Internal dispatcher to protect against starvation (#26816)

* Allow for dispatcher aliases and define a internal dispatcher
* Test checking dispatcher name
* MiMa for Dispatchers
* Migration guide entry
* No need to have custom dispatcher lookup logic in streams anymore
* Default dispatcher size and migration note about that
* Test checking exact config values...
* Typed receptionist on internal dispatcher
* All internal usages of system.dispatcher gone through

akka-actor-tests/src/test/scala/akka/actor/ActorSystemDispatcherSpec.scala

@@ -0,0 +1,167 @@
+/*
+ * Copyright (C) 2009-2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.actor
+
+import akka.ConfigurationException
+import akka.actor.setup.ActorSystemSetup
+import akka.dispatch.{ Dispatchers, ExecutionContexts }
+import akka.testkit.{ AkkaSpec, ImplicitSender, TestActors, TestProbe }
+import com.typesafe.config.ConfigFactory
+
+import scala.concurrent.ExecutionContext
+import scala.concurrent.duration._
+
+object ActorSystemDispatchersSpec {
+
+  class SnitchingExecutionContext(testActor: ActorRef, underlying: ExecutionContext) extends ExecutionContext {
+
+    def execute(runnable: Runnable): Unit = {
+      testActor ! "called"
+      underlying.execute(runnable)
+    }
+
+    def reportFailure(t: Throwable): Unit = {
+      testActor ! "failed"
+      underlying.reportFailure(t)
+    }
+  }
+
+}
+
+class ActorSystemDispatchersSpec extends AkkaSpec(ConfigFactory.parseString("""
+    dispatcher-loop-1 = "dispatcher-loop-2"
+    dispatcher-loop-2 = "dispatcher-loop-1"
+  """)) with ImplicitSender {
+
+  import ActorSystemDispatchersSpec._
+
+  "The ActorSystem" must {
+
+    "work with a passed in ExecutionContext" in {
+      val ecProbe = TestProbe()
+      val ec = new SnitchingExecutionContext(ecProbe.ref, ExecutionContexts.global())
+
+      val system2 = ActorSystem(name = "ActorSystemDispatchersSpec-passed-in-ec", defaultExecutionContext = Some(ec))
+
+      try {
+        val ref = system2.actorOf(Props(new Actor {
+          def receive = {
+            case "ping" => sender() ! "pong"
+          }
+        }))
+
+        val probe = TestProbe()
+
+        ref.tell("ping", probe.ref)
+
+        ecProbe.expectMsg(1.second, "called")
+        probe.expectMsg(1.second, "pong")
+      } finally {
+        shutdown(system2)
+      }
+    }
+
+    "not use passed in ExecutionContext if executor is configured" in {
+      val ecProbe = TestProbe()
+      val ec = new SnitchingExecutionContext(ecProbe.ref, ExecutionContexts.global())
+
+      val config = ConfigFactory.parseString("akka.actor.default-dispatcher.executor = \"fork-join-executor\"")
+      val system2 = ActorSystem(
+        name = "ActorSystemDispatchersSpec-ec-configured",
+        config = Some(config),
+        defaultExecutionContext = Some(ec))
+
+      try {
+        val ref = system2.actorOf(TestActors.echoActorProps)
+        val probe = TestProbe()
+
+        ref.tell("ping", probe.ref)
+
+        ecProbe.expectNoMessage(200.millis)
+        probe.expectMsg(1.second, "ping")
+      } finally {
+        shutdown(system2)
+      }
+    }
+
+    def userGuardianDispatcher(system: ActorSystem): String = {
+      val impl = system.asInstanceOf[ActorSystemImpl]
+      impl.guardian.asInstanceOf[ActorRefWithCell].underlying.asInstanceOf[ActorCell].dispatcher.id
+    }
+
+    "provide a single place to override the internal dispatcher" in {
+      val sys = ActorSystem(
+        "ActorSystemDispatchersSpec-override-internal-disp",
+        ConfigFactory.parseString("""
+             akka.actor.internal-dispatcher = akka.actor.default-dispatcher
+           """))
+      try {
+        // that the user guardian runs on the overriden dispatcher instead of internal
+        // isn't really a guarantee any internal actor has been made running on the right one
+        // but it's better than no test coverage at all
+        userGuardianDispatcher(sys) should ===("akka.actor.default-dispatcher")
+      } finally {
+        shutdown(sys)
+      }
+    }
+
+    "provide internal execution context instance through BootstrapSetup" in {
+      val ecProbe = TestProbe()
+      val ec = new SnitchingExecutionContext(ecProbe.ref, ExecutionContexts.global())
+
+      // using the default for internal dispatcher and passing a pre-existing execution context
+      val system2 =
+        ActorSystem(
+          name = "ActorSystemDispatchersSpec-passed-in-ec-for-internal",
+          config = Some(ConfigFactory.parseString("""
+            akka.actor.internal-dispatcher = akka.actor.default-dispatcher
+          """)),
+          defaultExecutionContext = Some(ec))
+
+      try {
+        val ref = system2.actorOf(Props(new Actor {
+          def receive = {
+            case "ping" => sender() ! "pong"
+          }
+        }).withDispatcher(Dispatchers.InternalDispatcherId))
+
+        val probe = TestProbe()
+
+        ref.tell("ping", probe.ref)
+
+        ecProbe.expectMsg(1.second, "called")
+        probe.expectMsg(1.second, "pong")
+      } finally {
+        shutdown(system2)
+      }
+    }
+
+    "use an internal dispatcher for the guardian by default" in {
+      userGuardianDispatcher(system) should ===("akka.actor.internal-dispatcher")
+    }
+
+    "use the default dispatcher by a user provided user guardian" in {
+      val sys = new ActorSystemImpl(
+        "ActorSystemDispatchersSpec-custom-user-guardian",
+        ConfigFactory.defaultReference(),
+        getClass.getClassLoader,
+        None,
+        Some(Props.empty),
+        ActorSystemSetup.empty)
+      sys.start()
+      try {
+        userGuardianDispatcher(sys) should ===("akka.actor.default-dispatcher")
+      } finally shutdown(sys)
+    }
+
+    "provide a good error on an dispatcher alias loop in the config" in {
+      intercept[ConfigurationException] {
+        system.dispatchers.lookup("dispatcher-loop-1")
+      }
+    }
+
+  }
+
+}

akka-actor-tests/src/test/scala/akka/actor/ActorSystemSpec.scala

@@ -11,15 +11,14 @@ import akka.actor.setup.ActorSystemSetup
 import akka.dispatch._
 import akka.japi.Util.immutableSeq
 import akka.pattern.ask
-import akka.testkit._
+import akka.testkit.{ TestKit, _ }
-import akka.testkit.TestKit
 import akka.util.Helpers.ConfigOps
 import akka.util.{ Switch, Timeout }
 import com.github.ghik.silencer.silent
 import com.typesafe.config.{ Config, ConfigFactory }
 
 import scala.concurrent.duration._
-import scala.concurrent.{ Await, ExecutionContext, Future }
+import scala.concurrent.{ Await, Future }
 import scala.language.postfixOps
 import scala.util.Properties
 
@@ -107,19 +106,6 @@ object ActorSystemSpec {
     override def dispatcher(): MessageDispatcher = instance
   }
 
-  class TestExecutionContext(testActor: ActorRef, underlying: ExecutionContext) extends ExecutionContext {
-
-    def execute(runnable: Runnable): Unit = {
-      testActor ! "called"
-      underlying.execute(runnable)
-    }
-
-    def reportFailure(t: Throwable): Unit = {
-      testActor ! "failed"
-      underlying.reportFailure(t)
-    }
-  }
-
   val config = s"""
       slow {
         type="${classOf[SlowDispatcher].getName}"
@@ -372,50 +358,6 @@ class ActorSystemSpec extends AkkaSpec(ActorSystemSpec.config) with ImplicitSend
       }
     }
 
-    "work with a passed in ExecutionContext" in {
-      val ecProbe = TestProbe()
-      val ec = new ActorSystemSpec.TestExecutionContext(ecProbe.ref, ExecutionContexts.global())
-
-      val system2 = ActorSystem(name = "default", defaultExecutionContext = Some(ec))
-
-      try {
-        val ref = system2.actorOf(Props(new Actor {
-          def receive = {
-            case "ping" => sender() ! "pong"
-          }
-        }))
-
-        val probe = TestProbe()
-
-        ref.tell("ping", probe.ref)
-
-        ecProbe.expectMsg(1.second, "called")
-        probe.expectMsg(1.second, "pong")
-      } finally {
-        shutdown(system2)
-      }
-    }
-
-    "not use passed in ExecutionContext if executor is configured" in {
-      val ecProbe = TestProbe()
-      val ec = new ActorSystemSpec.TestExecutionContext(ecProbe.ref, ExecutionContexts.global())
-
-      val config = ConfigFactory.parseString("akka.actor.default-dispatcher.executor = \"fork-join-executor\"")
-      val system2 = ActorSystem(name = "default", config = Some(config), defaultExecutionContext = Some(ec))
-
-      try {
-        val ref = system2.actorOf(TestActors.echoActorProps)
-        val probe = TestProbe()
-
-        ref.tell("ping", probe.ref)
-
-        ecProbe.expectNoMessage(200.millis)
-        probe.expectMsg(1.second, "ping")
-      } finally {
-        shutdown(system2)
-      }
-    }
-
     "not allow top-level actor creation with custom guardian" in {
       val sys = new ActorSystemImpl(
         "custom",

akka-actor-tests/src/test/scala/akka/config/ConfigSpec.scala

@@ -94,7 +94,7 @@ class ConfigSpec extends AkkaSpec(ConfigFactory.defaultReference(ActorSystem.fin
         {
           val pool = c.getConfig("fork-join-executor")
           pool.getInt("parallelism-min") should ===(8)
-          pool.getDouble("parallelism-factor") should ===(3.0)
+          pool.getDouble("parallelism-factor") should ===(1.0)
           pool.getInt("parallelism-max") should ===(64)
           pool.getString("task-peeking-mode") should be("FIFO")
         }

akka-actor-tests/src/test/scala/akka/dispatch/DispatcherShutdownSpec.scala

@@ -23,7 +23,9 @@ class DispatcherShutdownSpec extends WordSpec with Matchers {
           .dumpAllThreads(false, false)
           .toList
           .map(_.getThreadName)
-          .filter(_.startsWith("DispatcherShutdownSpec-akka.actor.default"))
+          .filter(name =>
+            name.startsWith("DispatcherShutdownSpec-akka.actor.default") || name.startsWith(
+              "DispatcherShutdownSpec-akka.actor.internal")) // nothing is run on default without any user actors started
           .size
 
       val system = ActorSystem("DispatcherShutdownSpec")

akka-actor-typed/src/main/scala/akka/actor/typed/Dispatchers.scala

@@ -4,6 +4,8 @@
 
 package akka.actor.typed
 
+import akka.annotation.InternalApi
+
 import scala.concurrent.ExecutionContextExecutor
 
 object Dispatchers {
@@ -13,6 +15,11 @@ object Dispatchers {
    * configuration of the default dispatcher.
    */
   final val DefaultDispatcherId = "akka.actor.default-dispatcher"
+
+  /**
+   * INTERNAL API
+   */
+  @InternalApi final val InternalDispatcherId = "akka.actor.internal-dispatcher"
 }
 
