diff --git a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/AsyncCallbackSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/AsyncCallbackSpec.scala
new file mode 100644
index 0000000000..a31a79efb5
--- /dev/null
+++ b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/AsyncCallbackSpec.scala
@@ -0,0 +1,242 @@
+/**
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package akka.stream.impl.fusing
+
+import akka.Done
+import akka.actor.ActorRef
+import akka.stream.stage._
+import akka.stream._
+import akka.stream.scaladsl.{ Keep, Sink, Source }
+import akka.stream.testkit.Utils.TE
+import akka.stream.testkit.{ TestPublisher, TestSubscriber }
+import akka.testkit.{ AkkaSpec, TestProbe }
+
+import scala.concurrent.{ Future, Promise }
+
+class AsyncCallbackSpec extends AkkaSpec {
+
+  implicit val materializer = ActorMaterializer(ActorMaterializerSettings(system).withFuzzing(false))
+
+  case object Started
+  case class Elem(n: Int)
+  case object Stopped
+
+  class AsyncCallbackGraphStage(probe: ActorRef, early: Option[AsyncCallback[AnyRef] ⇒ Unit] = None)
+    extends GraphStageWithMaterializedValue[FlowShape[Int, Int], AsyncCallback[AnyRef]] {
+
+    val in = Inlet[Int]("in")
+    val out = Outlet[Int]("out")
+    val shape = FlowShape(in, out)
+
+    def createLogicAndMaterializedValue(inheritedAttributes: Attributes): (GraphStageLogic, AsyncCallback[AnyRef]) = {
+      val logic = new GraphStageLogic(shape) {
+        val callback = getAsyncCallback((whatever: AnyRef) ⇒ {
+          whatever match {
+            case t: Throwable     ⇒ throw t
+            case "fail-the-stage" ⇒ failStage(new RuntimeException("failing the stage"))
+            case anythingElse     ⇒ probe ! anythingElse
+          }
+        })
+        early.foreach(cb ⇒ cb(callback))
+
+        override def preStart(): Unit = {
+          probe ! Started
+        }
+
+        override def postStop(): Unit = {
+          probe ! Stopped
+        }
+
+        setHandlers(in, out, new InHandler with OutHandler {
+          def onPush(): Unit = {
+            val n = grab(in)
+            probe ! Elem(n)
+            push(out, n)
+          }
+
+          def onPull(): Unit = {
+            pull(in)
+          }
+        })
+      }
+
+      (logic, logic.callback)
+    }
+  }
+
+  "The support for async callbacks" must {
+
+    "invoke without feedback, happy path" in {
+      val probe = TestProbe()
+      val in = TestPublisher.probe[Int]()
+      val out = TestSubscriber.probe[Int]()
+      val callback = Source.fromPublisher(in)
+        .viaMat(new AsyncCallbackGraphStage(probe.ref))(Keep.right)
+        .to(Sink.fromSubscriber(out))
+        .run()
+
+      probe.expectMsg(Started)
+      out.request(1)
+      in.expectRequest()
+
+      (0 to 10).foreach { n ⇒
+        val msg = "whatever" + n
+        callback.invoke(msg)
+        probe.expectMsg(msg)
+      }
+
+      in.sendComplete()
+      out.expectComplete()
+
+      probe.expectMsg(Stopped)
+    }
+
+    "invoke with feedback, happy path" in {
+      val probe = TestProbe()
+      val in = TestPublisher.probe[Int]()
+      val out = TestSubscriber.probe[Int]()
+      val callback = Source.fromPublisher(in)
+        .viaMat(new AsyncCallbackGraphStage(probe.ref))(Keep.right)
+        .to(Sink.fromSubscriber(out))
+        .run()
+
+      probe.expectMsg(Started)
+      out.request(1)
+      in.expectRequest()
+
+      (0 to 10).foreach { n ⇒
+        val msg = "whatever" + n
+        val feedbackF = callback.invokeWithFeedback(msg)
+        probe.expectMsg(msg)
+        feedbackF.futureValue should ===(Done)
+      }
+      in.sendComplete()
+      out.expectComplete()
+
+      probe.expectMsg(Stopped)
+    }
+
+    "fail the feedback future if stage is stopped" in {
+      val probe = TestProbe()
+      val callback = Source.empty
+        .viaMat(new AsyncCallbackGraphStage(probe.ref))(Keep.right)
+        .to(Sink.ignore)
+        .run()
+
