Hi Simon,
This documentation section explains this in more detail:
http://doc.akka.io/docs/akka-stream-and-http-experimental/2.0-M1/scala/stream-rate.html
Btw, Sink.head actually does not request more than one, however this is not
transitive and cannot be. Grouped cannot possibly know how many elements a
downstream ever needs, so it tries to prefetch and exploit concurrency.
-Endre
On Thu, Nov 12, 2015 at 6:39 PM, Simon Schäfer <m...@antoras.de> wrote:
> I just tried the following (on 2.0-M1):
>
> scala> Source(1 to 100).map{i ⇒ println(i);
> i}.grouped(10).runWith(Sink.head)
> 1
> 2
> 3
> 4
> 5
> 6
> 7
> 8
> 9
> 10
> 11
> res10: scala.concurrent.Future[scala.collection.immutable.Seq[Int]] =
> scala.concurrent.impl.Promise$DefaultPromise@2df79942
> 12
> 13
> 14
> 15
> 16
>
> scala> 17
> 18
> 19
> 20
> 21
> 22
> 23
> 24
> res10.onComplete(println)
> Success(Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10))
>
> A similar behavior can be seen for the take function:
>
> scala> Source(1 to 100).map{i ⇒ println(i);
> i}.take(10).runWith(Sink.fold(Vector[Int]())(_ :+ _))
> ...
>
>
> As one can see, the stream processes more elements than it has to. It is
> not a problem for me, but I would like to know if this behavior is spec'ed
> anywhere? Shouldn't the stream be able to say, just by looking at the
> arguments for grouped, take and the sink, how many elements should be
> produced?
>
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