Deprecate auto-fusing setting #22431

akka-docs/rst/java/stream/stream-composition.rst

@@ -283,7 +283,7 @@ the :class:`CompletionStage<Sink>` part, and wraps the other two values in a cus
 
 .. note::
   The nested structure in the above example is not necessary for combining the materialized values, it just
-  demonstrates how the two features work together. See :ref:`flow-combine-mat-java` for further examples
+  demonstrates how the two features work together. See :ref:`operator-fusion-java` for further examples
   of combining materialized values without nesting and hierarchy involved.
 
 Attributes

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

@@ -222,7 +222,7 @@ which will be running on the thread pools they have been configured to run on -
    Reusing *instances* of linear computation stages (Source, Sink, Flow) inside composite Graphs is legal,
    yet will materialize that stage multiple times.
 
-.. _flow-combine-mat-java:
+.. _operator-fusion-java:
 
 Operator Fusion
 ---------------

akka-docs/rst/project/migration-guide-2.4.x-2.5.x.rst

@@ -316,6 +316,15 @@ In 2.4 an explicit `async` marker (``AsyncBoundary`` attribute) had to be added.
 defined ``blocking-io-dispatcher`` as default followed by a ``map`` will now be separated by an async boundary, which was not
 the case in 2.4.
 
+Removal of the auto-fuse setting
+--------------------------------
+
+In 2.4 fusing stages together into the same actor could be completely disabled with the setting
+``akka.stream.materializer.auto-fusing``. The new materializer introduced in Akka 2.5 does not support disabling fusing,
+so this setting does not have any effect any more and has been deprecated. Running each stage in a stream on a separate
+actor can be done by adding explicit async boundaries around every stage. How to add asynchronous boundaries can be seen
+in :ref:`operator-fusion-java` (Java) and :ref:`operator-fusion-scala` (Scala).
+
 Remote
 ======
 

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

@@ -226,6 +226,8 @@ which will be running on the thread pools they have been configured to run on -
    Reusing *instances* of linear computation stages (Source, Sink, Flow) inside composite Graphs is legal,
    yet will materialize that stage multiple times.
 
+.. _operator-fusion-scala:
+
 Operator Fusion
 ---------------
 

akka-stream/src/main/resources/reference.conf

@@ -44,6 +44,7 @@ akka {
       # Enable automatic fusing of all graphs that are run. For short-lived streams
       # this may cause an initial runtime overhead, but most of the time fusing is
       # desirable since it reduces the number of Actors that are created.
+      # Deprecated, since Akka 2.5.0, setting does not have any effect.
       auto-fusing = on
 
       # Those stream elements which have explicit buffers (like mapAsync, mapAsyncUnordered,

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

@@ -433,6 +433,7 @@ final class ActorMaterializerSettings private (
    * this may cause an initial runtime overhead, but most of the time fusing is
    * desirable since it reduces the number of Actors that are created.
    */
+  @deprecated(since = "2.5.0", message = "Turning off fusing is no longer possible with the traversal based materializer")
   def withAutoFusing(enable: Boolean): ActorMaterializerSettings =
     if (enable == this.autoFusing) this
     else copy(autoFusing = enable)

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

@@ -347,6 +347,10 @@ case class PhasedFusingActorMaterializer(
     _logger.warning("Fuzzing mode is enabled on this system. If you see this warning on your production system then " +
       "set akka.stream.materializer.debug.fuzzing-mode to off.")
   }
+  if (!settings.autoFusing) {
+    _logger.warning("Deprecated setting auto-fusing set to false. Since Akka 2.5.0 it does not have any effect " +
+      "and streams are always fused.")
+  }
 
   override def shutdown(): Unit =
     if (haveShutDown.compareAndSet(false, true)) supervisor ! PoisonPill