!str #19732 Concise and consistent way to mark async boundaries

akka-docs/rst/java/code/docs/stream/FlowDocTest.java

@@ -287,8 +287,7 @@ public class FlowDocTest {
     
     //#flow-async
     Source.range(1, 3)
-        .map(x -> x + 1)
+        .map(x -> x + 1).async()
-        .withAttributes(Attributes.asyncBoundary())
         .map(x -> x * 2)
         .to(Sink.ignore());
     //#flow-async

akka-docs/rst/java/code/docs/stream/MigrationsJava.java

@@ -5,6 +5,7 @@ package docs.stream;
 
 import java.util.stream.Stream;
 
+import akka.NotUsed;
 import akka.japi.Pair;
 import akka.stream.javadsl.*;
 //#asPublisher-import
@@ -27,6 +28,11 @@ public class MigrationsJava {
     Sink.asPublisher(WITH_FANOUT);    // instead of Sink.asPublisher(true)
     Sink.asPublisher(WITHOUT_FANOUT); // instead of Sink.asPublisher(false)
     //#asPublisher
+
+    //#async
+    Flow<Integer, Integer, NotUsed> flow = Flow.of(Integer.class).map(n -> n + 1);
+    Source.range(1, 10).via(flow.async());
+    //#async
   }
 
 }

akka-docs/rst/java/stream/migration-guide-2.0-2.4-java.rst

@@ -104,6 +104,21 @@ Which is the same as using ``conflateWithSeed`` with an identity function::
 
    Flow.of(Integer.class).conflateWithSeed(x -> x, (a, b) -> a + b) // Add numbers while downstream is not ready
 
+
+``viaAsync`` and ``viaAsyncMat`` has been replaced with ``async()``
+-------------------------------------------------------------------
+``async()`` is available from ``Sink``, ``Source``, ``Flow`` and the sub flows. It provides a shortcut for
+setting the attribute ``Attributes.asyncBoundary`` on a flow. The existing methods ``Flow.viaAsync`` and
+``Flow.viaAsyncMat`` has been removed to make marking out asynchronous boundaries more consistent::
+
+    // This no longer works
+    source.viaAsync(flow)
+
+In Akka 2.4.x this will instead look lile this:
+
+.. includecode:: ../code/docs/stream/MigrationsJava.java#async
+
+
 Changed Sources / Sinks
 =======================
 

akka-docs/rst/java/stream/stream-flows-and-basics.rst

@@ -243,8 +243,9 @@ The first point can be countered by pre-fusing and then reusing a stream bluepri
 .. includecode:: ../code/docs/stream/FlowDocTest.java#explicit-fusing
 
 In order to balance the effects of the second and third bullet points you will have to insert asynchronous
-boundaries manually into your flows and graphs by way of adding ``Attributes.asyncBoundary`` to pieces that
+boundaries manually into your flows and graphs by way of adding ``Attributes.asyncBoundary`` using the method
-shall communicate with the rest of the graph in an asynchronous fashion.
+``async`` on ``Source``, ``Sink`` and ``Flow`` to pieces that shall communicate with the rest of the graph in
+an asynchronous fashion.
 
 .. includecode:: ../code/docs/stream/FlowDocTest.java#flow-async
 

akka-docs/rst/scala/code/docs/stream/FlowDocSpec.scala

@@ -237,11 +237,8 @@ class FlowDocSpec extends AkkaSpec {
 
   "defining asynchronous boundaries" in {
     //#flow-async
-    import akka.stream.Attributes.asyncBoundary
-
     Source(List(1, 2, 3))
-      .map(_ + 1)
+      .map(_ + 1).async
-      .withAttributes(asyncBoundary)
       .map(_ * 2)
       .to(Sink.ignore)
     //#flow-async

akka-docs/rst/scala/code/docs/stream/MigrationsScala.scala

@@ -23,6 +23,11 @@ class MigrationsScala extends AkkaSpec {
           })
         })
         //#expand-state
+
+        //#async
+        val flow = Flow[Int].map(_ + 1)
+        Source(1 to 10).via(flow.async)
+        //#async
       }
     }
   }

akka-docs/rst/scala/stream/migration-guide-2.0-2.4-scala.rst

@@ -91,6 +91,21 @@ Which is the same as using ``conflateWithSeed`` with an identity function
 
