diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowGroupBySpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowGroupBySpec.scala
index 30d8cda9aa..7e0885e5c8 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowGroupBySpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowGroupBySpec.scala
@@ -3,15 +3,22 @@
  */
 package akka.stream.scaladsl
 
+import java.util
+
 import akka.NotUsed
+import akka.actor.ActorSystem
+import akka.stream.Attributes._
+import akka.stream.impl.SinkModule
+import akka.stream.impl.StreamLayout.Module
+import akka.util.ByteString
+import scala.annotation.tailrec
+import scala.concurrent.{ Promise, Await }
 import scala.concurrent.duration._
-import akka.stream.ActorMaterializer
-import akka.stream.ActorMaterializerSettings
+import akka.stream._
 import akka.stream.Supervision.resumingDecider
 import akka.stream.testkit._
 import akka.stream.testkit.Utils._
 import org.reactivestreams.Publisher
-import akka.stream.ActorAttributes
 import org.scalatest.concurrent.ScalaFutures
 import org.scalactic.ConversionCheckedTripleEquals
 import org.scalatest.concurrent.PatienceConfiguration.Timeout
@@ -19,6 +26,8 @@ import akka.stream.testkit.scaladsl.TestSource
 import akka.stream.testkit.scaladsl.TestSink
 import akka.testkit.AkkaSpec
 
+import scala.concurrent.forkjoin.ThreadLocalRandom
+
 object FlowGroupBySpec {
 
   implicit class Lift[M](val f: SubFlow[Int, M, Source[Int, M]#Repr, RunnableGraph[M]]) extends AnyVal {
@@ -70,6 +79,12 @@ class FlowGroupBySpec extends AkkaSpec {
 
   }
 
+  def randomByteString(size: Int): ByteString = {
+    val a = new Array[Byte](size)
+    ThreadLocalRandom.current().nextBytes(a)
+    ByteString(a)
+  }
+
   "groupBy" must {
     "work in the happy case" in assertAllStagesStopped {
       new SubstreamsSupport(groupCount = 2) {
@@ -120,6 +135,18 @@ class FlowGroupBySpec extends AkkaSpec {
         .sortBy(_.head) should ===(List(List("Aaa", "Abb"), List("Bcc"), List("Cdd", "Cee")))
     }
 
+    "fail when key function return null" in {
+      val down = Source(List("Aaa", "Abb", "Bcc", "Cdd", "Cee"))
+        .groupBy(3, e ⇒ if (e.startsWith("A")) null else e.substring(0, 1))
+        .grouped(10)
+        .mergeSubstreams
+        .runWith(TestSink.probe[Seq[String]])
+      down.request(1)
+      val ex = down.expectError()
+      ex.getMessage.indexOf("Key cannot be null") should not be (-1)
+      ex.isInstanceOf[IllegalArgumentException] should be(true)
+    }
+
     "accept cancellation of substreams" in assertAllStagesStopped {
       new SubstreamsSupport(groupCount = 2) {
         StreamPuppet(getSubFlow(1).runWith(Sink.asPublisher(false))).cancel()
@@ -309,6 +336,137 @@ class FlowGroupBySpec extends AkkaSpec {
       s1.expectError(ex)
     }
 
+    "emit subscribe before completed" in assertAllStagesStopped {
+      val futureGroupSource =
+        Source.single(0)
+          .groupBy(1, elem ⇒ "all")
+          .prefixAndTail(0)
+          .map(_._2)
+          .concatSubstreams
+          .runWith(Sink.head)
+      val pub: Publisher[Int] = Await.result(futureGroupSource, 3.seconds).runWith(Sink.asPublisher(false))
+      val probe = TestSubscriber.manualProbe[Int]()
+      pub.subscribe(probe)
+      val sub = probe.expectSubscription()
+      sub.request(1)
+      probe.expectNext(0)
+      probe.expectComplete()
+
+    }
+
+    "work under fuzzing stress test" in assertAllStagesStopped {
+      val publisherProbe = TestPublisher.manualProbe[ByteString]()
+      val subscriber = TestSubscriber.manualProbe[ByteString]()
+
+      val publisher = Source.fromPublisher[ByteString](publisherProbe)
+        .groupBy(256, elem ⇒ elem.head).map(_.reverse).mergeSubstreams
+        .groupBy(256, elem ⇒ elem.head).map(_.reverse).mergeSubstreams
+        .runWith(Sink.asPublisher(false))
+      publisher.subscribe(subscriber)
+
+      val upstreamSubscription = publisherProbe.expectSubscription()
+      val downstreamSubscription = subscriber.expectSubscription()
+
+      downstreamSubscription.request(300)
+      for (i ← 1 to 300) {
+        val byteString = randomByteString(10)
+        upstreamSubscription.expectRequest()
+        upstreamSubscription.sendNext(byteString)
+        subscriber.expectNext() should ===(byteString)
+      }
+      upstreamSubscription.sendComplete()
+    }
+
+    "work with random demand" in assertAllStagesStopped {
+      val mat = ActorMaterializer(ActorMaterializerSettings(system)
+        .withInputBuffer(initialSize = 1, maxSize = 1))
+
+      var blockingNextElement: ByteString = null.asInstanceOf[ByteString]
+
+      val probes = new java.util.ArrayList[Promise[TestSubscriber.Probe[ByteString]]](100)
+      (0 to 99).foreach(_ ⇒ probes.add(Promise[TestSubscriber.Probe[ByteString]]()))
+
+      var probesWriterTop = 0
+      var probesReaderTop = 0
+
+      case class SubFlowState(probe: TestSubscriber.Probe[ByteString], hasDemand: Boolean, firstElement: ByteString)
+      val map = new util.HashMap[Int, SubFlowState]()
+
+      final class ProbeSink(val attributes: Attributes, shape: SinkShape[ByteString])(implicit system: ActorSystem) extends SinkModule[ByteString, TestSubscriber.Probe[ByteString]](shape) {
+        override def create(context: MaterializationContext) = {
+          val promise = probes.get(probesWriterTop)
+          val probe = TestSubscriber.probe[ByteString]()
+          promise.success(probe)
+          probesWriterTop += 1
+          (probe, probe)
+        }
+        override protected def newInstance(shape: SinkShape[ByteString]): SinkModule[ByteString, TestSubscriber.Probe[ByteString]] = new ProbeSink(attributes, shape)
+        override def withAttributes(attr: Attributes): Module = new ProbeSink(attr, amendShape(attr))
+      }
+
+      @tailrec
+      def randomDemand(): Unit = {
+        val nextIndex = ThreadLocalRandom.current().nextInt(0, map.size())
+        val key = new util.ArrayList(map.keySet()).get(nextIndex)
+        if (!map.get(key).hasDemand) {
+          val state = map.get(key)
+          map.put(key, SubFlowState(state.probe, true, state.firstElement))
+
+          state.probe.request(1)
+
+          //need to verify elements that are first element in subFlow or is in nextElement buffer before
+          // pushing next element from upstream
+          if (state.firstElement != null) {
+            state.probe.expectNext() should ===(state.firstElement)
+            map.put(key, SubFlowState(state.probe, false, null))
+            randomDemand()
+          } else if (blockingNextElement != null && Math.abs(blockingNextElement.head % 100) == key) {
+            state.probe.expectNext() should ===(blockingNextElement)
+            blockingNextElement = null
+            map.put(key, SubFlowState(state.probe, false, null))
+            randomDemand()
+          } else if (blockingNextElement != null) randomDemand()
+        } else randomDemand()
+      }
+
+      val publisherProbe = TestPublisher.manualProbe[ByteString]()
+      Source.fromPublisher[ByteString](publisherProbe)
+        .groupBy(100, elem ⇒ Math.abs(elem.head % 100)).to(new Sink(new ProbeSink(none, SinkShape(Inlet("ProbeSink.in"))))).run()(mat)
+
+      val upstreamSubscription = publisherProbe.expectSubscription()
+
+      for (i ← 1 to 400) {
+        val byteString = randomByteString(10)
+        val index = Math.abs(byteString.head % 100)
+
+        upstreamSubscription.expectRequest()
+        upstreamSubscription.sendNext(byteString)
+
+        if (map.get(index) == null) {
+          val probe: TestSubscriber.Probe[ByteString] = Await.result(probes.get(probesReaderTop).future, 300.millis)
+          probesReaderTop += 1
+          map.put(index, SubFlowState(probe, false, byteString))
+          //stream automatically requests next element
+        } else {
+          val state = map.get(index)
+          if (state.firstElement != null) { //first element in subFlow
+            if (!state.hasDemand) blockingNextElement = byteString
+            randomDemand()
+          } else if (state.hasDemand) {
+            if (blockingNextElement == null) {
+              state.probe.expectNext() should ===(byteString)
+              map.put(index, SubFlowState(state.probe, false, null))
+              randomDemand()
+            } else fail("INVALID CASE")
+          } else {
+            blockingNextElement = byteString
+            randomDemand()
+          }
+        }
+      }
+      upstreamSubscription.sendComplete()
+    }
+
   }
 