 /**

akka-actor-typed/src/main/scala/akka/actor/typed/receptionist/Receptionist.scala

@@ -4,14 +4,12 @@
 
 package akka.actor.typed.receptionist
 
-import akka.actor.typed.{ ActorRef, ActorSystem, Extension, ExtensionId }
+import akka.actor.typed.{ ActorRef, ActorSystem, Dispatchers, Extension, ExtensionId, ExtensionSetup, Props }
 import akka.actor.typed.internal.receptionist._
 import akka.annotation.DoNotInherit
+
 import scala.collection.JavaConverters._
 import scala.reflect.ClassTag
-
-import akka.actor.typed.ExtensionSetup
-import akka.actor.typed.Props
 import akka.annotation.InternalApi
 
 /**
@@ -51,7 +49,10 @@ abstract class Receptionist extends Extension {
       } else LocalReceptionist
 
     import akka.actor.typed.scaladsl.adapter._
-    system.internalSystemActorOf(provider.behavior, "receptionist", Props.empty)
+    system.internalSystemActorOf(
+      provider.behavior,
+      "receptionist",
+      Props.empty.withDispatcherFromConfig(Dispatchers.InternalDispatcherId))
   }
 }
 

akka-actor/src/main/mima-filters/2.5.x.backwards.excludes

@@ -1,3 +1,5 @@
+# excludes for 2.6
+
 ProblemFilters.exclude[MissingClassProblem]("akka.actor.Inbox$")
 ProblemFilters.exclude[MissingClassProblem]("akka.actor.Inbox")
 ProblemFilters.exclude[MissingClassProblem]("akka.actor.ActorDSL$")
@@ -13,3 +15,6 @@ ProblemFilters.exclude[DirectMissingMethodProblem]("akka.actor.ActorRefFactory.a
 ProblemFilters.exclude[DirectMissingMethodProblem]("akka.actor.ActorSystem.actorFor")
 ProblemFilters.exclude[DirectMissingMethodProblem]("akka.actor.ChildActorPath.this")
 ProblemFilters.exclude[MissingClassProblem]("akka.actor.dungeon.UndefinedUidActorRef")
+
+# Protect internals against starvation #23576
+ProblemFilters.exclude[DirectMissingMethodProblem]("akka.dispatch.Dispatchers.this")

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

@@ -451,7 +451,7 @@ akka {
         # The parallelism factor is used to determine thread pool size using the
         # following formula: ceil(available processors * factor). Resulting size
         # is then bounded by the parallelism-min and parallelism-max values.
-        parallelism-factor = 3.0
+        parallelism-factor = 1.0
 
         # Max number of threads to cap factor-based parallelism number to
         parallelism-max = 64
@@ -533,6 +533,20 @@ akka {
       mailbox-requirement = ""
     }
 
+    # Default separate internal dispatcher to run Akka internal tasks and actors on
+    # protecting them against starvation because of accidental blocking in user actors (which run on the
+    # default dispatcher)
+    internal-dispatcher {
+      type = "Dispatcher"
+      executor = "fork-join-executor"
+      throughput = 5
+      fork-join-executor {
+        parallelism-min = 4
+        parallelism-factor = 1.0
+        parallelism-max = 64
+      }
+    }
+
     default-blocking-io-dispatcher {
       type = "Dispatcher"
       executor = "thread-pool-executor"
@@ -855,11 +869,11 @@ akka {
 
       # Fully qualified config path which holds the dispatcher configuration
       # for the read/write worker actors
-      worker-dispatcher = "akka.actor.default-dispatcher"
+      worker-dispatcher = "akka.actor.internal-dispatcher"
 
       # Fully qualified config path which holds the dispatcher configuration
       # for the selector management actors
-      management-dispatcher = "akka.actor.default-dispatcher"
+      management-dispatcher = "akka.actor.internal-dispatcher"
 
       # Fully qualified config path which holds the dispatcher configuration
       # on which file IO tasks are scheduled
@@ -937,11 +951,11 @@ akka {
 
       # Fully qualified config path which holds the dispatcher configuration
       # for the read/write worker actors
-      worker-dispatcher = "akka.actor.default-dispatcher"
+      worker-dispatcher = "akka.actor.internal-dispatcher"
 
       # Fully qualified config path which holds the dispatcher configuration
       # for the selector management actors
-      management-dispatcher = "akka.actor.default-dispatcher"
+      management-dispatcher = "akka.actor.internal-dispatcher"
     }
 
     udp-connected {
@@ -993,18 +1007,18 @@ akka {
 
       # Fully qualified config path which holds the dispatcher configuration
       # for the read/write worker actors
-      worker-dispatcher = "akka.actor.default-dispatcher"
+      worker-dispatcher = "akka.actor.internal-dispatcher"
 
       # Fully qualified config path which holds the dispatcher configuration
       # for the selector management actors
-      management-dispatcher = "akka.actor.default-dispatcher"
+      management-dispatcher = "akka.actor.internal-dispatcher"
     }
 
     dns {
       # Fully qualified config path which holds the dispatcher configuration
       # for the manager and resolver router actors.
       # For actual router configuration see akka.actor.deployment./IO-DNS/*
-      dispatcher = "akka.actor.default-dispatcher"
+      dispatcher = "akka.actor.internal-dispatcher"
 
       # Name of the subconfig at path akka.io.dns, see inet-address below
       #

akka-actor/src/main/scala/akka/actor/ActorRefProvider.scala

@@ -5,21 +5,20 @@
 package akka.actor
 
 import akka.dispatch.sysmsg._
-import akka.dispatch.{ RequiresMessageQueue, UnboundedMessageQueueSemantics }
+import akka.dispatch.{ Mailboxes, RequiresMessageQueue, UnboundedMessageQueueSemantics }
 import akka.routing._
 import akka.event._
 import akka.util.Helpers
 import akka.util.Collections.EmptyImmutableSeq
+
 import scala.util.control.NonFatal
 import java.util.concurrent.atomic.AtomicLong
 
 import scala.concurrent.{ ExecutionContextExecutor, Future, Promise }
 import scala.annotation.implicitNotFound
-
 import akka.ConfigurationException
 import akka.annotation.DoNotInherit
 import akka.annotation.InternalApi
-import akka.dispatch.Mailboxes
 import akka.serialization.Serialization
 import akka.util.OptionVal
 
@@ -484,7 +483,7 @@ private[akka] class LocalActorRefProvider private[akka] (
    */
   protected def systemGuardianStrategy: SupervisorStrategy = SupervisorStrategy.defaultStrategy
 
-  private lazy val defaultDispatcher = system.dispatchers.defaultGlobalDispatcher
+  private def internalDispatcher = system.dispatchers.internalDispatcher
 
   private lazy val defaultMailbox = system.mailboxes.lookup(Mailboxes.DefaultMailboxId)
 
@@ -492,7 +491,7 @@ private[akka] class LocalActorRefProvider private[akka] (
     new LocalActorRef(
       system,
       Props(classOf[LocalActorRefProvider.Guardian], rootGuardianStrategy),
-      defaultDispatcher,
+      internalDispatcher,
       defaultMailbox,
       theOneWhoWalksTheBubblesOfSpaceTime,
       rootPath) {
@@ -511,10 +510,16 @@ private[akka] class LocalActorRefProvider private[akka] (
   override lazy val guardian: LocalActorRef = {
     val cell = rootGuardian.underlying
     cell.reserveChild("user")
+    // make user provided guardians not run on internal dispatcher
+    val dispatcher =
+      system.guardianProps match {
+        case None        => internalDispatcher
+        case Some(props) => system.dispatchers.lookup(props.dispatcher)
+      }
     val ref = new LocalActorRef(
       system,
       system.guardianProps.getOrElse(Props(classOf[LocalActorRefProvider.Guardian], guardianStrategy)),
-      defaultDispatcher,
+      dispatcher,
       defaultMailbox,
       rootGuardian,
       rootPath / "user")
@@ -529,7 +534,7 @@ private[akka] class LocalActorRefProvider private[akka] (
     val ref = new LocalActorRef(
       system,
       Props(classOf[LocalActorRefProvider.SystemGuardian], systemGuardianStrategy, guardian),
-      defaultDispatcher,
+      internalDispatcher,
       defaultMailbox,
       rootGuardian,
       rootPath / "system")

akka-actor/src/main/scala/akka/actor/ActorSystem.scala

@@ -319,7 +319,12 @@ object ActorSystem {
      */
     final val config: Config = {
       val config = cfg.withFallback(ConfigFactory.defaultReference(classLoader))
-      config.checkValid(ConfigFactory.defaultReference(classLoader), "akka")
+
+      config.checkValid(
+        ConfigFactory
+          .defaultReference(classLoader)
+          .withoutPath(Dispatchers.InternalDispatcherId), // allow this to be both string and config object
+        "akka")
       config
     }
 
@@ -840,7 +845,8 @@ private[akka] class ActorSystemImpl(
       dynamicAccess,
       settings,
       mailboxes,
-      defaultExecutionContext))
+      defaultExecutionContext),
+    log)
 
   val dispatcher: ExecutionContextExecutor = dispatchers.defaultGlobalDispatcher
 

akka-actor/src/main/scala/akka/actor/CoordinatedShutdown.scala

@@ -454,7 +454,7 @@ final class CoordinatedShutdown private[akka] (
    */
   def run(reason: Reason, fromPhase: Option[String]): Future[Done] = {
     if (runStarted.compareAndSet(None, Some(reason))) {
-      import system.dispatcher
+      implicit val ec = system.dispatchers.internalDispatcher
       val debugEnabled = log.isDebugEnabled
       def loop(remainingPhases: List[String]): Future[Done] = {
         remainingPhases match {

akka-actor/src/main/scala/akka/dispatch/Dispatchers.scala

@@ -6,11 +6,12 @@ package akka.dispatch
 
 import java.util.concurrent.{ ConcurrentHashMap, ThreadFactory }
 
-import com.typesafe.config.{ Config, ConfigFactory }
+import com.typesafe.config.{ Config, ConfigFactory, ConfigValueType }
 import akka.actor.{ ActorSystem, DynamicAccess, Scheduler }
 import akka.event.Logging.Warning
-import akka.event.EventStream
+import akka.event.{ EventStream, LoggingAdapter }
 import akka.ConfigurationException
+import akka.annotation.{ DoNotInherit, InternalApi }
 import akka.util.Helpers.ConfigOps
 import com.github.ghik.silencer.silent
 