    Flow[Int].conflateWithSeed(identity)(_ + _) // Add numbers while downstream is not ready
 
+
+``viaAsync`` and ``viaAsyncMat`` has been replaced with ``async``
+-----------------------------------------------------------------
+``async`` is available from ``Sink``, ``Source``, ``Flow`` and the sub flows. It provides a shortcut for
+setting the attribute ``Attributes.asyncBoundary`` on a flow. The existing methods ``Flow.viaAsync`` and
+``Flow.viaAsyncMat`` has been removed to make marking out asynchronous boundaries more consistent::
+
+    // This no longer works
+    source.viaAsync(flow)
+
+In Akka 2.4.x this will instead look lile this:
+
+.. includecode:: ../code/docs/stream/MigrationsScala.scala#async
+
+
 Changes in Akka HTTP
 ====================
 

akka-docs/rst/scala/stream/stream-flows-and-basics.rst

@@ -245,8 +245,9 @@ The first point can be countered by pre-fusing and then reusing a stream bluepri
 .. includecode:: ../code/docs/stream/FlowDocSpec.scala#explicit-fusing
 
 In order to balance the effects of the second and third bullet points you will have to insert asynchronous
-boundaries manually into your flows and graphs by way of adding ``Attributes.asyncBoundary`` to pieces that
+boundaries manually into your flows and graphs by way of adding ``Attributes.asyncBoundary`` using the method
-shall communicate with the rest of the graph in an asynchronous fashion.
+``async`` on ``Source``, ``Sink`` and ``Flow`` to pieces that shall communicate with the rest of the graph in an
+asynchronous fashion.
 
 .. includecode:: ../code/docs/stream/FlowDocSpec.scala#flow-async
 

akka-stream-tests/src/test/scala/akka/stream/FusingSpec.scala

@@ -95,7 +95,7 @@ class FusingSpec extends AkkaSpec with ScalaFutures with ConversionCheckedTriple
   "SubFusingActorMaterializer" must {
 
     "work with asynchronous boundaries in the subflows" in {
-      val async = Flow[Int].map(_ * 2).withAttributes(Attributes.asyncBoundary)
+      val async = Flow[Int].map(_ * 2).async
       Source(0 to 9)
         .map(_ * 10)
         .flatMapMerge(5, i ⇒ Source(i to (i + 9)).via(async))
@@ -110,7 +110,7 @@ class FusingSpec extends AkkaSpec with ScalaFutures with ConversionCheckedTriple
         val bus = GraphInterpreter.currentInterpreter.log.asInstanceOf[BusLogging]
         bus.logSource
       }
-      val async = Flow[Int].map(x ⇒ { testActor ! ref; x }).withAttributes(Attributes.asyncBoundary)
+      val async = Flow[Int].map(x ⇒ { testActor ! ref; x }).async
       Source(0 to 9)
         .map(x ⇒ { testActor ! ref; x })
         .flatMapMerge(5, i ⇒ Source.single(i).via(async))
@@ -132,7 +132,7 @@ class FusingSpec extends AkkaSpec with ScalaFutures with ConversionCheckedTriple
       val flow = Flow[Int].map(x ⇒ { testActor ! ref; x })
       Source(0 to 9)
         .map(x ⇒ { testActor ! ref; x })
-        .flatMapMerge(5, i ⇒ Source.single(i).viaAsync(flow))
+        .flatMapMerge(5, i ⇒ Source.single(i).via(flow.async))
         .grouped(1000)
         .runWith(Sink.head)
         .futureValue

akka-stream-tests/src/test/scala/akka/stream/scaladsl/BidiFlowSpec.scala

@@ -114,7 +114,7 @@ class BidiFlowSpec extends AkkaSpec with ConversionCheckedTripleEquals {
 
