diff --git a/akka-http-core/src/main/scala/akka/http/impl/util/StreamUtils.scala b/akka-http-core/src/main/scala/akka/http/impl/util/StreamUtils.scala
index 0a20fc95d9..0379ea38d9 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/util/StreamUtils.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/util/StreamUtils.scala
@@ -8,7 +8,6 @@ import java.util.concurrent.atomic.{ AtomicBoolean, AtomicReference }
 import akka.NotUsed
 import akka.http.scaladsl.model.RequestEntity
 import akka.stream._
-import akka.stream.impl.StreamLayout.Module
 import akka.stream.impl.fusing.GraphStages.SimpleLinearGraphStage
 import akka.stream.impl.{ PublisherSink, SinkModule, SourceModule }
 import akka.stream.scaladsl._
@@ -187,7 +186,7 @@ private[http] object StreamUtils {
     override protected def newInstance(shape: SinkShape[In]): SinkModule[In, Publisher[In]] =
       new OneTimePublisherSink[In](attributes, shape, cell)
 
-    override def withAttributes(attr: Attributes): Module =
+    override def withAttributes(attr: Attributes): OneTimePublisherSink[In] =
       new OneTimePublisherSink[In](attr, amendShape(attr), cell)
   }
   /** A copy of SubscriberSource that allows access to the subscriber through the cell but can only materialized once */
@@ -212,7 +211,7 @@ private[http] object StreamUtils {
 
     override protected def newInstance(shape: SourceShape[Out]): SourceModule[Out, Subscriber[Out]] =
       new OneTimeSubscriberSource[Out](attributes, shape, cell)
-    override def withAttributes(attr: Attributes): Module =
+    override def withAttributes(attr: Attributes): OneTimeSubscriberSource[Out] =
       new OneTimeSubscriberSource[Out](attr, amendShape(attr), cell)
   }
 
diff --git a/akka-http-core/src/test/java/akka/http/javadsl/model/JavaApiTestCases.java b/akka-http-core/src/test/java/akka/http/javadsl/model/JavaApiTestCases.java
index 3143313c88..250dc9d391 100644
--- a/akka-http-core/src/test/java/akka/http/javadsl/model/JavaApiTestCases.java
+++ b/akka-http-core/src/test/java/akka/http/javadsl/model/JavaApiTestCases.java
@@ -4,7 +4,6 @@
 
 package akka.http.javadsl.model;
 
-import akka.http.impl.util.Util;
 import akka.http.javadsl.model.headers.*;
 import akka.japi.Pair;
 
diff --git a/akka-stream-tests/src/test/scala/akka/stream/impl/StreamLayoutSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/impl/StreamLayoutSpec.scala
index d174b2e145..71c766438e 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/impl/StreamLayoutSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/impl/StreamLayoutSpec.scala
@@ -11,12 +11,10 @@ import akka.stream._
 class StreamLayoutSpec extends AkkaSpec {
   import StreamLayout._
 
-  def testAtomic(inPortCount: Int, outPortCount: Int): Module = new Module {
+  def testAtomic(inPortCount: Int, outPortCount: Int): Module = new AtomicModule {
     override val shape = AmorphousShape(List.fill(inPortCount)(Inlet("")), List.fill(outPortCount)(Outlet("")))
     override def replaceShape(s: Shape): Module = ???
 
-    override def subModules: Set[Module] = Set.empty
-
     override def carbonCopy: Module = ???
 
     override def attributes: Attributes = Attributes.none
@@ -174,7 +172,7 @@ class StreamLayoutSpec extends AkkaSpec {
     var publishers = Vector.empty[TestPublisher]
     var subscribers = Vector.empty[TestSubscriber]
 
-    override protected def materializeAtomic(atomic: Module, effectiveAttributes: Attributes,
+    override protected def materializeAtomic(atomic: AtomicModule, effectiveAttributes: Attributes,
                                              matVal: java.util.Map[Module, Any]): Unit = {
       for (inPort ← atomic.inPorts) {
         val subscriber = TestSubscriber(atomic, inPort)
diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphMatValueSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphMatValueSpec.scala
index 3fd0327754..ad0e503ee3 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphMatValueSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphMatValueSpec.scala
@@ -12,109 +12,162 @@ import akka.testkit.AkkaSpec
 
 class GraphMatValueSpec extends AkkaSpec {
 
-  val settings = ActorMaterializerSettings(system)
-    .withInputBuffer(initialSize = 2, maxSize = 16)
-
-  implicit val materializer = ActorMaterializer(settings)
-
   import GraphDSL.Implicits._
 
-  "A Graph with materialized value" must {
+  val foldSink = Sink.fold[Int, Int](0)(_ + _)
 
-    val foldSink = Sink.fold[Int, Int](0)(_ + _)
+  "A Graph with materialized value" when {
 
-    "expose the materialized value as source" in {
-      val sub = TestSubscriber.manualProbe[Int]()
-      val f = RunnableGraph.fromGraph(GraphDSL.create(foldSink) { implicit b ⇒
-        fold ⇒
-          Source(1 to 10) ~> fold
-          b.materializedValue.mapAsync(4)(identity) ~> Sink.fromSubscriber(sub)
-          ClosedShape
-      }).run()
+    for (autoFusing ← Seq(true, false)) {
+      val settings = ActorMaterializerSettings(system)
+        .withInputBuffer(initialSize = 2, maxSize = 16)
+        .withAutoFusing(autoFusing)
+      implicit val materializer = ActorMaterializer(settings)
 
-      val r1 = Await.result(f, 3.seconds)
-      sub.expectSubscription().request(1)
-      val r2 = sub.expectNext()
+      s"using autoFusing=$autoFusing" must {
 
-      r1 should ===(r2)
-    }
+        "expose the materialized value as source" in {
+          val sub = TestSubscriber.manualProbe[Int]()
+          val f = RunnableGraph.fromGraph(GraphDSL.create(foldSink) { implicit b ⇒
+            fold ⇒
+              Source(1 to 10) ~> fold
+              b.materializedValue.mapAsync(4)(identity) ~> Sink.fromSubscriber(sub)
+              ClosedShape
+          }).run()
 
-    "expose the materialized value as source multiple times" in {
-      val sub = TestSubscriber.manualProbe[Int]()
+          val r1 = Await.result(f, 3.seconds)
+          sub.expectSubscription().request(1)
+          val r2 = sub.expectNext()
 
-      val f = RunnableGraph.fromGraph(GraphDSL.create(foldSink) { implicit b ⇒
-        fold ⇒
-          val zip = b.add(ZipWith[Int, Int, Int](_ + _))
-          Source(1 to 10) ~> fold
-          b.materializedValue.mapAsync(4)(identity) ~> zip.in0
-          b.materializedValue.mapAsync(4)(identity) ~> zip.in1
+          r1 should ===(r2)
+        }
 
-          zip.out ~> Sink.fromSubscriber(sub)
-          ClosedShape
-      }).run()
+        "expose the materialized value as source multiple times" in {
+          val sub = TestSubscriber.manualProbe[Int]()
 
-      val r1 = Await.result(f, 3.seconds)
-      sub.expectSubscription().request(1)
-      val r2 = sub.expectNext()
+          val f = RunnableGraph.fromGraph(GraphDSL.create(foldSink) { implicit b ⇒
+            fold ⇒
+              val zip = b.add(ZipWith[Int, Int, Int](_ + _))
+              Source(1 to 10) ~> fold
+              b.materializedValue.mapAsync(4)(identity) ~> zip.in0
+              b.materializedValue.mapAsync(4)(identity) ~> zip.in1
 
-      r1 should ===(r2 / 2)
-    }
+              zip.out ~> Sink.fromSubscriber(sub)
+              ClosedShape
+          }).run()
 
-    // Exposes the materialized value as a stream value
-    val foldFeedbackSource: Source[Future[Int], Future[Int]] = Source.fromGraph(GraphDSL.create(foldSink) { implicit b ⇒
-      fold ⇒
-        Source(1 to 10) ~> fold
-        SourceShape(b.materializedValue)
-    })
+          val r1 = Await.result(f, 3.seconds)
+          sub.expectSubscription().request(1)
+          val r2 = sub.expectNext()
 
-    "allow exposing the materialized value as port" in {
-      val (f1, f2) = foldFeedbackSource.mapAsync(4)(identity).map(_ + 100).toMat(Sink.head)(Keep.both).run()
-      Await.result(f1, 3.seconds) should ===(55)
-      Await.result(f2, 3.seconds) should ===(155)
-    }
+          r1 should ===(r2 / 2)
+        }
 
-    "allow exposing the materialized value as port even if wrapped and the final materialized value is Unit" in {
-      val noMatSource: Source[Int, Unit] = foldFeedbackSource.mapAsync(4)(identity).map(_ + 100).mapMaterializedValue((_) ⇒ ())
-      Await.result(noMatSource.runWith(Sink.head), 3.seconds) should ===(155)
-    }
-
-    "work properly with nesting and reusing" in {
-      val compositeSource1 = Source.fromGraph(GraphDSL.create(foldFeedbackSource, foldFeedbackSource)(Keep.both) { implicit b ⇒
-        (s1, s2) ⇒
-          val zip = b.add(ZipWith[Int, Int, Int](_ + _))
-
-          s1.out.mapAsync(4)(identity) ~> zip.in0
-          s2.out.mapAsync(4)(identity).map(_ * 100) ~> zip.in1
-          SourceShape(zip.out)
-      })
-
-      val compositeSource2 = Source.fromGraph(GraphDSL.create(compositeSource1, compositeSource1)(Keep.both) { implicit b ⇒
-        (s1, s2) ⇒
-          val zip = b.add(ZipWith[Int, Int, Int](_ + _))
-          s1.out ~> zip.in0
-          s2.out.map(_ * 10000) ~> zip.in1
-          SourceShape(zip.out)
-      })
-
-      val (((f1, f2), (f3, f4)), result) = compositeSource2.toMat(Sink.head)(Keep.both).run()
-
-      Await.result(result, 3.seconds) should ===(55555555)
-      Await.result(f1, 3.seconds) should ===(55)
-      Await.result(f2, 3.seconds) should ===(55)
-      Await.result(f3, 3.seconds) should ===(55)
-      Await.result(f4, 3.seconds) should ===(55)
-
-    }
-
-    "work also when the source’s module is copied" in {
-      val foldFlow: Flow[Int, Int, Future[Int]] = Flow.fromGraph(GraphDSL.create(Sink.fold[Int, Int](0)(_ + _)) {
-        implicit builder ⇒
+        // Exposes the materialized value as a stream value
+        val foldFeedbackSource: Source[Future[Int], Future[Int]] = Source.fromGraph(GraphDSL.create(foldSink) { implicit b ⇒
           fold ⇒
-            FlowShape(fold.in, builder.materializedValue.mapAsync(4)(identity).outlet)
-      })
+            Source(1 to 10) ~> fold
+            SourceShape(b.materializedValue)
+        })
 
