Future.sequence 中使用的未来失败 - 报告哪个失败以及何时报告？

Question

Multiple futures can be combined into one using Future.sequence . The result is obvious when all of them succeed - it is the sequence of the results, which is available once all of them have completed. What is however not clear, and I did not find it documented, is what happens when some (perhaps even multiple) futures fail. My Scastie experiment seems to indicate that as soon as any future fails, the failure is reported immediately.

import scala.concurrent._
import scala.util._

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration.Duration

val f1 = Future {
  Thread.sleep(600)
  println("f1 done")
}
val f2 = Future {
  Thread.sleep(400)
  println("f2 done")
  throw new Exception("Failed")
}
val f3 = Future {
  Thread.sleep(200)
  println("f3 done")
  throw new Exception("Failed")
}

val results = Future.sequence(Seq(f1, f2, f3))

results.onComplete {
  case Success(_) =>
    println("Seq completed")
  case Failure(_) =>
    println("Seq failed")
}

Await.result(results, Duration.Inf)

Gives result:

f3 done

Seq failed

Are there any guarantees or specification regarding which failure is reported and when once any of the futures fails?

Answer 1

Which future is returned is an implementation detail.

The current implementation of Future.sequence in turn depends on the implementation of zipWith , which implementations of the Future trait are freely allowed to override, so it's not just a dependence on Future.sequence 's implementation but also on whatever library is giving you Future s (and things like Future.successful / Future.failed can behave differently from a Future { } or even a Future derived from something like a ZIO or a Cats-Effect IO or an Akka ask).

In short, correctness should never depend on which failure is returned from Future.sequence : all you can depend on is that at least one of the futures failed.