     "suitably override attribute handling methods" in {
       import Attributes._
-      val b: BidiFlow[Int, Long, ByteString, String, NotUsed] = bidi.withAttributes(name("")).addAttributes(asyncBoundary).named("")
+      val b: BidiFlow[Int, Long, ByteString, String, NotUsed] = bidi.withAttributes(name("")).async.named("")
     }
 
   }

akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowSpec.scala

@@ -596,7 +596,7 @@ class FlowSpec extends AkkaSpec(ConfigFactory.parseString("akka.actor.debug.rece
 
     "suitably override attribute handling methods" in {
       import Attributes._
-      val f: Flow[Int, Int, NotUsed] = Flow[Int].withAttributes(asyncBoundary).addAttributes(none).named("")
+      val f: Flow[Int, Int, NotUsed] = Flow[Int].async.addAttributes(none).named("")
     }
   }
 

akka-stream-tests/src/test/scala/akka/stream/scaladsl/SinkSpec.scala

@@ -127,7 +127,7 @@ class SinkSpec extends AkkaSpec with ConversionCheckedTripleEquals with ScalaFut
 
     "suitably override attribute handling methods" in {
       import Attributes._
-      val s: Sink[Int, Future[Int]] = Sink.head[Int].withAttributes(asyncBoundary).addAttributes(none).named("")
+      val s: Sink[Int, Future[Int]] = Sink.head[Int].async.addAttributes(none).named("")
     }
 
     "support contramap" in {

akka-stream-tests/src/test/scala/akka/stream/scaladsl/SourceSpec.scala

@@ -272,7 +272,7 @@ class SourceSpec extends AkkaSpec with DefaultTimeout with ScalaFutures {
   "A Source" must {
     "suitably override attribute handling methods" in {
       import Attributes._
-      val s: Source[Int, NotUsed] = Source.single(42).withAttributes(asyncBoundary).addAttributes(none).named("")
+      val s: Source[Int, NotUsed] = Source.single(42).async.addAttributes(none).named("")
     }
   }
 

akka-stream/src/main/scala/akka/stream/Graph.scala

@@ -26,5 +26,10 @@ trait Graph[+S <: Shape, +M] {
 
   def named(name: String): Graph[S, M] = withAttributes(Attributes.name(name))
 
+  /**
+   * Put an asynchronous boundary around this `Graph`
+   */
+  def async: Graph[S, M] = addAttributes(Attributes.asyncBoundary)
+
   def addAttributes(attr: Attributes): Graph[S, M] = withAttributes(module.attributes and attr)
 }

akka-stream/src/main/scala/akka/stream/impl/SubFlowImpl.scala

@@ -34,6 +34,8 @@ class SubFlowImpl[In, Out, Mat, F[+_], C](val subFlow: Flow[In, Out, NotUsed],
   override def named(name: String): SubFlow[Out, Mat, F, C] =
     new SubFlowImpl[In, Out, Mat, F, C](subFlow.named(name), mergeBackFunction, finishFunction)
 
+  override def async: Repr[Out] = new SubFlowImpl[In, Out, Mat, F, C](subFlow.async, mergeBackFunction, finishFunction)
+
   override def mergeSubstreamsWithParallelism(breadth: Int): F[Out] = mergeBackFunction(subFlow, breadth)
 
   def to[M](sink: Graph[SinkShape[Out], M]): C = finishFunction(subFlow.to(sink))

akka-stream/src/main/scala/akka/stream/javadsl/Flow.scala

@@ -125,47 +125,6 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
   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)))
 