+      probe.expectMsg(Started)
+      probe.expectMsg(Stopped)
+
+      val feedbakF = callback.invokeWithFeedback("whatever")
+      feedbakF.failed.futureValue shouldBe a[StreamDetachedException]
+    }
+
+    "invoke early" in {
+      val probe = TestProbe()
+      val in = TestPublisher.probe[Int]()
+      val callback = Source.fromPublisher(in)
+        .viaMat(new AsyncCallbackGraphStage(
+          probe.ref,
+          Some(asyncCb ⇒ asyncCb.invoke("early"))
+        ))(Keep.right)
+        .to(Sink.ignore)
+        .run()
+
+      // and deliver in order
+      callback.invoke("later")
+
+      probe.expectMsg(Started)
+      probe.expectMsg("early")
+      probe.expectMsg("later")
+
+      in.sendComplete()
+      probe.expectMsg(Stopped)
+
+    }
+
+    "invoke with feedback early" in {
+      val probe = TestProbe()
+      val earlyFeedback = Promise[Done]()
+      val in = TestPublisher.probe[Int]()
+      val callback = Source.fromPublisher(in)
+        .viaMat(new AsyncCallbackGraphStage(
+          probe.ref,
+          Some(asyncCb ⇒ earlyFeedback.completeWith(asyncCb.invokeWithFeedback("early")))
+        ))(Keep.right)
+        .to(Sink.ignore)
+        .run()
+
+      // and deliver in order
+      val laterFeedbackF = callback.invokeWithFeedback("later")
+
+      probe.expectMsg(Started)
+      probe.expectMsg("early")
+      earlyFeedback.future.futureValue should ===(Done)
+
+      probe.expectMsg("later")
+      laterFeedbackF.futureValue should ===(Done)
+
+      in.sendComplete()
+      probe.expectMsg(Stopped)
+    }
+
+    "accept concurrent input" in {
+      val probe = TestProbe()
+      val in = TestPublisher.probe[Int]()
+      val callback = Source.fromPublisher(in)
+        .viaMat(new AsyncCallbackGraphStage(probe.ref))(Keep.right)
+        .to(Sink.ignore)
+        .run()
+
+      import system.dispatcher
+      val feedbacks = (1 to 100).map { n ⇒
+        Future {
+          callback.invokeWithFeedback(n.toString)
+        }.flatMap(d ⇒ d)
+      }
+
+      probe.expectMsg(Started)
+      Future.sequence(feedbacks).futureValue should have size (100)
+      (1 to 100).map(_ ⇒ probe.expectMsgType[String]).toSet should have size (100)
+
+      in.sendComplete()
+      probe.expectMsg(Stopped)
+    }
+
+    "fail the feedback if the handler throws" in {
+      val probe = TestProbe()
+      val in = TestPublisher.probe()
+      val callback = Source.fromPublisher(in)
+        .viaMat(new AsyncCallbackGraphStage(probe.ref))(Keep.right)
+        .to(Sink.ignore)
+        .run()
+
+      probe.expectMsg(Started)
+      callback.invokeWithFeedback("happy-case").futureValue should ===(Done)
+      probe.expectMsg("happy-case")
+
+      val feedbackF = callback.invokeWithFeedback(TE("oh my gosh, whale of a wash!"))
+      val failure = feedbackF.failed.futureValue
+      failure shouldBe a[TE]
+      failure.getMessage should ===("oh my gosh, whale of a wash!")
+
+      in.expectCancellation()
+    }
+
+    "fail the feedback if the handler fails the stage" in {
+      val probe = TestProbe()
+      val callback = Source.empty
+        .viaMat(new AsyncCallbackGraphStage(probe.ref))(Keep.right)
+        .to(Sink.ignore)
+        .run()
+
+      probe.expectMsg(Started)
+      probe.expectMsg(Stopped)
+
+      val feedbakF = callback.invokeWithFeedback("fail-the-stage")
+      val failure = feedbakF.failed.futureValue
+      failure shouldBe a[StreamDetachedException] // we can't capture the exception in this case
+    }
+
+  }
+}
diff --git a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala
index c83fdb725f..14de8732d1 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala
@@ -11,6 +11,7 @@ import akka.stream.impl.fusing.GraphInterpreter.{ Connection, DownstreamBoundary
 import akka.stream.stage.{ GraphStage, GraphStageLogic, InHandler, OutHandler, _ }
 import akka.stream.testkit.StreamSpec
 import akka.stream.testkit.Utils.TE
+import akka.util.OptionVal
 