-      Await.result(Source(1 to 10).via(foldFlow).runWith(Sink.head), 3.seconds) should ===(55)
+        "allow exposing the materialized value as port" in {
+          val (f1, f2) = foldFeedbackSource.mapAsync(4)(identity).map(_ + 100).toMat(Sink.head)(Keep.both).run()
+          Await.result(f1, 3.seconds) should ===(55)
+          Await.result(f2, 3.seconds) should ===(155)
+        }
+
+        "allow exposing the materialized value as port even if wrapped and the final materialized value is Unit" in {
+          val noMatSource: Source[Int, Unit] = foldFeedbackSource.mapAsync(4)(identity).map(_ + 100).mapMaterializedValue((_) ⇒ ())
+          Await.result(noMatSource.runWith(Sink.head), 3.seconds) should ===(155)
+        }
+
+        "work properly with nesting and reusing" in {
+          val compositeSource1 = Source.fromGraph(GraphDSL.create(foldFeedbackSource, foldFeedbackSource)(Keep.both) { implicit b ⇒
+            (s1, s2) ⇒
+              val zip = b.add(ZipWith[Int, Int, Int](_ + _))
+
+              s1.out.mapAsync(4)(identity) ~> zip.in0
+              s2.out.mapAsync(4)(identity).map(_ * 100) ~> zip.in1
+              SourceShape(zip.out)
+          })
+
+          val compositeSource2 = Source.fromGraph(GraphDSL.create(compositeSource1, compositeSource1)(Keep.both) { implicit b ⇒
+            (s1, s2) ⇒
+              val zip = b.add(ZipWith[Int, Int, Int](_ + _))
+              s1.out ~> zip.in0
+              s2.out.map(_ * 10000) ~> zip.in1
+              SourceShape(zip.out)
+          })
+
+          val (((f1, f2), (f3, f4)), result) = compositeSource2.toMat(Sink.head)(Keep.both).run()
+
+          Await.result(result, 3.seconds) should ===(55555555)
+          Await.result(f1, 3.seconds) should ===(55)
+          Await.result(f2, 3.seconds) should ===(55)
+          Await.result(f3, 3.seconds) should ===(55)
+          Await.result(f4, 3.seconds) should ===(55)
+
+        }
+
+        "work also when the source’s module is copied" in {
+          val foldFlow: Flow[Int, Int, Future[Int]] = Flow.fromGraph(GraphDSL.create(foldSink) {
+            implicit builder ⇒
+              fold ⇒
+                FlowShape(fold.in, builder.materializedValue.mapAsync(4)(identity).outlet)
+          })
+
+          Await.result(Source(1 to 10).via(foldFlow).runWith(Sink.head), 3.seconds) should ===(55)
+        }
+
+        "work also when the source’s module is copied and the graph is extended before using the matValSrc" in {
+          val foldFlow: Flow[Int, Int, Future[Int]] = Flow.fromGraph(GraphDSL.create(foldSink) {
+            implicit builder ⇒
+              fold ⇒
+                val map = builder.add(Flow[Future[Int]].mapAsync(4)(identity))
+                builder.materializedValue ~> map
+                FlowShape(fold.in, map.outlet)
+          })
+
+          Await.result(Source(1 to 10).via(foldFlow).runWith(Sink.head), 3.seconds) should ===(55)
+        }
+
+        "perform side-effecting transformations even when not used as source" in {
+          var done = false
+          val g = GraphDSL.create() { implicit b ⇒
+            import GraphDSL.Implicits._
+            Source.empty.mapMaterializedValue(_ ⇒ done = true) ~> Sink.ignore
+            ClosedShape
+          }
+          val r = RunnableGraph.fromGraph(GraphDSL.create(Sink.ignore) { implicit b ⇒
+            (s) ⇒
+              b.add(g)
+              Source(1 to 10) ~> s
+              ClosedShape
+          })
+          r.run().futureValue should ===(akka.Done)
+          done should ===(true)
+        }
+
+        "produce NotUsed when not importing materialized value" in {
+          val source = Source.fromGraph(GraphDSL.create() { implicit b ⇒
+            SourceShape(b.materializedValue)
+          })
+          source.runWith(Sink.seq).futureValue should ===(List(akka.NotUsed))
+        }
+
+        "produce NotUsed when starting from Flow.via" in {
+          Source.empty.viaMat(Flow[Int].map(_ * 2))(Keep.right).to(Sink.ignore).run() should ===(akka.NotUsed)
+        }
+
+        "produce NotUsed when starting from Flow.via with transformation" in {
+          var done = false
+          Source.empty.viaMat(
+            Flow[Int].via(Flow[Int].mapMaterializedValue(_ ⇒ done = true)))(Keep.right)
+            .to(Sink.ignore).run() should ===(akka.NotUsed)
+          done should ===(true)
+        }
+
+      }
     }
-
   }
 }
diff --git a/akka-stream/src/main/boilerplate/akka/stream/scaladsl/GraphApply.scala.template b/akka-stream/src/main/boilerplate/akka/stream/scaladsl/GraphApply.scala.template
index d72a72c1cf..5282c5ca83 100644
--- a/akka-stream/src/main/boilerplate/akka/stream/scaladsl/GraphApply.scala.template
+++ b/akka-stream/src/main/boilerplate/akka/stream/scaladsl/GraphApply.scala.template
@@ -61,6 +61,8 @@ trait GraphApply {
 private[stream] object GraphApply {
   final class GraphImpl[S <: Shape, Mat](override val shape: S, private[stream] override val module: StreamLayout.Module)
     extends Graph[S, Mat] {
+    
+    override def toString: String = s"Graph($shape, $module)"
 
     override def withAttributes(attr: Attributes): Graph[S, Mat] =
       new GraphImpl(shape, module.withAttributes(attr))
diff --git a/akka-stream/src/main/scala/akka/stream/Fusing.scala b/akka-stream/src/main/scala/akka/stream/Fusing.scala
index d90abac97f..6ff1277fd5 100644
--- a/akka-stream/src/main/scala/akka/stream/Fusing.scala
+++ b/akka-stream/src/main/scala/akka/stream/Fusing.scala
@@ -31,11 +31,7 @@ object Fusing {
    * implementations based on [[akka.stream.stage.GraphStage]]) and not forbidden
    * via [[akka.stream.Attributes#AsyncBoundary]].
    */
-  def aggressive[S <: Shape, M](g: Graph[S, M]): FusedGraph[S, M] =
-    g match {
-      case fg: FusedGraph[_, _] ⇒ fg
-      case _                    ⇒ Impl.aggressive(g)
-    }
+  def aggressive[S <: Shape, M](g: Graph[S, M]): FusedGraph[S, M] = Impl.aggressive(g)
 
   /**
    * A fused graph of the right shape, containing a [[FusedModule]] which
@@ -48,6 +44,14 @@ object Fusing {
     override def withAttributes(attr: Attributes) = copy(module = module.withAttributes(attr))
   }
 
+  object FusedGraph {
+    def unapply[S <: Shape, M](g: Graph[S, M]): Option[(FusedModule, S)] =
+      g.module match {
+        case f: FusedModule => Some((f, g.shape))
+        case _              => None
+      }
+  }
+
   /**
    * When fusing a [[Graph]] a part of the internal stage wirings are hidden within
    * [[akka.stream.impl.fusing.GraphInterpreter#GraphAssembly]] objects that are
diff --git a/akka-stream/src/main/scala/akka/stream/Shape.scala b/akka-stream/src/main/scala/akka/stream/Shape.scala
index 5e9259cfd9..1afe64802f 100644
--- a/akka-stream/src/main/scala/akka/stream/Shape.scala
+++ b/akka-stream/src/main/scala/akka/stream/Shape.scala
@@ -215,6 +215,8 @@ object ClosedShape extends ClosedShape {
    * Java API: obtain ClosedShape instance
    */
   def getInstance: ClosedShape = this
+
+  override def toString: String = "ClosedShape"
 }
 
 /**
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 82f20a16ff..e4f3bcdd0a 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
@@ -11,7 +11,7 @@ import akka.event.Logging
 import akka.dispatch.Dispatchers
 import akka.pattern.ask
 import akka.stream._
-import akka.stream.impl.StreamLayout.Module
+import akka.stream.impl.StreamLayout.{ Module, AtomicModule }
 import akka.stream.impl.fusing.{ ActorGraphInterpreter, GraphModule }
 import akka.stream.impl.io.TLSActor
 import akka.stream.impl.io.TlsModule
@@ -97,7 +97,8 @@ private[akka] case class ActorMaterializerImpl(system: ActorSystem,
         name
       }
 
-      override protected def materializeAtomic(atomic: Module, effectiveAttributes: Attributes, matVal: ju.Map[Module, Any]): Unit = {
+      override protected def materializeAtomic(atomic: AtomicModule, effectiveAttributes: Attributes, matVal: ju.Map[Module, Any]): Unit = {
+        if (MaterializerSession.Debug) println(s"materializing $atomic")
 
         def newMaterializationContext() =
           new MaterializationContext(ActorMaterializerImpl.this, effectiveAttributes, stageName(effectiveAttributes))
diff --git a/akka-stream/src/main/scala/akka/stream/impl/FlowModule.scala b/akka-stream/src/main/scala/akka/stream/impl/FlowModule.scala
index 4789f9e987..0188d83898 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/FlowModule.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/FlowModule.scala
@@ -5,11 +5,12 @@ 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.Module {
+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
@@ -18,6 +19,6 @@ private[stream] trait FlowModule[In, Out, Mat] extends StreamLayout.Module {
   val outPort = Outlet[Out]("Flow.out")
   override val shape = new FlowShape(inPort, outPort)
 
-  override def subModules: Set[Module] = Set.empty
+  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/Modules.scala b/akka-stream/src/main/scala/akka/stream/impl/Modules.scala
index c3b1e232df..ae5771f15d 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/Modules.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/Modules.scala
@@ -6,29 +6,30 @@ package akka.stream.impl
 import akka.NotUsed
 import akka.actor._
 import akka.stream._
-import akka.stream.impl.StreamLayout.Module
+import akka.stream.impl.StreamLayout.AtomicModule
 import org.reactivestreams._
-
 import scala.annotation.unchecked.uncheckedVariance
 import scala.concurrent.Promise
+import akka.event.Logging
 
 /**
  * INTERNAL API
  */
-private[akka] abstract class SourceModule[+Out, +Mat](val shape: SourceShape[Out]) extends Module {
+private[akka] abstract class SourceModule[+Out, +Mat](val shape: SourceShape[Out]) extends AtomicModule {
+
+  protected def label: String = Logging.simpleName(this)
+  final override def toString: String = f"$label [${System.identityHashCode(this)}%08x]"
 
   def create(context: MaterializationContext): (Publisher[Out] @uncheckedVariance, Mat)
 
-  override def replaceShape(s: Shape): Module =
+  override def replaceShape(s: Shape): AtomicModule =
     if (s != shape) throw new UnsupportedOperationException("cannot replace the shape of a Source, you need to wrap it in a Graph for that")
     else this
 
   // This is okay since the only caller of this method is right below.
   protected def newInstance(shape: SourceShape[Out] @uncheckedVariance): SourceModule[Out, Mat]
 
-  override def carbonCopy: Module = newInstance(SourceShape(shape.out.carbonCopy()))
-
-  override def subModules: Set[Module] = Set.empty
+  override def carbonCopy: AtomicModule = newInstance(SourceShape(shape.out.carbonCopy()))
 
   protected def amendShape(attr: Attributes): SourceShape[Out] = {
     val thisN = attributes.nameOrDefault(null)
@@ -52,7 +53,7 @@ private[akka] final class SubscriberSource[Out](val attributes: Attributes, shap
   }
 
   override protected def newInstance(shape: SourceShape[Out]): SourceModule[Out, Subscriber[Out]] = new SubscriberSource[Out](attributes, shape)
-  override def withAttributes(attr: Attributes): Module = new SubscriberSource[Out](attr, amendShape(attr))
+  override def withAttributes(attr: Attributes): AtomicModule = new SubscriberSource[Out](attr, amendShape(attr))
 }
 
 /**
@@ -63,22 +64,26 @@ private[akka] final class SubscriberSource[Out](val attributes: Attributes, shap
  * back-pressure upstream.
  */
 private[akka] final class PublisherSource[Out](p: Publisher[Out], val attributes: Attributes, shape: SourceShape[Out]) extends SourceModule[Out, NotUsed](shape) {
+
+  override protected def label: String = s"PublisherSource($p)"
+
   override def create(context: MaterializationContext) = (p, NotUsed)
 
   override protected def newInstance(shape: SourceShape[Out]): SourceModule[Out, NotUsed] = new PublisherSource[Out](p, attributes, shape)
-  override def withAttributes(attr: Attributes): Module = new PublisherSource[Out](p, attr, amendShape(attr))
+  override def withAttributes(attr: Attributes): AtomicModule = new PublisherSource[Out](p, attr, amendShape(attr))
 }
 
 /**
  * INTERNAL API
  */
 private[akka] final class MaybeSource[Out](val attributes: Attributes, shape: SourceShape[Out]) extends SourceModule[Out, Promise[Option[Out]]](shape) {
+
   override def create(context: MaterializationContext) = {
     val p = Promise[Option[Out]]()
     new MaybePublisher[Out](p, attributes.nameOrDefault("MaybeSource"))(context.materializer.executionContext) → p
   }
   override protected def newInstance(shape: SourceShape[Out]): SourceModule[Out, Promise[Option[Out]]] = new MaybeSource[Out](attributes, shape)
-  override def withAttributes(attr: Attributes): Module = new MaybeSource(attr, amendShape(attr))
+  override def withAttributes(attr: Attributes): AtomicModule = new MaybeSource(attr, amendShape(attr))
 }
 