-  /**
-   * Transform this [[Flow]] by appending the given processing steps, ensuring
-   * that an `asyncBoundary` attribute is set around those steps.
-   * {{{
-   *     +----------------------------+
-   *     | Resulting Flow             |
-   *     |                            |
-   *     |  +------+        +------+  |
-   *     |  |      |        |      |  |
-   * In ~~> | this | ~Out~> | flow | ~~> T
-   *     |  |      |        |      |  |
-   *     |  +------+        +------+  |
-   *     +----------------------------+
-   * }}}
-   * The materialized value of the combined [[Flow]] will be the materialized
-   * value of the current flow (ignoring the other Flow’s value), use
-   * `viaMat` if a different strategy is needed.
-   */
-  def viaAsync[T, M](flow: Graph[FlowShape[Out, T], M]): javadsl.Flow[In, T, Mat] =
-    new Flow(delegate.viaAsync(flow))
-
-  /**
-   * Transform this [[Flow]] by appending the given processing steps, ensuring
-   * that an `asyncBoundary` attribute is set around those steps.
-   * {{{
-   *     +----------------------------+
-   *     | Resulting Flow             |
-   *     |                            |
-   *     |  +------+        +------+  |
-   *     |  |      |        |      |  |
-   * In ~~> | this | ~Out~> | flow | ~~> T
-   *     |  |      |        |      |  |
-   *     |  +------+        +------+  |
-   *     +----------------------------+
-   * }}}
-   * The `combine` function is used to compose the materialized values of this flow and that
-   * flow into the materialized value of the resulting Flow.
-   */
-  def viaAsyncMat[T, M, M2](flow: Graph[FlowShape[Out, T], M], combine: function.Function2[Mat, M, M2]): javadsl.Flow[In, T, M2] =
-    new Flow(delegate.viaAsyncMat(flow)(combinerToScala(combine)))
-
   /**
    * Connect this [[Flow]] to a [[Sink]], concatenating the processing steps of both.
    * {{{
@@ -1664,6 +1623,12 @@ final class Flow[-In, +Out, +Mat](delegate: scaladsl.Flow[In, Out, Mat]) extends
   override def named(name: String): javadsl.Flow[In, Out, Mat] =
     new Flow(delegate.named(name))
 
+  /**
+   * Put an asynchronous boundary around this `Flow`
+   */
+  override def async: javadsl.Flow[In, Out, Mat] =
+    new Flow(delegate.async)
+
   /**
    * Logs elements flowing through the stream as well as completion and erroring.
    *

akka-stream/src/main/scala/akka/stream/javadsl/Sink.scala

@@ -306,4 +306,11 @@ final class Sink[-In, +Mat](delegate: scaladsl.Sink[In, Mat]) extends Graph[Sink
    */
   override def named(name: String): javadsl.Sink[In, Mat] =
     new Sink(delegate.named(name))
+
+  /**
+   * Put an asynchronous boundary around this `Sink`
+   */
+  override def async: javadsl.Sink[In, Mat] =
+    new Sink(delegate.async)
+
 }

akka-stream/src/main/scala/akka/stream/javadsl/Source.scala

@@ -376,47 +376,6 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
   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)))
 
-  /**
-   * Transform this [[Source]] by appending the given processing stages, ensuring
-   * that an `asyncBoundary` attribute is set around those steps.
-   * {{{
-   *     +----------------------------+
-   *     | Resulting Source           |
-   *     |                            |
-   *     |  +------+        +------+  |
-   *     |  |      |        |      |  |
-   *     |  | this | ~Out~> | flow | ~~> T
-   *     |  |      |        |      |  |
-   *     |  +------+        +------+  |
-   *     +----------------------------+
-   * }}}
-   * The materialized value of the combined [[Flow]] will be the materialized
-   * value of the current flow (ignoring the other Flow’s value), use
-   * `viaMat` if a different strategy is needed.
-   */
-  def viaAsync[T, M](flow: Graph[FlowShape[Out, T], M]): javadsl.Source[T, Mat] =
-    new Source(delegate.viaAsync(flow))
-
-  /**
-   * Transform this [[Source]] by appending the given processing stages, ensuring
-   * that an `asyncBoundary` attribute is set around those steps.
-   * {{{
-   *     +----------------------------+
-   *     | Resulting Source           |
-   *     |                            |
-   *     |  +------+        +------+  |
-   *     |  |      |        |      |  |
-   *     |  | this | ~Out~> | flow | ~~> T
-   *     |  |      |        |      |  |
-   *     |  +------+        +------+  |
-   *     +----------------------------+
-   * }}}
-   * The `combine` function is used to compose the materialized values of this flow and that
-   * flow into the materialized value of the resulting Flow.
-   */
-  def viaAsyncMat[T, M, M2](flow: Graph[FlowShape[Out, T], M], combine: function.Function2[Mat, M, M2]): javadsl.Source[T, M2] =
-    new Source(delegate.viaAsyncMat(flow)(combinerToScala(combine)))
-
   /**
    * Connect this [[Source]] to a [[Sink]], concatenating the processing steps of both.
    * {{{
@@ -1808,6 +1767,12 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
   override def named(name: String): javadsl.Source[Out, Mat] =
     new Source(delegate.named(name))
 
