diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl2/FlowGraph.scala b/akka-stream/src/main/scala/akka/stream/scaladsl2/FlowGraph.scala
index 4afede1fb2..eb7aaee529 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl2/FlowGraph.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl2/FlowGraph.scala
@@ -14,24 +14,35 @@ import akka.stream.impl2.Ast
 
 /**
  * Fan-in and fan-out vertices in the [[FlowGraph]] implements
- * this marker interface.
+ * this marker interface. Edges may end at a `JunctionInPort`.
  */
-sealed trait Junction[T] extends JunctionInPort[T] with JunctionOutPort[T] {
-  override def port: Int = FlowGraphInternal.UnlabeledPort
-  override def vertex: FlowGraphInternal.Vertex
-  type NextT = T
-  override def next = this
+sealed trait JunctionInPort[T] {
+  private[akka] def port: Int = FlowGraphInternal.UnlabeledPort
+  private[akka] def vertex: FlowGraphInternal.Vertex
+  type NextT
+  private[akka] def next: JunctionOutPort[NextT]
 }
 
-sealed trait JunctionInPort[T] {
-  def port: Int = FlowGraphInternal.UnlabeledPort
-  def vertex: FlowGraphInternal.Vertex
-  type NextT
-  def next: JunctionOutPort[NextT]
-}
+/**
+ * Fan-in and fan-out vertices in the [[FlowGraph]] implements
+ * this marker interface. Edges may start at a `JunctionOutPort`.
+ */
 sealed trait JunctionOutPort[T] {
-  def port: Int = FlowGraphInternal.UnlabeledPort
-  def vertex: FlowGraphInternal.Vertex
+  private[akka] def port: Int = FlowGraphInternal.UnlabeledPort
+  private[akka] def vertex: FlowGraphInternal.Vertex
+}
+
+/**
+ * INTERNAL API
+ *
+ * Fan-in and fan-out vertices in the [[FlowGraph]] implements
+ * this marker interface.
+ */
+private[akka] sealed trait Junction[T] extends JunctionInPort[T] with JunctionOutPort[T] {
+  override private[akka] def port: Int = FlowGraphInternal.UnlabeledPort
+  override private[akka] def vertex: FlowGraphInternal.Vertex
+  override type NextT = T
+  override private[akka] def next = this
 }
 
 object Merge {
@@ -58,7 +69,7 @@ object Merge {
  * and one output flow/sink to the `Merge` vertex.
  */
 final class Merge[T](override val name: Option[String]) extends FlowGraphInternal.InternalVertex with Junction[T] {
-  override val vertex = this
+  override private[akka] val vertex = this
   override val minimumInputCount: Int = 2
   override val maximumInputCount: Int = Int.MaxValue
   override val minimumOutputCount: Int = 1
@@ -87,11 +98,11 @@ object Broadcast {
  * two downstream subscribers have been established.
  */
 final class Broadcast[T](override val name: Option[String]) extends FlowGraphInternal.InternalVertex with Junction[T] {
-  override val vertex = this
-  override val minimumInputCount: Int = 1
-  override val maximumInputCount: Int = 1
-  override val minimumOutputCount: Int = 2
-  override val maximumOutputCount: Int = Int.MaxValue
+  override private[akka] def vertex = this
+  override def minimumInputCount: Int = 1
+  override def maximumInputCount: Int = 1
+  override def minimumOutputCount: Int = 2
+  override def maximumOutputCount: Int = Int.MaxValue
 }
 
 object Zip {
@@ -111,17 +122,17 @@ object Zip {
    */
   def apply[A, B](name: String): Zip[A, B] = new Zip[A, B](Some(name))
 
-  class Left[A, B] private[akka] (val vertex: Zip[A, B]) extends JunctionInPort[A] {
-    override val port = 0
+  class Left[A, B] private[akka] (private[akka] val vertex: Zip[A, B]) extends JunctionInPort[A] {
+    override private[akka] def port = 0
     type NextT = (A, B)
-    override def next = vertex.out
+    override private[akka] def next = vertex.out
   }
-  class Right[A, B] private[akka] (val vertex: Zip[A, B]) extends JunctionInPort[B] {
-    override val port = 1
+  class Right[A, B] private[akka] (private[akka] val vertex: Zip[A, B]) extends JunctionInPort[B] {
+    override private[akka] def port = 1
     type NextT = (A, B)
-    override def next = vertex.out
+    override private[akka] def next = vertex.out
   }
-  class Out[A, B] private[akka] (val vertex: Zip[A, B]) extends JunctionOutPort[(A, B)]
+  class Out[A, B] private[akka] (private[akka] val vertex: Zip[A, B]) extends JunctionOutPort[(A, B)]
 }
 
 /**
@@ -134,10 +145,10 @@ final class Zip[A, B](override val name: Option[String]) extends FlowGraphIntern
   val right = new Zip.Right(this)
   val out = new Zip.Out(this)
 