 }
diff --git a/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala b/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
index e4f3bcdd0a..ebb2454597 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
@@ -112,11 +112,10 @@ private[akka] case class ActorMaterializerImpl(system: ActorSystem,
             assignPort(source.shape.out, pub.asInstanceOf[Publisher[Any]])
             matVal.put(atomic, mat)
 
-          // FIXME: Remove this, only stream-of-stream ops need it
-          case stage: StageModule ⇒
-            val (processor, mat) = processorFor(stage, effectiveAttributes, effectiveSettings(effectiveAttributes))
+          case stage: ProcessorModule[_, _, _] ⇒
+            val (processor, mat) = stage.createProcessor()
             assignPort(stage.inPort, processor)
-            assignPort(stage.outPort, processor)
+            assignPort(stage.outPort, processor.asInstanceOf[Publisher[Any]])
             matVal.put(atomic, mat)
 
           case tls: TlsModule ⇒ // TODO solve this so TlsModule doesn't need special treatment here
@@ -175,16 +174,6 @@ private[akka] case class ActorMaterializerImpl(system: ActorSystem,
         }
       }
 
-      // FIXME: Remove this, only stream-of-stream ops need it
-      private def processorFor(op: StageModule,
-                               effectiveAttributes: Attributes,
-                               effectiveSettings: ActorMaterializerSettings): (Processor[Any, Any], Any) = op match {
-        case DirectProcessor(processorFactory, _) ⇒ processorFactory()
-        case _ ⇒
-          val (opprops, mat) = ActorProcessorFactory.props(ActorMaterializerImpl.this, op, effectiveAttributes)
-          ActorProcessorFactory[Any, Any](
-            actorOf(opprops, stageName(effectiveAttributes), effectiveSettings.dispatcher)) -> mat
-      }
     }
 
     session.materialize().asInstanceOf[Mat]
@@ -294,27 +283,3 @@ private[akka] class StreamSupervisor(settings: ActorMaterializerSettings, haveSh
   override def postStop(): Unit = haveShutDown.set(true)
 }
 