+  /**
+   * Put an asynchronous boundary around this `Source`
+   */
+  override def async: javadsl.Source[Out, Mat] =
+    new Source(delegate.async)
+
   /**
    * Logs elements flowing through the stream as well as completion and erroring.
    *

akka-stream/src/main/scala/akka/stream/javadsl/SubFlow.scala

@@ -84,29 +84,6 @@ class SubFlow[-In, +Out, +Mat](delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Flo
   def via[T, M](flow: Graph[FlowShape[Out, T], M]): SubFlow[In, T, Mat] =
     new SubFlow(delegate.via(flow))
 
-  /**
-   * Transform this [[Flow]] by appending the given processing steps, ensuring
-   * that an `asyncBoundary` attribute is set around those steps.
-   *
-   * {{{
-   *     +----------------------------+
-   *     | Resulting Flow             |
-   *     |                            |
-   *     |  +------+        +------+  |
-   *     |  |      |        |      |  |
-   * In ~~> | this | ~Out~> | flow | ~~> T
-   *     |  |      |        |      |  |
-   *     |  +------+        +------+  |
-   *     +----------------------------+
-   * }}}
-   *
-   * The materialized value of the combined [[Flow]] will be the materialized
-   * value of the current flow (ignoring the other Flow’s value), use
-   * [[Flow#viaMat viaMat]] if a different strategy is needed.
-   */
-  def viaAsync[T, M](flow: Graph[FlowShape[Out, T], M]): SubFlow[In, T, Mat] =
-    new SubFlow(delegate.viaAsync(flow))
-
   /**
    * Connect this [[SubFlow]] to a [[Sink]], concatenating the processing steps of both.
    * This means that all sub-flows that result from the previous sub-stream operator
@@ -1191,6 +1168,12 @@ class SubFlow[-In, +Out, +Mat](delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Flo
   def named(name: String): SubFlow[In, Out, Mat] =
     new SubFlow(delegate.named(name))
 
+  /**
+   * Put an asynchronous boundary around this `SubFlow`
+   */
+  def async: SubFlow[In, Out, Mat] =
+    new SubFlow(delegate.async)
+
   /**
    * Logs elements flowing through the stream as well as completion and erroring.
    *

akka-stream/src/main/scala/akka/stream/javadsl/SubSource.scala

@@ -82,27 +82,6 @@ class SubSource[+Out, +Mat](delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Source
   def via[T, M](flow: Graph[FlowShape[Out, T], M]): SubSource[T, Mat] =
     new SubSource(delegate.via(flow))
 
-  /**
-   * Transform this [[SubSource]] by appending the given processing steps, ensuring
-   * that an `asyncBoundary` attribute is set around those steps.
-   * {{{
-   *     +----------------------------+
-   *     | Resulting Source           |
-   *     |                            |
-   *     |  +------+        +------+  |
-   *     |  |      |        |      |  |
-   *     |  | this | ~Out~> | flow | ~~> T
-   *     |  |      |        |      |  |
-   *     |  +------+        +------+  |
-   *     +----------------------------+
-   * }}}
-   * The materialized value of the combined [[Flow]] will be the materialized
-   * value of the current flow (ignoring the other Flow’s value), use
-   * [[Flow#viaMat viaMat]] if a different strategy is needed.
-   */
-  def viaAsync[T, M](flow: Graph[FlowShape[Out, T], M]): SubSource[T, Mat] =
-    new SubSource(delegate.viaAsync(flow))
-
   /**
    * Connect this [[SubSource]] to a [[Sink]], concatenating the processing steps of both.
    * This means that all sub-flows that result from the previous sub-stream operator
@@ -1188,6 +1167,12 @@ class SubSource[+Out, +Mat](delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Source
   def named(name: String): SubSource[Out, Mat] =
     new SubSource(delegate.named(name))
 