@@ -32,6 +33,7 @@ trait DispatcherPrerequisites {
 /**
  * INTERNAL API
  */
+@InternalApi
 private[akka] final case class DefaultDispatcherPrerequisites(
     threadFactory: ThreadFactory,
     eventStream: EventStream,
@@ -49,6 +51,21 @@ object Dispatchers {
    * configuration of the default dispatcher.
    */
   final val DefaultDispatcherId = "akka.actor.default-dispatcher"
+
+  /**
+   * The id of a default dispatcher to use for operations known to be blocking. Note that
+   * for optimal performance you will want to isolate different blocking resources
+   * on different thread pools.
+   */
+  final val DefaultBlockingDispatcherId: String = "akka.actor.default-blocking-io-dispatcher"
+
+  /**
+   * INTERNAL API
+   */
+  @InternalApi
+  private[akka] final val InternalDispatcherId = "akka.actor.internal-dispatcher"
+
+  private val MaxDispatcherAliasDepth = 20
 }
 
 /**
@@ -56,10 +73,19 @@ object Dispatchers {
  * for different environments. Use the `lookup` method to create
  * a dispatcher as specified in configuration.
  *
+ * A dispatcher config can also be an alias, in that case it is a config string value pointing
+ * to the actual dispatcher config.
+ *
  * Look in `akka.actor.default-dispatcher` section of the reference.conf
  * for documentation of dispatcher options.
+ *
+ * Not for user instantiation or extension
  */
-class Dispatchers(val settings: ActorSystem.Settings, val prerequisites: DispatcherPrerequisites) {
+@DoNotInherit
+class Dispatchers @InternalApi private[akka] (
+    val settings: ActorSystem.Settings,
+    val prerequisites: DispatcherPrerequisites,
+    logger: LoggingAdapter) {
 
   import Dispatchers._
 
@@ -75,13 +101,23 @@ class Dispatchers(val settings: ActorSystem.Settings, val prerequisites: Dispatc
 
   private val dispatcherConfigurators = new ConcurrentHashMap[String, MessageDispatcherConfigurator]
 
+  /**
+   * INTERNAL API
+   */
+  private[akka] val internalDispatcher = lookup(Dispatchers.InternalDispatcherId)
+
   /**
    * Returns a dispatcher as specified in configuration. Please note that this
-   * method _may_ create and return a NEW dispatcher, _every_ call.
+   * method _may_ create and return a NEW dispatcher, _every_ call (depending on the `MessageDispatcherConfigurator` /
+   * dispatcher config the id points to).
+   *
+   * A dispatcher id can also be an alias. In the case it is a string value in the config it is treated as the id
+   * of the actual dispatcher config to use. If several ids leading to the same actual dispatcher config is used only one
+   * instance is created. This means that for dispatchers you expect to be shared they will be.
    *
    * Throws ConfigurationException if the specified dispatcher cannot be found in the configuration.
    */
-  def lookup(id: String): MessageDispatcher = lookupConfigurator(id).dispatcher()
+  def lookup(id: String): MessageDispatcher = lookupConfigurator(id, 0).dispatcher()
 
   /**
    * Checks that the configuration provides a section for the given dispatcher.
@@ -91,15 +127,37 @@ class Dispatchers(val settings: ActorSystem.Settings, val prerequisites: Dispatc
    */
   def hasDispatcher(id: String): Boolean = dispatcherConfigurators.containsKey(id) || cachingConfig.hasPath(id)
 
-  private def lookupConfigurator(id: String): MessageDispatcherConfigurator = {
+  private def lookupConfigurator(id: String, depth: Int): MessageDispatcherConfigurator = {
+    if (depth > MaxDispatcherAliasDepth)
+      throw new ConfigurationException(
+        s"Didn't find a concrete dispatcher config after following $MaxDispatcherAliasDepth, " +
+        s"is there a loop in your config? last looked for id was $id")
     dispatcherConfigurators.get(id) match {
       case null =>
         // It doesn't matter if we create a dispatcher configurator that isn't used due to concurrent lookup.
         // That shouldn't happen often and in case it does the actual ExecutorService isn't
         // created until used, i.e. cheap.
-        val newConfigurator =
+
-          if (cachingConfig.hasPath(id)) configuratorFrom(config(id))
+        val newConfigurator: MessageDispatcherConfigurator =
-          else throw new ConfigurationException(s"Dispatcher [$id] not configured")
+          if (cachingConfig.hasPath(id)) {
+            val valueAtPath = cachingConfig.getValue(id)
+            valueAtPath.valueType() match {
+              case ConfigValueType.STRING =>
+                // a dispatcher key can be an alias of another dispatcher, if it is a string
+                // we treat that string value as the id of a dispatcher to lookup, it will be stored
+                // both under the actual id and the alias id in the 'dispatcherConfigurators' cache
+                val actualId = valueAtPath.unwrapped().asInstanceOf[String]
+                logger.debug("Dispatcher id [{}] is an alias, actual dispatcher will be [{}]", id, actualId)
+                lookupConfigurator(actualId, depth + 1)
+
+              case ConfigValueType.OBJECT =>
+                configuratorFrom(config(id))
+              case unexpected =>
+                throw new ConfigurationException(
+                  s"Expected either a dispatcher config or an alias at [$id] but found [$unexpected]")
+
+            }
+          } else throw new ConfigurationException(s"Dispatcher [$id] not configured")
 
         dispatcherConfigurators.putIfAbsent(id, newConfigurator) match {
           case null     => newConfigurator

akka-cluster-metrics/src/main/resources/reference.conf

@@ -22,8 +22,7 @@
 # Provides periodic statistics collection and publication throughout the cluster.
 akka.cluster.metrics {
   # Full path of dispatcher configuration key.
-  # Use "" for default key `akka.actor.default-dispatcher`.
+  dispatcher = "akka.actor.default-dispatcher"
-  dispatcher = ""
   # How long should any actor wait before starting the periodic tasks.
   periodic-tasks-initial-delay = 1s
   # Sigar native library extract location.

akka-cluster-metrics/src/main/scala/akka/cluster/metrics/ClusterMetricsSettings.scala

@@ -5,7 +5,6 @@
 package akka.cluster.metrics
 
 import com.typesafe.config.Config
-import akka.dispatch.Dispatchers
 import scala.concurrent.duration.FiniteDuration
 import akka.util.Helpers.Requiring
 import akka.util.Helpers.ConfigOps
@@ -19,10 +18,7 @@ case class ClusterMetricsSettings(config: Config) {
   private val cc = config.getConfig("akka.cluster.metrics")
 
   // Extension.
-  val MetricsDispatcher: String = cc.getString("dispatcher") match {
+  val MetricsDispatcher: String = cc.getString("dispatcher")
-    case "" => Dispatchers.DefaultDispatcherId
-    case id => id
-  }
   val PeriodicTasksInitialDelay: FiniteDuration = cc.getMillisDuration("periodic-tasks-initial-delay")
   val NativeLibraryExtractFolder: String = cc.getString("native-library-extract-folder")
 

akka-cluster-sharding/src/main/resources/reference.conf

@@ -157,11 +157,10 @@ akka.cluster.sharding {
   }
 
   # The id of the dispatcher to use for ClusterSharding actors.
-  # If not specified default dispatcher is used.
   # If specified you need to define the settings of the actual dispatcher.
   # This dispatcher for the entity actors is defined by the user provided
   # Props, i.e. this dispatcher is not used for the entity actors.
-  use-dispatcher = ""
+  use-dispatcher = "akka.actor.internal-dispatcher"
 
   # Config path of the lease that each shard must acquire before starting entity actors
   # default is no lease

akka-cluster-sharding/src/main/scala/akka/cluster/sharding/ClusterSharding.scala

@@ -31,7 +31,6 @@ import akka.cluster.ClusterSettings.DataCenter
 import akka.cluster.ddata.Replicator
 import akka.cluster.ddata.ReplicatorSettings
 import akka.cluster.singleton.ClusterSingletonManager
-import akka.dispatch.Dispatchers
 import akka.event.Logging
 import akka.pattern.BackoffOpts
 import akka.pattern.ask
@@ -179,10 +178,7 @@ class ClusterSharding(system: ExtendedActorSystem) extends Extension {
   private lazy val guardian: ActorRef = {
     val guardianName: String =
       system.settings.config.getString("akka.cluster.sharding.guardian-name")
-    val dispatcher = system.settings.config.getString("akka.cluster.sharding.use-dispatcher") match {
+    val dispatcher = system.settings.config.getString("akka.cluster.sharding.use-dispatcher")
-      case "" => Dispatchers.DefaultDispatcherId
-      case id => id
-    }
     system.systemActorOf(Props[ClusterShardingGuardian].withDispatcher(dispatcher), guardianName)
   }
 

akka-cluster-tools/src/main/resources/reference.conf

@@ -33,9 +33,8 @@ akka.cluster.pub-sub {
   send-to-dead-letters-when-no-subscribers = on
   
   # The id of the dispatcher to use for DistributedPubSubMediator actors. 
-  # If not specified default dispatcher is used.
   # If specified you need to define the settings of the actual dispatcher.
-  use-dispatcher = ""
+  use-dispatcher = "akka.actor.internal-dispatcher"
 }
 # //#pub-sub-ext-config
 
@@ -74,10 +73,9 @@ akka.cluster.client.receptionist {
   # after this time of inactivity.
   response-tunnel-receive-timeout = 30s
   
-  # The id of the dispatcher to use for ClusterReceptionist actors. 
+  # The id of the dispatcher to use for ClusterReceptionist actors.
-  # If not specified default dispatcher is used.
   # If specified you need to define the settings of the actual dispatcher.
-  use-dispatcher = ""
+  use-dispatcher = "akka.actor.internal-dispatcher"
 
   # How often failure detection heartbeat messages should be received for
   # each ClusterClient

akka-cluster-tools/src/main/scala/akka/cluster/client/ClusterClient.scala

@@ -36,7 +36,6 @@ import akka.routing.ConsistentHash
 import akka.routing.MurmurHash
 import com.typesafe.config.Config
 import akka.remote.DeadlineFailureDetector
-import akka.dispatch.Dispatchers
 import akka.util.MessageBuffer
 import akka.util.ccompat._
 import scala.collection.immutable.{ HashMap, HashSet }
@@ -596,10 +595,7 @@ final class ClusterClientReceptionist(system: ExtendedActorSystem) extends Exten
       system.deadLetters
     else {
       val name = config.getString("name")
-      val dispatcher = config.getString("use-dispatcher") match {
+      val dispatcher = config.getString("use-dispatcher")
-        case "" => Dispatchers.DefaultDispatcherId
-        case id => id
-      }
       // important to use val mediator here to activate it outside of ClusterReceptionist constructor
       val mediator = pubSubMediator
       system.systemActorOf(

akka-cluster-tools/src/main/scala/akka/cluster/pubsub/DistributedPubSubMediator.scala

