Scala期货 - 内建超时？ [英] Scala Futures - built in timeout?

问题描述

有一个方面的未来，我不完全明白从官方教程ref。 http://docs.scala-lang.org/overviews/core/futures.html

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

scala中的期货是否具有某种内置的超时机制？让我们说下面的例子是一个5千兆字节的文本文件...隐含的Implicits.global范围最终导致onFailure以非阻塞方式触发，还是可以定义？没有一种默认的超时时间，这不意味着可能不会成功也不会失败会永远火？

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)
}

推荐答案

当你使用阻塞来获得 Future 的结果时，你只能得到超时行为。如果你想使用非阻塞回调 onComplete ， onSuccess 或 onFailure ，那么你将不得不滚动自己的超时处理。 Akka已经建立了超时处理请求/响应（？）之间的消息传递，但不确定是否要开始使用Akka。 FWIW，在Akka中，对于超时处理，它们通过 Future.firstCompletedOf 组成两个 Futures ，一个代表实际异步任务和表示超时的任务。如果超时定时器（通过 HashedWheelTimer ）首先弹出，则会导致异步回调失败。

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非常简化的例子滚动自己的可能会像这样的东西。首先，一个用于安排超时的对象：

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)
  }
}

一个函数取一个Future并添加超时行为：

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
}

c $ c> HashedWheelTimer 我在这里使用是从Netty。

Note that the HashedWheelTimer I am using here is from Netty.

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