+  /**
+   * Put an asynchronous boundary around this `SubSource`
+   */
+  def async: SubSource[Out, Mat] =
+    new SubSource(delegate.async)
+
   /**
    * Logs elements flowing through the stream as well as completion and erroring.
    *

akka-stream/src/main/scala/akka/stream/scaladsl/BidiFlow.scala

@@ -152,7 +152,10 @@ final class BidiFlow[-I1, +O1, -I2, +O2, +Mat](private[stream] override val modu
    * Add a ``name`` attribute to this Flow.
    */
   override def named(name: String): BidiFlow[I1, O1, I2, O2, Mat] =
-    withAttributes(Attributes.name(name))
+    addAttributes(Attributes.name(name))
+
+  override def async: BidiFlow[I1, O1, I2, O2, Mat] =
+    addAttributes(Attributes.asyncBoundary)
 }
 
 object BidiFlow {

akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala

@@ -214,7 +214,7 @@ final class Flow[-In, +Out, +Mat](private[stream] override val module: Module)
    */
   override def withAttributes(attr: Attributes): Repr[Out] =
     if (isIdentity) this
-    else new Flow(module.withAttributes(attr).nest())
+    else new Flow(module.withAttributes(attr))
 
   /**
    * Add the given attributes to this Flow. Further calls to `withAttributes`
@@ -227,7 +227,12 @@ final class Flow[-In, +Out, +Mat](private[stream] override val module: Module)
   /**
    * Add a ``name`` attribute to this Flow.
    */
-  override def named(name: String): Repr[Out] = withAttributes(Attributes.name(name))
+  override def named(name: String): Repr[Out] = addAttributes(Attributes.name(name))
+
+  /**
+   * Put an asynchronous boundary around this `Flow`
+   */
+  override def async: Repr[Out] = addAttributes(Attributes.asyncBoundary)
 
   /**
    * Connect the `Source` to this `Flow` and then connect it to the `Sink` and run it. The returned tuple contains
@@ -390,26 +395,6 @@ trait FlowOps[+Out, +Mat] {
    */
   def via[T, Mat2](flow: Graph[FlowShape[Out, T], Mat2]): Repr[T]
 
-  /**
-   * Transform this [[Flow]] by appending the given processing steps, ensuring
-   * that an `asyncBoundary` attribute is set around those steps.
-   * {{{
-   *     +----------------------------+
-   *     | Resulting Flow             |
-   *     |                            |
-   *     |  +------+        +------+  |
-   *     |  |      |        |      |  |
-   * In ~~> | this | ~Out~> | flow | ~~> T
-   *     |  |      |        |      |  |
-   *     |  +------+        +------+  |
-   *     +----------------------------+
-   * }}}
-   * The materialized value of the combined [[Flow]] will be the materialized
-   * value of the current flow (ignoring the other Flow’s value), use
-   * [[Flow#viaMat viaMat]] if a different strategy is needed.
-   */
-  def viaAsync[T, Mat2](flow: Graph[FlowShape[Out, T], Mat2]): Repr[T] = via(flow.addAttributes(Attributes.asyncBoundary))
-
   /**
    * Recover allows to send last element on failure and gracefully complete the stream
    * Since the underlying failure signal onError arrives out-of-band, it might jump over existing elements.
@@ -1743,6 +1728,8 @@ trait FlowOps[+Out, +Mat] {
 
   def named(name: String): Repr[Out]
 
+  def async: Repr[Out]
+
   /** INTERNAL API */
   private[scaladsl] def andThen[T](op: SymbolicStage[Out, T]): Repr[T] =
     via(SymbolicGraphStage(op))
@@ -1787,26 +1774,6 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
    */
   def viaMat[T, Mat2, Mat3](flow: Graph[FlowShape[Out, T], Mat2])(combine: (Mat, Mat2) ⇒ Mat3): ReprMat[T, Mat3]
 