 /**
@@ -95,7 +100,7 @@ private[akka] final class ActorPublisherSource[Out](props: Props, val attributes
 
   override protected def newInstance(shape: SourceShape[Out]): SourceModule[Out, ActorRef] =
     new ActorPublisherSource[Out](props, attributes, shape)
-  override def withAttributes(attr: Attributes): Module = new ActorPublisherSource(props, attr, amendShape(attr))
+  override def withAttributes(attr: Attributes): AtomicModule = new ActorPublisherSource(props, attr, amendShape(attr))
 }
 
 /**
@@ -105,6 +110,8 @@ private[akka] final class ActorRefSource[Out](
   bufferSize: Int, overflowStrategy: OverflowStrategy, val attributes: Attributes, shape: SourceShape[Out])
   extends SourceModule[Out, ActorRef](shape) {
 
+  override protected def label: String = s"ActorRefSource($bufferSize, $overflowStrategy)"
+
   override def create(context: MaterializationContext) = {
     val mat = ActorMaterializer.downcast(context.materializer)
     val ref = mat.actorOf(context, ActorRefSourceActor.props(bufferSize, overflowStrategy, mat.settings))
@@ -113,6 +120,6 @@ private[akka] final class ActorRefSource[Out](
 
   override protected def newInstance(shape: SourceShape[Out]): SourceModule[Out, ActorRef] =
     new ActorRefSource[Out](bufferSize, overflowStrategy, attributes, shape)
-  override def withAttributes(attr: Attributes): Module =
+  override def withAttributes(attr: Attributes): AtomicModule =
     new ActorRefSource(bufferSize, overflowStrategy, attr, amendShape(attr))
 }
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 b1e0b5a32e..ad8780e16a 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/Sinks.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/Sinks.scala
@@ -8,7 +8,7 @@ import akka.actor.{ ActorRef, Props }
 import akka.stream.Attributes.InputBuffer
 import akka.stream._
 import akka.stream.impl.Stages.DefaultAttributes
-import akka.stream.impl.StreamLayout.Module
+import akka.stream.impl.StreamLayout.AtomicModule
 import akka.stream.stage._
 import org.reactivestreams.{ Publisher, Subscriber }
 import scala.annotation.unchecked.uncheckedVariance
@@ -20,11 +20,12 @@ import java.util.concurrent.CompletionStage
 import scala.compat.java8.FutureConverters._
 import scala.compat.java8.OptionConverters._
 import java.util.Optional
+import akka.event.Logging
 
 /**
  * INTERNAL API
  */
-private[akka] abstract class SinkModule[-In, Mat](val shape: SinkShape[In]) extends Module {
+private[akka] abstract class SinkModule[-In, Mat](val shape: SinkShape[In]) extends AtomicModule {
 
   /**
    * Create the Subscriber or VirtualPublisher that consumes the incoming
@@ -35,16 +36,14 @@ private[akka] abstract class SinkModule[-In, Mat](val shape: SinkShape[In]) exte
    */
   def create(context: MaterializationContext): (AnyRef, Mat)
 
-  override def replaceShape(s: Shape): Module =
+  override def replaceShape(s: Shape): AtomicModule =
     if (s != shape) throw new UnsupportedOperationException("cannot replace the shape of a Sink, you need to wrap it in a Graph for that")
     else this
 
   // This is okay since we the only caller of this method is right below.
   protected def newInstance(s: SinkShape[In] @uncheckedVariance): SinkModule[In, Mat]
 
-  override def carbonCopy: Module = newInstance(SinkShape(shape.in.carbonCopy()))
-
-  override def subModules: Set[Module] = Set.empty
+  override def carbonCopy: AtomicModule = newInstance(SinkShape(shape.in.carbonCopy()))
 
   protected def amendShape(attr: Attributes): SinkShape[In] = {
     val thisN = attributes.nameOrDefault(null)
@@ -53,6 +52,10 @@ private[akka] abstract class SinkModule[-In, Mat](val shape: SinkShape[In]) exte
     if ((thatN eq null) || thisN == thatN) shape
     else shape.copy(in = Inlet(thatN + ".in"))
   }
+
+  protected def label: String = Logging.simpleName(this)
+  final override def toString: String = f"$label [${System.identityHashCode(this)}%08x]"
+
 }
 
 /**
@@ -64,8 +67,6 @@ private[akka] abstract class SinkModule[-In, Mat](val shape: SinkShape[In]) exte
  */
 private[akka] class PublisherSink[In](val attributes: Attributes, shape: SinkShape[In]) extends SinkModule[In, Publisher[In]](shape) {
 
-  override def toString: String = "PublisherSink"
-
   /*
    * This method is the reason why SinkModule.create may return something that is
    * not a Subscriber: a VirtualPublisher is used in order to avoid the immediate
@@ -77,7 +78,7 @@ private[akka] class PublisherSink[In](val attributes: Attributes, shape: SinkSha
   }
 
   override protected def newInstance(shape: SinkShape[In]): SinkModule[In, Publisher[In]] = new PublisherSink[In](attributes, shape)
-  override def withAttributes(attr: Attributes): Module = new PublisherSink[In](attr, amendShape(attr))
+  override def withAttributes(attr: Attributes): AtomicModule = new PublisherSink[In](attr, amendShape(attr))
 }
 
 /**
@@ -100,7 +101,7 @@ private[akka] final class FanoutPublisherSink[In](
   override protected def newInstance(shape: SinkShape[In]): SinkModule[In, Publisher[In]] =
     new FanoutPublisherSink[In](attributes, shape)
 
-  override def withAttributes(attr: Attributes): Module =
+  override def withAttributes(attr: Attributes): AtomicModule =
     new FanoutPublisherSink[In](attr, amendShape(attr))
 }
 
@@ -118,8 +119,7 @@ private[akka] final class SinkholeSink(val attributes: Attributes, shape: SinkSh
   }
 
   override protected def newInstance(shape: SinkShape[Any]): SinkModule[Any, Future[Done]] = new SinkholeSink(attributes, shape)
-  override def withAttributes(attr: Attributes): Module = new SinkholeSink(attr, amendShape(attr))
-  override def toString: String = "SinkholeSink"
+  override def withAttributes(attr: Attributes): AtomicModule = new SinkholeSink(attr, amendShape(attr))
 }
 
 /**
@@ -131,8 +131,7 @@ private[akka] final class SubscriberSink[In](subscriber: Subscriber[In], val att
   override def create(context: MaterializationContext) = (subscriber, NotUsed)
 
   override protected def newInstance(shape: SinkShape[In]): SinkModule[In, NotUsed] = new SubscriberSink[In](subscriber, attributes, shape)
-  override def withAttributes(attr: Attributes): Module = new SubscriberSink[In](subscriber, attr, amendShape(attr))
-  override def toString: String = "SubscriberSink"
+  override def withAttributes(attr: Attributes): AtomicModule = new SubscriberSink[In](subscriber, attr, amendShape(attr))
 }
 
 /**
@@ -142,8 +141,7 @@ private[akka] final class SubscriberSink[In](subscriber: Subscriber[In], val att
 private[akka] final class CancelSink(val attributes: Attributes, shape: SinkShape[Any]) extends SinkModule[Any, NotUsed](shape) {
   override def create(context: MaterializationContext): (Subscriber[Any], NotUsed) = (new CancellingSubscriber[Any], NotUsed)
   override protected def newInstance(shape: SinkShape[Any]): SinkModule[Any, NotUsed] = new CancelSink(attributes, shape)
-  override def withAttributes(attr: Attributes): Module = new CancelSink(attr, amendShape(attr))
-  override def toString: String = "CancelSink"
+  override def withAttributes(attr: Attributes): AtomicModule = new CancelSink(attr, amendShape(attr))
 }
 
 /**
@@ -159,8 +157,7 @@ private[akka] final class ActorSubscriberSink[In](props: Props, val attributes:
   }
 
   override protected def newInstance(shape: SinkShape[In]): SinkModule[In, ActorRef] = new ActorSubscriberSink[In](props, attributes, shape)
-  override def withAttributes(attr: Attributes): Module = new ActorSubscriberSink[In](props, attr, amendShape(attr))
-  override def toString: String = "ActorSubscriberSink"
+  override def withAttributes(attr: Attributes): AtomicModule = new ActorSubscriberSink[In](props, attr, amendShape(attr))
 }
 
 /**
@@ -180,9 +177,8 @@ private[akka] final class ActorRefSink[In](ref: ActorRef, onCompleteMessage: Any
 
   override protected def newInstance(shape: SinkShape[In]): SinkModule[In, NotUsed] =
     new ActorRefSink[In](ref, onCompleteMessage, attributes, shape)
-  override def withAttributes(attr: Attributes): Module =
+  override def withAttributes(attr: Attributes): AtomicModule =
     new ActorRefSink[In](ref, onCompleteMessage, attr, amendShape(attr))
-  override def toString: String = "ActorRefSink"
 }
 
 private[akka] final class LastOptionStage[T] extends GraphStageWithMaterializedValue[SinkShape[T], Future[Option[T]]] {
@@ -257,6 +253,8 @@ private[akka] final class HeadOptionStage[T] extends GraphStageWithMaterializedV
 private[akka] final class SeqStage[T] extends GraphStageWithMaterializedValue[SinkShape[T], Future[immutable.Seq[T]]] {
   val in = Inlet[T]("seq.in")
 
+  override def toString: String = "SeqStage"
+
   override val shape: SinkShape[T] = SinkShape.of(in)
 
   override protected def initialAttributes: Attributes = DefaultAttributes.seqSink
@@ -302,6 +300,8 @@ final private[stream] class QueueSink[T]() extends GraphStageWithMaterializedVal
   override def initialAttributes = DefaultAttributes.queueSink
   override val shape: SinkShape[T] = SinkShape.of(in)
 
+  override def toString: String = "QueueSink"
+
   override def createLogicAndMaterializedValue(inheritedAttributes: Attributes) = {
     val stageLogic = new GraphStageLogic(shape) with CallbackWrapper[Requested[T]] {
       type Received[E] = Try[Option[E]]
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 ac9e344abd..34043f90c1 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/Stages.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/Stages.scala
@@ -208,6 +208,7 @@ private[stream] object Stages {
 
   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 {
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 4510fcc105..8fb58a9b4e 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/StreamLayout.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/StreamLayout.scala
@@ -19,6 +19,7 @@ import scala.collection.JavaConverters._
 import akka.stream.impl.fusing.GraphStageModule
 import akka.stream.impl.fusing.GraphStages.MaterializedValueSource
 import akka.stream.impl.fusing.GraphModule
+import akka.event.Logging
 
 /**
  * INTERNAL API
@@ -104,9 +105,21 @@ object StreamLayout {
     if (problems.nonEmpty && !doPrint) throw new IllegalStateException(s"module inconsistent, found ${problems.size} problems")
   }
 
-  // TODO: Materialization order
-  // TODO: Special case linear composites
-  // TODO: Cycles
+  object IgnorableMatValComp {
+    def apply(comp: MaterializedValueNode): Boolean =
+      comp match {
+        case Atomic(module)            ⇒ IgnorableMatValComp(module)
+        case _: Combine | _: Transform ⇒ false
+        case Ignore                    ⇒ true
+      }
+    def apply(module: Module): Boolean =
+      module match {
+        case _: AtomicModule | EmptyModule        ⇒ true
+        case CopiedModule(_, _, module)           ⇒ IgnorableMatValComp(module)
+        case CompositeModule(_, _, _, _, comp, _) ⇒ IgnorableMatValComp(comp)
+        case FusedModule(_, _, _, _, comp, _, _)  ⇒ IgnorableMatValComp(comp)
+      }
+  }
 
   sealed trait MaterializedValueNode {
     /*
@@ -121,14 +134,14 @@ object StreamLayout {
     override def toString: String = s"Combine($dep1,$dep2)"
   }
   case class Atomic(module: Module) extends MaterializedValueNode {
-    override def toString: String = s"Atomic(${module.attributes.nameOrDefault(module.getClass.getName)}[${module.hashCode}])"
+    override def toString: String = f"Atomic(${module.attributes.nameOrDefault(module.getClass.getName)}[${System.identityHashCode(module)}%08x])"
   }
   case class Transform(f: Any ⇒ Any, dep: MaterializedValueNode) extends MaterializedValueNode {
     override def toString: String = s"Transform($dep)"
   }
   case object Ignore extends MaterializedValueNode
 
-  trait Module {
+  sealed trait Module {
 
     def shape: Shape
     /**
@@ -241,19 +254,30 @@ object StreamLayout {
       require(that ne this, "A module cannot be added to itself. You should pass a separate instance to compose().")
       require(!subModules(that), "An existing submodule cannot be added again. All contained modules must be unique.")
 