@@ -29,7 +29,6 @@ import akka.routing.BroadcastRoutingLogic
 
 import scala.collection.immutable.TreeMap
 import com.typesafe.config.Config
-import akka.dispatch.Dispatchers
 
 object DistributedPubSubSettings {
 
@@ -930,10 +929,7 @@ class DistributedPubSub(system: ExtendedActorSystem) extends Extension {
       system.deadLetters
     else {
       val name = system.settings.config.getString("akka.cluster.pub-sub.name")
-      val dispatcher = system.settings.config.getString("akka.cluster.pub-sub.use-dispatcher") match {
+      val dispatcher = system.settings.config.getString("akka.cluster.pub-sub.use-dispatcher")
-        case "" => Dispatchers.DefaultDispatcherId
-        case id => id
-      }
       system.systemActorOf(DistributedPubSubMediator.props(settings).withDispatcher(dispatcher), name)
     }
   }

akka-cluster-tools/src/main/scala/akka/cluster/singleton/ClusterSingletonManager.scala

@@ -34,6 +34,7 @@ import akka.coordination.lease.LeaseUsageSettings
 import akka.pattern.ask
 import akka.util.Timeout
 import akka.coordination.lease.scaladsl.{ Lease, LeaseProvider }
+import akka.dispatch.Dispatchers
 import com.github.ghik.silencer.silent
 
 import scala.util.control.NonFatal
@@ -163,7 +164,9 @@ object ClusterSingletonManager {
    * Scala API: Factory method for `ClusterSingletonManager` [[akka.actor.Props]].
    */
   def props(singletonProps: Props, terminationMessage: Any, settings: ClusterSingletonManagerSettings): Props =
-    Props(new ClusterSingletonManager(singletonProps, terminationMessage, settings)).withDeploy(Deploy.local)
+    Props(new ClusterSingletonManager(singletonProps, terminationMessage, settings))
+      .withDispatcher(Dispatchers.InternalDispatcherId)
+      .withDeploy(Deploy.local)
 
   /**
    * INTERNAL API

akka-cluster-tools/src/main/scala/akka/cluster/singleton/ClusterSingletonProxy.scala

@@ -6,6 +6,7 @@ package akka.cluster.singleton
 
 import akka.actor._
 import akka.cluster.{ Cluster, Member, MemberStatus }
+
 import scala.collection.immutable
 import akka.cluster.ClusterEvent._
 import akka.cluster.ClusterEvent.MemberRemoved
@@ -13,6 +14,7 @@ import akka.cluster.ClusterEvent.MemberUp
 import akka.actor.RootActorPath
 import akka.cluster.ClusterEvent.CurrentClusterState
 import akka.cluster.ClusterEvent.MemberExited
+
 import scala.concurrent.duration._
 import scala.language.postfixOps
 import com.typesafe.config.Config
@@ -21,6 +23,7 @@ import akka.event.Logging
 import akka.util.MessageBuffer
 import akka.cluster.ClusterSettings
 import akka.cluster.ClusterSettings.DataCenter
+import akka.dispatch.Dispatchers
 
 object ClusterSingletonProxySettings {
 
@@ -127,7 +130,9 @@ object ClusterSingletonProxy {
    * @param settings see [[ClusterSingletonProxySettings]]
    */
   def props(singletonManagerPath: String, settings: ClusterSingletonProxySettings): Props =
-    Props(new ClusterSingletonProxy(singletonManagerPath, settings)).withDeploy(Deploy.local)
+    Props(new ClusterSingletonProxy(singletonManagerPath, settings))
+      .withDispatcher(Dispatchers.InternalDispatcherId)
+      .withDeploy(Deploy.local)
 
   private case object TryToIdentifySingleton extends NoSerializationVerificationNeeded
 

akka-cluster-typed/src/main/scala/akka/cluster/ddata/typed/scaladsl/DistributedData.scala

@@ -4,12 +4,8 @@
 
 package akka.cluster.ddata.typed.scaladsl
 
-import akka.actor.typed.ActorSystem
+import akka.actor.typed.{ ActorRef, ActorSystem, Extension, ExtensionId, Props }
-import akka.actor.typed.Extension
-import akka.actor.typed.ExtensionId
-import akka.actor.typed.ActorRef
 import akka.actor.ExtendedActorSystem
-import akka.actor.typed.Props
 import akka.cluster.{ ddata => dd }
 import akka.cluster.ddata.SelfUniqueAddress
 
@@ -50,7 +46,10 @@ class DistributedData(system: ActorSystem[_]) extends Extension {
       val underlyingReplicator = dd.DistributedData(untypedSystem).replicator
       val replicatorBehavior = Replicator.behavior(settings, underlyingReplicator)
 
-      system.internalSystemActorOf(replicatorBehavior, ReplicatorSettings.name(system), Props.empty)
+      system.internalSystemActorOf(
+        replicatorBehavior,
+        ReplicatorSettings.name(system),
+        Props.empty.withDispatcherFromConfig(settings.dispatcher))
     }
 
   /**

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

@@ -148,10 +148,9 @@ akka {
     # Disable with "off".
     publish-stats-interval = off
 
-    # The id of the dispatcher to use for cluster actors. If not specified
+    # The id of the dispatcher to use for cluster actors.
-    # default dispatcher is used.
     # If specified you need to define the settings of the actual dispatcher.
-    use-dispatcher = ""
+    use-dispatcher = "akka.actor.internal-dispatcher"
 
     # Gossip to random node with newer or older state information, if any with
     # this probability. Otherwise Gossip to any random live node.

akka-cluster/src/main/scala/akka/cluster/ClusterSettings.scala

@@ -12,7 +12,6 @@ import scala.concurrent.duration.Duration
 import akka.actor.Address
 import akka.actor.AddressFromURIString
 import akka.annotation.InternalApi
-import akka.dispatch.Dispatchers
 import akka.util.Helpers.{ toRootLowerCase, ConfigOps, Requiring }
 
 import scala.concurrent.duration.FiniteDuration
@@ -179,10 +178,7 @@ final class ClusterSettings(val config: Config, val systemName: String) {
   val RunCoordinatedShutdownWhenDown: Boolean = cc.getBoolean("run-coordinated-shutdown-when-down")
   val JmxEnabled: Boolean = cc.getBoolean("jmx.enabled")
   val JmxMultiMbeansInSameEnabled: Boolean = cc.getBoolean("jmx.multi-mbeans-in-same-jvm")
-  val UseDispatcher: String = cc.getString("use-dispatcher") match {
+  val UseDispatcher: String = cc.getString("use-dispatcher")
-    case "" => Dispatchers.DefaultDispatcherId
-    case id => id
-  }
   val GossipDifferentViewProbability: Double = cc.getDouble("gossip-different-view-probability")
   val ReduceGossipDifferentViewProbability: Int = cc.getInt("reduce-gossip-different-view-probability")
   val SchedulerTickDuration: FiniteDuration = cc.getMillisDuration("scheduler.tick-duration")

akka-cluster/src/test/scala/akka/cluster/ClusterConfigSpec.scala

@@ -48,7 +48,7 @@ class ClusterConfigSpec extends AkkaSpec {
       SelfDataCenter should ===("default")
       Roles should ===(Set(ClusterSettings.DcRolePrefix + "default"))
       JmxEnabled should ===(true)
-      UseDispatcher should ===(Dispatchers.DefaultDispatcherId)
+      UseDispatcher should ===(Dispatchers.InternalDispatcherId)
       GossipDifferentViewProbability should ===(0.8 +- 0.0001)
       ReduceGossipDifferentViewProbability should ===(400)
       SchedulerTickDuration should ===(33 millis)

akka-coordination/src/main/scala/akka/coordination/lease/javadsl/LeaseProvider.scala

@@ -36,6 +36,6 @@ class LeaseProvider(system: ExtendedActorSystem) extends Extension {
    */
   def getLease(leaseName: String, configPath: String, ownerName: String): Lease = {
     val scalaLease = delegate.getLease(leaseName, configPath, ownerName)
-    new LeaseAdapter(scalaLease)(system.dispatcher)
+    new LeaseAdapter(scalaLease)(system.dispatchers.internalDispatcher)
   }
 }

akka-discovery/src/main/scala/akka/discovery/aggregate/AggregateServiceDiscovery.scala

@@ -55,7 +55,7 @@ private[akka] final class AggregateServiceDiscovery(system: ExtendedActorSystem)
     val serviceDiscovery = Discovery(system)
     settings.discoveryMethods.map(mech => (mech, serviceDiscovery.loadServiceDiscovery(mech)))
   }
-  private implicit val ec = system.dispatcher
+  private implicit val ec = system.dispatchers.internalDispatcher
 
   /**
    * Each discovery method is given the resolveTimeout rather than reducing it each time between methods.

akka-discovery/src/main/scala/akka/discovery/dns/DnsServiceDiscovery.scala

@@ -84,7 +84,7 @@ private[akka] class DnsServiceDiscovery(system: ExtendedActorSystem) extends Ser
   // (eventually visible)
   private var asyncDnsCache: OptionVal[AsyncDnsCache] = OptionVal.None
 
-  import system.dispatcher
+  private implicit val ec = system.dispatchers.internalDispatcher
 
   dns.ask(AsyncDnsManager.GetCache)(Timeout(30.seconds)).onComplete {
     case Success(cache: AsyncDnsCache) =>

akka-distributed-data/src/main/resources/reference.conf

@@ -26,10 +26,9 @@ akka.cluster.distributed-data {
   # the replicas. Next chunk will be transferred in next round of gossip.
   max-delta-elements = 1000
   
-  # The id of the dispatcher to use for Replicator actors. If not specified
+  # The id of the dispatcher to use for Replicator actors.
-  # default dispatcher is used.
   # If specified you need to define the settings of the actual dispatcher.
-  use-dispatcher = ""
+  use-dispatcher = "akka.actor.internal-dispatcher"
 
