I'm converting Future code to IO. I have code similar to this
def doSomething: Future[Foo] = {
Future {
//some code the result of which we don't care about
}
Future {
//Foo
}
}
And then at the end of the program, I doSomething.unsafeRunSync. How do I convert these Futures to IOs while maintaining the fire-and-forget functionality of the first Future? In using IO's async API, I am worried about accidentally blocking the thread when I later call unsafeRunSync on doSomething.
A solution that uses only cats-effect could use IO.start. This, combined with the fact that you will then never join the resulting Fiber, will look something like this:
import cats.effect._
import cats.implicits._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration._
object ExampleApp extends App{
val fireAndForget =
IO(println("Side effect pre-sleep")) *>
IO.sleep(2.seconds) *>
IO(println("Side effect post-sleep"))
val toBeUsed = IO{
println("Inside second one")
42
}
val result = for {
fiber <- IO.shift *> fireAndForget.start
res <- toBeUsed.handleErrorWith { error =>
// This is just in case you 'toBeUsed' can actually fail,
// and you might want to cancel the original side-effecting IO
fiber.cancel *> IO.raiseError(error) }
} yield res
println(result.unsafeRunSync())
println("Waiting 3 seconds...")
IO.sleep(3.seconds).unsafeRunSync()
println("Done")
}
This will print (most of the times) something similar to:
Side effect pre-sleep
Inside second one
42 // Up until here, will be printed right away
Waiting 3 seconds... // It will then be waiting a while
Side effect post-sleep // ...at which point the side effecting code terminates
Done
Finally, here are more details about Fiber and IO.shift
I believe that you need to wrap the first Future in such a way that it completes immediately. We ignore exeptions, or catch them however, but they are contained within its own thread. The parameter cb is the promise that needs to complete; so we short-circuit the completion by providing a value immediately.
def firstFuture(implicit ec: ExecutionContext): IO[Unit] = {
IO.async[Unit] { cb =>
ec.execute(() => {
try {
//some code the result of which we don't care about
} catch {
}
})
cb(Right(()))
}
}
In the for-comprehension, the firstFuture will complete immediately even though its thread will have a long-running task active on it.
def doSomething(implicit ec: ExecutionContext): IO[Foo] = {
for {
_ <- firstFuture
IO.async[Foo] { fb =>
// Foo
}
}
}
There are many questions on SO that combine Futures with Timeout. To be honest, I haven't completely understood how to use them. But it seems I have stumbled upon a problem where I will have to (or maybe not).
I want to throw a TimeoutException if a statement takes more than say 1 minute.To be more clear, currently, this statement tries to get a response from a server but does not throw if the server is not setup. It currently looks like this:
//proper import of exceptions
case class ServerException(exception: Throwable) extends Exception(exception)
//Code that instantiates client and post
val response = try {
client.execute(post)
} catch {
case e#(_: IOException | _: ClientProtocolException) => throw new ServerException(e)
}
To mitigate this problem, I want to introduce a timeout. How do I introduce timeout to this statement such that it throws if no response is got within one minute, else it instantiates response and the program continues as it is?
It's not available in scala Futures. You can switch to scalaz Task - it's a bit different abstraction for async/delayed computations. You can read awesome documentation for it here: http://timperrett.com/2014/07/20/scalaz-task-the-missing-documentation/
import java.util.concurrent.Executors
import scalaz.concurrent.Task
import scala.concurrent.duration._
implicit val scheduledThreadPool =
Executors.newScheduledThreadPool(5)
def executeRequest(req: Request): Task[Response] = ???
val withTimeOut: Task[Response] =
executeRequest(req).timed(1.minute)
Update
Btw you can easily transform your Future to Task, for example it Future is coming from 3rd party lib
object Future2Task {
implicit class Transformer[+T](fut: => Future[T]) {
def toTask(implicit ec: scala.concurrent.ExecutionContext): Task[T] = {
import scala.util.{Failure, Success}
import scalaz.syntax.either._
Task.async {
register =>
fut.onComplete {
case Success(v) => register(v.right)
case Failure(ex) => register(ex.left)
}
}
}
}
}
Timeouts are usually implemented by having an asynchronous timer act as the timeout signal and completing the future in question whenever it or the timer completes.