-/**
- * INTERNAL API
- */
-private[akka] object ActorProcessorFactory {
-  import akka.stream.impl.Stages._
-
-  def props(materializer: ActorMaterializer, op: StageModule, parentAttributes: Attributes): (Props, Any) = {
-    val att = parentAttributes and op.attributes
-    // USE THIS TO AVOID CLOSING OVER THE MATERIALIZER BELOW
-    // Also, otherwise the attributes will not affect the settings properly!
-    val settings = materializer.effectiveSettings(att)
-    op match {
-      case GroupBy(maxSubstreams, f, _) ⇒ (GroupByProcessorImpl.props(settings, maxSubstreams, f), ())
-      case DirectProcessor(p, m)        ⇒ throw new AssertionError("DirectProcessor cannot end up in ActorProcessorFactory")
-    }
-  }
-
-  def apply[I, O](impl: ActorRef): ActorProcessor[I, O] = {
-    val p = new ActorProcessor[I, O](impl)
-    // Resolve cyclic dependency with actor. This MUST be the first message no matter what.
-    impl ! ExposedPublisher(p.asInstanceOf[ActorPublisher[Any]])
-    p
-  }
-}
diff --git a/akka-stream/src/main/scala/akka/stream/impl/FlowModule.scala b/akka-stream/src/main/scala/akka/stream/impl/FlowModule.scala
deleted file mode 100644
index 0188d83898..0000000000
--- a/akka-stream/src/main/scala/akka/stream/impl/FlowModule.scala
+++ /dev/null
@@ -1,24 +0,0 @@
-/**
- * Copyright (C) 2015-2016 Lightbend Inc. <http://www.lightbend.com>
- */
-package akka.stream.impl
-
-import akka.stream._
-import akka.stream.impl.StreamLayout.Module
-import akka.event.Logging
-
-/**
- * INTERNAL API
- */
-private[stream] trait FlowModule[In, Out, Mat] extends StreamLayout.AtomicModule {
-  override def replaceShape(s: Shape) =
-    if (s != shape) throw new UnsupportedOperationException("cannot replace the shape of a FlowModule")
-    else this
-
-  val inPort = Inlet[In]("Flow.in")
-  val outPort = Outlet[Out]("Flow.out")
-  override val shape = new FlowShape(inPort, outPort)
-
-  protected def label: String = Logging.simpleName(this)
-  final override def toString: String = f"$label [${System.identityHashCode(this)}%08x]"
-}
diff --git a/akka-stream/src/main/scala/akka/stream/impl/GroupByProcessorImpl.scala b/akka-stream/src/main/scala/akka/stream/impl/GroupByProcessorImpl.scala
deleted file mode 100644
index 95e3cf14ce..0000000000
--- a/akka-stream/src/main/scala/akka/stream/impl/GroupByProcessorImpl.scala
+++ /dev/null
@@ -1,101 +0,0 @@
-/**
- * Copyright (C) 2009-2016 Lightbend Inc. <http://www.lightbend.com>
- */
-package akka.stream.impl
-
-import scala.util.control.NonFatal
-import akka.actor.{ Deploy, Props }
-import akka.stream.ActorMaterializerSettings
-import akka.stream.Supervision
-import akka.stream.scaladsl.Source
-
-/**
- * INTERNAL API
- */
-private[akka] object GroupByProcessorImpl {
-  def props(settings: ActorMaterializerSettings, maxSubstreams: Int, keyFor: Any ⇒ Any): Props =
-    Props(new GroupByProcessorImpl(settings, maxSubstreams, keyFor)).withDeploy(Deploy.local)
-
-  private case object Drop
-}
-
-/**
- * INTERNAL API
- */
-private[akka] class GroupByProcessorImpl(settings: ActorMaterializerSettings, val maxSubstreams: Int, val keyFor: Any ⇒ Any)
-  extends MultiStreamOutputProcessor(settings) {
-
-  import MultiStreamOutputProcessor._
-  import GroupByProcessorImpl.Drop
-
-  val decider = settings.supervisionDecider
-  val keyToSubstreamOutput = collection.mutable.Map.empty[Any, SubstreamOutput]
-
-  var pendingSubstreamOutput: SubstreamOutput = _
-
-  // No substream is open yet. If downstream cancels now, we are complete
-  val waitFirst = TransferPhase(primaryInputs.NeedsInput && primaryOutputs.NeedsDemand) { () ⇒
-    val elem = primaryInputs.dequeueInputElement()
-    tryKeyFor(elem) match {
-      case Drop ⇒
-      case key  ⇒ nextPhase(openSubstream(elem, key))
-    }
-  }
-
-  // some substreams are open now. If downstream cancels, we still continue until the substreams are closed
-  val waitNext = TransferPhase(primaryInputs.NeedsInput) { () ⇒
-    val elem = primaryInputs.dequeueInputElement()
-    tryKeyFor(elem) match {
-      case Drop ⇒
-      case key ⇒
-        keyToSubstreamOutput.get(key) match {
-          case Some(substream) if substream.isOpen ⇒ nextPhase(dispatchToSubstream(elem, keyToSubstreamOutput(key)))
-          case None if primaryOutputs.isOpen       ⇒ nextPhase(openSubstream(elem, key))
-          case _                                   ⇒ // stay
-        }
-    }
-  }
-
-  private def tryKeyFor(elem: Any): Any =
-    try keyFor(elem) catch {
-      case NonFatal(e) if decider(e) != Supervision.Stop ⇒
-        if (settings.debugLogging)
-          log.debug("Dropped element [{}] due to exception from groupBy function: {}", elem, e.getMessage)
-        Drop
-    }
-
-  def openSubstream(elem: Any, key: Any): TransferPhase = TransferPhase(primaryOutputs.NeedsDemandOrCancel) { () ⇒
-    if (primaryOutputs.isClosed) {
-      // Just drop, we do not open any more substreams
-      nextPhase(waitNext)
-    } else {
-      if (keyToSubstreamOutput.size == maxSubstreams)
-        throw new IllegalStateException(s"cannot open substream for key '$key': too many substreams open")
-      val substreamOutput = createSubstreamOutput()
-      val substreamFlow = Source.fromPublisher[Any](substreamOutput)
-      primaryOutputs.enqueueOutputElement(substreamFlow)
-      keyToSubstreamOutput(key) = substreamOutput
-      nextPhase(dispatchToSubstream(elem, substreamOutput))
-    }
-  }
-
-  def dispatchToSubstream(elem: Any, substream: SubstreamOutput): TransferPhase = {
-    pendingSubstreamOutput = substream
-    TransferPhase(substream.NeedsDemand) { () ⇒
-      substream.enqueueOutputElement(elem)
-      pendingSubstreamOutput = null
-      nextPhase(waitNext)
-    }
-  }
-
-  initialPhase(1, waitFirst)
-
-  override def invalidateSubstreamOutput(substream: SubstreamKey): Unit = {
-    if ((pendingSubstreamOutput ne null) && substream == pendingSubstreamOutput.key) {
-      pendingSubstreamOutput = null
-      nextPhase(waitNext)
-    }
-    super.invalidateSubstreamOutput(substream)
-  }
-
-}
diff --git a/akka-stream/src/main/scala/akka/stream/impl/Sinks.scala b/akka-stream/src/main/scala/akka/stream/impl/Sinks.scala
index 3929c9f39c..3fb9bbe8ab 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/Sinks.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/Sinks.scala
@@ -5,7 +5,7 @@ package akka.stream.impl
 
 import akka.stream.impl.QueueSink.{ Output, Pull }
 import akka.{ Done, NotUsed }
-import akka.actor.{ ActorRef, Props }
+import akka.actor.{ ActorRef, Actor, Props }
 import akka.stream.Attributes.InputBuffer
 import akka.stream._
 import akka.stream.impl.Stages.DefaultAttributes
@@ -100,10 +100,12 @@ private[akka] final class FanoutPublisherSink[In](
 
   override def create(context: MaterializationContext): (Subscriber[In], Publisher[In]) = {
     val actorMaterializer = ActorMaterializer.downcast(context.materializer)
-    val fanoutProcessor = ActorProcessorFactory[In, In](
-      actorMaterializer.actorOf(
-        context,
-        FanoutProcessorImpl.props(actorMaterializer.effectiveSettings(attributes))))
+    val impl = actorMaterializer.actorOf(
+      context,
+      FanoutProcessorImpl.props(actorMaterializer.effectiveSettings(attributes)))
+    val fanoutProcessor = new ActorProcessor[In, In](impl)
+    impl ! ExposedPublisher(fanoutProcessor.asInstanceOf[ActorPublisher[Any]])
+    // Resolve cyclic dependency with actor. This MUST be the first message no matter what.
     (fanoutProcessor, fanoutProcessor)
   }
 
diff --git a/akka-stream/src/main/scala/akka/stream/impl/Stages.scala b/akka-stream/src/main/scala/akka/stream/impl/Stages.scala
index fad770ed7c..b943026fe7 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/Stages.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/Stages.scala
@@ -134,12 +134,6 @@ private[stream] object Stages {
 
   import DefaultAttributes._
 
-  // FIXME: To be deprecated as soon as stream-of-stream operations are stages
-  sealed trait StageModule extends FlowModule[Any, Any, Any] {
-    def withAttributes(attributes: Attributes): StageModule
-    override def carbonCopy: Module = withAttributes(attributes)
-  }
-
   /*
    * Stage that is backed by a GraphStage but can be symbolically introspected
    */
@@ -189,14 +183,4 @@ private[stream] object Stages {
     override def create(attr: Attributes): Stage[T, T] = fusing.Buffer(size, overflowStrategy)
   }
 