   # How often the Replicator checks for pruning of data associated with
   # removed cluster nodes. If this is set to 'off' the pruning feature will

akka-distributed-data/src/main/scala/akka/cluster/ddata/DurableStore.scala

@@ -243,7 +243,7 @@ final class LmdbDurableStore(config: Config) extends Actor with ActorLogging {
           dbPut(OptionVal.None, key, data)
         } else {
           if (pending.isEmpty)
-            context.system.scheduler.scheduleOnce(writeBehindInterval, self, WriteBehind)(context.system.dispatcher)
+            context.system.scheduler.scheduleOnce(writeBehindInterval, self, WriteBehind)(context.dispatcher)
           pending.put(key, data)
         }
         reply match {

akka-distributed-data/src/main/scala/akka/cluster/ddata/Replicator.scala

@@ -80,10 +80,7 @@ object ReplicatorSettings {
    * the default configuration `akka.cluster.distributed-data`.
    */
   def apply(config: Config): ReplicatorSettings = {
-    val dispatcher = config.getString("use-dispatcher") match {
+    val dispatcher = config.getString("use-dispatcher")
-      case "" => Dispatchers.DefaultDispatcherId
-      case id => id
-    }
 
     val pruningInterval = toRootLowerCase(config.getString("pruning-interval")) match {
       case "off" | "false" => Duration.Zero
@@ -299,7 +296,7 @@ final class ReplicatorSettings(
 
   def withDispatcher(dispatcher: String): ReplicatorSettings = {
     val d = dispatcher match {
-      case "" => Dispatchers.DefaultDispatcherId
+      case "" => Dispatchers.InternalDispatcherId
       case id => id
     }
     copy(dispatcher = d)

akka-distributed-data/src/main/scala/akka/cluster/ddata/protobuf/ReplicatorMessageSerializer.scala

@@ -162,7 +162,7 @@ class ReplicatorMessageSerializer(val system: ExtendedActorSystem)
   system.scheduler.schedule(cacheTimeToLive, cacheTimeToLive / 2) {
     readCache.evict()
     writeCache.evict()
-  }(system.dispatcher)
+  }(system.dispatchers.internalDispatcher)
 
   private val writeAckBytes = dm.Empty.getDefaultInstance.toByteArray
   private val dummyAddress = UniqueAddress(Address("a", "b", "c", 2552), 1L)

akka-docs/src/main/paradox/cluster-usage.md

@@ -877,37 +877,10 @@ akka.cluster.log-info-verbose = on
 <a id="cluster-dispatcher"></a>
 ### Cluster Dispatcher
 
-Under the hood the cluster extension is implemented with actors and it can be necessary
+Under the hood the cluster extension is implemented with actors. To protect them against
-to create a bulkhead for those actors to avoid disturbance from other actors. Especially
+disturbance from user actors they are by default run on the internal dispatcher configured
-the heartbeating actors that is used for failure detection can generate false positives
+under `akka.actor.internal-dispatcher`. The cluster actors can potentially be isolated even
-if they are not given a chance to run at regular intervals.
+further onto their own dispatcher using the setting `akka.cluster.use-dispatcher`.
-For this purpose you can define a separate dispatcher to be used for the cluster actors:
-
-```
-akka.cluster.use-dispatcher = cluster-dispatcher
-
-cluster-dispatcher {
-  type = "Dispatcher"
-  executor = "fork-join-executor"
-  fork-join-executor {
-    parallelism-min = 2
-    parallelism-max = 4
-  }
-}
-```
-
-@@@ note
-
-Normally it should not be necessary to configure a separate dispatcher for the Cluster.
-The default-dispatcher should be sufficient for performing the Cluster tasks, i.e. `akka.cluster.use-dispatcher`
-should not be changed. If you have Cluster related problems when using the default-dispatcher that is typically an
-indication that you are running blocking or CPU intensive actors/tasks on the default-dispatcher.
-Use dedicated dispatchers for such actors/tasks instead of running them on the default-dispatcher,
-because that may starve system internal tasks.
-Related config properties: `akka.cluster.use-dispatcher = akka.cluster.cluster-dispatcher`.
-Corresponding default values: `akka.cluster.use-dispatcher =`.
-
-@@@
 
 ### Configuration Compatibility Check
 

akka-docs/src/main/paradox/dispatchers.md

@@ -25,6 +25,13 @@ dispatchers in this ActorSystem. If no ExecutionContext is given, it will fallba
 `akka.actor.default-dispatcher.default-executor.fallback`. By default this is a "fork-join-executor", which
 gives excellent performance in most cases.
 
+## Internal dispatcher
+
+To protect the internal Actors that is spawned by the various Akka modules, a separate internal dispatcher is used by default.
+The internal dispatcher can be tuned in a fine grained way with the setting `akka.actor.internal-dispatcher`, it can also
+be replaced by another dispatcher by making `akka.actor.internal-dispatcher` an @ref[alias](#dispatcher-aliases).
+ 
+
 <a id="dispatcher-lookup"></a>
 ## Looking up a Dispatcher
 
@@ -184,6 +191,19 @@ is used for `PinnedDispatcher` to keep resource usage down in case of idle actor
 thread all the time you need to add `thread-pool-executor.allow-core-timeout=off` to the
 configuration of the `PinnedDispatcher`.
 
+## Dispatcher aliases
+
+When a dispatcher is looked up, and the given setting contains a string rather than a dispatcher config block, 
+the lookup will treat it as an alias, and follow that string to an alternate location for a dispatcher config.
+If the dispatcher config is referenced both through an alias and through the absolute path only one dispatcher will
+be used and shared among the two ids.
+
+Example: configuring `internal-dispatcher` to be an alias for `default-dispatcher`: 
+
+```
+akka.actor.internal-dispatcher = akka.actor.default-dispatcher
+``` 
+
 ## Blocking Needs Careful Management
 
 In some cases it is unavoidable to do blocking operations, i.e. to put a thread

akka-docs/src/main/paradox/project/migration-guide-2.5.x-2.6.x.md

@@ -34,6 +34,34 @@ Use plain `system.actorOf` instead of the DSL to create Actors if you have been
 
 `actorFor` has been deprecated since `2.2`. Use `ActorSelection` instead.
 
+## Internal dispatcher introduced
+
+To protect the Akka internals against starvation when user code blocks the default dispatcher (for example by accidental
+use of blocking APIs from actors) a new internal dispatcher has been added. All of Akka's internal, non-blocking actors
+now run on the internal dispatcher by default.
+
+The dispatcher can be configured through `akka.actor.internal-dispatcher`.
+
+For maximum performance, you might want to use a single shared dispatcher for all non-blocking,
+asynchronous actors, user actors and Akka internal actors. In that case, can configure the
+`akka.actor.internal-dispatcher` with a string value of `akka.actor.default-dispatcher`.
+This reinstantiates the behavior from previous Akka versions but also removes the isolation between
+user and Akka internals. So, use at your own risk!
+
+Several `use-dispatcher` configuration settings that previously accepted an empty value to fall back to the default
+dispatcher has now gotten an explicit value of `akka.actor.internal-dispatcher` and no longer accept an empty 
+string as value. If such an empty value is used in your `application.conf` the same result is achieved by simply removing
+that entry completely and having the default apply.
+
+For more details about configuring dispatchers, see the @ref[Dispatchers](../dispatchers.md)
+
+## Default dispatcher size
+
+Previously the factor for the default dispatcher was set a bit high (`3.0`) to give some extra threads in case of accidental
+blocking and protect a bit against starving the internal actors. Since the internal actors are now on a separate dispatcher
+the default dispatcher has been adjusted down to `1.0` which means the number of threads will be one per core, but at least
+`8` and at most `64`. This can be tuned using the individual settings in `akka.actor.default-dispatcher.fork-join-executor`.
+
 ## Default remoting is now Artery TCP
 
 @ref[Artery TCP](../remoting-artery.md) is now the default remoting implementation.

akka-remote/src/main/scala/akka/remote/Remoting.scala

@@ -146,6 +146,8 @@ private[remote] class Remoting(_system: ExtendedActorSystem, _provider: RemoteAc
 
   import provider.remoteSettings._
 
+  private implicit val ec = system.dispatchers.lookup(Dispatcher)
+
   val transportSupervisor = system.systemActorOf(configureDispatcher(Props[TransportSupervisor]), "transports")
 
   override def localAddressForRemote(remote: Address): Address =
@@ -167,7 +169,6 @@ private[remote] class Remoting(_system: ExtendedActorSystem, _provider: RemoteAc
           endpointManager = None
         }
 
-        import system.dispatcher
         (manager ? ShutdownAndFlush)
           .mapTo[Boolean]
           .andThen {
@@ -252,7 +253,6 @@ private[remote] class Remoting(_system: ExtendedActorSystem, _provider: RemoteAc
 
   override def managementCommand(cmd: Any): Future[Boolean] = endpointManager match {
     case Some(manager) =>
-      import system.dispatcher
       implicit val timeout = CommandAckTimeout
       (manager ? ManagementCommand(cmd)).map { case ManagementCommandAck(status) => status }
     case None =>

akka-remote/src/main/scala/akka/remote/artery/ArteryTransport.scala

@@ -504,7 +504,7 @@ private[remote] abstract class ArteryTransport(_system: ExtendedActorSystem, _pr
     system.scheduler.schedule(removeAfter, interval) {
       if (!isShutdown)
         associationRegistry.removeUnusedQuarantined(removeAfter)
-    }(system.dispatcher)
+    }(system.dispatchers.internalDispatcher)
   }
 
   // Select inbound lane based on destination to preserve message order,
@@ -559,11 +559,12 @@ private[remote] abstract class ArteryTransport(_system: ExtendedActorSystem, _pr
                     val a = association(from.address)
                     // make sure uid is same for active association
                     if (a.associationState.uniqueRemoteAddressValue().contains(from)) {
-                      import system.dispatcher
+
-                      a.changeActorRefCompression(table).foreach { _ =>
+                      a.changeActorRefCompression(table)
-                        a.sendControl(ActorRefCompressionAdvertisementAck(localAddress, table.version))
+                        .foreach { _ =>
-                        system.eventStream.publish(Events.ReceivedActorRefCompressionTable(from, table))
+                          a.sendControl(ActorRefCompressionAdvertisementAck(localAddress, table.version))
-                      }
+                          system.eventStream.publish(Events.ReceivedActorRefCompressionTable(from, table))
+                        }(system.dispatchers.internalDispatcher)
                     }
                   } else
                     log.debug(
@@ -590,11 +591,11 @@ private[remote] abstract class ArteryTransport(_system: ExtendedActorSystem, _pr
                     val a = association(from.address)
                     // make sure uid is same for active association
                     if (a.associationState.uniqueRemoteAddressValue().contains(from)) {
-                      import system.dispatcher
+                      a.changeClassManifestCompression(table)
-                      a.changeClassManifestCompression(table).foreach { _ =>
+                        .foreach { _ =>
-                        a.sendControl(ClassManifestCompressionAdvertisementAck(localAddress, table.version))
+                          a.sendControl(ClassManifestCompressionAdvertisementAck(localAddress, table.version))
-                        system.eventStream.publish(Events.ReceivedClassManifestCompressionTable(from, table))
+                          system.eventStream.publish(Events.ReceivedClassManifestCompressionTable(from, table))
-                      }
+                        }(system.dispatchers.internalDispatcher)
                     }
                   } else
                     log.debug(
@@ -681,7 +682,7 @@ private[remote] abstract class ArteryTransport(_system: ExtendedActorSystem, _pr
             "remoteFlushOnShutdown")
           flushingPromise.future
         }
-      implicit val ec = system.dispatcher
+      implicit val ec = system.dispatchers.internalDispatcher
       flushing.recover { case _ => Done }.flatMap(_ => internalShutdown())
     } else {
       Future.successful(Done)
@@ -689,7 +690,7 @@ private[remote] abstract class ArteryTransport(_system: ExtendedActorSystem, _pr
   }
 