-      val modules1 = if (this.isSealed) Set(this) else this.subModules
-      val modules2 = if (that.isSealed) Set(that) else that.subModules
+      val modulesLeft = if (this.isSealed) Set(this) else this.subModules
+      val modulesRight = if (that.isSealed) Set(that) else that.subModules
 
-      val matComputation1 = if (this.isSealed) Atomic(this) else this.materializedValueComputation
-      val matComputation2 = if (that.isSealed) Atomic(that) else that.materializedValueComputation
+      val matCompLeft = if (this.isSealed) Atomic(this) else this.materializedValueComputation
+      val matCompRight = if (that.isSealed) Atomic(that) else that.materializedValueComputation
+
+      val mat =
+        {
+          val comp =
+            if (f == scaladsl.Keep.left) {
+              if (IgnorableMatValComp(matCompRight)) matCompLeft else null
+            } else if (f == scaladsl.Keep.right) {
+              if (IgnorableMatValComp(matCompLeft)) matCompRight else null
+            } else null
+          if (comp == null) Combine(f.asInstanceOf[(Any, Any) ⇒ Any], matCompLeft, matCompRight)
+          else comp
+        }
 
       CompositeModule(
-        modules1 ++ modules2,
+        modulesLeft ++ modulesRight,
         AmorphousShape(shape.inlets ++ that.shape.inlets, shape.outlets ++ that.shape.outlets),
         downstreams ++ that.downstreams,
         upstreams ++ that.upstreams,
-        // would like to optimize away this allocation for Keep.{left,right} but that breaks side-effecting transformations
-        Combine(f.asInstanceOf[(Any, Any) ⇒ Any], matComputation1, matComputation2),
+        mat,
         Attributes.none)
     }
 
@@ -314,7 +338,7 @@ object StreamLayout {
     final override def equals(obj: scala.Any): Boolean = super.equals(obj)
   }
 
-  object EmptyModule extends Module {
+  case object EmptyModule extends Module {
     override def shape = ClosedShape
     override def replaceShape(s: Shape) =
       if (s != shape) throw new UnsupportedOperationException("cannot replace the shape of the EmptyModule")
@@ -357,7 +381,7 @@ object StreamLayout {
 
     override def isCopied: Boolean = true
 
-    override def toString: String = s"$copyOf (copy)"
+    override def toString: String = f"[${System.identityHashCode(this)}%08x] copy of $copyOf"
   }
 
   final case class CompositeModule(
@@ -379,13 +403,13 @@ object StreamLayout {
     override def withAttributes(attributes: Attributes): Module = copy(attributes = attributes)
 
     override def toString =
-      s"""
+      f"""CompositeModule [${System.identityHashCode(this)}%08x]
          |  Name: ${this.attributes.nameOrDefault("unnamed")}
          |  Modules:
-         |    ${subModules.iterator.map(m ⇒ m.attributes.nameLifted.getOrElse(m.toString.replaceAll("\n", "\n    "))).mkString("\n    ")}
+         |    ${subModules.iterator.map(m ⇒ s"(${m.attributes.nameLifted.getOrElse("unnamed")}) ${m.toString.replaceAll("\n", "\n    ")}").mkString("\n    ")}
          |  Downstreams: ${downstreams.iterator.map { case (in, out) ⇒ s"\n    $in -> $out" }.mkString("")}
          |  Upstreams: ${upstreams.iterator.map { case (out, in) ⇒ s"\n    $out -> $in" }.mkString("")}
-         |""".stripMargin
+         |  MatValue: $materializedValueComputation""".stripMargin
   }
 
   object CompositeModule {
@@ -414,13 +438,24 @@ object StreamLayout {
     override def withAttributes(attributes: Attributes): FusedModule = copy(attributes = attributes)
 
     override def toString =
-      s"""
-        |  Name: ${this.attributes.nameOrDefault("unnamed")}
-        |  Modules:
-        |    ${subModules.iterator.map(m ⇒ m.attributes.nameLifted.getOrElse(m.toString.replaceAll("\n", "\n    "))).mkString("\n    ")}
-        |  Downstreams: ${downstreams.iterator.map { case (in, out) ⇒ s"\n    $in -> $out" }.mkString("")}
-        |  Upstreams: ${upstreams.iterator.map { case (out, in) ⇒ s"\n    $out -> $in" }.mkString("")}
-        |""".stripMargin
+      f"""FusedModule [${System.identityHashCode(this)}%08x]
+         |  Name: ${this.attributes.nameOrDefault("unnamed")}
+         |  Modules:
+         |    ${subModules.iterator.map(m ⇒ m.attributes.nameLifted.getOrElse(m.toString.replaceAll("\n", "\n    "))).mkString("\n    ")}
+         |  Downstreams: ${downstreams.iterator.map { case (in, out) ⇒ s"\n    $in -> $out" }.mkString("")}
+         |  Upstreams: ${upstreams.iterator.map { case (out, in) ⇒ s"\n    $out -> $in" }.mkString("")}
+         |  MatValue: $materializedValueComputation""".stripMargin
+  }
+
+  /**
+   * This is the only extension point for the sealed type hierarchy: composition
+   * (i.e. the module tree) is managed strictly within this file, only leaf nodes
+   * may be declared elsewhere.
+   */
+  abstract class AtomicModule extends Module {
+    final override def subModules: Set[Module] = Set.empty
+    final override def downstreams: Map[OutPort, InPort] = super.downstreams
+    final override def upstreams: Map[InPort, OutPort] = super.upstreams
   }
 }
 
@@ -718,6 +753,8 @@ private[stream] abstract class MaterializerSession(val topLevel: StreamLayout.Mo
     new ju.HashMap[InPort, AnyRef] :: Nil
   private var publishersStack: List[ju.Map[OutPort, Publisher[Any]]] =
     new ju.HashMap[OutPort, Publisher[Any]] :: Nil
+  private var matValSrcStack: List[ju.Map[MaterializedValueNode, List[MaterializedValueSource[Any]]]] =
+    new ju.HashMap[MaterializedValueNode, List[MaterializedValueSource[Any]]] :: Nil
 
   /*
    * Please note that this stack keeps track of the scoped modules wrapped in CopiedModule but not the CopiedModule
@@ -732,13 +769,16 @@ private[stream] abstract class MaterializerSession(val topLevel: StreamLayout.Mo
   private def subscribers: ju.Map[InPort, AnyRef] = subscribersStack.head
   private def publishers: ju.Map[OutPort, Publisher[Any]] = publishersStack.head
   private def currentLayout: Module = moduleStack.head
+  private def matValSrc: ju.Map[MaterializedValueNode, List[MaterializedValueSource[Any]]] = matValSrcStack.head
 
   // Enters a copied module and establishes a scope that prevents internals to leak out and interfere with copies
   // of the same module.
   // We don't store the enclosing CopiedModule itself as state since we don't use it anywhere else than exit and enter
   private def enterScope(enclosing: CopiedModule): Unit = {
+    if (MaterializerSession.Debug) println(f"entering scope [${System.identityHashCode(enclosing)}%08x]")
     subscribersStack ::= new ju.HashMap
     publishersStack ::= new ju.HashMap
+    matValSrcStack ::= new ju.HashMap
     moduleStack ::= enclosing.copyOf
   }
 
@@ -747,12 +787,16 @@ private[stream] abstract class MaterializerSession(val topLevel: StreamLayout.Mo
   // leading to port identity collisions)
   // We don't store the enclosing CopiedModule itself as state since we don't use it anywhere else than exit and enter
   private def exitScope(enclosing: CopiedModule): Unit = {
+    if (MaterializerSession.Debug) println(f"exiting scope [${System.identityHashCode(enclosing)}%08x]")
     val scopeSubscribers = subscribers
     val scopePublishers = publishers
     subscribersStack = subscribersStack.tail
     publishersStack = publishersStack.tail
+    matValSrcStack = matValSrcStack.tail
     moduleStack = moduleStack.tail
 
+    if (MaterializerSession.Debug) println(s"  subscribers = $scopeSubscribers\n  publishers = $scopePublishers")
+
     // When we exit the scope of a copied module,  pick up the Subscribers/Publishers belonging to exposed ports of
     // the original module and assign them to the copy ports in the outer scope that we will return to
     enclosing.copyOf.shape.inlets.iterator.zip(enclosing.shape.inlets.iterator).foreach {
@@ -765,6 +809,7 @@ private[stream] abstract class MaterializerSession(val topLevel: StreamLayout.Mo
   }
 
   final def materialize(): Any = {
+    if (MaterializerSession.Debug) println(s"beginning materialization of $topLevel")
     require(topLevel ne EmptyModule, "An empty module cannot be materialized (EmptyModule was given)")
     require(
       topLevel.isRunnable,
@@ -789,7 +834,6 @@ private[stream] abstract class MaterializerSession(val topLevel: StreamLayout.Mo
   protected def mergeAttributes(parent: Attributes, current: Attributes): Attributes =
     parent and current
 
-  private val matValSrc: ju.Map[MaterializedValueNode, List[MaterializedValueSource[Any]]] = new ju.HashMap
   def registerSrc(ms: MaterializedValueSource[Any]): Unit = {
     if (MaterializerSession.Debug) println(s"registering source $ms")
     matValSrc.get(ms.computation) match {
@@ -801,53 +845,60 @@ private[stream] abstract class MaterializerSession(val topLevel: StreamLayout.Mo
   protected def materializeModule(module: Module, effectiveAttributes: Attributes): Any = {
     val materializedValues: ju.Map[Module, Any] = new ju.HashMap
 
+    if (MaterializerSession.Debug) println(f"entering module [${System.identityHashCode(module)}%08x] (${Logging.simpleName(module)})")
+
     for (submodule ← module.subModules) {
       val subEffectiveAttributes = mergeAttributes(effectiveAttributes, submodule.attributes)
       submodule match {
-        case GraphStageModule(shape, attributes, mv: MaterializedValueSource[_]) ⇒
-          val copy = mv.copySrc.asInstanceOf[MaterializedValueSource[Any]]
-          registerSrc(copy)
-          materializeAtomic(copy.module, subEffectiveAttributes, materializedValues)
-        case atomic if atomic.isAtomic ⇒
+        case atomic: AtomicModule ⇒
           materializeAtomic(atomic, subEffectiveAttributes, materializedValues)
         case copied: CopiedModule ⇒
           enterScope(copied)
           materializedValues.put(copied, materializeModule(copied, subEffectiveAttributes))
           exitScope(copied)
-        case composite ⇒
+        case composite @ (_: CompositeModule | _: FusedModule) ⇒
           materializedValues.put(composite, materializeComposite(composite, subEffectiveAttributes))
+        case EmptyModule => // nothing to do or say
       }
     }
 
     if (MaterializerSession.Debug) {
-      println("RESOLVING")
-      println(s"  module = $module")
-      println(s"  computation = ${module.materializedValueComputation}")
+      println(f"resolving module [${System.identityHashCode(module)}%08x] computation ${module.materializedValueComputation}")
       println(s"  matValSrc = $matValSrc")
-      println(s"  matVals = $materializedValues")
+      println(s"  matVals =\n    ${materializedValues.asScala.map(p ⇒ "%08x".format(System.identityHashCode(p._1)) -> p._2).mkString("\n    ")}")
     }
-    resolveMaterialized(module.materializedValueComputation, materializedValues, "  ")
+
+    val ret = resolveMaterialized(module.materializedValueComputation, materializedValues, 2)
+    while (!matValSrc.isEmpty) {
+      val node = matValSrc.keySet.iterator.next()
+      if (MaterializerSession.Debug) println(s"  delayed computation of $node")
+      resolveMaterialized(node, materializedValues, 4)
+    }
+
+    if (MaterializerSession.Debug) println(f"exiting module [${System.identityHashCode(module)}%08x]")
+
+    ret
   }
 
   protected def materializeComposite(composite: Module, effectiveAttributes: Attributes): Any = {
     materializeModule(composite, effectiveAttributes)
   }
 