-  // FIXME: These are not yet proper stages, therefore they use the deprecated StageModule infrastructure
-
-  final case class GroupBy(maxSubstreams: Int, f: Any ⇒ Any, attributes: Attributes = groupBy) extends StageModule {
-    override def withAttributes(attributes: Attributes) = copy(attributes = attributes)
-    override protected def label: String = s"GroupBy($maxSubstreams)"
-  }
-
-  final case class DirectProcessor(p: () ⇒ (Processor[Any, Any], Any), attributes: Attributes = processor) extends StageModule {
-    override def withAttributes(attributes: Attributes) = copy(attributes = attributes)
-  }
 }
diff --git a/akka-stream/src/main/scala/akka/stream/impl/StreamLayout.scala b/akka-stream/src/main/scala/akka/stream/impl/StreamLayout.scala
index 0fa9bb9047..d91a783f4a 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/StreamLayout.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/StreamLayout.scala
@@ -7,6 +7,7 @@ import java.util.concurrent.atomic.{ AtomicReference }
 import java.{ util ⇒ ju }
 import akka.NotUsed
 import akka.stream.impl.MaterializerSession.MaterializationPanic
+import akka.stream.impl.Stages.DefaultAttributes
 import akka.stream.impl.StreamLayout.Module
 import akka.stream.scaladsl.Keep
 import akka.stream._
@@ -935,3 +936,20 @@ private[stream] abstract class MaterializerSession(val topLevel: StreamLayout.Mo
     }
 