I believe Akka has a such a timer, but it's pretty simple to roll your own:
object ConcurrencyUtil {
// creates a Future that will complete after a specified duration
object Delay {
def apply(d: Duration): Future[Unit] = {
val p = Promise[Unit]()
val t = new Timer
t.schedule(new TimerTask {
override def run(): Unit = p.success()
}, d.toMillis)
p.future
}
}
implicit class FutureExtensions[T](future: Future[T]) {
def timeout(timeout: Duration) = Future.firstCompletedOf(Seq(
Delay(timeout).map(_ => throw new TimeoutException()),
future
))
}
}
Now you can compose timeout with your future like this:
import ConcurrencyUtil._
val f = someTaskReturningAFuture.timeout(1.minute)
What is now if the task has not completed within 1 minute, the delay will fire, get mapped to throwing a TimeoutException and complete the future f as failed.
Note: This does not address cancellation, i.e. the other future, while no longer being listened for will continue to exist and if it's executing something, continue to execute.
Just trying to get my first futures use up and running and doing a test similar to an example outlined in the Akka in Action MEAP book. I want to call a web service and return the result in a future. I am using scalaxb to access the web service. I have outlined the code below but when I run it the app terminates without waiting for the response from the service. Maybe someone can tell me what I am missing?
import scala.util._
import control.NonFatal
import scala.concurrent._
import ExecutionContext.Implicits.global
object Test {
val service = (new MyServiceBindings with scalaxb.Soap11Clients with scalaxb.DispatchHttpClients {}).service
def test = {
val f = future {
service.someCall() match {
case Right(resp) => resp
case Left(fault) => throw new Exception("Fault: " + fault)}
}
}
f.onComplete {
case Success(resp) => println("Resp: " + resp)
case Failure(NonFatal(e)) => println("Fail: " + e)
}
}
def main(args: Array[String]): Unit = {
test
}
}
It terminates because the main thread that is executed inside your Test has completed. The threads that are used internally by the Dispatch library don't keep the program from exiting.
You would need to wait on the future since this is the only thing your test app is doing. Put this after the onComplete statement.
import scala.concurrent.duration._
Await.ready(f, 10.seconds)
Now bear in mind that this is bad practice usually. You need it here because your test app is doing nothing else, but in a real app, you wouldn't want to block after each futures call, as that would negate the point of using futures.
You can patch this in the main funtion by adding the following logic to your test application:
def main(args: Array[String]) : Unit = {
// Assuming test is a future that did not complete yet.
while(!test.isCompleted) {
Thread.sleep(100)
}
// Software exits here only after future is completed.
}
Or Better if you have many futures you can implement it this way:
// Assuming you have --> listOfFutures = ArrayBuffer[Future]
def main(args: Array[String]) : Unit = {
while(!listOfFutures.isEmpty) {
listOfFutures.foreach {
future =>
if(future.isCompleted) {
listOfFutures -= future
}
}
//checks which futures are completed every half-a-second
Thread.sleep(500)
}
// Program exits after all futures have been completed.
}
I want to make a request to a server asynchronously involving actors. Say I have 2 actors:
class SessionRetriever extends Actor {
import SessionRetriever._
def receiver = {
Get =>
val s = getSessionIdFromServer() // 1
sender ! Result(s) // 2
}
def getSessionIdFromServer(): String = { ... } // 3
}
object SessionRetriever {
object Get
object Result(s: String)
}
And
class RequestSender extends Actor {
val sActor = context actorOf Props[SessionRetriever]
def receiver = {
// get session id
val sesId = sActor ! SessionRetriever.Get
val res = sendRequestToServer(sesId)
logToFile(res)
context shutdown sActor
}
def sendRequestToServer(sessionId: String): String = { .... }
}
My questions:
val s = getSessionIdFromServer() // 1
sender ! Result(s) // 2
1) getSessionIdFromServer() does a synchronous request to the server. I think it would be much better make a request asynchronous, correct? So it will return Future[String] instead of a plain String.
2) How do I make asynchronous: by using an AsyncHttpClient (if I recall correctly its name) or by wrapping its synchronous body into Future { } ?
3) Should I use blocking { } block ? If yes, then where exactly: inside its body or here val s = blocking { getSessionIdFromServer() } ?
P.S. I'd like not to use async { } and await { } at this point because they are quite high level functions and after all they are build on top on Futures.
you might try this non-blocking way
def receive = {
Get =>
//assume getSessionIdFromServer() run aysnchronize
val f: Future[String] = getSessionIdFromServer()
val client = sender //keep it local to use when future back
f onComplete {
case Success(rep) => client ! Result(rep)
case Failure(ex) => client ! Failed(ex)
}
}
1) If getSessionIdFromServer() is blocking then you should execute it asynchronously from your receive function, otherwise your actor will block each time it receives a new request and will always wait until it receives a new session before processing the next request.