   private def internalShutdown(): Future[Done] = {
-    import system.dispatcher
+    implicit val ec = system.dispatchers.internalDispatcher
 
     killSwitch.abort(ShutdownSignal)
     topLevelFlightRecorder.loFreq(Transport_KillSwitchPulled, NoMetaData)
@@ -722,7 +723,7 @@ private[remote] abstract class ArteryTransport(_system: ExtendedActorSystem, _pr
    * Will complete successfully even if one of the stream completion futures failed
    */
   private def streamsCompleted: Future[Done] = {
-    implicit val ec = system.dispatcher
+    implicit val ec = system.dispatchers.internalDispatcher
     for {
       _ <- Future.traverse(associationRegistry.allAssociations)(_.streamsCompleted)
       _ <- Future.sequence(streamMatValues.get().valuesIterator.map {

akka-remote/src/main/scala/akka/remote/artery/Association.scala

@@ -196,7 +196,7 @@ private[remote] class Association(
     updateOutboundCompression(c => c.clearCompression())
 
   private def updateOutboundCompression(action: OutboundCompressionAccess => Future[Done]): Future[Done] = {
-    import transport.system.dispatcher
+    implicit val ec = transport.system.dispatchers.internalDispatcher
     val c = outboundCompressionAccess
     if (c.isEmpty) Future.successful(Done)
     else if (c.size == 1) action(c.head)
@@ -276,7 +276,7 @@ private[remote] class Association(
         // clear outbound compression, it's safe to do that several times if someone else
         // completes handshake at same time, but it's important to clear it before
         // we signal that the handshake is completed (uniqueRemoteAddressPromise.trySuccess)
-        import transport.system.dispatcher
+        implicit val ec = transport.system.dispatchers.internalDispatcher
         clearOutboundCompression().map { _ =>
           current.uniqueRemoteAddressPromise.trySuccess(peer)
           current.uniqueRemoteAddressValue() match {
@@ -572,7 +572,7 @@ private[remote] class Association(
     stopQuarantinedTimer.set(Some(transport.system.scheduler.scheduleOnce(advancedSettings.StopQuarantinedAfterIdle) {
       if (associationState.isQuarantined())
         abortQuarantined()
-    }(transport.system.dispatcher)))
+    }(transport.system.dispatchers.internalDispatcher)))
   }
 
   private def abortQuarantined(): Unit = {
@@ -803,7 +803,7 @@ private[remote] class Association(
 
       val (queueValues, compressionAccessValues, laneCompletedValues) = values.unzip3
 
-      import transport.system.dispatcher
+      implicit val ec = transport.system.dispatchers.internalDispatcher
 
       // tear down all parts if one part fails or completes
       Future.firstCompletedOf(laneCompletedValues).failed.foreach { reason =>

akka-remote/src/main/scala/akka/remote/artery/aeron/ArteryAeronUdpTransport.scala

@@ -255,7 +255,7 @@ private[remote] class ArteryAeronUdpTransport(_system: ExtendedActorSystem, _pro
   private def startAeronErrorLog(): Unit = {
     aeronErrorLog = new AeronErrorLog(new File(aeronDir, CncFileDescriptor.CNC_FILE), log)
     val lastTimestamp = new AtomicLong(0L)
-    import system.dispatcher
+    implicit val ec = system.dispatchers.internalDispatcher
     aeronErrorLogTask = system.scheduler.schedule(3.seconds, 5.seconds) {
       if (!isShutdown) {
         val newLastTimestamp = aeronErrorLog.logErrors(log, lastTimestamp.get)
@@ -265,7 +265,7 @@ private[remote] class ArteryAeronUdpTransport(_system: ExtendedActorSystem, _pro
   }
 
   private def startAeronCounterLog(): Unit = {
-    import system.dispatcher
+    implicit val ec = system.dispatchers.internalDispatcher
     aeronCounterTask = system.scheduler.schedule(5.seconds, 5.seconds) {
       if (!isShutdown && log.isDebugEnabled) {
         aeron.countersReader.forEach(new MetaData() {
@@ -379,7 +379,7 @@ private[remote] class ArteryAeronUdpTransport(_system: ExtendedActorSystem, _pro
             }
             .to(immutable.Vector)
 
-        import system.dispatcher
+        implicit val ec = system.dispatchers.internalDispatcher
 
         // tear down the upstream hub part if downstream lane fails
         // lanes are not completed with success by themselves so we don't have to care about onSuccess
@@ -420,19 +420,20 @@ private[remote] class ArteryAeronUdpTransport(_system: ExtendedActorSystem, _pro
   }
 
   override protected def shutdownTransport(): Future[Done] = {
-    import system.dispatcher
+    taskRunner
-    taskRunner.stop().map { _ =>
+      .stop()
-      topLevelFlightRecorder.loFreq(Transport_Stopped, NoMetaData)
+      .map { _ =>
-      if (aeronErrorLogTask != null) {
+        topLevelFlightRecorder.loFreq(Transport_Stopped, NoMetaData)
-        aeronErrorLogTask.cancel()
+        if (aeronErrorLogTask != null) {
-        topLevelFlightRecorder.loFreq(Transport_AeronErrorLogTaskStopped, NoMetaData)
+          aeronErrorLogTask.cancel()
-      }
+          topLevelFlightRecorder.loFreq(Transport_AeronErrorLogTaskStopped, NoMetaData)
-      if (aeron != null) aeron.close()
+        }
-      if (aeronErrorLog != null) aeronErrorLog.close()
+        if (aeron != null) aeron.close()
-      if (mediaDriver.get.isDefined) stopMediaDriver()
+        if (aeronErrorLog != null) aeronErrorLog.close()
+        if (mediaDriver.get.isDefined) stopMediaDriver()
 
-      Done
+        Done
-    }
+      }(system.dispatchers.internalDispatcher)
   }
 
 }

akka-remote/src/main/scala/akka/remote/artery/tcp/ArteryTcpTransport.scala

@@ -389,7 +389,7 @@ private[remote] class ArteryTcpTransport(
             }
             .to(immutable.Vector)
 
-        import system.dispatcher
+        implicit val ec = system.dispatchers.internalDispatcher
 
         // tear down the upstream hub part if downstream lane fails
         // lanes are not completed with success by themselves so we don't have to care about onSuccess
@@ -433,7 +433,7 @@ private[remote] class ArteryTcpTransport(
   }
 
   override protected def shutdownTransport(): Future[Done] = {
-    import system.dispatcher
+    implicit val ec = system.dispatchers.internalDispatcher
     inboundKillSwitch.shutdown()
     unbind().map { _ =>
       topLevelFlightRecorder.loFreq(Transport_Stopped, NoMetaData)
@@ -444,7 +444,7 @@ private[remote] class ArteryTcpTransport(
   private def unbind(): Future[Done] = {
     serverBinding match {
       case Some(binding) =>
-        import system.dispatcher
+        implicit val ec = system.dispatchers.internalDispatcher
         for {
           b <- binding
           _ <- b.unbind()

akka-remote/src/main/scala/akka/remote/transport/AbstractTransportAdapter.scala

@@ -156,7 +156,7 @@ object ActorTransportAdapter {
 }
 
 abstract class ActorTransportAdapter(wrappedTransport: Transport, system: ActorSystem)
-    extends AbstractTransportAdapter(wrappedTransport)(system.dispatcher) {
+    extends AbstractTransportAdapter(wrappedTransport)(system.dispatchers.internalDispatcher) {
 
   import ActorTransportAdapter._
 

akka-remote/src/main/scala/akka/remote/transport/FailureInjectorTransportAdapter.scala

@@ -61,7 +61,7 @@ private[remote] object FailureInjectorTransportAdapter {
 private[remote] class FailureInjectorTransportAdapter(
     wrappedTransport: Transport,
     val extendedSystem: ExtendedActorSystem)
-    extends AbstractTransportAdapter(wrappedTransport)(extendedSystem.dispatcher)
+    extends AbstractTransportAdapter(wrappedTransport)(extendedSystem.dispatchers.internalDispatcher)
     with AssociationEventListener {
 
   private def rng = ThreadLocalRandom.current()

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

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2009-2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.stream
+
+import akka.dispatch.Dispatchers
+import akka.stream.testkit.StreamSpec
+
+class StreamDispatcherSpec extends StreamSpec {
+
+  "The default blocking io dispatcher for streams" must {
+
+    "be the same as the default blocking io dispatcher for actors" in {
+      val streamIoDispatcher = system.dispatchers.lookup(ActorAttributes.IODispatcher.dispatcher)
+      val actorIoDispatcher = system.dispatchers.lookup(Dispatchers.DefaultBlockingDispatcherId)
+
+      streamIoDispatcher shouldBe theSameInstanceAs(actorIoDispatcher)
+    }
+
+  }
+
+  "The deprecated default stream io dispatcher" must {
+    "be the same as the default blocking io dispatcher for actors" in {
+      // in case it is still used
+      val streamIoDispatcher = system.dispatchers.lookup("akka.stream.default-blocking-io-dispatcher")
+      val actorIoDispatcher = system.dispatchers.lookup(Dispatchers.DefaultBlockingDispatcherId)
+
+      streamIoDispatcher shouldBe theSameInstanceAs(actorIoDispatcher)
+    }
+
+  }
+}

akka-stream-tests/src/test/scala/akka/stream/io/FileSinkSpec.scala

@@ -8,7 +8,7 @@ import java.nio.file.StandardOpenOption.{ CREATE, WRITE }
 import java.nio.file._
 
 import akka.actor.ActorSystem
-import akka.dispatch.ExecutionContexts
+import akka.dispatch.{ Dispatchers, ExecutionContexts }
 import akka.stream.impl.PhasedFusingActorMaterializer
 import akka.stream.impl.StreamSupervisor
 import akka.stream.impl.StreamSupervisor.Children
@@ -157,7 +157,7 @@ class FileSinkSpec extends StreamSpec(UnboundedMailboxConfig) {
 
     "use dedicated blocking-io-dispatcher by default" in assertAllStagesStopped {
       targetFile { f =>
-        val sys = ActorSystem("dispatcher-testing", UnboundedMailboxConfig)
+        val sys = ActorSystem("FileSinkSpec-dispatcher-testing-1", UnboundedMailboxConfig)
         val materializer = ActorMaterializer()(sys)
         try {
           Source.fromIterator(() => Iterator.continually(TestByteStrings.head)).runWith(FileIO.toPath(f))(materializer)
@@ -167,14 +167,15 @@ class FileSinkSpec extends StreamSpec(UnboundedMailboxConfig) {
             .supervisor
             .tell(StreamSupervisor.GetChildren, testActor)
           val ref = expectMsgType[Children].children.find(_.path.toString contains "fileSink").get
-          assertDispatcher(ref, "akka.stream.default-blocking-io-dispatcher")
+          // haven't figured out why this returns the aliased id rather than the id, but the stage is going away so whatever
+          assertDispatcher(ref, Dispatchers.DefaultBlockingDispatcherId)
         } finally shutdown(sys)
       }
     }
 