-  protected def materializeAtomic(atomic: Module, effectiveAttributes: Attributes, matVal: ju.Map[Module, Any]): Unit
+  protected def materializeAtomic(atomic: AtomicModule, effectiveAttributes: Attributes, matVal: ju.Map[Module, Any]): Unit
 
-  private def resolveMaterialized(matNode: MaterializedValueNode, matVal: ju.Map[Module, Any], indent: String): Any = {
-    if (MaterializerSession.Debug) println(indent + matNode)
+  private def resolveMaterialized(matNode: MaterializedValueNode, matVal: ju.Map[Module, Any], spaces: Int): Any = {
+    if (MaterializerSession.Debug) println(" " * spaces + matNode)
     val ret = matNode match {
       case Atomic(m)          ⇒ matVal.get(m)
-      case Combine(f, d1, d2) ⇒ f(resolveMaterialized(d1, matVal, indent + "  "), resolveMaterialized(d2, matVal, indent + "  "))
-      case Transform(f, d)    ⇒ f(resolveMaterialized(d, matVal, indent + "  "))
+      case Combine(f, d1, d2) ⇒ f(resolveMaterialized(d1, matVal, spaces + 2), resolveMaterialized(d2, matVal, spaces + 2))
+      case Transform(f, d)    ⇒ f(resolveMaterialized(d, matVal, spaces + 2))
       case Ignore             ⇒ NotUsed
     }
-    if (MaterializerSession.Debug) println(indent + s"result = $ret")
+    if (MaterializerSession.Debug) println(" " * spaces + s"result = $ret")
     matValSrc.remove(matNode) match {
       case null ⇒ // nothing to do
       case srcs ⇒
-        if (MaterializerSession.Debug) println(indent + s"triggering sources $srcs")
+        if (MaterializerSession.Debug) println(" " * spaces + s"triggering sources $srcs")
         srcs.foreach(_.setValue(ret))
     }
     ret
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 dbb16227aa..59faa28470 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
@@ -10,7 +10,7 @@ import akka.event.Logging
 import akka.stream._
 import akka.stream.impl._
 import akka.stream.impl.ReactiveStreamsCompliance._
-import akka.stream.impl.StreamLayout.{ CompositeModule, CopiedModule, Module }
+import akka.stream.impl.StreamLayout.{ CompositeModule, CopiedModule, Module, AtomicModule }
 import akka.stream.impl.fusing.GraphInterpreter.{ DownstreamBoundaryStageLogic, UpstreamBoundaryStageLogic, GraphAssembly }
 import akka.stream.stage.{ GraphStageLogic, InHandler, OutHandler }
 import org.reactivestreams.{ Subscriber, Subscription }
@@ -23,9 +23,9 @@ import scala.annotation.tailrec
 /**
  * INTERNAL API
  */
-private[stream] case class GraphModule(assembly: GraphAssembly, shape: Shape, attributes: Attributes,
-                                       matValIDs: Array[Module]) extends Module {
-  override def subModules: Set[Module] = Set.empty
+private[stream] final case class GraphModule(assembly: GraphAssembly, shape: Shape, attributes: Attributes,
+                                             matValIDs: Array[Module]) extends AtomicModule {
+
   override def withAttributes(newAttr: Attributes): Module = copy(attributes = newAttr)
 
   override final def carbonCopy: Module = CopiedModule(shape.deepCopy(), Attributes.none, this)
@@ -34,7 +34,12 @@ private[stream] case class GraphModule(assembly: GraphAssembly, shape: Shape, at
     if (newShape != shape) CompositeModule(this, newShape)
     else this
 
-  override def toString: String = s"GraphModule\n  ${assembly.toString.replace("\n", "\n  ")}\n  shape=$shape, attributes=$attributes"
+  override def toString: String =
+    s"""GraphModule
+       |  ${assembly.toString.replace("\n", "\n  ")}
+       |  shape=$shape, attributes=$attributes
+       |  matVals=
+       |    ${matValIDs.mkString("\n    ")}""".stripMargin
 }
 
 /**
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/Fusing.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/Fusing.scala
index ca1fd38af2..273e0cb672 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/Fusing.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/Fusing.scala
@@ -27,7 +27,14 @@ private[stream] object Fusing {
   /**
    * Fuse everything that is not forbidden via AsyncBoundary attribute.
    */
-  def aggressive[S <: Shape, M](g: Graph[S, M]): FusedGraph[S, M] = {
+  def aggressive[S <: Shape, M](g: Graph[S, M]): FusedGraph[S, M] =
+    g match {
+      case fg: FusedGraph[_, _]      ⇒ fg
+      case FusedGraph(module, shape) => FusedGraph(module, shape)
+      case _                         ⇒ doAggressive(g)
+    }
+
+  private def doAggressive[S <: Shape, M](g: Graph[S, M]): FusedGraph[S, M] = {
     val struct = new BuildStructuralInfo
     /*
      * First perform normalization by descending the module tree and recording
@@ -153,6 +160,7 @@ private[stream] object Fusing {
         }
 
         pos += 1
+      case _ => throw new IllegalArgumentException("unexpected module structure")
     }
 
     val outsB2 = new Array[Outlet[_]](insB2.size)
@@ -178,6 +186,7 @@ private[stream] object Fusing {
             }
         }
         pos += 1
+      case _ => throw new IllegalArgumentException("unexpected module structure")
     }
 
     /*
@@ -207,7 +216,10 @@ private[stream] object Fusing {
     copyToArray(outOwnersB3.iterator, outOwners, outStart)
 
     // FIXME attributes should contain some naming info and async boundary where needed
-    val firstModule = group.iterator.next()
+    val firstModule = group.iterator.next() match {
+      case c: CopiedModule => c
+      case _               => throw new IllegalArgumentException("unexpected module structure")
+    }
     val async = if (isAsync(firstModule)) Attributes(AsyncBoundary) else Attributes.none
     val disp = dispatcher(firstModule) match {
       case None    ⇒ Attributes.none
@@ -253,7 +265,7 @@ private[stream] object Fusing {
       case _ if m.isAtomic     ⇒ true // non-GraphStage atomic or has AsyncBoundary
       case _                   ⇒ m.attributes.contains(AsyncBoundary)
     }
-    if (Debug) log(s"entering ${m.getClass} (hash=${m.hashCode}, async=$async, name=${m.attributes.nameLifted}, dispatcher=${dispatcher(m)})")
+    if (Debug) log(s"entering ${m.getClass} (hash=${struct.hash(m)}, async=$async, name=${m.attributes.nameLifted}, dispatcher=${dispatcher(m)})")
     val localGroup =
       if (async) struct.newGroup(indent)
       else openGroup
@@ -315,6 +327,7 @@ private[stream] object Fusing {
               struct.registerInternals(newShape, indent)
 
               copy
+            case _ => throw new IllegalArgumentException("unexpected module structure")
           }
           val newgm = gm.copy(shape = oldShape.copyFromPorts(oldIns.toList, oldOuts.toList), matValIDs = newids)
           // make sure to add all the port mappings from old GraphModule Shape to new shape
@@ -356,7 +369,7 @@ private[stream] object Fusing {
             subMatBuilder ++= res
           }
           val subMat = subMatBuilder.result()
-          if (Debug) log(subMat.map(p ⇒ s"${p._1.getClass.getName}[${p._1.hashCode}] -> ${p._2}").mkString("subMat\n  " + "  " * indent, "\n  " + "  " * indent, ""))
+          if (Debug) log(subMat.map(p ⇒ s"${p._1.getClass.getName}[${struct.hash(p._1)}] -> ${p._2}").mkString("subMat\n  " + "  " * indent, "\n  " + "  " * indent, ""))
           // we need to remove all wirings that this module copied from nested modules so that we
           // don’t do wirings twice
           val oldDownstreams = m match {
@@ -370,17 +383,17 @@ private[stream] object Fusing {
           // now rewrite the materialized value computation based on the copied modules and their computation nodes
           val matNodeMapping: ju.Map[MaterializedValueNode, MaterializedValueNode] = new ju.HashMap
           val newMat = rewriteMat(subMat, m.materializedValueComputation, matNodeMapping)
+          if (Debug) log(matNodeMapping.asScala.map(p ⇒ s"${p._1} -> ${p._2}").mkString("matNodeMapping\n  " + "  " * indent, "\n  " + "  " * indent, ""))
           // and finally rewire all MaterializedValueSources to their new computation nodes
           val matSrcs = struct.exitMatCtx()
           matSrcs.foreach { c ⇒
-            if (Debug) log(s"materialized value source: ${struct.hash(c)}")
-            val ms = c.copyOf match {
-              case g: GraphStageModule ⇒ g.stage.asInstanceOf[MaterializedValueSource[Any]]
-            }
+            val ms = c.copyOf.asInstanceOf[GraphStageModule].stage.asInstanceOf[MaterializedValueSource[Any]]
             val mapped = ms.computation match {
               case Atomic(sub) ⇒ subMat(sub)
+              case Ignore      => Ignore
               case other       ⇒ matNodeMapping.get(other)
             }
+            if (Debug) log(s"materialized value source: ${c.copyOf} -> $mapped")
             require(mapped != null, s"mismatch:\n  ${ms.computation}\n  ${m.materializedValueComputation}")
             val newSrc = new MaterializedValueSource[Any](mapped, ms.out)
             val replacement = CopiedModule(c.shape, c.attributes, newSrc.module)
@@ -692,7 +705,7 @@ private[stream] object Fusing {
   /**
    * Determine whether the given CopiedModule has an AsyncBoundary attribute.
    */
-  private def isAsync(m: Module): Boolean = m match {
+  private def isAsync(m: CopiedModule): Boolean = m match {
     case CopiedModule(_, inherited, orig) ⇒
       val attr = inherited and orig.attributes
       attr.contains(AsyncBoundary)
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 07bac7504b..751a9ebd2a 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
@@ -191,14 +191,17 @@ private[akka] object GraphInterpreter {
       (inHandlers, outHandlers, logics)
     }
 
-    override def toString: String =
+    override def toString: String = {
+      val stageList = stages.iterator.zip(originalAttributes.iterator).map {
+        case (stage, attr) ⇒ s"${stage.module}    [${attr.attributeList.mkString(", ")}]"
+      }
       "GraphAssembly\n  " +
-        stages.mkString("Stages: [", ",", "]") + "\n  " +
-        originalAttributes.mkString("Attributes: [", ",", "]") + "\n  " +
-        ins.mkString("Inlets: [", ",", "]") + "\n  " +
-        inOwners.mkString("InOwners: [", ",", "]") + "\n  " +
-        outs.mkString("Outlets: [", ",", "]") + "\n  " +
-        outOwners.mkString("OutOwners: [", ",", "]")
+        stageList.mkString("[ ", "\n    ", "\n  ]") + "\n  " +
+        ins.mkString("[", ",", "]") + "\n  " +
+        inOwners.mkString("[", ",", "]") + "\n  " +
+        outs.mkString("[", ",", "]") + "\n  " +
+        outOwners.mkString("[", ",", "]")
+    }
   }
 
   object GraphAssembly {
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphStages.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphStages.scala
index 37030d83a9..c02fd1d3c6 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphStages.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphStages.scala
@@ -24,20 +24,18 @@ import scala.util.Try
  */
 private[akka] final case class GraphStageModule(shape: Shape,
                                                 attributes: Attributes,
-                                                stage: GraphStageWithMaterializedValue[Shape, Any]) extends Module {
+                                                stage: GraphStageWithMaterializedValue[Shape, Any]) extends AtomicModule {
   override def carbonCopy: Module = CopiedModule(shape.deepCopy(), Attributes.none, this)
 
   override def replaceShape(s: Shape): Module =
     if (s != shape) CompositeModule(this, s)
     else this
 
-  override def subModules: Set[Module] = Set.empty
-
   override def withAttributes(attributes: Attributes): Module =
     if (attributes ne this.attributes) new GraphStageModule(shape, attributes, stage)
     else this
 