2) Using a Future will "move" the blocking operation to a different thread. So, your actor will not be blocked and will be able to keep processing incoming requests - that's good -, however you are still blocking a thread - not so great. Using the AsyncHttpClient is a good idea. You can explore other non-blocking httpClient, like PlayWebService.
3) I am not quite familiar with blocking so not sure I should advise anything here. From what I understand, it will tell the thread pool that the operation is blocking and that it should spawn a temporary new thread to handle it - this avoids having all your workers being blocked. Again, if you do that your actor will not blocked, but you are still blocking a thread while getting the session from the server.
To summarize: just use an async http client in getSessionIdFromServer if it is possible. Otherwise, use either Future{} or blocking.
To do an asynchronous call with AsyncHttpClient you could deal with the java Future via a scala Promise.
import scala.concurrent.Future
import com.ning.http.client.AsyncHttpClient
import scala.concurrent.Promise
import java.util.concurrent.Executor
object WebClient {
private val client = new AsyncHttpClient
case class BadStatus(status: Int) extends RuntimeException
def get(url: String)(implicit exec: Executor): Future[String] = {
val f = client.prepareGet(url).execute();
val p = Promise[String]()
f.addListener(new Runnable {
def run = {
val response = f.get
if (response.getStatusCode / 100 < 4)
p.success(response.getResponseBodyExcerpt(131072))
else p.failure(BadStatus(response.getStatusCode))
}
}, exec)
p.future
}
def shutdown(): Unit = client.close()
}
object WebClientTest extends App {
import scala.concurrent.ExecutionContext.Implicits.global
WebClient get "http://www.google.com/" map println foreach (_ => WebClient.shutdown())
}
And then deal with the future completion via a callback.
Credit for the code to the awesome reactive programming course at Coursera.
there is an aspect of futures that I do not exactly understand from the official tutorial ref. http://docs.scala-lang.org/overviews/core/futures.html
Do futures in scala have a built in time-out mechanism of some kind? Let's say the example below was a 5 gigabyte text file... does the implied scope of "Implicits.global" eventually cause onFailure to fire in a non-blocking way or can that be defined? And without a default time-out of some kind, wouldn't that imply it's possible neither success nor failure would ever fire?
import scala.concurrent._
import ExecutionContext.Implicits.global
val firstOccurence: Future[Int] = future {
val source = scala.io.Source.fromFile("myText.txt")
source.toSeq.indexOfSlice("myKeyword")
}
firstOccurence onSuccess {
case idx => println("The keyword first appears at position: " + idx)
}
firstOccurence onFailure {
case t => println("Could not process file: " + t.getMessage)
}
You only get timeout behavior when you use blocking to get the results of the Future. If you want to use the non-blocking callbacks onComplete, onSuccess or onFailure, then you would have to roll your own timeout handling. Akka has built in timeout handling for request/response (?) messaging between actors, but not sure if you want to start using Akka. FWIW, in Akka, for timeout handling, they compose two Futures together via Future.firstCompletedOf, one which represents the actual async task and one that represents the timeout. If the timeout timer (via a HashedWheelTimer) pops first, you get a failure on the async callback.
A very simplified example of rolling your own might go something like this. First, an object for scheduling timeouts:
import org.jboss.netty.util.{HashedWheelTimer, TimerTask, Timeout}
import java.util.concurrent.TimeUnit
import scala.concurrent.duration.Duration
import scala.concurrent.Promise
import java.util.concurrent.TimeoutException
object TimeoutScheduler{
val timer = new HashedWheelTimer(10, TimeUnit.MILLISECONDS)
def scheduleTimeout(promise:Promise[_], after:Duration) = {
timer.newTimeout(new TimerTask{
def run(timeout:Timeout){
promise.failure(new TimeoutException("Operation timed out after " + after.toMillis + " millis"))
}
}, after.toNanos, TimeUnit.NANOSECONDS)
}
}
Then a function to take a Future and add timeout behavior to it:
import scala.concurrent.{Future, ExecutionContext, Promise}
import scala.concurrent.duration.Duration
def withTimeout[T](fut:Future[T])(implicit ec:ExecutionContext, after:Duration) = {
val prom = Promise[T]()
val timeout = TimeoutScheduler.scheduleTimeout(prom, after)
val combinedFut = Future.firstCompletedOf(List(fut, prom.future))
fut onComplete{case result => timeout.cancel()}
combinedFut
}
Note that the HashedWheelTimer I am using here is from Netty.
