# Flow.mapAsyncUnordered synchronization

**URL:** <https://discuss.akka.io/t/flow-mapasyncunordered-synchronization/1606>\
**Category:** Akka Streams & Alpakka\
**Created:** [July 19, 2018, 1:47am UTC](https://discuss.akka.io/t/flow-mapasyncunordered-synchronization/1606 "2018-07-19T01:47:35Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![jknight12882](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.akka.io/jknight12882/32/696_2.png) [@jknight12882](https://discuss.akka.io/u/jknight12882)\
**Post date:** [July 19, 2018, 1:47am UTC](https://discuss.akka.io/t/flow-mapasyncunordered-synchronization/1606/1 "2018-07-19T01:47:35Z")

</div>

I need to synchronize a `Flow.mapAsyncUnordered` such that it does not pull until the future has completed. I have a cyclical graph where the next element depends on the results of the previous, therefore I need to way until the downstream has finished before pulling again. `Flow.mapAsyncUnordered` currently always buffers at least one element, so it is pulling too early. I have come up with the following:

```auto
val ref: ActorRef = <some dummy actor>

Sink.fromGraph(GraphDSL.create() { implicit b =>
  import GraphDSL.Implicits._

  val broadcast = b.add(Broadcast[DummyMessage](2))
  val zip = b.add(Zip[DummyMessage, Done])
  val flow = b.add(Flow[DummyMessage].mapAsyncUnordered(1)(ref ? _))

  broadcast ~> zip.in0
  broadcast ~> flow ~> zip.in1
                       zip.out ~> Sink.ignore

  SinkShape(broadcast.in)
})

```

Is there a better way to pull messages one at a time? I had also considered the following:

```auto
Flow[DummyMessage].flatMapConcat(m => Source.fromFuture(ref ? m)).to(Sink.ignore)

```

Thanks  
Jeff