 }
+
+/**
+  * INTERNAL API
+  */
+private[akka] final case class ProcessorModule[In, Out, Mat](val createProcessor: () ⇒ (Processor[In, Out], Mat),
+                                                             attributes: Attributes = DefaultAttributes.processor) extends StreamLayout.AtomicModule {
+  val inPort = Inlet[In]("ProcessorModule.in")
+  val outPort = Outlet[Out]("ProcessorModule.out")
+  override val shape = new FlowShape(inPort, outPort)
+
+  override def replaceShape(s: Shape) = if (s != shape) throw new UnsupportedOperationException("cannot replace the shape of a FlowModule")
+  else this
+
+  override def withAttributes(attributes: Attributes) = copy(attributes = attributes)
+  override def carbonCopy: Module = withAttributes(attributes)
+  override def toString: String = f"ProcessorModule [${System.identityHashCode(this)}%08x]"
+}
diff --git a/akka-stream/src/main/scala/akka/stream/impl/StreamOfStreamProcessors.scala b/akka-stream/src/main/scala/akka/stream/impl/StreamOfStreamProcessors.scala
deleted file mode 100644
index e038b398e6..0000000000
--- a/akka-stream/src/main/scala/akka/stream/impl/StreamOfStreamProcessors.scala
+++ /dev/null
@@ -1,226 +0,0 @@
-/**
- * Copyright (C) 2009-2016 Lightbend Inc. <http://www.lightbend.com>
- */
-package akka.stream.impl
-
-import java.util.concurrent.atomic.AtomicReference
-import akka.actor._
-import akka.stream.ActorMaterializerSettings
-import org.reactivestreams.{ Publisher, Subscriber, Subscription }
-import scala.collection.mutable
-
-/**
- * INTERNAL API
- */
-private[akka] object MultiStreamOutputProcessor {
-  final case class SubstreamKey(id: Long)
-  final case class SubstreamRequestMore(substream: SubstreamKey, demand: Long) extends DeadLetterSuppression with NoSerializationVerificationNeeded
-  final case class SubstreamCancel(substream: SubstreamKey) extends DeadLetterSuppression with NoSerializationVerificationNeeded
-  final case class SubstreamSubscribe(substream: SubstreamKey, subscriber: Subscriber[Any]) extends DeadLetterSuppression with NoSerializationVerificationNeeded
-  final case class SubstreamSubscriptionTimeout(substream: SubstreamKey) extends DeadLetterSuppression with NoSerializationVerificationNeeded
-
-  class SubstreamSubscription(val parent: ActorRef, val substreamKey: SubstreamKey) extends Subscription {
-    override def request(elements: Long): Unit = parent ! SubstreamRequestMore(substreamKey, elements)
-    override def cancel(): Unit = parent ! SubstreamCancel(substreamKey)
-    override def toString = "SubstreamSubscription" + System.identityHashCode(this)
-  }
-
-  object SubstreamOutput {
-    sealed trait PublisherState
-    sealed trait CompletedState extends PublisherState
-    case object Open extends PublisherState
-    final case class Attached(sub: Subscriber[Any]) extends PublisherState
-    case object Completed extends CompletedState
-    case object Cancelled extends CompletedState
-    final case class Failed(e: Throwable) extends CompletedState
-  }
-
-  class SubstreamOutput(val key: SubstreamKey, actor: ActorRef, pump: Pump, subscriptionTimeout: Cancellable)
-    extends SimpleOutputs(actor, pump) with Publisher[Any] {
-    import ReactiveStreamsCompliance._
-
-    import SubstreamOutput._
-
-    private val subscription = new SubstreamSubscription(actor, key)
-    private val state = new AtomicReference[PublisherState](Open)
-
-    override def subreceive: SubReceive =
-      throw new UnsupportedOperationException("Substream outputs are managed in a dedicated receive block")
-
-    def isAttached = state.get().isInstanceOf[Attached]
-
-    def enqueueOutputDemand(demand: Long): Unit = {
-      downstreamDemand += demand
-      pump.pump()
-    }
-
-    override def error(e: Throwable): Unit = {
-      if (!downstreamCompleted) {
-        closePublisher(Failed(e))
-        downstreamCompleted = true
-      }
-    }
-
-    override def cancel(): Unit = {
-      if (!downstreamCompleted) {
-        closePublisher(Cancelled)
-        downstreamCompleted = true
-      }
-    }
-
-    override def complete(): Unit = {
-      if (!downstreamCompleted) {
-        closePublisher(Completed)
-        downstreamCompleted = true
-      }
-    }
-
-    private def closePublisher(withState: CompletedState): Unit = {
-      subscriptionTimeout.cancel()
-      state.getAndSet(withState) match {
-        case _: CompletedState ⇒ throw new IllegalStateException("Attempted to double shutdown publisher")
-        case Attached(sub) ⇒
-          if (subscriber eq null) tryOnSubscribe(sub, CancelledSubscription)
-          closeSubscriber(sub, withState)
-        case Open ⇒ // No action needed
-      }
-    }
-
-    private def closeSubscriber(s: Subscriber[Any], withState: CompletedState): Unit = withState match {
-      case Completed                ⇒ tryOnComplete(s)
-      case Cancelled                ⇒ // nothing to do
-      case Failed(e: SpecViolation) ⇒ // nothing to do
-      case Failed(e)                ⇒ tryOnError(s, e)
-    }
-
-    override def subscribe(s: Subscriber[_ >: Any]): Unit = {
-      requireNonNullSubscriber(s)
-      subscriptionTimeout.cancel()
-      if (state.compareAndSet(Open, Attached(s))) actor ! SubstreamSubscribe(key, s)
-      else {
-        state.get() match {
-          case _: Attached | Cancelled ⇒
-            rejectAdditionalSubscriber(s, "Substream publisher")
-          case c: CompletedState ⇒
-            tryOnSubscribe(s, CancelledSubscription)
-            closeSubscriber(s, c)
-          case Open ⇒
-            throw new IllegalStateException("Publisher cannot become open after being used before")
-        }
-      }
-    }
-
-    def attachSubscriber(s: Subscriber[Any]): Unit =
-      if (subscriber eq null) {
-        subscriber = s
-        tryOnSubscribe(subscriber, subscription)
-      } else
-        rejectAdditionalSubscriber(s, "Substream publisher")
-  }
-}
-
-/**
- * INTERNAL API
- */
-private[akka] trait MultiStreamOutputProcessorLike extends Pump with StreamSubscriptionTimeoutSupport {
-  this: Actor with ActorLogging ⇒
-
-  import MultiStreamOutputProcessor._
-  import StreamSubscriptionTimeoutSupport._
-
-  protected def nextId(): Long
-
-  // stream keys will be removed from this map on cancellation/subscription-timeout, never assume a key is present
-  private val substreamOutputs = mutable.Map.empty[SubstreamKey, SubstreamOutput]
-
-  protected def createSubstreamOutput(): SubstreamOutput = {
-    val id = SubstreamKey(nextId())
-    val cancellable = scheduleSubscriptionTimeout(self, SubstreamSubscriptionTimeout(id))
-    val output = new SubstreamOutput(id, self, this, cancellable)
-    substreamOutputs(output.key) = output
-    output
-  }
-
-  protected def invalidateSubstreamOutput(substream: SubstreamKey): Unit = {
-    cancelSubstreamOutput(substream)
-    pump()
-  }
-
-  protected def cancelSubstreamOutput(substream: SubstreamKey): Unit = {
-    substreamOutputs.get(substream) match {
-      case Some(sub) ⇒
-        sub.cancel()
-        substreamOutputs -= substream
-      case _ ⇒ // ignore, already completed...
-    }
-  }
-
-  protected def completeSubstreamOutput(substream: SubstreamKey): Unit = {
-    substreamOutputs.get(substream) match {
-      case Some(sub) ⇒
-        sub.complete()
-        substreamOutputs -= substream
-      case _ ⇒ // ignore, already completed...
-    }
-  }
-
-  protected def failOutputs(e: Throwable): Unit = {
-    substreamOutputs.values foreach (_.error(e))
-  }
-
-  protected def finishOutputs(): Unit = {
-    substreamOutputs.values foreach (_.complete())
-  }
-
-  val outputSubstreamManagement: Receive = {
-    case SubstreamRequestMore(key, demand) ⇒
-      substreamOutputs.get(key) match {
-        case Some(sub) ⇒
-          if (demand < 1) // According to Reactive Streams Spec 3.9, with non-positive demand must yield onError
-            sub.error(ReactiveStreamsCompliance.numberOfElementsInRequestMustBePositiveException)
-          else
-            sub.enqueueOutputDemand(demand)
-        case _ ⇒ // ignore...
-      }
-    case SubstreamSubscribe(key, subscriber) ⇒ substreamOutputs.get(key) match {
-      case Some(sub) ⇒ sub.attachSubscriber(subscriber)
-      case _         ⇒ // ignore...
-    }
-    case SubstreamSubscriptionTimeout(key) ⇒ substreamOutputs.get(key) match {
-      case Some(sub) if !sub.isAttached ⇒ subscriptionTimedOut(sub)
-      case _                            ⇒ // ignore...
-    }
-    case SubstreamCancel(key) ⇒
-      invalidateSubstreamOutput(key)
-  }
-
-  override protected def handleSubscriptionTimeout(target: Publisher[_], cause: Exception) = target match {
-    case s: SubstreamOutput ⇒
-      s.error(cause)
-      s.attachSubscriber(CancelingSubscriber)
-    case _ ⇒ // ignore
-  }
-}
-
-/**
- * INTERNAL API
- */
-private[akka] abstract class MultiStreamOutputProcessor(_settings: ActorMaterializerSettings) extends ActorProcessorImpl(_settings) with MultiStreamOutputProcessorLike {
-  private var _nextId = 0L
-  protected def nextId(): Long = { _nextId += 1; _nextId }
-
-  override val subscriptionTimeoutSettings = _settings.subscriptionTimeoutSettings
-
-  override protected def fail(e: Throwable): Unit = {
-    failOutputs(e)
-    super.fail(e)
-  }
-
-  override def pumpFinished(): Unit = {
-    finishOutputs()
-    super.pumpFinished()
-  }
-
-  override def activeReceive: Receive = primaryInputs.subreceive orElse primaryOutputs.subreceive orElse outputSubstreamManagement
-}
-
diff --git a/akka-stream/src/main/scala/akka/stream/impl/SubFlowImpl.scala b/akka-stream/src/main/scala/akka/stream/impl/SubFlowImpl.scala
index 61de42a8dd..4cb2c28f89 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/SubFlowImpl.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/SubFlowImpl.scala
@@ -19,9 +19,6 @@ class SubFlowImpl[In, Out, Mat, F[+_], C](val subFlow: Flow[In, Out, NotUsed],
                                           finishFunction: Sink[In, NotUsed] ⇒ C)
   extends SubFlow[Out, Mat, F, C] {
 
-  override def deprecatedAndThen[U](op: Stages.StageModule): SubFlow[U, Mat, F, C] =
-    new SubFlowImpl[In, U, Mat, F, C](subFlow.deprecatedAndThen(op), mergeBackFunction, finishFunction)
-
   override def via[T, Mat2](flow: Graph[FlowShape[Out, T], Mat2]): Repr[T] =
     new SubFlowImpl[In, T, Mat, F, C](subFlow.via(flow), mergeBackFunction, finishFunction)
 
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/StreamOfStreams.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/StreamOfStreams.scala
index 13627664e8..5e7b3cec69 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/StreamOfStreams.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/StreamOfStreams.scala
@@ -5,19 +5,21 @@ package akka.stream.impl.fusing
 
 import java.util.concurrent.atomic.AtomicReference
 import akka.NotUsed
+import akka.stream.ActorAttributes.SupervisionStrategy
 import akka.stream._
 import akka.stream.impl.Stages.DefaultAttributes
 import akka.stream.impl.SubscriptionTimeoutException
 import akka.stream.stage._
 import akka.stream.scaladsl._
 import akka.stream.actor.ActorSubscriberMessage
-import scala.collection.immutable
+import scala.collection.{ mutable, immutable }
 import scala.concurrent.duration.FiniteDuration
 import scala.util.control.NonFatal
 import scala.annotation.tailrec
 import akka.stream.impl.PublisherSource
 import akka.stream.impl.CancellingSubscriber
 import akka.stream.impl.{ Buffer ⇒ BufferImpl }
+import scala.collection.JavaConversions._
 