All of these answers require additional dependencies. I decided to write a version using java.util.Timer which is an efficient way to run a function in the future, in this case to trigger a timeout.
Blog post with more details here
Using this with Scala's Promise, we can make a Future with timeout as follows:
package justinhj.concurrency
import java.util.concurrent.TimeoutException
import java.util.{Timer, TimerTask}
import scala.concurrent.duration.FiniteDuration
import scala.concurrent.{ExecutionContext, Future, Promise}
import scala.language.postfixOps
object FutureUtil {
// All Future's that use futureWithTimeout will use the same Timer object
// it is thread safe and scales to thousands of active timers
// The true parameter ensures that timeout timers are daemon threads and do not stop
// the program from shutting down
val timer: Timer = new Timer(true)
/**
* Returns the result of the provided future within the given time or a timeout exception, whichever is first
* This uses Java Timer which runs a single thread to handle all futureWithTimeouts and does not block like a
* Thread.sleep would
* #param future Caller passes a future to execute
* #param timeout Time before we return a Timeout exception instead of future's outcome
* #return Future[T]
*/
def futureWithTimeout[T](future : Future[T], timeout : FiniteDuration)(implicit ec: ExecutionContext): Future[T] = {
// Promise will be fulfilled with either the callers Future or the timer task if it times out
val p = Promise[T]
// and a Timer task to handle timing out
val timerTask = new TimerTask() {
def run() : Unit = {
p.tryFailure(new TimeoutException())
}
}
// Set the timeout to check in the future
timer.schedule(timerTask, timeout.toMillis)
future.map {
a =>
if(p.trySuccess(a)) {
timerTask.cancel()
}
}
.recover {
case e: Exception =>
if(p.tryFailure(e)) {
timerTask.cancel()
}
}
p.future
}
}
I've just created a TimeoutFuture class for a coworker:
TimeoutFuture
package model
import scala.concurrent._
import scala.concurrent.duration._
import play.libs.Akka
import play.api.libs.concurrent.Execution.Implicits._
object TimeoutFuture {
def apply[A](timeout: FiniteDuration)(block: => A): Future[A] = {
val prom = promise[A]
// timeout logic
Akka.system.scheduler.scheduleOnce(timeout) {
prom tryFailure new java.util.concurrent.TimeoutException
}
// business logic
Future {
prom success block
}
prom.future
}
}
Usage
val future = TimeoutFuture(10 seconds) {
// do stuff here
}
future onComplete {
case Success(stuff) => // use "stuff"
case Failure(exception) => // catch exception (either TimeoutException or an exception inside the given block)
}
Notes:
Assumes Play! framework (but it's easy enough to adapt)
Every piece of code runs in the same ExecutionContext which may not be ideal.
Play framework contains Promise.timeout so you can write code like following
private def get(): Future[Option[Boolean]] = {
val timeoutFuture = Promise.timeout(None, Duration("1s"))
val mayBeHaveData = Future{
// do something
Some(true)
}
// if timeout occurred then None will be result of method
Future.firstCompletedOf(List(mayBeHaveData, timeoutFuture))
}
I'm quite surprise this is not standard in Scala. My versions is short and has no dependencies
import scala.concurrent.Future
sealed class TimeoutException extends RuntimeException
object FutureTimeout {
import scala.concurrent.ExecutionContext.Implicits.global
implicit class FutureTimeoutLike[T](f: Future[T]) {
def withTimeout(ms: Long): Future[T] = Future.firstCompletedOf(List(f, Future {
Thread.sleep(ms)
throw new TimeoutException
}))
lazy val withTimeout: Future[T] = withTimeout(2000) // default 2s timeout
}
}
Usage example
import FutureTimeout._
Future { /* do smth */ } withTimeout
If you want the writer (promise holder) to be the one who controls the timeout logic, use akka.pattern.after, in the following way:
val timeout = akka.pattern.after(10 seconds, system.scheduler)(Future.failed(new TimeoutException(s"timed out during...")))
Future.firstCompletedOf(Seq(promiseRef.future, timeout))
This way, if your promise completion logic never takes place, your caller's future will still be completed at some point with a failure.
You can specify the timeout when you wait on the future:
For scala.concurrent.Future, the result method lets you specify a timeout.