-  /**
-   * Transform this [[Flow]] by appending the given processing steps, ensuring
-   * that an `asyncBoundary` attribute is set around those steps.
-   * {{{
-   *     +----------------------------+
-   *     | Resulting Flow             |
-   *     |                            |
-   *     |  +------+        +------+  |
-   *     |  |      |        |      |  |
-   * In ~~> | this | ~Out~> | flow | ~~> T
-   *     |  |      |        |      |  |
-   *     |  +------+        +------+  |
-   *     +----------------------------+
-   * }}}
-   * The `combine` function is used to compose the materialized values of this flow and that
-   * flow into the materialized value of the resulting Flow.
-   */
-  def viaAsyncMat[T, Mat2, Mat3](flow: Graph[FlowShape[Out, T], Mat2])(combine: (Mat, Mat2) ⇒ Mat3): ReprMat[T, Mat3] =
-    viaMat(flow.addAttributes(Attributes.asyncBoundary))(combine)
-
   /**
    * Connect this [[Flow]] to a [[Sink]], concatenating the processing steps of both.
    * {{{

akka-stream/src/main/scala/akka/stream/scaladsl/Graph.scala

@@ -1007,14 +1007,14 @@ object GraphDSL extends GraphApply {
     private class PortOpsImpl[+Out](override val outlet: Outlet[Out @uncheckedVariance], b: Builder[_])
       extends PortOps[Out] {
 
-      override def withAttributes(attr: Attributes): Repr[Out] =
+      override def withAttributes(attr: Attributes): Repr[Out] = throw settingAttrNotSupported
-        throw new UnsupportedOperationException("Cannot set attributes on chained ops from a junction output port")
+      override def addAttributes(attr: Attributes): Repr[Out] = throw settingAttrNotSupported
+      override def named(name: String): Repr[Out] = throw settingAttrNotSupported
+      override def async: Repr[Out] = throw settingAttrNotSupported
 
-      override def addAttributes(attr: Attributes): Repr[Out] =
+      private def settingAttrNotSupported =
-        throw new UnsupportedOperationException("Cannot set attributes on chained ops from a junction output port")
+        new UnsupportedOperationException("Cannot set attributes on chained ops from a junction output port")
 
-      override def named(name: String): Repr[Out] =
-        throw new UnsupportedOperationException("Cannot set attributes on chained ops from a junction output port")
 
       override def importAndGetPort(b: Builder[_]): Outlet[Out @uncheckedVariance] = outlet
 

akka-stream/src/main/scala/akka/stream/scaladsl/Sink.scala

@@ -72,7 +72,12 @@ final class Sink[-In, +Mat](private[stream] override val module: Module)
   /**
    * Add a ``name`` attribute to this Flow.
    */
-  override def named(name: String): Sink[In, Mat] = withAttributes(Attributes.name(name))
+  override def named(name: String): Sink[In, Mat] = addAttributes(Attributes.name(name))
+
+  /**
+   * Put an asynchronous boundary around this `Sink`
+   */
+  override def async: Sink[In, Mat] = addAttributes(Attributes.asyncBoundary)
 
   /** Converts this Scala DSL element to it's Java DSL counterpart. */
   def asJava: javadsl.Sink[In, Mat] = new javadsl.Sink(this)

akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala

@@ -145,7 +145,12 @@ final class Source[+Out, +Mat](private[stream] override val module: Module)
   /**
    * Add a ``name`` attribute to this Flow.
    */
-  override def named(name: String): Repr[Out] = withAttributes(Attributes.name(name))
+  override def named(name: String): Repr[Out] = addAttributes(Attributes.name(name))
+
+  /**
+   * Put an asynchronous boundary around this `Source`
+   */
+  override def async: Repr[Out] = addAttributes(Attributes.asyncBoundary)
 
   /** Converts this Scala DSL element to it's Java DSL counterpart. */
   def asJava: javadsl.Source[Out, Mat] = new javadsl.Source(this)