 /**
  * INTERNAL API
@@ -198,8 +200,7 @@ final class PrefixAndTail[T](n: Int) extends GraphStage[FlowShape[T, (immutable.
       // Otherwise substream is open, ignore
     }
 
-    setHandler(in, this)
-    setHandler(out, this)
+    setHandlers(in, out, this)
   }
 
   override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = new PrefixAndTailLogic(shape)
@@ -207,6 +208,179 @@ final class PrefixAndTail[T](n: Int) extends GraphStage[FlowShape[T, (immutable.
   override def toString: String = s"PrefixAndTail($n)"
 }
 
+/**
+ * INTERNAL API
+ */
+final class GroupBy[T, K](maxSubstreams: Int, keyFor: T ⇒ K) extends GraphStage[FlowShape[T, Source[T, NotUsed]]] {
+  val in: Inlet[T] = Inlet("GroupBy.in")
+  val out: Outlet[Source[T, NotUsed]] = Outlet("GroupBy.out")
+  override val shape: FlowShape[T, Source[T, NotUsed]] = FlowShape(in, out)
+  override def initialAttributes = DefaultAttributes.groupBy
+
+  override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = new TimerGraphStageLogic(shape) with OutHandler with InHandler {
+    parent ⇒
+    lazy val decider = inheritedAttributes.get[SupervisionStrategy].map(_.decider).getOrElse(Supervision.stoppingDecider)
+    private val activeSubstreamsMap = new java.util.HashMap[Any, SubstreamSource]()
+    private val closedSubstreams = new java.util.HashSet[Any]()
+    private var timeout: FiniteDuration = _
+    private var substreamWaitingToBePushed: Option[SubstreamSource] = None
+    private var nextElementKey: K = null.asInstanceOf[K]
+    private var nextElementValue: T = null.asInstanceOf[T]
+    private var _nextId = 0
+    private val substreamsJustStared = new java.util.HashSet[Any]()
+    private var firstPushCounter: Int = 0
+
+    private def nextId(): Long = { _nextId += 1; _nextId }
+
+    private def hasNextElement = nextElementKey != null
+
+    private def clearNextElement(): Unit = {
+      nextElementKey = null.asInstanceOf[K]
+      nextElementValue = null.asInstanceOf[T]
+    }
+
+    private def tryCompleteAll(): Boolean =
+      if (activeSubstreamsMap.isEmpty || (!hasNextElement && firstPushCounter == 0)) {
+        for (value ← activeSubstreamsMap.values()) value.complete()
+        completeStage()
+        true
+      } else false
+
+    private def fail(ex: Throwable): Unit = {
+      for (value ← activeSubstreamsMap.values()) value.fail(ex)
+      failStage(ex)
+    }
+
+    private def needToPull: Boolean = !(hasBeenPulled(in) || isClosed(in) || hasNextElement)
+
+    override def preStart(): Unit =
+      timeout = ActorMaterializer.downcast(interpreter.materializer).settings.subscriptionTimeoutSettings.timeout
+
+    override def onPull(): Unit = {
+      substreamWaitingToBePushed match {
+        case Some(substreamSource) ⇒
+          push(out, Source.fromGraph(substreamSource.source))
+          scheduleOnce(substreamSource.key, timeout)
+          substreamWaitingToBePushed = None
+        case None ⇒
+          if (hasNextElement) {
+            val subSubstreamSource = activeSubstreamsMap.get(nextElementKey)
+            if (subSubstreamSource.isAvailable) {
+              subSubstreamSource.push(nextElementValue)
+              clearNextElement()
+            }
+          } else tryPull(in)
+      }
+    }
+
+    override def onUpstreamFailure(ex: Throwable): Unit = fail(ex)
+
+    override def onDownstreamFinish(): Unit =
+      if (activeSubstreamsMap.isEmpty) completeStage() else setKeepGoing(true)
+
+    override def onPush(): Unit = try {
+      val elem = grab(in)
+      val key = keyFor(elem)
+      require(key != null, "Key cannot be null")
+      val substreamSource = activeSubstreamsMap.get(key)
+      if (substreamSource != null) {
+        if (substreamSource.isAvailable) substreamSource.push(elem)
+        else {
+          nextElementKey = key
+          nextElementValue = elem
+        }
+      } else {
+        if (activeSubstreamsMap.size == maxSubstreams)
+          fail(new IllegalStateException(s"Cannot open substream for key '$key': too many substreams open"))
+        else if (closedSubstreams.contains(key) && !hasBeenPulled(in))
+          pull(in)
+        else runSubstream(key, elem)
+      }
+    } catch {
+      case NonFatal(ex) ⇒
+        decider(ex) match {
+          case Supervision.Stop                         ⇒ fail(ex)
+          case Supervision.Resume | Supervision.Restart ⇒ if (!hasBeenPulled(in)) pull(in)
+        }
+    }
+
+    override def onUpstreamFinish(): Unit = {
+      if (!tryCompleteAll()) setKeepGoing(true)
+    }
+
+    private def runSubstream(key: K, value: T): Unit = {
+      val substreamSource = new SubstreamSource("GroupBySource " + nextId, key, value)
+      activeSubstreamsMap.put(key, substreamSource)
+      firstPushCounter += 1
+      if (isAvailable(out)) {
+        push(out, Source.fromGraph(substreamSource.source))
+        scheduleOnce(key, timeout)
+        substreamWaitingToBePushed = None
+      } else {
+        setKeepGoing(true)
+        substreamsJustStared.add(substreamSource)
+        substreamWaitingToBePushed = Some(substreamSource)
+      }
+    }
+
+    override protected def onTimer(timerKey: Any): Unit = {
+      val substreamSource = activeSubstreamsMap.get(timerKey)
+      if (substreamSource != null) {
+        substreamSource.timeout(timeout)
+        closedSubstreams.add(timerKey)
+        activeSubstreamsMap.remove(timerKey)
+        if (isClosed(in)) tryCompleteAll()
+      }
+    }
+
+    setHandlers(in, out, this)
+
+    private class SubstreamSource(name: String, val key: K, var firstElement: T) extends SubSourceOutlet[T](name) with OutHandler {
+      def firstPush(): Boolean = firstElement != null
+      def hasNextForSubSource = hasNextElement && nextElementKey == key
+      private def completeSubStream(): Unit = {
+        complete()
+        activeSubstreamsMap.remove(key)
+        closedSubstreams.add(key)
+      }
+
+      private def tryCompleteHandler(): Unit = {
+        if (parent.isClosed(in) && !hasNextForSubSource) {
+          completeSubStream()
+          tryCompleteAll()
+        }
+      }
+
+      override def onPull(): Unit = {
+        cancelTimer(key)
+        if (firstPush) {
+          firstPushCounter -= 1
+          push(firstElement)
+          firstElement = null.asInstanceOf[T]
+          substreamsJustStared.remove(this)
+          if (substreamsJustStared.isEmpty) setKeepGoing(false)
+        } else if (hasNextForSubSource) {
+          push(nextElementValue)
+          clearNextElement()
+        } else if (needToPull) pull(in)
+
+        tryCompleteHandler()
+      }
+
+      override def onDownstreamFinish(): Unit = {
+        if (hasNextElement && nextElementKey == key) clearNextElement()
+        if (firstPush()) firstPushCounter -= 1
+        completeSubStream()
+        if (parent.isClosed(in)) tryCompleteAll() else if (needToPull) pull(in)
+      }
+
+      setHandler(this)
+    }
+  }
+
+  override def toString: String = "GroupBy"
+
+}
 /**
  * INTERNAL API
  */
@@ -246,7 +420,7 @@ final class Split[T](decision: Split.SplitDecision, p: T ⇒ Boolean, substreamC
 