 import scala.collection.{ Map ⇒ SMap }
 import scala.language.existentials
@@ -252,7 +253,7 @@ trait GraphInterpreterSpecKit extends StreamSpec {
         logger,
         logics,
         connections,
-        onAsyncInput = (_, _, _) ⇒ (),
+        onAsyncInput = (_, _, _, _) ⇒ (),
         fuzzingMode = false,
         context = null)
       _interpreter.init(null)
diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/HubSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/HubSpec.scala
index 0df497012e..6f66c9f9ca 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/HubSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/HubSpec.scala
@@ -365,6 +365,34 @@ class HubSpec extends StreamSpec {
       }
     }
 
+    "handle cancelled Sink" in assertAllStagesStopped {
+      val in = TestPublisher.probe[Int]()
+      val hubSource = Source.fromPublisher(in).runWith(BroadcastHub.sink(4))
+
+      val out = TestSubscriber.probe[Int]()
+
+      hubSource.runWith(Sink.cancelled)
+      hubSource.runWith(Sink.fromSubscriber(out))
+
+      out.ensureSubscription()
+
+      out.request(10)
+      in.expectRequest()
+      in.sendNext(1)
+      out.expectNext(1)
+      in.sendNext(2)
+      out.expectNext(2)
+      in.sendNext(3)
+      out.expectNext(3)
+      in.sendNext(4)
+      out.expectNext(4)
+      in.sendNext(5)
+      out.expectNext(5)
+
+      in.sendComplete()
+      out.expectComplete()
+    }
+
   }
 
   "PartitionHub" must {
diff --git a/akka-stream/src/main/mima-filters/2.5.6.backwards.excludes b/akka-stream/src/main/mima-filters/2.5.6.backwards.excludes
index 07e820af4b..fe984a3f4a 100644
--- a/akka-stream/src/main/mima-filters/2.5.6.backwards.excludes
+++ b/akka-stream/src/main/mima-filters/2.5.6.backwards.excludes
@@ -21,3 +21,13 @@ ProblemFilters.exclude[MissingClassProblem]("akka.stream.stage.CallbackWrapper")
 
 # Optimize TCP stream writes
 ProblemFilters.exclude[Problem]("akka.stream.impl.io.*")
+
+# #23953 fixes to async callback with feedback
+ProblemFilters.exclude[MissingTypesProblem]("akka.stream.impl.fusing.GraphInterpreterShell$AsyncInput$")
+ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.impl.fusing.GraphInterpreterShell#AsyncInput.copy")
+ProblemFilters.exclude[IncompatibleResultTypeProblem]("akka.stream.impl.fusing.GraphInterpreter.onAsyncInput")
+ProblemFilters.exclude[IncompatibleMethTypeProblem]("akka.stream.impl.fusing.GraphInterpreter.this")
+ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.impl.fusing.GraphInterpreterShell#AsyncInput.apply")
+ProblemFilters.exclude[IncompatibleResultTypeProblem]("akka.stream.impl.fusing.GraphInterpreterShell#AsyncInput.copy$default$4")
+ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.impl.fusing.GraphInterpreterShell#AsyncInput.this")
+ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.impl.fusing.GraphInterpreter.runAsyncInput")
\ No newline at end of file
diff --git a/akka-stream/src/main/scala/akka/stream/StreamDetachedException.scala b/akka-stream/src/main/scala/akka/stream/StreamDetachedException.scala
index 73888b1882..0fb53554e7 100644
--- a/akka-stream/src/main/scala/akka/stream/StreamDetachedException.scala
+++ b/akka-stream/src/main/scala/akka/stream/StreamDetachedException.scala
@@ -9,6 +9,10 @@ import scala.util.control.NoStackTrace
  * This exception signals that materialized value is already detached from stream. This usually happens
  * when stream is completed and an ActorSystem is shut down while materialized object is still available.
  */
-final class StreamDetachedException
-  extends RuntimeException("Stream is terminated. Materialized value is detached.")
-  with NoStackTrace
+final class StreamDetachedException(message: String)
+  extends RuntimeException(message)
+  with NoStackTrace {
+
+  def this() = this("Stream is terminated. Materialized value is detached.")
+
+}
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala
index 620b37bab5..9f235c3b1c 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala
@@ -7,6 +7,7 @@ import java.util
 import java.util.concurrent.TimeoutException
 import java.util.concurrent.atomic.AtomicReference
 
