将多个 Future[Seq] 连接成一个 Future[Seq]

Question

没有 Future，这就是我如何将所有较小的 Seq 组合成一个带有 flatmap

的大 Seq

category.getCategoryUrlKey(id: Int):Seq[Meta] // main method
val appDomains: Seq[Int]

val categories:Seq[Meta] = appDomains.flatMap(category.getCategoryUrlKey(_))

现在方法 getCategoryUrlKey 可能会失败。我在前面放了一个断路器，以避免在 maxFailures 之后为下一个元素调用它。现在断路器不是 return Seq 而是 Future[Seq]

lazy val breaker = new akka.pattern.CircuitBreaker(...)

private def getMeta(appDomainId: Int): Future[Seq[Meta]] = {
  breaker.withCircuitBreaker {
    category.getCategoryUrlKey(appDomainId)
  }
}

如何遍历列表 appDomains 并将结果组合成一个单一的 Future[Seq] ，可能成为 Seq ？

如果适用函数式编程，有没有不用临时变量直接转换的方法？

Answer 1

使用 Future.sequence

压缩期货序列

Future.sequence将Seq[Future[T]]转换为Future[Seq[T]]

在你的例子中 T 是 Seq。在序列操作之后，你将得到 Seq[Seq[T]]。所以在序列操作之后使用 flatten 将其展平。

def squashFutures[T](list: Seq[Future[Seq[T]]]): Future[Seq[T]] =
  Future.sequence(list).map(_.flatten)

您的代码变为

Future.sequence(appDomains.map(getMeta)).map(_.flatten)

Answer 2

从 TraversableOnce[Future[A]] 到 Future[TraversableOnce[A]]

val categories = Future.successful(appDomains).flatMap(seq => {
    val fs = seq.map(i => getMeta(i))
    val sequenced = Future.sequence(fs)
    sequenced.map(_.flatten)
})

Future.successful(appDomains) 将 appDomains 提升到 Future

希望对您有所帮助。

Answer 3

val metaSeqFutureSeq = appDomains.map(i => getMeta(i))
// Seq[Future[Seq[Meta]]]

val metaSeqSeqFuture = Future.sequence(metaSeqFutureSeq)
// Future[Seq[Seq[Meta]]]
// NOTE :: this future will fail if any of the futures in the sequence fails

val metaSeqFuture = metaSeqSeqFuture.map(seq => seq.flatten)
// Future[Seq[Meta]]

如果你想拒绝唯一失败的未来而保留成功的未来，那么我们就必须有点创造性，用承诺来构建我们的未来。

import java.util.concurrent.locks.ReentrantLock

import scala.collection.mutable.ArrayBuffer
import scala.concurrent.{Future, Promise}
import scala.util.{Failure, Success}

def futureSeqToOptionSeqFuture[T](futureSeq: Seq[Future[T]]): Future[Seq[Option[T]]] = {
  val promise = Promise[Seq[Option[T]]]()

  var remaining = futureSeq.length

  val result = ArrayBuffer[Option[T]]()
  result ++ futureSeq.map(_ => None)

  val resultLock = new ReentrantLock()

  def handleFutureResult(option: Option[T], index: Int): Unit = {
    resultLock.lock()
    result(index) = option
    remaining = remaining - 1
    if (remaining == 0) {
      promise.success(result)
    }
    resultLock.unlock()
  }

  futureSeq.zipWithIndex.foreach({ case (future, index) => future.onComplete({
    case Success(t) => handleFutureResult(Some(t), index)
    case Failure(ex) => handleFutureResult(None, index)
  }) })

  promise.future
}

val metaSeqFutureSeq = appDomains.map(i => getMeta(i))
// Seq[Future[Seq[Meta]]]

val metaSeqOptionSeqFuture = futureSeqToOptionSeqFuture(metaSeqFutureSeq)
// Future[Seq[Option[Seq[Meta]]]]

val metaSeqFuture = metaSeqSeqFuture.map(seq => seq.flatten.flatten)
// Future[Seq[Meta]]

将多个 Future[Seq] 连接成一个 Future[Seq]

Concatenate many Future[Seq] into one Future[Seq]

scala

future

seq

circuit-breaker

使用 Future.sequence

从 TraversableOnce[Future[A]] 到 Future[TraversableOnce[A]]