     private var timeout: FiniteDuration = _
     private var substreamSource: SubSourceOutlet[T] = null
-    private var substreamPushed = false
+    private var substreamWaitingToBePushed = false
     private var substreamCancelled = false
 
     override def preStart(): Unit = {
@@ -256,16 +430,16 @@ final class Split[T](decision: Split.SplitDecision, p: T ⇒ Boolean, substreamC
     setHandler(out, new OutHandler {
       override def onPull(): Unit = {
         if (substreamSource eq null) pull(in)
-        else if (!substreamPushed) {
+        else if (!substreamWaitingToBePushed) {
           push(out, Source.fromGraph(substreamSource.source))
           scheduleOnce(SubscriptionTimer, timeout)
-          substreamPushed = true
+          substreamWaitingToBePushed = true
         }
       }
 
       override def onDownstreamFinish(): Unit = {
         // If the substream is already cancelled or it has not been handed out, we can go away
-        if (!substreamPushed || substreamCancelled) completeStage()
+        if (!substreamWaitingToBePushed || substreamCancelled) completeStage()
       }
     })
 
@@ -300,8 +474,8 @@ final class Split[T](decision: Split.SplitDecision, p: T ⇒ Boolean, substreamC
         if (isAvailable(out)) {
           push(out, Source.fromGraph(substreamSource.source))
           scheduleOnce(SubscriptionTimer, timeout)
-          substreamPushed = true
-        } else substreamPushed = false
+          substreamWaitingToBePushed = true
+        } else substreamWaitingToBePushed = false
       }
     }
 
@@ -381,6 +555,8 @@ final class Split[T](decision: Split.SplitDecision, p: T ⇒ Boolean, substreamC
 
     }
   }
+  override def toString: String = "Split"
+
 }
 
 /**
@@ -406,11 +582,13 @@ final class SubSink[T](name: String, externalCallback: ActorSubscriberMessage 
 
   private val status = new AtomicReference[AnyRef]
 
-  def pullSubstream(): Unit = status.get match {
-    case f: AsyncCallback[Any] @unchecked ⇒ f.invoke(RequestOne)
-    case null ⇒
-      if (!status.compareAndSet(null, RequestOne))
-        status.get.asInstanceOf[Command ⇒ Unit](RequestOne)
+  def pullSubstream(): Unit = {
+    status.get match {
+      case f: AsyncCallback[Any] @unchecked ⇒ f.invoke(RequestOne)
+      case null ⇒
+        if (!status.compareAndSet(null, RequestOne))
+          status.get.asInstanceOf[Command ⇒ Unit](RequestOne)
+    }
   }
 
   def cancelSubstream(): Unit = status.get match {
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
index bf896f55e6..f546754483 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
@@ -1088,6 +1088,8 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * is [[akka.stream.Supervision#resume]] or [[akka.stream.Supervision#restart]]
    * the element is dropped and the stream and substreams continue.
    *
+   * Function `f`  MUST NOT return `null`. This will throw exception and trigger supervision decision mechanism.
+   *
    * '''Emits when''' an element for which the grouping function returns a group that has not yet been created.
    * Emits the new group
    *
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
index 2551439d63..1a7bd530cf 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
@@ -3,10 +3,10 @@
  */
 package akka.stream.scaladsl
 
-import akka.event.LoggingAdapter
+import akka.event.{Logging, LoggingAdapter}
 import akka.stream._
 import akka.Done
-import akka.stream.impl.Stages.{ DirectProcessor, StageModule }
+import akka.stream.impl.Stages.DefaultAttributes
 import akka.stream.impl.StreamLayout.Module
 import akka.stream.impl._
 import akka.stream.impl.fusing._
@@ -205,21 +205,6 @@ final class Flow[-In, +Out, +Mat](private[stream] override val module: Module)
       .replaceShape(FlowShape(ins(1), outs.head)))
   }
 
-  /** INTERNAL API */
-  // FIXME: Only exists to keep old stuff alive
-  private[stream] override def deprecatedAndThen[U](op: StageModule): Repr[U] = {
-    //No need to copy here, op is a fresh instance
-    if (this.isIdentity) new Flow(op).asInstanceOf[Repr[U]]
-    else new Flow(module.fuse(op, shape.out, op.inPort).replaceShape(FlowShape(shape.in, op.outPort)))
-  }
-
-  // FIXME: Only exists to keep old stuff alive
-  private[akka] def deprecatedAndThenMat[U, Mat2, O >: Out](processorFactory: () ⇒ (Processor[O, U], Mat2)): ReprMat[U, Mat2] = {
-    val op = DirectProcessor(processorFactory.asInstanceOf[() ⇒ (Processor[Any, Any], Any)])
-    if (this.isIdentity) new Flow(op).asInstanceOf[ReprMat[U, Mat2]]
-    else new Flow[In, U, Mat2](module.fuse(op, shape.out, op.inPort, Keep.right).replaceShape(FlowShape(shape.in, op.outPort)))
-  }
-
   /**
    * Change the attributes of this [[Flow]] to the given ones and seal the list
    * of attributes. This means that further calls will not be able to remove these
@@ -293,9 +278,8 @@ object Flow {
   /**
    * Creates a Flow from a Reactive Streams [[org.reactivestreams.Processor]] and returns a materialized value.
    */
-  def fromProcessorMat[I, O, Mat](processorFactory: () ⇒ (Processor[I, O], Mat)): Flow[I, O, Mat] = {
-    Flow[I].deprecatedAndThenMat(processorFactory)
-  }
+  def fromProcessorMat[I, O, M](processorFactory: () ⇒ (Processor[I, O], M)): Flow[I, O, M] =
+    new Flow(ProcessorModule(processorFactory))
 
