div completed with $res")

Awaiting our hero

An application that needs a response quickly can demand the future to respond quickly using Await with a timeout duration.

import scala.concurrent.Await
val result = Await.result(longDivision, 1.second)

There are a few drawbacks to this approach.

  • you are giving up any result composability you get with Futures
  • you are blocking the current thread until the future is satisfied
  • you are now back to a world where throwing exceptions is the norm

a Complete solution

Undelay, provides combinator which allows you do specify a maximum duration of time a future has to complete is operation.

import undelay._
val shortDivision = longDivision.within(1.second)
shortDivision.onComplete {
  case res => println(s"should division completed with result of $res")

The undelay package defines an implicit value class called Complete which takes a single future argument. Complete instances may declare a finite duration suitable to complete your task with the within(deadline) method. By default, the future will be failed with a TimeoutException. You may optional provide your own exception defining function which takes the provided duration and returns a suitable Throwable. Calling within will not block your current thread, nor cost you future compatibility, nor throw an exception in your current thread.

val recovery = shortDivsion.recover {
  case e: TimeoutException => checkNeighborsAnswer()
recovery.onComplete {
  case res => println(s"we got $res quickly without blocking and without interrupting the current thread")

The wildcard import above is there only for the aesthetic of making it look like you can call within on a Scala future. If this is not your thing, you may be more explicit in your travels.

val shortDivision = undelay.Complete(longDivision).within(1.second)
shortDivision.onComplete {
  case res => println(s"should division completed with result of $res")

Doug Tangren (softprops) 2014

You can’t perform that action at this time.