+import akka.Done
 import akka.actor._
 import akka.annotation.InternalApi
 import akka.event.Logging
@@ -20,6 +21,7 @@ import org.reactivestreams.{ Publisher, Subscriber, Subscription }
 
 import scala.annotation.tailrec
 import scala.collection.immutable
+import scala.concurrent.Promise
 import scala.util.control.NonFatal
 
 /**
@@ -447,15 +449,26 @@ import scala.util.control.NonFatal
   private var self: ActorRef = _
   lazy val log = Logging(mat.system.eventStream, self)
 
-  final case class AsyncInput(shell: GraphInterpreterShell, logic: GraphStageLogic, evt: Any, handler: (Any) ⇒ Unit) extends BoundaryEvent {
+  /**
+   * @param promise Will be completed upon processing the event, or failed if processing the event throws
+   *                if the event isn't ever processed the promise (the stage stops) is failed elsewhere
+   */
+  final case class AsyncInput(
+    shell:   GraphInterpreterShell,
+    logic:   GraphStageLogic,
+    evt:     Any,
+    promise: OptionVal[Promise[Done]],
+    handler: (Any) ⇒ Unit) extends BoundaryEvent {
     override def execute(eventLimit: Int): Int = {
       if (!waitingForShutdown) {
-        interpreter.runAsyncInput(logic, evt, handler)
+        interpreter.runAsyncInput(logic, evt, promise, handler)
         if (eventLimit == 1 && interpreter.isSuspended) {
           sendResume(true)
           0
         } else runBatch(eventLimit - 1)
-      } else eventLimit
+      } else {
+        eventLimit
+      }
     }
   }
 
@@ -481,8 +494,8 @@ import scala.util.control.NonFatal
   private var enqueueToShortCircuit: (Any) ⇒ Unit = _
 
   lazy val interpreter: GraphInterpreter = new GraphInterpreter(mat, log, logics, connections,
-    (logic, event, handler) ⇒ {
-      val asyncInput = AsyncInput(this, logic, event, handler)
+    (logic, event, promise, handler) ⇒ {
+      val asyncInput = AsyncInput(this, logic, event, promise, handler)
       val currentInterpreter = GraphInterpreter.currentInterpreterOrNull
       if (currentInterpreter == null || (currentInterpreter.context ne self))
         self ! asyncInput
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala
index 442219591a..f0fa43f950 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala
@@ -9,8 +9,11 @@ import akka.stream.stage._
 import akka.stream._
 import java.util.concurrent.ThreadLocalRandom
 
+import akka.Done
 import akka.annotation.InternalApi
+import akka.util.OptionVal
 
+import scala.concurrent.Promise
 import scala.util.control.NonFatal
 
 /**
@@ -191,7 +194,7 @@ import scala.util.control.NonFatal
   val log:          LoggingAdapter,
   val logics:       Array[GraphStageLogic], // Array of stage logics
   val connections:  Array[GraphInterpreter.Connection],
-  val onAsyncInput: (GraphStageLogic, Any, (Any) ⇒ Unit) ⇒ Unit,
+  val onAsyncInput: (GraphStageLogic, Any, OptionVal[Promise[Done]], (Any) ⇒ Unit) ⇒ Unit,
   val fuzzingMode:  Boolean,
   val context:      ActorRef) {
 