   /**
    * Returns a `Flow` which outputs all its inputs.
@@ -1210,6 +1194,8 @@ trait FlowOps[+Out, +Mat] {
    * is [[akka.stream.Supervision.Resume]] or [[akka.stream.Supervision.Restart]]
    * the element is dropped and the stream and substreams continue.
    *
+   * Function `f`  MUST NOT return `null`. This will throw exception and trigger supervision decision mechanism.
+   *
    * '''Emits when''' an element for which the grouping function returns a group that has not yet been created.
    * Emits the new group
    *
@@ -1223,16 +1209,15 @@ trait FlowOps[+Out, +Mat] {
    *        that are supported; if more distinct keys are encountered then the stream fails
    */
   def groupBy[K](maxSubstreams: Int, f: Out ⇒ K): SubFlow[Out, Mat, Repr, Closed] = {
-    implicit def mat = GraphInterpreter.currentInterpreter.materializer
     val merge = new SubFlowImpl.MergeBack[Out, Repr] {
       override def apply[T](flow: Flow[Out, T, NotUsed], breadth: Int): Repr[T] =
-        deprecatedAndThen[Source[Out, NotUsed]](GroupBy(maxSubstreams, f.asInstanceOf[Any ⇒ Any]))
+        via(new GroupBy(maxSubstreams, f))
           .map(_.via(flow))
           .via(new FlattenMerge(breadth))
     }
     val finish: (Sink[Out, NotUsed]) ⇒ Closed = s ⇒
-      deprecatedAndThen[Source[Out, NotUsed]](GroupBy(maxSubstreams, f.asInstanceOf[Any ⇒ Any]))
-        .to(Sink.foreach(_.runWith(s)))
+      via(new GroupBy(maxSubstreams, f))
+        .to(Sink.foreach(_.runWith(s)(GraphInterpreter.currentInterpreter.materializer)))
     new SubFlowImpl(Flow[Out], merge, finish)
   }
 
@@ -1849,8 +1834,6 @@ trait FlowOps[+Out, +Mat] {
   /** INTERNAL API */
   private[scaladsl] def andThen[T](op: SymbolicStage[Out, T]): Repr[T] =
     via(SymbolicGraphStage(op))
-
-  private[scaladsl] def deprecatedAndThen[U](op: StageModule): Repr[U]
 }
 
 /**
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Graph.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Graph.scala
index edd6419571..e7c67f23b0 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Graph.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Graph.scala
@@ -8,7 +8,7 @@ import akka.stream._
 import akka.stream.impl._
 import akka.stream.impl.fusing.GraphStages
 import akka.stream.impl.fusing.GraphStages.MaterializedValueSource
-import akka.stream.impl.Stages.{ DefaultAttributes, StageModule }
+import akka.stream.impl.Stages.DefaultAttributes
 import akka.stream.impl.StreamLayout._
 import akka.stream.scaladsl.Partition.PartitionOutOfBoundsException
 import akka.stream.stage.{ OutHandler, InHandler, GraphStageLogic, GraphStage }
@@ -977,13 +977,6 @@ object GraphDSL extends GraphApply {
       source.out
     }
 
-    private[stream] def deprecatedAndThen(port: OutPort, op: StageModule): Unit = {
-      moduleInProgress =
-        moduleInProgress
-          .compose(op)
-          .wire(port, op.inPort)
-    }
-
     private[stream] def module: Module = moduleInProgress
 
     /** Converts this Scala DSL element to it's Java DSL counterpart. */
@@ -1118,11 +1111,6 @@ object GraphDSL extends GraphApply {
       override def via[T, Mat2](flow: Graph[FlowShape[Out, T], Mat2]): Repr[T] =
         super.~>(flow)(b)
 
-      override private[scaladsl] def deprecatedAndThen[U](op: StageModule): Repr[U] = {
-        b.deprecatedAndThen(outlet, op)
-        new PortOpsImpl(op.shape.out.asInstanceOf[Outlet[U]], b)
-      }
-
       def to[Mat2](sink: Graph[SinkShape[Out], Mat2]): Closed = {
         super.~>(sink)(b)
       }
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
index 5d2c4b8c14..62cba7bba1 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
@@ -3,10 +3,10 @@
  */
 package akka.stream.scaladsl
 
+import akka.stream.impl.Stages.DefaultAttributes
 import akka.{ Done, NotUsed }
 import akka.actor.{ ActorRef, Cancellable, Props }
 import akka.stream.actor.ActorPublisher
-import akka.stream.impl.Stages.{ DefaultAttributes, StageModule }
 import akka.stream.impl.StreamLayout.Module
 import akka.stream.impl.fusing.GraphStages
 import akka.stream.impl.fusing.GraphStages._
@@ -74,15 +74,6 @@ final class Source[+Out, +Mat](private[stream] override val module: Module)
   override def mapMaterializedValue[Mat2](f: Mat ⇒ Mat2): ReprMat[Out, Mat2] =
     new Source[Out, Mat2](module.transformMaterializedValue(f.asInstanceOf[Any ⇒ Any]))
 
-  /** INTERNAL API */
-  override private[scaladsl] def deprecatedAndThen[U](op: StageModule): Repr[U] = {
-    // No need to copy here, op is a fresh instance
-    new Source(
-      module
-        .fuse(op, shape.out, op.inPort)
-        .replaceShape(SourceShape(op.outPort)))
-  }
-
   /**
    * Connect this `Source` to a `Sink` and run it. The returned value is the materialized value
    * of the `Sink`, e.g. the `Publisher` of a [[akka.stream.scaladsl.Sink#publisher]].
diff --git a/project/MiMa.scala b/project/MiMa.scala
index b9c2b17b04..6b8f0b7314 100644
--- a/project/MiMa.scala
+++ b/project/MiMa.scala
@@ -747,6 +747,14 @@ object MiMa extends AutoPlugin {
         ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.http.scaladsl.DefaultSSLContextCreation.validateAndWarnAboutLooseSettings")
       ),
       "2.4.4" -> Seq(
+
+        //#20229 migrate GroupBy to GraphStage
+        ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.scaladsl.GraphDSL#Builder.deprecatedAndThen"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.scaladsl.Flow.deprecatedAndThen"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.scaladsl.Flow.deprecatedAndThenMat"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.scaladsl.Source.deprecatedAndThen"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.scaladsl.FlowOps.deprecatedAndThen"),
+
         // #20080, #20081 remove race condition on HTTP client
         ProblemFilters.exclude[DirectMissingMethodProblem]("akka.http.scaladsl.Http#HostConnectionPool.gatewayFuture"),
         ProblemFilters.exclude[IncompatibleMethTypeProblem]("akka.http.scaladsl.Http#HostConnectionPool.copy"),