     "allow overriding the dispatcher using Attributes" in assertAllStagesStopped {
       targetFile { f =>
-        val sys = ActorSystem("dispatcher-testing", UnboundedMailboxConfig)
+        val sys = ActorSystem("FileSinkSpec-dispatcher-testing-2", UnboundedMailboxConfig)
         val materializer = ActorMaterializer()(sys)
 
         try {

akka-stream-tests/src/test/scala/akka/stream/io/FileSourceSpec.scala

@@ -9,6 +9,7 @@ import java.nio.file.{ Files, NoSuchFileException }
 import java.util.Random
 
 import akka.actor.ActorSystem
+import akka.dispatch.Dispatchers
 import akka.stream.IOResult._
 import akka.stream._
 import akka.stream.impl.{ PhasedFusingActorMaterializer, StreamSupervisor }
@@ -251,7 +252,7 @@ class FileSourceSpec extends StreamSpec(UnboundedMailboxConfig) {
           .supervisor
           .tell(StreamSupervisor.GetChildren, testActor)
         val ref = expectMsgType[Children].children.find(_.path.toString contains "fileSource").get
-        try assertDispatcher(ref, "akka.stream.default-blocking-io-dispatcher")
+        try assertDispatcher(ref, ActorAttributes.IODispatcher.dispatcher)
         finally p.cancel()
       } finally shutdown(sys)
     }

akka-stream-tests/src/test/scala/akka/stream/io/InputStreamSinkSpec.scala

@@ -24,6 +24,8 @@ import akka.util.ByteString
 import scala.concurrent.duration._
 import java.util.concurrent.ThreadLocalRandom
 
+import akka.dispatch.Dispatchers
+
 import scala.concurrent.{ Await, Future }
 import scala.util.control.NoStackTrace
 
@@ -223,7 +225,7 @@ class InputStreamSinkSpec extends StreamSpec(UnboundedMailboxConfig) {
           .supervisor
           .tell(StreamSupervisor.GetChildren, testActor)
         val ref = expectMsgType[Children].children.find(_.path.toString contains "inputStreamSink").get
-        assertDispatcher(ref, "akka.stream.default-blocking-io-dispatcher")
+        assertDispatcher(ref, ActorAttributes.IODispatcher.dispatcher)
       } finally shutdown(sys)
     }
 

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

@@ -7,6 +7,7 @@ package akka.stream.scaladsl
 import java.util.concurrent.{ CompletionStage, TimeUnit }
 
 import akka.actor.ActorSystem
+import akka.dispatch.Dispatchers
 import akka.{ Done, NotUsed }
 import akka.stream.Attributes._
 import akka.stream._
@@ -282,8 +283,8 @@ class AttributesSpec
         Source
           .fromGraph(
             // directly on stage
-            new ThreadNameSnitchingStage("akka.stream.default-blocking-io-dispatcher").addAttributes(
+            new ThreadNameSnitchingStage(ActorAttributes.IODispatcher.dispatcher)
-              ActorAttributes.dispatcher("my-dispatcher")))
+              .addAttributes(ActorAttributes.dispatcher("my-dispatcher")))
           .runWith(Sink.head)
           .futureValue
 
@@ -293,20 +294,20 @@ class AttributesSpec
     "use the most specific dispatcher when another one is defined on a surrounding composed graph" in {
       val dispatcher =
         Source
-          .fromGraph(new ThreadNameSnitchingStage("akka.stream.default-blocking-io-dispatcher"))
+          .fromGraph(new ThreadNameSnitchingStage(ActorAttributes.IODispatcher.dispatcher))
           .map(identity)
           // this is now for the composed source -> flow graph
           .addAttributes(ActorAttributes.dispatcher("my-dispatcher"))
           .runWith(Sink.head)
           .futureValue
 
-      dispatcher should startWith("AttributesSpec-akka.stream.default-blocking-io-dispatcher")
+      dispatcher should startWith(s"AttributesSpec-${Dispatchers.DefaultBlockingDispatcherId}")
     }
 
     "not change dispatcher from one defined on a surrounding graph" in {
       val dispatcher =
         Source
-          .fromGraph(new ThreadNameSnitchingStage("akka.stream.default-blocking-io-dispatcher"))
+          .fromGraph(new ThreadNameSnitchingStage(ActorAttributes.IODispatcher.dispatcher))
           // this already introduces an async boundary here
           .map(identity)
           // this is now just for map since there already is one in-between stage and map
@@ -315,13 +316,13 @@ class AttributesSpec
           .runWith(Sink.head)
           .futureValue
 
-      dispatcher should startWith("AttributesSpec-akka.stream.default-blocking-io-dispatcher")
+      dispatcher should startWith(s"AttributesSpec-${Dispatchers.DefaultBlockingDispatcherId}")
     }
 
     "change dispatcher when defined directly on top of the async boundary" in {
       val dispatcher =
         Source
-          .fromGraph(new ThreadNameSnitchingStage("akka.stream.default-blocking-io-dispatcher"))
+          .fromGraph(new ThreadNameSnitchingStage(ActorAttributes.IODispatcher.dispatcher))
           .async
           .withAttributes(ActorAttributes.dispatcher("my-dispatcher"))
           .runWith(Sink.head)
@@ -333,7 +334,7 @@ class AttributesSpec
     "change dispatcher when defined on the async call" in {
       val dispatcher =
         Source
-          .fromGraph(new ThreadNameSnitchingStage("akka.stream.default-blocking-io-dispatcher"))
+          .fromGraph(new ThreadNameSnitchingStage(ActorAttributes.IODispatcher.dispatcher))
           .async("my-dispatcher")
           .runWith(Sink.head)
           .futureValue
@@ -411,7 +412,7 @@ class AttributesSpec
     "not change dispatcher from one defined on a surrounding graph" in {
       val dispatcherF =
         javadsl.Source
-          .fromGraph(new ThreadNameSnitchingStage("akka.stream.default-blocking-io-dispatcher"))
+          .fromGraph(new ThreadNameSnitchingStage(ActorAttributes.IODispatcher.dispatcher))
           // this already introduces an async boundary here
           .detach
           // this is now just for map since there already is one in-between stage and map
@@ -421,13 +422,13 @@ class AttributesSpec
 
       val dispatcher = dispatcherF.toCompletableFuture.get(remainingOrDefault.toMillis, TimeUnit.MILLISECONDS)
 
-      dispatcher should startWith("AttributesSpec-akka.stream.default-blocking-io-dispatcher")
+      dispatcher should startWith(s"AttributesSpec-${Dispatchers.DefaultBlockingDispatcherId}")
     }
 
     "change dispatcher when defined directly on top of the async boundary" in {
       val dispatcherF =
         javadsl.Source
-          .fromGraph(new ThreadNameSnitchingStage("akka.stream.default-blocking-io-dispatcher"))
+          .fromGraph(new ThreadNameSnitchingStage(ActorAttributes.IODispatcher.dispatcher))
           .async
           .withAttributes(ActorAttributes.dispatcher("my-dispatcher"))
           .runWith(javadsl.Sink.head(), materializer)
@@ -507,12 +508,12 @@ class AttributesSpec
       try {
         val dispatcher =
           Source
-            .fromGraph(new ThreadNameSnitchingStage("akka.stream.default-blocking-io-dispatcher"))
+            .fromGraph(new ThreadNameSnitchingStage(ActorAttributes.IODispatcher.dispatcher))
             .runWith(Sink.head)(myDispatcherMaterializer)
             .futureValue
 
         // should not override stage specific dispatcher
-        dispatcher should startWith("AttributesSpec-akka.stream.default-blocking-io-dispatcher")
+        dispatcher should startWith("AttributesSpec-akka.actor.default-blocking-io-dispatcher")
 
       } finally {
         myDispatcherMaterializer.shutdown()
@@ -565,7 +566,7 @@ class AttributesSpec
       val threadName =
         Source.fromGraph(new ThreadNameSnitchingStage(None).addAttributes(Attributes(IODispatcher))).runWith(Sink.head)
 
-      threadName.futureValue should startWith("AttributesSpec-akka.stream.default-blocking-io-dispatcher")
+      threadName.futureValue should startWith("AttributesSpec-akka.actor.default-blocking-io-dispatcher")
     }
 
     "allow for specifying a custom default io-dispatcher" in {
@@ -586,19 +587,6 @@ class AttributesSpec
         TestKit.shutdownActorSystem(system)
       }
     }
-
-    "resolve the dispatcher attribute" in {
-      import ActorAttributes._
-
-      Dispatcher.resolve(dispatcher("my-dispatcher"), materializer.settings) should be("my-dispatcher")
-    }
-
-    "resolve the blocking io dispatcher attribute" in {
-      import ActorAttributes._
-
-      Dispatcher.resolve(Attributes(IODispatcher), materializer.settings) should be(
-        "akka.stream.default-blocking-io-dispatcher")
-    }
   }
 
 }

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

@@ -7,6 +7,7 @@ package akka.stream.scaladsl
 import java.util.stream.Collectors
 
 import akka.actor.ActorSystem
+import akka.dispatch.Dispatchers
 import akka.stream._
 import akka.stream.impl.{ PhasedFusingActorMaterializer, StreamSupervisor }
 import akka.stream.impl.StreamSupervisor.Children
@@ -80,7 +81,7 @@ class SinkAsJavaStreamSpec extends StreamSpec(UnboundedMailboxConfig) {
           .supervisor
           .tell(StreamSupervisor.GetChildren, testActor)
         val ref = expectMsgType[Children].children.find(_.path.toString contains "asJavaStream").get
-        assertDispatcher(ref, "akka.stream.default-blocking-io-dispatcher")
+        assertDispatcher(ref, ActorAttributes.IODispatcher.dispatcher)
       } finally shutdown(sys)
     }
   }

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

@@ -8,6 +8,7 @@ import java.util.concurrent.atomic.AtomicInteger
 
 import akka.Done
 import akka.actor.ActorSystem
+import akka.dispatch.Dispatchers
 import akka.stream.impl.StreamSupervisor.Children
 import akka.stream.impl.{ PhasedFusingActorMaterializer, StreamSupervisor }
 import akka.stream.testkit.Utils._
@@ -325,7 +326,7 @@ class UnfoldResourceAsyncSourceSpec extends StreamSpec(UnboundedMailboxConfig) {
           .supervisor
           .tell(StreamSupervisor.GetChildren, testActor)
         val ref = expectMsgType[Children].children.find(_.path.toString contains "unfoldResourceSourceAsync").get
-        assertDispatcher(ref, "akka.stream.default-blocking-io-dispatcher")
+        assertDispatcher(ref, ActorAttributes.IODispatcher.dispatcher)
       } finally shutdown(sys)
     }
 

