How to Enable Source.Queue Backpressure

I'm using host-level API with a queue.

  private val (queueSource, connectionPool) = Source.queue[(HttpRequest, Promise[HttpResponse])](queueSize, OverflowStrategy.backpressure).async
    .viaMat(poolFlow)(Keep.both)
    .toMat(
      Sink.foreach({
        case ((Success(resp), p)) =>
          p.success(resp)
        case ((Failure(e), p)) => p.failure(e)
      })
    )(Keep.left)
    .run()

I have a lot of request racing for connections in the connection pool but I get the following error:

 java.lang.IllegalStateException: You have to wait for previous offer to be resolved to send another request
    at akka.stream.impl.QueueSource$$anon$1.akka$stream$impl$QueueSource$$anon$$bufferElem(QueueSource.scala:84)
    at akka.stream.impl.QueueSource$$anon$1$$anonfun$1.apply(QueueSource.scala:94)
    at akka.stream.impl.QueueSource$$anon$1$$anonfun$1.apply(QueueSource.scala:91)
    at akka.stream.impl.fusing.GraphInterpreter.runAsyncInput(GraphInterpreter.scala:447)
    at akka.stream.impl.fusing.GraphInterpreterShell$AsyncInput.execute(ActorGraphInterpreter.scala:464)
    at akka.stream.impl.fusing.GraphInterpreterShell.processEvent(ActorGraphInterpreter.scala:559)
    at akka.stream.impl.fusing.ActorGraphInterpreter.akka$stream$impl$fusing$ActorGraphInterpreter$$processEvent(ActorGraphInterpreter.scala:741)
    at akka.stream.impl.fusing.ActorGraphInterpreter$$anonfun$receive$1.applyOrElse(ActorGraphInterpreter.scala:756)
    at akka.actor.Actor$class.aroundReceive(Actor.scala:517)
    at akka.stream.impl.fusing.ActorGraphInterpreter.aroundReceive(ActorGraphInterpreter.scala:666)
    at akka.actor.ActorCell.receiveMessage(ActorCell.scala:527)
    at akka.actor.ActorCell.invoke(ActorCell.scala:496)
    at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:257)
    at akka.dispatch.Mailbox.run(Mailbox.scala:224)
    at akka.dispatch.Mailbox.exec(Mailbox.scala:234)
    at akka.dispatch.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
    at akka.dispatch.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
    at akka.dispatch.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
    at akka.dispatch.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)

I tried adding .async but back pressure still does not kick in. What does the error above mean and how to go about investigating the problem?

You are already constructing a Source with the Source.queue object method so I don't think it is possible to directly apply back pressure to whatever functionality is calling queue.offer. However, your problem can likely be solved in a different way.

Different OverflowStrategy

You could change the strategy to something like OverflowStrategy.dropHead or OverflowStrategy.dropTail. If your queueSize is large enough compared to the rate of queue.offer invocations then this will probably suite your needs.