-  override val minimumInputCount: Int = 2
-  override val maximumInputCount: Int = 2
-  override val minimumOutputCount: Int = 1
-  override val maximumOutputCount: Int = 1
+  override def minimumInputCount: Int = 2
+  override def maximumInputCount: Int = 2
+  override def minimumOutputCount: Int = 1
+  override def maximumOutputCount: Int = 1
 }
 
 object UndefinedSink {
@@ -162,10 +173,10 @@ object UndefinedSink {
  * be replaced with [[FlowGraphBuilder#attachSink]].
  */
 final class UndefinedSink[T](override val name: Option[String]) extends FlowGraphInternal.InternalVertex {
-  override val minimumInputCount: Int = 1
-  override val maximumInputCount: Int = 1
-  override val minimumOutputCount: Int = 0
-  override val maximumOutputCount: Int = 0
+  override def minimumInputCount: Int = 1
+  override def maximumInputCount: Int = 1
+  override def minimumOutputCount: Int = 0
+  override def maximumOutputCount: Int = 0
 }
 
 object UndefinedSource {
@@ -190,10 +201,10 @@ object UndefinedSource {
  * be replaced with [[FlowGraphBuilder#attachSource]].
  */
 final class UndefinedSource[T](override val name: Option[String]) extends FlowGraphInternal.InternalVertex {
-  override val minimumInputCount: Int = 0
-  override val maximumInputCount: Int = 0
-  override val minimumOutputCount: Int = 1
-  override val maximumOutputCount: Int = 1
+  override def minimumInputCount: Int = 0
+  override def maximumInputCount: Int = 0
+  override def minimumOutputCount: Int = 1
+  override def maximumOutputCount: Int = 1
 }
 
 /**
@@ -201,7 +212,7 @@ final class UndefinedSource[T](override val name: Option[String]) extends FlowGr
  */
 private[akka] object FlowGraphInternal {
 
-  val UnlabeledPort = -1
+  def UnlabeledPort = -1
 
   sealed trait Vertex
   case class SourceVertex(source: Source[_]) extends Vertex {
@@ -369,7 +380,7 @@ class FlowGraphBuilder private (graph: Graph[FlowGraphInternal.Vertex, LkDiEdge]
           case Some(node) ⇒
             require(
               (node.inDegree + 1) <= iv.maximumInputCount,
-              s"${node.value}) must have at most ${iv.maximumInputCount} incoming edges")
+              s"${node.value} must have at most ${iv.maximumInputCount} incoming edges")
           case _ ⇒ // ok
         }
       case _ ⇒ // ok, no checks here
@@ -383,7 +394,7 @@ class FlowGraphBuilder private (graph: Graph[FlowGraphInternal.Vertex, LkDiEdge]
           case Some(node) ⇒
             require(
               (node.outDegree + 1) <= iv.maximumOutputCount,
-              s"${node.value}) must have at most ${iv.maximumOutputCount} outgoing edges")
+              s"${node.value} must have at most ${iv.maximumOutputCount} outgoing edges")
           case _ ⇒ // ok
         }
       case _ ⇒ // ok, no checks here
diff --git a/akka-stream/src/test/scala/akka/stream/scaladsl2/FlowGraphCompileSpec.scala b/akka-stream/src/test/scala/akka/stream/scaladsl2/FlowGraphCompileSpec.scala
index 8185f47e6d..6c4e8e1cc4 100644
--- a/akka-stream/src/test/scala/akka/stream/scaladsl2/FlowGraphCompileSpec.scala
+++ b/akka-stream/src/test/scala/akka/stream/scaladsl2/FlowGraphCompileSpec.scala
@@ -89,7 +89,7 @@ class FlowGraphCompileSpec extends AkkaSpec {
           val merge = Merge[String]
           val bcast1 = Broadcast[String]
           val bcast2 = Broadcast[String]
-          val feedbackLoopBuffer = FlowFrom[String].buffer(10, OverflowStrategy.DropBuffer)
+          val feedbackLoopBuffer = FlowFrom[String].buffer(10, OverflowStrategy.dropBuffer)
           b.
             addEdge(in1, f1, merge).
             addEdge(merge, f2, bcast1).
@@ -108,7 +108,7 @@ class FlowGraphCompileSpec extends AkkaSpec {
         val merge = Merge[String]
         val bcast1 = Broadcast[String]
         val bcast2 = Broadcast[String]
-        val feedbackLoopBuffer = FlowFrom[String].buffer(10, OverflowStrategy.DropBuffer)
+        val feedbackLoopBuffer = FlowFrom[String].buffer(10, OverflowStrategy.dropBuffer)
         import FlowGraphImplicits._
         in1 ~> f1 ~> merge ~> f2 ~> bcast1 ~> f3 ~> out1
         bcast1 ~> feedbackLoopBuffer ~> bcast2 ~> f5 ~> merge