-  override def toString: String = stage.toString
+  override def toString: String = f"GraphStage($stage) [${System.identityHashCode(this)}%08x]"
 }
 
 /**
@@ -133,6 +131,7 @@ object GraphStages {
 
     override val initialAttributes = Attributes.name("breaker")
     override val shape = FlowShape(Inlet[Any]("breaker.in"), Outlet[Any]("breaker.out"))
+    override def toString: String = "Breaker"
 
     override def createLogicAndMaterializedValue(attr: Attributes) = {
       val promise = Promise[Breaker]
@@ -167,6 +166,7 @@ object GraphStages {
     override val shape = BidiShape(
       Inlet[Any]("breaker.in1"), Outlet[Any]("breaker.out1"),
       Inlet[Any]("breaker.in2"), Outlet[Any]("breaker.out2"))
+    override def toString: String = "BidiBreaker"
 
     override def createLogicAndMaterializedValue(attr: Attributes) = {
       val promise = Promise[Breaker]
@@ -215,21 +215,6 @@ object GraphStages {
 
   def bidiBreaker[T1, T2]: Graph[BidiShape[T1, T1, T2, T2], Future[Breaker]] = BidiBreaker.asInstanceOf[Graph[BidiShape[T1, T1, T2, T2], Future[Breaker]]]
 
-  private object TickSource {
-    class TickSourceCancellable(cancelled: AtomicBoolean) extends Cancellable {
-      private val cancelPromise = Promise[Done]()
-
-      def cancelFuture: Future[Done] = cancelPromise.future
-
-      override def cancel(): Boolean = {
-        if (!isCancelled) cancelPromise.trySuccess(Done)
-        true
-      }
-
-      override def isCancelled: Boolean = cancelled.get()
-    }
-  }
-
   private object TerminationWatcher extends GraphStageWithMaterializedValue[FlowShape[Any, Any], Future[Done]] {
     val in = Inlet[Any]("terminationWatcher.in")
     val out = Outlet[Any]("terminationWatcher.out")
@@ -269,6 +254,21 @@ object GraphStages {
   def terminationWatcher[T]: GraphStageWithMaterializedValue[FlowShape[T, T], Future[Done]] =
     TerminationWatcher.asInstanceOf[GraphStageWithMaterializedValue[FlowShape[T, T], Future[Done]]]
 
+  private object TickSource {
+    class TickSourceCancellable(cancelled: AtomicBoolean) extends Cancellable {
+      private val cancelPromise = Promise[Done]()
+
+      def cancelFuture: Future[Done] = cancelPromise.future
+
+      override def cancel(): Boolean = {
+        if (!isCancelled) cancelPromise.trySuccess(Done)
+        true
+      }
+
+      override def isCancelled: Boolean = cancelled.get()
+    }
+  }
+
   final class TickSource[T](initialDelay: FiniteDuration, interval: FiniteDuration, tick: T)
     extends GraphStageWithMaterializedValue[SourceShape[T], Cancellable] {
     override val shape = SourceShape(Outlet[T]("TickSource.out"))
@@ -302,7 +302,7 @@ object GraphStages {
       (logic, cancellable)
     }
 
-    override def toString: String = "TickSource"
+    override def toString: String = s"TickSource($initialDelay, $interval, $tick)"
   }
 
   /**
diff --git a/akka-stream/src/main/scala/akka/stream/impl/io/IOSinks.scala b/akka-stream/src/main/scala/akka/stream/impl/io/IOSinks.scala
index 58f91094d0..ebe6d55500 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/io/IOSinks.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/io/IOSinks.scala
@@ -22,6 +22,8 @@ import scala.concurrent.{ Future, Promise }
 private[akka] final class FileSink(f: File, options: Set[StandardOpenOption], val attributes: Attributes, shape: SinkShape[ByteString])
   extends SinkModule[ByteString, Future[IOResult]](shape) {
 
+  override protected def label: String = s"FileSink($f, $options)"
+
   override def create(context: MaterializationContext) = {
     val materializer = ActorMaterializer.downcast(context.materializer)
     val settings = materializer.effectiveSettings(context.effectiveAttributes)
@@ -68,4 +70,3 @@ private[akka] final class OutputStreamSink(createOutput: () ⇒ OutputStream, va
   override def withAttributes(attr: Attributes): Module =
     new OutputStreamSink(createOutput, attr, amendShape(attr), autoFlush)
 }
-
diff --git a/akka-stream/src/main/scala/akka/stream/impl/io/IOSources.scala b/akka-stream/src/main/scala/akka/stream/impl/io/IOSources.scala
index 855f241e7f..2a080cdba9 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/io/IOSources.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/io/IOSources.scala
@@ -41,6 +41,8 @@ private[akka] final class FileSource(f: File, chunkSize: Int, val attributes: At
 
   override def withAttributes(attr: Attributes): Module =
     new FileSource(f, chunkSize, attr, amendShape(attr))
+
+  override protected def label: String = s"FileSource($f, $chunkSize)"
 }
 
 /**
diff --git a/akka-stream/src/main/scala/akka/stream/impl/io/InputStreamPublisher.scala b/akka-stream/src/main/scala/akka/stream/impl/io/InputStreamPublisher.scala
index 4d601836ac..7527b4ace5 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/io/InputStreamPublisher.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/io/InputStreamPublisher.scala
@@ -28,8 +28,8 @@ private[akka] object InputStreamPublisher {
 
 /** INTERNAL API */
 private[akka] class InputStreamPublisher(is: InputStream, completionPromise: Promise[IOResult], chunkSize: Int)
-    extends akka.stream.actor.ActorPublisher[ByteString]
-    with ActorLogging {
+  extends akka.stream.actor.ActorPublisher[ByteString]
+  with ActorLogging {
 
   // TODO possibly de-duplicate with FilePublisher?
 
diff --git a/akka-stream/src/main/scala/akka/stream/impl/io/InputStreamSinkStage.scala b/akka-stream/src/main/scala/akka/stream/impl/io/InputStreamSinkStage.scala
index 93b0eed1a2..f0ac7c9a1e 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/io/InputStreamSinkStage.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/io/InputStreamSinkStage.scala
@@ -140,7 +140,8 @@ private[akka] class InputStreamAdapter(sharedBuffer: BlockingQueue[StreamToAdapt
                 case Failed(ex) ⇒
                   isStageAlive = false
                   throw new IOException(ex)
-                case null ⇒ throw new IOException("Timeout on waiting for new data")
+                case null        ⇒ throw new IOException("Timeout on waiting for new data")
+                case Initialized ⇒ throw new IllegalStateException("message 'Initialized' must come first")
               }
             } catch {
               case ex: InterruptedException ⇒ throw new IOException(ex)
@@ -215,4 +216,3 @@ private[akka] class InputStreamAdapter(sharedBuffer: BlockingQueue[StreamToAdapt
     }
   }
 }
-
diff --git a/akka-stream/src/main/scala/akka/stream/impl/io/TlsModule.scala b/akka-stream/src/main/scala/akka/stream/impl/io/TlsModule.scala
index 84bcb032ee..1fce7c240b 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/io/TlsModule.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/io/TlsModule.scala
@@ -3,7 +3,7 @@ package akka.stream.impl.io
 import javax.net.ssl.SSLContext
 
 import akka.stream._
-import akka.stream.impl.StreamLayout.{ CompositeModule, Module }
+import akka.stream.impl.StreamLayout.{ CompositeModule, AtomicModule }
 import akka.stream.TLSProtocol._
 import akka.util.ByteString
 
@@ -15,11 +15,10 @@ private[akka] final case class TlsModule(plainIn: Inlet[SslTlsOutbound], plainOu
                                          shape: Shape, attributes: Attributes,
                                          sslContext: SSLContext,
                                          firstSession: NegotiateNewSession,
-                                         role: TLSRole, closing: TLSClosing, hostInfo: Option[(String, Int)]) extends Module {
-  override def subModules: Set[Module] = Set.empty
+                                         role: TLSRole, closing: TLSClosing, hostInfo: Option[(String, Int)]) extends AtomicModule {
 
-  override def withAttributes(att: Attributes): Module = copy(attributes = att)
-  override def carbonCopy: Module =
+  override def withAttributes(att: Attributes): TlsModule = copy(attributes = att)
+  override def carbonCopy: TlsModule =
     TlsModule(attributes, sslContext, firstSession, role, closing, hostInfo)
 
   override def replaceShape(s: Shape) =
@@ -27,6 +26,8 @@ private[akka] final case class TlsModule(plainIn: Inlet[SslTlsOutbound], plainOu
       shape.requireSamePortsAs(s)
       CompositeModule(this, s)
     } else this
+
+  override def toString: String = f"TlsModule($firstSession, $role, $closing, $hostInfo) [${System.identityHashCode(this)}%08x]"
 }
 
 /**
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 540dd45fb0..027a866a59 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala
@@ -75,6 +75,8 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
   override def shape: FlowShape[In, Out] = delegate.shape
   private[stream] def module: StreamLayout.Module = delegate.module
 
+  override def toString: String = delegate.toString
+
   /** Converts this Flow to its Scala DSL counterpart */
   def asScala: scaladsl.Flow[In, Out, Mat] = delegate
 
@@ -119,6 +121,9 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * }}}
    * The `combine` function is used to compose the materialized values of this flow and that
    * flow into the materialized value of the resulting Flow.
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def viaMat[T, M, M2](flow: Graph[FlowShape[Out, T], M], combine: function.Function2[Mat, M, M2]): javadsl.Flow[In, T, M2] =
     new Flow(delegate.viaMat(flow)(combinerToScala(combine)))
@@ -158,6 +163,9 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * }}}
    * The `combine` function is used to compose the materialized values of this flow and that
    * Sink into the materialized value of the resulting Sink.
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def toMat[M, M2](sink: Graph[SinkShape[Out], M], combine: function.Function2[Mat, M, M2]): javadsl.Sink[In, M2] =
     new Sink(delegate.toMat(sink)(combinerToScala(combine)))
@@ -189,6 +197,9 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * }}}
    * The `combine` function is used to compose the materialized values of this flow and that
    * Flow into the materialized value of the resulting Flow.
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def joinMat[M, M2](flow: Graph[FlowShape[Out, In], M], combine: function.Function2[Mat, M, M2]): javadsl.RunnableGraph[M2] =
     RunnableGraph.fromGraph(delegate.joinMat(flow)(combinerToScala(combine)))
@@ -228,6 +239,9 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * }}}
    * The `combine` function is used to compose the materialized values of this flow and that
    * [[BidiFlow]] into the materialized value of the resulting [[Flow]].
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def joinMat[I2, O2, Mat2, M](bidi: Graph[BidiShape[Out, O2, I2, In], Mat2], combine: function.Function2[Mat, Mat2, M]): Flow[I2, O2, M] =
     new Flow(delegate.joinMat(bidi)(combinerToScala(combine)))
@@ -1246,6 +1260,9 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    *
    * If this [[Flow]] gets upstream error - no elements from the given [[Source]] will be pulled.
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#concat]]
    */
   def concatMat[T >: Out, M, M2](that: Graph[SourceShape[T], M], matF: function.Function2[Mat, M, M2]): javadsl.Flow[In, T, M2] =
@@ -1282,7 +1299,10 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    *
    * If the given [[Source]] gets upstream error - no elements from this [[Flow]] will be pulled.
    *
-   * @see [[#prepend]].
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
+   * @see [[#prepend]]
    */
   def prependMat[T >: Out, M, M2](that: Graph[SourceShape[T], M], matF: function.Function2[Mat, M, M2]): javadsl.Flow[In, T, M2] =
     new Flow(delegate.prependMat(that)(combinerToScala(matF)))
@@ -1306,6 +1326,9 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * Attaches the given [[Sink]] to this [[Flow]], meaning that elements that passes
    * through will also be sent to the [[Sink]].
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#alsoTo]]
    */
   def alsoToMat[M2, M3](that: Graph[SinkShape[Out], M2],
@@ -1349,7 +1372,10 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    *
    * If this [[Flow]] or [[Source]] gets upstream error - stream completes with failure.
    *
-   * @see [[#interleave]].
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
+   * @see [[#interleave]]
    */
   def interleaveMat[T >: Out, M, M2](that: Graph[SourceShape[T], M], segmentSize: Int,
                                      matF: function.Function2[Mat, M, M2]): javadsl.Flow[In, T, M2] =
@@ -1389,6 +1415,9 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * Merge the given [[Source]] to this [[Flow]], taking elements as they arrive from input streams,
    * picking randomly when several elements ready.
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#merge]]
    */
   def mergeMat[T >: Out, M, M2](that: Graph[SourceShape[T], M],
@@ -1399,6 +1428,9 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * Merge the given [[Source]] to this [[Flow]], taking elements as they arrive from input streams,
    * picking randomly when several elements ready.
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#merge]]
    */
   def mergeMat[T >: Out, M, M2](that: Graph[SourceShape[T], M],
@@ -1431,6 +1463,9 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * waiting for elements, this merge will block when one of the inputs does not have more elements (and
    * does not complete).
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#mergeSorted]].
    */
   def mergeSortedMat[U >: Out, Mat2, Mat3](that: Graph[SourceShape[U], Mat2], comp: Comparator[U],
@@ -1454,12 +1489,15 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
   /**
    * Combine the elements of current [[Flow]] and the given [[Source]] into a stream of tuples.
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#zip]]
    */
   def zipMat[T, M, M2](that: Graph[SourceShape[T], M],
                        matF: function.Function2[Mat, M, M2]): javadsl.Flow[In, Out @uncheckedVariance Pair T, M2] =
     this.viaMat(Flow.fromGraph(GraphDSL.create(that,
-      new function.Function2[GraphDSL.Builder[M], SourceShape[T], FlowShape[Out, Out @ uncheckedVariance Pair T]] {
+      new function.Function2[GraphDSL.Builder[M], SourceShape[T], FlowShape[Out, Out @uncheckedVariance Pair T]] {
         def apply(b: GraphDSL.Builder[M], s: SourceShape[T]): FlowShape[Out, Out @uncheckedVariance Pair T] = {
           val zip: FanInShape2[Out, T, Out Pair T] = b.add(Zip.create[Out, T])
           b.from(s).toInlet(zip.in1)
@@ -1487,6 +1525,9 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
    * Put together the elements of current [[Flow]] and the given [[Source]]
    * into a stream of combined elements using a combiner function.
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#zipWith]]
    */
   def zipWithMat[Out2, Out3, M, M2](that: Graph[SourceShape[Out2], M],
@@ -1792,6 +1833,9 @@ object RunnableGraph {
   private final class RunnableGraphAdapter[Mat](runnable: scaladsl.RunnableGraph[Mat]) extends RunnableGraph[Mat] {
     def shape = ClosedShape
     def module = runnable.module
+
+    override def toString: String = runnable.toString
+
     override def mapMaterializedValue[Mat2](f: function.Function[Mat, Mat2]): RunnableGraphAdapter[Mat2] =
       new RunnableGraphAdapter(runnable.mapMaterializedValue(f.apply _))
 