@@ -432,7 +435,7 @@ import scala.util.control.NonFatal
     eventsRemaining
   }
 
-  def runAsyncInput(logic: GraphStageLogic, evt: Any, handler: (Any) ⇒ Unit): Unit =
+  def runAsyncInput(logic: GraphStageLogic, evt: Any, promise: OptionVal[Promise[Done]], handler: (Any) ⇒ Unit): Unit =
     if (!isStageCompleted(logic)) {
       if (GraphInterpreter.Debug) println(s"$Name ASYNC $evt ($handler) [$logic]")
       val currentInterpreterHolder = _currentInterpreter.get()
@@ -440,9 +443,13 @@ import scala.util.control.NonFatal
       currentInterpreterHolder(0) = this
       try {
         activeStage = logic
-        try handler(evt)
-        catch {
-          case NonFatal(ex) ⇒ logic.failStage(ex)
+        try {
+          handler(evt)
+          if (promise.isDefined) promise.get.success(Done)
+        } catch {
+          case NonFatal(ex) ⇒
+            if (promise.isDefined) promise.get.failure(ex)
+            logic.failStage(ex)
         }
         afterStageHasRun(logic)
       } finally currentInterpreterHolder(0) = previousInterpreter
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Hub.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Hub.scala
index 96edf32dc2..6a93a396e6 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Hub.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Hub.scala
@@ -7,14 +7,13 @@ import java.util
 import java.util.concurrent.atomic.{ AtomicLong, AtomicReference }
 
 import akka.NotUsed
-import akka.dispatch.AbstractNodeQueue
+import akka.dispatch.{ AbstractNodeQueue, ExecutionContexts }
 import akka.stream._
 import akka.stream.stage._
 
 import scala.annotation.tailrec
 import scala.concurrent.{ Future, Promise }
 import scala.util.{ Failure, Success, Try }
-import java.util.Arrays
 import java.util.concurrent.ConcurrentHashMap
 import java.util.concurrent.atomic.AtomicInteger
 import java.util.concurrent.atomic.AtomicReferenceArray
@@ -424,12 +423,19 @@ private[akka] class BroadcastHub[T](bufferSize: Int) extends GraphStageWithMater
             val startFrom = head
             activeConsumers += 1
             addConsumer(consumer, startFrom)
-            consumer.callback.invoke(Initialize(startFrom))
+            // in case the consumer is already stopped we need to undo registration
+            implicit val ec = materializer.executionContext
+            consumer.callback.invokeWithFeedback(Initialize(startFrom)).onFailure {
+              case _: StreamDetachedException ⇒
+                callbackPromise.future.foreach(callback ⇒
+                  callback.invoke(UnRegister(consumer.id, startFrom, startFrom))
+                )
+            }
           }
 
         case UnRegister(id, previousOffset, finalOffset) ⇒
-          activeConsumers -= 1
-          val consumer = findAndRemoveConsumer(id, previousOffset)
+          if (findAndRemoveConsumer(id, previousOffset) != null)
+            activeConsumers -= 1
           if (activeConsumers == 0) {
             if (isClosed(in)) completeStage()
             else if (head != finalOffset) {
@@ -443,14 +449,15 @@ private[akka] class BroadcastHub[T](bufferSize: Int) extends GraphStageWithMater
               if (!hasBeenPulled(in)) pull(in)
             }
           } else checkUnblock(previousOffset)
+
         case Advance(id, previousOffset) ⇒
           val newOffset = previousOffset + DemandThreshold
-          // Move the consumer from its last known offest to its new one. Check if we are unblocked.
+          // Move the consumer from its last known offset to its new one. Check if we are unblocked.
           val consumer = findAndRemoveConsumer(id, previousOffset)
           addConsumer(consumer, newOffset)
           checkUnblock(previousOffset)
         case NeedWakeup(id, previousOffset, currentOffset) ⇒
-          // Move the consumer from its last known offest to its new one. Check if we are unblocked.
+          // Move the consumer from its last known offset to its new one. Check if we are unblocked.
           val consumer = findAndRemoveConsumer(id, previousOffset)
           addConsumer(consumer, currentOffset)
 