For scala.actors.Future, Futures.awaitAll lets you specify a timeout.
I do not think there is a timeout built-in the execution of a Future.
Nobody's mentioned akka-streams, yet. The flows have an easy completionTimeout method, and applying that on a single-source stream works like a Future.
But, akka-streams also does cancellation so it can actually end the source from running, i.e. it signals the timeout to the source.
Monix Task has timeout support
import monix.execution.Scheduler.Implicits.global
import monix.eval._
import scala.concurrent.duration._
import scala.concurrent.TimeoutException
val source = Task("Hello!").delayExecution(10.seconds)
// Triggers error if the source does not complete in 3 seconds after runOnComplete
val timedOut = source.timeout(3.seconds)
timedOut.runOnComplete(r => println(r))
//=> Failure(TimeoutException)
This version works without using any external timer (just Await.result)
import scala.concurrent._
import scala.concurrent.duration.FiniteDuration
object TimeoutFuture {
def apply[A](
timeout: FiniteDuration
)(block: => A)(implicit executor: ExecutionContext): Future[A] =
try {
Future { Await.result(Future { block }, timeout) }
} catch {
case _: TimeoutException => Future.failed(new TimeoutException(s"Timed out after ${timeout.toString}"))
}
}
I'm using this version (based on Play example above) which uses Akka system dispatcher:
object TimeoutFuture {
def apply[A](system: ActorSystem, timeout: FiniteDuration)(block: => A): Future[A] = {
implicit val executionContext = system.dispatcher
val prom = Promise[A]
// timeout logic
system.scheduler.scheduleOnce(timeout) {
prom tryFailure new java.util.concurrent.TimeoutException
}
// business logic
Future {
try {
prom success block
} catch {
case t: Throwable => prom tryFailure t
}
}
prom.future
}
}
The simplest way to specify timeout on Future IMO is scala's built in mechanism using scala.concurrent.Await.ready This will throw a TimeoutException if the Future takes longer than the specified timeout. Otherwise, it will return the Future itself.
Here is a simple contrived example
import scala.concurrent.ExecutionContext.Implicits._
import scala.concurrent.duration._
val f1: Future[Int] = Future {
Thread.sleep(1100)
5
}
val fDoesntTimeout: Future[Int] = Await.ready(f1, 2000 milliseconds)
val f: Future[Int] = Future {
Thread.sleep(1100)
5
}
val fTimesOut: Future[Int] = Await.ready(f, 100 milliseconds)
You can wait for a future to finish by making use of Await.
import scala.concurrent.duration._
import scala.concurrent.{Await, Future}
val meaningOfLife: Int = Await.result(Future(42), 1.nano)
println (meaningOfLife)
The above prints 42
You may need an implicit ExecutionContext available in which case, just add:
import scala.concurrent.ExecutionContext.Implicits.global
Another way to do it is to use Coeval from monix. This method does not work in all situations, and you can read all about it here.
The basic idea is that sometimes a future does not really take any time and is returning the result of a synchronous function call or value, so this future can be evaluated on the current thread. This is also useful for testing and mocking futures. Also you don't have to specify a timeout which is expected, but still nice to not have to worry about that.
You start by transforming the future into a Task and wrap that task in a Coeval then cross your fingers as you wait to see what you get. This is a very simple example to show how it works:
You need an implicit Scheduler to be able to use it:
import monix.execution.Scheduler.Implicits.global
Coeval(Task.fromFuture(Future (42)).runSyncStep).value() match {
case Right(v) => println(v)
case Left(task) => println("Task did not finish")
}
The above completes and prints 42 to the console.
Coeval(Task.fromFuture(Future {
scala.concurrent.blocking {
42
}
}).runSyncStep).value() match {
case Right(v) => println(v)
case Left(task) => println("Task did not finish")
}
This example prints Task did not finish:
You can simply run the future to completion without giving any timeout interval by setting the timeout to infinite as below:
**import scala.concurrent.duration._
Await.result(run(executionContext), Duration.Inf)**
run function can be as below :
def run(implicit ec: ExecutionContext) = {
val list = Seq(
Future { println("start 1"); Thread.sleep(1000); println("stop 1")},
Future { println("start 2"); Thread.sleep(2000); println("stop 2")},
Future { println("start 3"); Thread.sleep(3000); println("stop 3")},
Future { println("start 4"); Thread.sleep(4000); println("stop 4")},
Future { println("start 5"); Thread.sleep(5000); println("stop 5")}
)
Future.sequence(list)
}