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Sink.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Sink.scala
index 68d8ba151f..18335aff7d 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Sink.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Sink.scala
@@ -256,6 +256,8 @@ final class Sink[-In, +Mat](delegate: scaladsl.Sink[In, Mat]) extends Graph[Sink
   override def shape: SinkShape[In] = delegate.shape
   private[stream] def module: StreamLayout.Module = delegate.module
 
+  override def toString: String = delegate.toString
+
   /** Converts this Sink to its Scala DSL counterpart */
   def asScala: scaladsl.Sink[In, Mat] = delegate
 
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala b/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
index dcd39491ba..4bb92f3451 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/Source.scala
@@ -323,6 +323,8 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
 
   private[stream] def module: StreamLayout.Module = delegate.module
 
+  override def toString: String = delegate.toString
+
   /** Converts this Java DSL element to its Scala DSL counterpart. */
   def asScala: scaladsl.Source[Out, Mat] = delegate
 
@@ -367,6 +369,9 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
    * }}}
    * The `combine` function is used to compose the materialized values of this flow and that
    * flow into the materialized value of the resulting Flow.
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def viaMat[T, M, M2](flow: Graph[FlowShape[Out, T], M], combine: function.Function2[Mat, M, M2]): javadsl.Source[T, M2] =
     new Source(delegate.viaMat(flow)(combinerToScala(combine)))
@@ -406,6 +411,9 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
    * }}}
    * The `combine` function is used to compose the materialized values of this flow and that
    * Sink into the materialized value of the resulting Sink.
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def toMat[M, M2](sink: Graph[SinkShape[Out], M], combine: function.Function2[Mat, M, M2]): javadsl.RunnableGraph[M2] =
     RunnableGraph.fromGraph(delegate.toMat(sink)(combinerToScala(combine)))
@@ -470,6 +478,9 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
    *
    * If this [[Source]] gets upstream error - no elements from the given [[Source]] will be pulled.
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#concat]].
    */
   def concatMat[T >: Out, M, M2](that: Graph[SourceShape[T], M],
@@ -507,6 +518,9 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
    *
    * If the given [[Source]] gets upstream error - no elements from this [[Source]] will be pulled.
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#prepend]].
    */
   def prependMat[T >: Out, M, M2](that: Graph[SourceShape[T], M],
@@ -532,6 +546,9 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
    * Attaches the given [[Sink]] to this [[Flow]], meaning that elements that passes
    * through will also be sent to the [[Sink]].
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#alsoTo]]
    */
   def alsoToMat[M2, M3](that: Graph[SinkShape[Out], M2],
@@ -574,6 +591,9 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
    *
    * If one of sources gets upstream error - stream completes with failure.
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#interleave]].
    */
   def interleaveMat[T >: Out, M, M2](that: Graph[SourceShape[T], M], segmentSize: Int,
@@ -599,6 +619,9 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
    * Merge the given [[Source]] to the current one, taking elements as they arrive from input streams,
    * picking randomly when several elements ready.
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#merge]].
    */
   def mergeMat[T >: Out, M, M2](that: Graph[SourceShape[T], M],
@@ -630,6 +653,9 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
    * waiting for elements, this merge will block when one of the inputs does not have more elements (and
    * does not complete).
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#mergeSorted]].
    */
   def mergeSortedMat[U >: Out, Mat2, Mat3](that: Graph[SourceShape[U], Mat2], comp: util.Comparator[U],
@@ -653,6 +679,9 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
   /**
    * Combine the elements of current [[Source]] and the given one into a stream of tuples.
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#zip]].
    */
   def zipMat[T, M, M2](that: Graph[SourceShape[T], M],
@@ -679,6 +708,9 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
    * Put together the elements of current [[Source]] and the given one
    * into a stream of combined elements using a combiner function.
    *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
+   *
    * @see [[#zipWith]].
    */
   def zipWithMat[Out2, Out3, M, M2](that: Graph[SourceShape[Out2], M],
diff --git a/akka-stream/src/main/scala/akka/stream/javadsl/package.scala b/akka-stream/src/main/scala/akka/stream/javadsl/package.scala
index cbf1188e02..6de66e5854 100644
--- a/akka-stream/src/main/scala/akka/stream/javadsl/package.scala
+++ b/akka-stream/src/main/scala/akka/stream/javadsl/package.scala
@@ -6,6 +6,8 @@ package akka.stream
 package object javadsl {
   def combinerToScala[M1, M2, M](f: akka.japi.function.Function2[M1, M2, M]): (M1, M2) ⇒ M =
     f match {
+      case x if x eq Keep.left   ⇒ scaladsl.Keep.left.asInstanceOf[(M1, M2) ⇒ M]
+      case x if x eq Keep.right  ⇒ scaladsl.Keep.right.asInstanceOf[(M1, M2) ⇒ M]
       case s: Function2[_, _, _] ⇒ s.asInstanceOf[(M1, M2) ⇒ M]
       case other                 ⇒ other.apply _
     }
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 8da794ede0..d49a36a4f3 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala
@@ -30,6 +30,8 @@ final class Flow[-In, +Out, +Mat](private[stream] override val module: Module)
 
   override val shape: FlowShape[In, Out] = module.shape.asInstanceOf[FlowShape[In, Out]]
 
+  override def toString: String = s"Flow($shape, $module)"
+
   override type Repr[+O] = Flow[In @uncheckedVariance, O, Mat @uncheckedVariance]
   override type ReprMat[+O, +M] = Flow[In @uncheckedVariance, O, M]
 
@@ -42,8 +44,14 @@ final class Flow[-In, +Out, +Mat](private[stream] override val module: Module)
 
   override def viaMat[T, Mat2, Mat3](flow: Graph[FlowShape[Out, T], Mat2])(combine: (Mat, Mat2) ⇒ Mat3): Flow[In, T, Mat3] =
     if (this.isIdentity) {
-      Flow.fromGraph(flow.asInstanceOf[Graph[FlowShape[In, T], Mat2]])
-        .mapMaterializedValue(combine(NotUsed.asInstanceOf[Mat], _))
+      import Predef.Map.empty
+      import StreamLayout.{ CompositeModule, Ignore, IgnorableMatValComp, Transform, Atomic, Combine }
+      val m = flow.module
+      val mat =
+        if (combine == Keep.left) {
+          if (IgnorableMatValComp(m)) Ignore else Transform(_ => NotUsed, Atomic(m))
+        } else Combine(combine.asInstanceOf[(Any, Any) => Any], Ignore, Atomic(m))
+      new Flow(CompositeModule(Set(m), m.shape, empty, empty, mat, Attributes.none))
     } else {
       val flowCopy = flow.module.carbonCopy
       new Flow(
@@ -86,11 +94,14 @@ final class Flow[-In, +Out, +Mat](private[stream] override val module: Module)
    * }}}
    * The `combine` function is used to compose the materialized values of this flow and that
    * Sink into the materialized value of the resulting Sink.
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def toMat[Mat2, Mat3](sink: Graph[SinkShape[Out], Mat2])(combine: (Mat, Mat2) ⇒ Mat3): Sink[In, Mat3] = {
     if (isIdentity)
       Sink.fromGraph(sink.asInstanceOf[Graph[SinkShape[In], Mat2]])
-        .mapMaterializedValue(combine(().asInstanceOf[Mat], _))
+        .mapMaterializedValue(combine(NotUsed.asInstanceOf[Mat], _))
     else {
       val sinkCopy = sink.module.carbonCopy
       new Sink(
@@ -132,6 +143,9 @@ final class Flow[-In, +Out, +Mat](private[stream] override val module: Module)
    * }}}
    * The `combine` function is used to compose the materialized values of this flow and that
    * Flow into the materialized value of the resulting Flow.
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def joinMat[Mat2, Mat3](flow: Graph[FlowShape[Out, In], Mat2])(combine: (Mat, Mat2) ⇒ Mat3): RunnableGraph[Mat3] = {
     val flowCopy = flow.module.carbonCopy
@@ -176,6 +190,9 @@ final class Flow[-In, +Out, +Mat](private[stream] override val module: Module)
    * }}}
    * The `combine` function is used to compose the materialized values of this flow and that
    * [[BidiFlow]] into the materialized value of the resulting [[Flow]].
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def joinMat[I2, O2, Mat2, M](bidi: Graph[BidiShape[Out, O2, I2, In], Mat2])(combine: (Mat, Mat2) ⇒ M): Flow[I2, O2, M] = {
     val copy = bidi.module.carbonCopy
@@ -261,8 +278,6 @@ final class Flow[-In, +Out, +Mat](private[stream] override val module: Module)
 
   /** Converts this Scala DSL element to it's Java DSL counterpart. */
   def asJava: javadsl.Flow[In, Out, Mat] = new javadsl.Flow(this)
-
-  override def toString = s"""Flow(${module})"""
 }
 
 object Flow {
@@ -410,23 +425,23 @@ trait FlowOps[+Out, +Mat] {
   def recover[T >: Out](pf: PartialFunction[Throwable, T]): Repr[T] = andThen(Recover(pf))
 