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

@@ -10,6 +10,7 @@ import java.nio.file.Files
 import java.util.concurrent.atomic.AtomicInteger
 
 import akka.actor.ActorSystem
+import akka.dispatch.Dispatchers
 import akka.stream.ActorAttributes._
 import akka.stream.Supervision._
 import akka.stream.impl.StreamSupervisor.Children
@@ -163,7 +164,7 @@ class UnfoldResourceSourceSpec extends StreamSpec(UnboundedMailboxConfig) {
           .supervisor
           .tell(StreamSupervisor.GetChildren, testActor)
         val ref = expectMsgType[Children].children.find(_.path.toString contains "unfoldResourceSource").get
-        try assertDispatcher(ref, "akka.stream.default-blocking-io-dispatcher")
+        try assertDispatcher(ref, ActorAttributes.IODispatcher.dispatcher)
         finally p.cancel()
       } finally shutdown(sys)
     }

akka-stream/src/main/mima-filters/2.5.x.backwards.excludes

@@ -62,10 +62,12 @@ ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.stream.scaladsl.FlowO
 ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.stream.scaladsl.FlowOps.zipLatestWithGraph")
 ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.stream.scaladsl.FlowOps.zipLatestWith")
 
-## 2.6
+# dispatcher aliases made internal streams dispatcher resolve superfluous #26775
+ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.ActorAttributes#Dispatcher.resolve")
 
 # #24372 No Future/CompletionStage in StreamRefs
-# FIXME why was change not detected?
+# no filter because MiMa doesn't check the generic signature
+# https://github.com/lightbend/migration-manager/issues/40
 
 # 26188 remove Timed
 ProblemFilters.exclude[MissingClassProblem]("akka.stream.extra.Timed$TimedFlowContext")

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

@@ -14,11 +14,13 @@ akka {
       max-input-buffer-size = 16
 
       # Fully qualified config path which holds the dispatcher configuration
-      # to be used by ActorMaterializer when creating Actors.
+      # or full dispatcher configuration to be used by ActorMaterializer when creating Actors.
-      # When this value is left empty, the default-dispatcher will be used.
+      dispatcher = "akka.actor.default-dispatcher"
-      dispatcher = ""
 
-      blocking-io-dispatcher = "akka.stream.default-blocking-io-dispatcher"
+      # Fully qualified config path which holds the dispatcher configuration
+      # or full dispatcher configuration to be used by stream operators that
+      # perform blocking operations
+      blocking-io-dispatcher = "akka.actor.default-blocking-io-dispatcher"
 
       # Cleanup leaked publishers and subscribers when they are not used within a given
       # deadline
@@ -122,21 +124,12 @@ akka {
       //#stream-ref
     }
 
-    # Deprecated, use akka.stream.materializer.blocking-io-dispatcher, this setting
+    # Deprecated, left here to not break Akka HTTP which refers to it
-    # was never applied because of bug #24357
+    blocking-io-dispatcher = "akka.actor.default-blocking-io-dispatcher"
-    # It must still have a valid value because used from Akka HTTP.
-    blocking-io-dispatcher = "akka.stream.default-blocking-io-dispatcher"
-
-    default-blocking-io-dispatcher {
-      type = "Dispatcher"
-      executor = "thread-pool-executor"
-      throughput = 1
-
-      thread-pool-executor {
-        fixed-pool-size = 16
-      }
-    }
 
+    # Deprecated, will not be used unless user code refer to it, use 'akka.stream.materializer.blocking-io-dispatcher'
+    # instead, or if from code, prefer the 'ActorAttributes.IODispatcher' attribute
+    default-blocking-io-dispatcher = "akka.actor.default-blocking-io-dispatcher"
   }
 
   # configure overrides to ssl-configuration here (to be used by akka-streams, and akka-http – i.e. when serving https connections)

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

@@ -405,33 +405,8 @@ object ActorAttributes {
   import Attributes._
   final case class Dispatcher(dispatcher: String) extends MandatoryAttribute
 
-  object Dispatcher {
-
-    /**
-     * INTERNAL API
-     * Resolves the dispatcher's name with a fallback to the default blocking IO dispatcher.
-     * Note that `IODispatcher.dispatcher` is not used here as the config used to create [[ActorMaterializerSettings]]
-     * is not easily accessible, instead the name is taken from `settings.blockingIoDispatcher`
-     */
-    @InternalApi
-    private[akka] def resolve(attributes: Attributes, settings: ActorMaterializerSettings): String =
-      attributes.mandatoryAttribute[Dispatcher] match {
-        case IODispatcher           => settings.blockingIoDispatcher
-        case Dispatcher(dispatcher) => dispatcher
-      }
-
-    /**
-     * INTERNAL API
-     * Resolves the dispatcher name with a fallback to the default blocking IO dispatcher.
-     */
-    @InternalApi
-    private[akka] def resolve(context: MaterializationContext): String =
-      resolve(context.effectiveAttributes, ActorMaterializerHelper.downcast(context.materializer).settings)
-  }
-
   final case class SupervisionStrategy(decider: Supervision.Decider) extends MandatoryAttribute
 
-  // this is actually a config key that needs reading and itself will contain the actual dispatcher name
   val IODispatcher: Dispatcher = ActorAttributes.Dispatcher("akka.stream.materializer.blocking-io-dispatcher")
 
   /**

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

@@ -8,16 +8,7 @@ import java.util
 import java.util.concurrent.atomic.AtomicBoolean
 
 import akka.NotUsed
-import akka.actor.{
+import akka.actor.{ ActorContext, ActorRef, ActorRefFactory, ActorSystem, Cancellable, ExtendedActorSystem, PoisonPill }
-  ActorContext,
-  ActorRef,
-  ActorRefFactory,
-  ActorSystem,
-  Cancellable,
-  Deploy,
-  ExtendedActorSystem,
-  PoisonPill
-}
 import akka.annotation.{ DoNotInherit, InternalApi }
 import akka.dispatch.Dispatchers
 import akka.event.{ Logging, LoggingAdapter }
@@ -421,14 +412,10 @@ private final case class SavedIslandData(
     Attributes(
       Attributes.InputBuffer(settings.initialInputBufferSize, settings.maxInputBufferSize) ::
       ActorAttributes.SupervisionStrategy(settings.supervisionDecider) ::
-      ActorAttributes.Dispatcher(if (settings.dispatcher == Deploy.NoDispatcherGiven) Dispatchers.DefaultDispatcherId
+      ActorAttributes.Dispatcher(settings.dispatcher) :: Nil)
-      else settings.dispatcher) :: Nil)
   }
 
-  override lazy val executionContext: ExecutionContextExecutor = dispatchers.lookup(settings.dispatcher match {
+  override lazy val executionContext: ExecutionContextExecutor = dispatchers.lookup(settings.dispatcher)
-    case Deploy.NoDispatcherGiven => Dispatchers.DefaultDispatcherId
-    case other                    => other
-  })
 
   override def schedulePeriodically(
       initialDelay: FiniteDuration,
@@ -782,7 +769,7 @@ private final case class SavedIslandData(
       case _ =>
         val props = ActorGraphInterpreter
           .props(shell)
-          .withDispatcher(ActorAttributes.Dispatcher.resolve(effectiveAttributes, settings))
+          .withDispatcher(effectiveAttributes.mandatoryAttribute[ActorAttributes.Dispatcher].dispatcher)
 
         val actorName = fullIslandName match {
           case OptionVal.Some(n) => n
@@ -933,7 +920,7 @@ private final case class SavedIslandData(
   def materializeAtomic(mod: AtomicModule[Shape, Any], attributes: Attributes): (NotUsed, Any) = {
     val tls = mod.asInstanceOf[TlsModule]
 
-    val dispatcher = ActorAttributes.Dispatcher.resolve(attributes, materializer.settings)
+    val dispatcher = attributes.mandatoryAttribute[ActorAttributes.Dispatcher].dispatcher
     val maxInputBuffer = attributes.mandatoryAttribute[Attributes.InputBuffer].max
 
     val props =

akka-stream/src/main/scala/akka/stream/impl/io/IOSinks.scala

@@ -38,8 +38,9 @@ import scala.concurrent.{ Future, Promise }
 
     val ioResultPromise = Promise[IOResult]()
     val props = FileSubscriber.props(f, ioResultPromise, maxInputBufferSize, startPosition, options)
-
+    val ref = materializer.actorOf(
-    val ref = materializer.actorOf(context, props.withDispatcher(Dispatcher.resolve(context)))
+      context,
+      props.withDispatcher(context.effectiveAttributes.mandatoryAttribute[Dispatcher].dispatcher))
 
     (akka.stream.actor.ActorSubscriber[ByteString](ref), ioResultPromise.future)
   }
@@ -72,7 +73,7 @@ import scala.concurrent.{ Future, Promise }
 
     val props = OutputStreamSubscriber
       .props(os, ioResultPromise, maxInputBufferSize, autoFlush)
-      .withDispatcher(Dispatcher.resolve(context))
+      .withDispatcher(context.effectiveAttributes.mandatoryAttribute[Dispatcher].dispatcher)
 
     val ref = materializer.actorOf(context, props)
     (akka.stream.actor.ActorSubscriber[ByteString](ref), ioResultPromise.future)

akka-stream/src/main/scala/akka/stream/impl/io/IOSources.scala

@@ -159,7 +159,9 @@ private[akka] final class FileSource(path: Path, chunkSize: Int, startPosition:
     val pub = try {
       val is = createInputStream() // can throw, i.e. FileNotFound
 
-      val props = InputStreamPublisher.props(is, ioResultPromise, chunkSize).withDispatcher(Dispatcher.resolve(context))
+      val props = InputStreamPublisher
+        .props(is, ioResultPromise, chunkSize)
+        .withDispatcher(context.effectiveAttributes.mandatoryAttribute[Dispatcher].dispatcher)
 
       val ref = materializer.actorOf(context, props)
       akka.stream.actor.ActorPublisher[ByteString](ref)

akka-stream/src/main/scala/akka/stream/snapshot/MaterializerState.scala

@@ -33,8 +33,7 @@ object MaterializerState {
   def streamSnapshots(mat: Materializer): Future[immutable.Seq[StreamSnapshot]] = {
     mat match {
       case impl: PhasedFusingActorMaterializer =>
-        import impl.system.dispatcher
+        requestFromSupervisor(impl.supervisor)(impl.system.dispatchers.internalDispatcher)
-        requestFromSupervisor(impl.supervisor)
     }
   }