diff --git a/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala b/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
index 1e2aaa4909..bda38d6eef 100644
--- a/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
+++ b/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
@@ -1061,8 +1061,8 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
     @tailrec
     final private[stage] def onStart(): Unit = {
       (currentState.getAndSet(Initializing): @unchecked) match {
-        case Pending(l) ⇒ l.reverse.foreach(ack ⇒ {
-          onAsyncInput(ack.e)
+        case Pending(l) ⇒ l.reverse.foreach(evt ⇒ {
+          onAsyncInput(evt.e, evt.handlingPromise)
         })
       }
       if (!currentState.compareAndSet(Initializing, Initialized)) {
@@ -1082,10 +1082,12 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
       waitingForProcessing.clear()
     }
 
-    private def onAsyncInput(event: T) = interpreter.onAsyncInput(GraphStageLogic.this, event, handler.asInstanceOf[Any ⇒ Unit])
+    private def onAsyncInput(event: T, promise: OptionVal[Promise[Done]]) = {
+      interpreter.onAsyncInput(GraphStageLogic.this, event, promise, handler.asInstanceOf[Any ⇒ Unit])
+    }
 
     private def sendEvent(event: T, promise: Promise[Done]): Promise[Done] = {
-      onAsyncInput(event)
+      onAsyncInput(event, OptionVal.Some(promise))
       currentState.get() match {
         case Completed ⇒ failPromiseOnComplete(promise)
         case _         ⇒ promise
@@ -1123,7 +1125,7 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
 
     private def failPromiseOnComplete(promise: Promise[Done]): Promise[Done] = {
       waitingForProcessing.remove(promise)
-      promise.tryFailure(new StreamDetachedException())
+      promise.tryFailure(new StreamDetachedException("Stage stopped before async invocation was processed"))
       promise
     }
 
@@ -1132,14 +1134,14 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
       @tailrec
       def internalInvoke(event: T): Unit = currentState.get() match {
         // started - can just send message to stream
-        case Initialized       ⇒ onAsyncInput(event)
+        case Initialized       ⇒ onAsyncInput(event, OptionVal.None)
         // not started yet
         case list @ Pending(l) ⇒ if (!currentState.compareAndSet(list, Pending(Event(event, OptionVal.None) :: l))) internalInvoke(event)
         // initializing is in progress in another thread (initializing thread is managed by akka)
         case Initializing ⇒ if (!currentState.compareAndSet(Initializing, Pending(Event(event, OptionVal.None) :: Nil))) {
           (currentState.get(): @unchecked) match {
             case list @ Pending(l) ⇒ if (!currentState.compareAndSet(list, Pending(Event(event, OptionVal.None) :: l))) internalInvoke(event)
-            case Initialized       ⇒ onAsyncInput(event)
+            case Initialized       ⇒ onAsyncInput(event, OptionVal.None)
           }
         }
         case Completed ⇒ // do nothing here as stream is completed
@@ -1401,13 +1403,22 @@ trait AsyncCallback[T] {
   /**
    * Dispatch an asynchronous notification. This method is thread-safe and
    * may be invoked from external execution contexts.
+   *
+   * For cases where it is important to know if the notification was ever processed or not
+   * see [AsyncCallback#invokeWithFeedback]]
    */
   def invoke(t: T): Unit
   /**
-   * Dispatch an asynchronous notification.
-   * This method is thread-safe and may be invoked from external execution contexts.
-   * Promise in `HasCallbackPromise` will fail if stream is already closed or closed before
-   * being able to process the event
+   * Dispatch an asynchronous notification. This method is thread-safe and
+   * may be invoked from external execution contexts.
+   *
+   * The method returns directly and the returned future is then completed once the event
+   * has been handled by the stage, if the event triggers an exception from the handler the future
+   * is failed with that exception and finally if the stage was stopped before the event has been
+   * handled the future is failed with `StreamDetachedException`.
+   *
+   * The handling of the returned future incurs a slight overhead, so for cases where it does not matter
+   * to the invoking logic see [[AsyncCallback#invoke]]
    */
   def invokeWithFeedback(t: T): Future[Done]
 }