   /**
-    * RecoverWith allows to switch to alternative Source on flow failure. It will stay in effect after
-    * a failure has been recovered so that each time there is a failure it is fed into the `pf` and a new
-    * Source may be materialized.
-    *
-    * Since the underlying failure signal onError arrives out-of-band, it might jump over existing elements.
-    * This stage can recover the failure signal, but not the skipped elements, which will be dropped.
-    *
-    * '''Emits when''' element is available from the upstream or upstream is failed and element is available
-    * from alternative Source
-    *
-    * '''Backpressures when''' downstream backpressures
-    *
-    * '''Completes when''' upstream completes or upstream failed with exception pf can handle
-    *
-    * '''Cancels when''' downstream cancels
-    *
-    */
+   * RecoverWith allows to switch to alternative Source on flow failure. It will stay in effect after
+   * a failure has been recovered so that each time there is a failure it is fed into the `pf` and a new
+   * Source may be materialized.
+   *
+   * Since the underlying failure signal onError arrives out-of-band, it might jump over existing elements.
+   * This stage can recover the failure signal, but not the skipped elements, which will be dropped.
+   *
+   * '''Emits when''' element is available from the upstream or upstream is failed and element is available
+   * from alternative Source
+   *
+   * '''Backpressures when''' downstream backpressures
+   *
+   * '''Completes when''' upstream completes or upstream failed with exception pf can handle
+   *
+   * '''Cancels when''' downstream cancels
+   *
+   */
   def recoverWith[T >: Out](pf: PartialFunction[Throwable, Graph[SourceShape[T], NotUsed]]): Repr[T] =
     via(new RecoverWith(pf))
 
@@ -466,27 +481,27 @@ trait FlowOps[+Out, +Mat] {
   def mapConcat[T](f: Out ⇒ immutable.Iterable[T]): Repr[T] = statefulMapConcat(() => f)
 
   /**
-    * Transform each input element into an `Iterable` of output elements that is
-    * then flattened into the output stream. The transformation is meant to be stateful,
-    * which is enabled by creating the transformation function anew for every materialization —
-    * the returned function will typically close over mutable objects to store state between
-    * invocations. For the stateless variant see [[FlowOps.mapConcat]].
-    *
-    * The returned `Iterable` MUST NOT contain `null` values,
-    * as they are illegal as stream elements - according to the Reactive Streams specification.
-    *
-    * '''Emits when''' the mapping function returns an element or there are still remaining elements
-    * from the previously calculated collection
-    *
-    * '''Backpressures when''' downstream backpressures or there are still remaining elements from the
-    * previously calculated collection
-    *
-    * '''Completes when''' upstream completes and all remaining elements has been emitted
-    *
-    * '''Cancels when''' downstream cancels
-    *
-    * See also [[FlowOps.mapConcat]]
-    */
+   * Transform each input element into an `Iterable` of output elements that is
+   * then flattened into the output stream. The transformation is meant to be stateful,
+   * which is enabled by creating the transformation function anew for every materialization —
+   * the returned function will typically close over mutable objects to store state between
+   * invocations. For the stateless variant see [[FlowOps.mapConcat]].
+   *
+   * The returned `Iterable` MUST NOT contain `null` values,
+   * as they are illegal as stream elements - according to the Reactive Streams specification.
+   *
+   * '''Emits when''' the mapping function returns an element or there are still remaining elements
+   * from the previously calculated collection
+   *
+   * '''Backpressures when''' downstream backpressures or there are still remaining elements from the
+   * previously calculated collection
+   *
+   * '''Completes when''' upstream completes and all remaining elements has been emitted
+   *
+   * '''Cancels when''' downstream cancels
+   *
+   * See also [[FlowOps.mapConcat]]
+   */
   def statefulMapConcat[T](f: () ⇒ Out ⇒ immutable.Iterable[T]): Repr[T] =
     via(new StatefulMapConcat(f))
 
@@ -1813,6 +1828,9 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    * }}}
    * The `combine` function is used to compose the materialized values of this flow and that
    * flow into the materialized value of the resulting Flow.
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def viaMat[T, Mat2, Mat3](flow: Graph[FlowShape[Out, T], Mat2])(combine: (Mat, Mat2) ⇒ Mat3): ReprMat[T, Mat3]
 
@@ -1831,6 +1849,9 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    * }}}
    * The `combine` function is used to compose the materialized values of this flow and that
    * Sink into the materialized value of the resulting Sink.
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def toMat[Mat2, Mat3](sink: Graph[SinkShape[Out], Mat2])(combine: (Mat, Mat2) ⇒ Mat3): ClosedMat[Mat3]
 
@@ -1838,6 +1859,9 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    * Combine the elements of current flow and the given [[Source]] into a stream of tuples.
    *
    * @see [[#zip]].
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def zipMat[U, Mat2, Mat3](that: Graph[SourceShape[U], Mat2])(matF: (Mat, Mat2) ⇒ Mat3): ReprMat[(Out, U), Mat3] =
     viaMat(zipGraph(that))(matF)
@@ -1847,6 +1871,9 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    * into a stream of combined elements using a combiner function.
    *
    * @see [[#zipWith]].
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def zipWithMat[Out2, Out3, Mat2, Mat3](that: Graph[SourceShape[Out2], Mat2])(combine: (Out, Out2) ⇒ Out3)(matF: (Mat, Mat2) ⇒ Mat3): ReprMat[Out3, Mat3] =
     viaMat(zipWithGraph(that)(combine))(matF)
@@ -1856,6 +1883,9 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    * picking randomly when several elements ready.
    *
    * @see [[#merge]].
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def mergeMat[U >: Out, Mat2, Mat3](that: Graph[SourceShape[U], Mat2], eagerComplete: Boolean = false)(matF: (Mat, Mat2) ⇒ Mat3): ReprMat[U, Mat3] =
     viaMat(mergeGraph(that, eagerComplete))(matF)
@@ -1870,6 +1900,9 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    * If it gets error from one of upstreams - stream completes with failure.
    *
    * @see [[#interleave]].
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def interleaveMat[U >: Out, Mat2, Mat3](that: Graph[SourceShape[U], Mat2], request: Int)(matF: (Mat, Mat2) ⇒ Mat3): ReprMat[U, Mat3] =
     viaMat(interleaveGraph(that, request))(matF)
@@ -1882,6 +1915,9 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    * does not complete).
    *
    * @see [[#mergeSorted]].
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def mergeSortedMat[U >: Out, Mat2, Mat3](that: Graph[SourceShape[U], Mat2])(matF: (Mat, Mat2) ⇒ Mat3)(implicit ord: Ordering[U]): ReprMat[U, Mat3] =
     viaMat(mergeSortedGraph(that))(matF)
@@ -1897,6 +1933,9 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    * If this [[Flow]] gets upstream error - no elements from the given [[Source]] will be pulled.
    *
    * @see [[#concat]].
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def concatMat[U >: Out, Mat2, Mat3](that: Graph[SourceShape[U], Mat2])(matF: (Mat, Mat2) ⇒ Mat3): ReprMat[U, Mat3] =
     viaMat(concatGraph(that))(matF)
@@ -1912,6 +1951,9 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    * If the given [[Source]] gets upstream error - no elements from this [[Flow]] will be pulled.
    *
    * @see [[#prepend]].
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def prependMat[U >: Out, Mat2, Mat3](that: Graph[SourceShape[U], Mat2])(matF: (Mat, Mat2) ⇒ Mat3): ReprMat[U, Mat3] =
     viaMat(prependGraph(that))(matF)
@@ -1921,6 +1963,9 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    * through will also be sent to the [[Sink]].
    *
    * @see [[#alsoTo]]
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def alsoToMat[Mat2, Mat3](that: Graph[SinkShape[Out], Mat2])(matF: (Mat, Mat2) ⇒ Mat3): ReprMat[Out, Mat3] =
     viaMat(alsoToGraph(that))(matF)
@@ -1930,6 +1975,9 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    * The Future completes with success when received complete message from upstream or cancel
    * from downstream. It fails with the same error when received error message from
    * downstream.
+   *
+   * It is recommended to use the internally optimized `Keep.left` and `Keep.right` combiners
+   * where appropriate instead of manually writing functions that pass through one of the values.
    */
   def watchTermination[Mat2]()(matF: (Mat, Future[Done]) ⇒ Mat2): ReprMat[Out, Mat2] =
     viaMat(GraphStages.terminationWatcher)(matF)
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 caca92f7b5..cbdc2d69dd 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, StageModule }
 import akka.stream.impl.StreamLayout._
 import akka.stream.scaladsl.Partition.PartitionOutOfBoundsException
 import akka.stream.stage.{ OutHandler, InHandler, GraphStageLogic, GraphStage }
@@ -862,6 +862,20 @@ object GraphDSL extends GraphApply {
      * @return The outlet that will emit the materialized value.
      */
     def materializedValue: Outlet[M @uncheckedVariance] = {
+      /*
+       * This brings the graph into a homogenous shape: if only one `add` has
+       * been performed so far, the moduleInProgress will be a CopiedModule
+       * that upon the next `composeNoMat` will be wrapped together with the
+       * MaterializedValueSource into a CompositeModule, leading to its
+       * relevant computation being an Atomic() for the CopiedModule. This is
+       * what we must reference, and we can only get this reference if we
+       * create that computation up-front: just making one up will not work
+       * because that computation node would not be part of the tree and
+       * the source would not be triggered.
+       */
+      if (moduleInProgress.isInstanceOf[CopiedModule]) {
+        moduleInProgress = CompositeModule(moduleInProgress, moduleInProgress.shape)
+      }
       val source = new MaterializedValueSource[M](moduleInProgress.materializedValueComputation)
       moduleInProgress = moduleInProgress.composeNoMat(source.module)
       source.out
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala
index f480cbe518..1d32708bfd 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala
@@ -27,6 +27,8 @@ final class Sink[-In, +Mat](private[stream] override val module: Module)
 
   override val shape: SinkShape[In] = module.shape.asInstanceOf[SinkShape[In]]
 
+  override def toString: String = s"Sink($shape, $module)"
+
   /**
    * Transform this Sink by applying a function to each *incoming* upstream element before
    * it is passed to the [[Sink]]
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 fcdcae35dc..ec08270095 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
@@ -38,6 +38,8 @@ final class Source[+Out, +Mat](private[stream] override val module: Module)
 
   override val shape: SourceShape[Out] = module.shape.asInstanceOf[SourceShape[Out]]
 
+  override def toString: String = s"Source($shape, $module)"
+
   override def via[T, Mat2](flow: Graph[FlowShape[Out, T], Mat2]): Repr[T] = viaMat(flow)(Keep.left)
 
   override def viaMat[T, Mat2, Mat3](flow: Graph[FlowShape[Out, T], Mat2])(combine: (Mat, Mat2) ⇒ Mat3): Source[T, Mat3] = {
diff --git a/project/MiMa.scala b/project/MiMa.scala
index adba135c98..4705b753ed 100644
--- a/project/MiMa.scala
+++ b/project/MiMa.scala
@@ -683,7 +683,21 @@ object MiMa extends AutoPlugin {
         ProblemFilters.exclude[FinalClassProblem]("akka.http.scaladsl.marshalling.Marshal$UnacceptableResponseContentTypeException"),
 
         // #20009 internal and shouldn't have been public
-        ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.impl.QueueSource.completion")
+        ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.impl.QueueSource.completion"),
+
+        // #20015 simplify materialized value computation tree
+        ProblemFilters.exclude[FinalMethodProblem]("akka.stream.impl.StreamLayout#AtomicModule.subModules"),
+        ProblemFilters.exclude[FinalMethodProblem]("akka.stream.impl.StreamLayout#AtomicModule.downstreams"),
+        ProblemFilters.exclude[FinalMethodProblem]("akka.stream.impl.StreamLayout#AtomicModule.upstreams"),
+        ProblemFilters.exclude[FinalMethodProblem]("akka.stream.impl.Stages#DirectProcessor.toString"),
+        ProblemFilters.exclude[IncompatibleMethTypeProblem]("akka.stream.impl.MaterializerSession.materializeAtomic"),
+        ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.stream.impl.MaterializerSession.materializeAtomic"),
+        ProblemFilters.exclude[MissingTypesProblem]("akka.stream.impl.Stages$StageModule"),
+        ProblemFilters.exclude[FinalMethodProblem]("akka.stream.impl.Stages#GroupBy.toString"),
+        ProblemFilters.exclude[MissingTypesProblem]("akka.stream.impl.FlowModule"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.impl.FlowModule.subModules"),
+        ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.stream.impl.FlowModule.label"),
+        ProblemFilters.exclude[FinalClassProblem]("akka.stream.impl.fusing.GraphModule")
       )
     )
   }