akka-streams.conf

#####################################
# Akka Stream Reference Config File #
#####################################

akka.version = "2.4.3"

akka {

  stream {

    # Default flow materializer settings
    materializer {

      # Initial size of buffers used in stream elements
      initial-input-buffer-size = 4
      # Maximum size of buffers used in stream elements
      max-input-buffer-size = 16

      # Fully qualified config path which holds the dispatcher configuration
      # to be used by FlowMaterialiser when creating Actors.
      # When this value is left empty, the default-dispatcher will be used.
      dispatcher = ""

      # Cleanup leaked publishers and subscribers when they are not used within a given
      # deadline
      subscription-timeout {
        # when the subscription timeout is reached one of the following strategies on
        # the "stale" publisher:
        # cancel - cancel it (via `onError` or subscribing to the publisher and
        #          `cancel()`ing the subscription right away
        # warn   - log a warning statement about the stale element (then drop the
        #          reference to it)
        # noop   - do nothing (not recommended)
        mode = cancel

        # time after which a subscriber / publisher is considered stale and eligible
        # for cancelation (see `akka.stream.subscription-timeout.mode`)
        timeout = 5s
      }
      
      # Enable additional troubleshooting logging at DEBUG log level
      debug-logging = off

      # Maximum number of elements emitted in batch if downstream signals large demand
      output-burst-limit = 1000
      
      # 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.
      auto-fusing = on

      # Those stream elements which have explicit buffers (like mapAsync, mapAsyncUnordered,
      # buffer, flatMapMerge, Source.actorRef, Source.queue, etc.) will preallocate a fixed
      # buffer upon stream materialization if the requested buffer size is less than this
      # configuration parameter. The default is very high because failing early is better
      # than failing under load.
      #
      # Buffers sized larger than this will dynamically grow/shrink and consume more memory
      # per element than the fixed size buffers.
      max-fixed-buffer-size = 1000000000

      # Maximum number of sync messages that actor can process for stream to substream communication.
      # Parameter allows to interrupt synchronous processing to get upsteam/downstream messages.
      # Allows to accelerate message processing that happening withing same actor but keep system responsive.
      sync-processing-limit = 1000

      debug {
        # Enables the fuzzing mode which increases the chance of race conditions
        # by aggressively reordering events and making certain operations more
        # concurrent than usual.
        # This setting is for testing purposes, NEVER enable this in a production
        # environment!
        # To get the best results, try combining this setting with a throughput
        # of 1 on the corresponding dispatchers.
        fuzzing-mode = off
      }
    }

    # Fully qualified config path which holds the dispatcher configuration
    # to be used by FlowMaterialiser when creating Actors for IO operations,
    # such as FileSource, FileSink and others.
    blocking-io-dispatcher = "akka.stream.default-blocking-io-dispatcher"

    default-blocking-io-dispatcher {
      type = "Dispatcher"
      executor = "thread-pool-executor"
      throughput = 1

      thread-pool-executor {
        core-pool-size-min = 2
        core-pool-size-factor = 2.0
        core-pool-size-max = 16
      }
    }
  }

  # configure overrides to ssl-configuration here (to be used by akka-streams, and akka-http – i.e. when serving https connections)
  ssl-config {
    # due to still supporting JDK6 in this release
    # TODO once JDK 8 is required switch this to TLSv1.2 (or remove entirely, leave up to ssl-config to pick)
    protocol = "TLSv1"
  }

  # Home directory of Akka, modules in the deploy directory will be loaded
  home = ""

  # Loggers to register at boot time (akka.event.Logging$DefaultLogger logs
  # to STDOUT)
  loggers = ["akka.event.Logging$DefaultLogger"]
  
  # Filter of log events that is used by the LoggingAdapter before 
  # publishing log events to the eventStream. It can perform
  # fine grained filtering based on the log source. The default
  # implementation filters on the `loglevel`.
  # FQCN of the LoggingFilter. The Class of the FQCN must implement 
  # akka.event.LoggingFilter and have a public constructor with
  # (akka.actor.ActorSystem.Settings, akka.event.EventStream) parameters.
  logging-filter = "akka.event.DefaultLoggingFilter"

  # Specifies the default loggers dispatcher
  loggers-dispatcher = "akka.actor.default-dispatcher"

  # Loggers are created and registered synchronously during ActorSystem
  # start-up, and since they are actors, this timeout is used to bound the
  # waiting time
  logger-startup-timeout = 5s

  # Log level used by the configured loggers (see "loggers") as soon
  # as they have been started; before that, see "stdout-loglevel"
  # Options: OFF, ERROR, WARNING, INFO, DEBUG
  loglevel = "INFO"

  # Log level for the very basic logger activated during ActorSystem startup.
  # This logger prints the log messages to stdout (System.out).
  # Options: OFF, ERROR, WARNING, INFO, DEBUG
  stdout-loglevel = "WARNING"

  # Log the complete configuration at INFO level when the actor system is started.
  # This is useful when you are uncertain of what configuration is used.
  log-config-on-start = off

  # Log at info level when messages are sent to dead letters.
  # Possible values:
  # on: all dead letters are logged
  # off: no logging of dead letters
  # n: positive integer, number of dead letters that will be logged
  log-dead-letters = 10

  # Possibility to turn off logging of dead letters while the actor system
  # is shutting down. Logging is only done when enabled by 'log-dead-letters'
  # setting.
  log-dead-letters-during-shutdown = on

  # List FQCN of extensions which shall be loaded at actor system startup.
  # Should be on the format: 'extensions = ["foo", "bar"]' etc.
  # See the Akka Documentation for more info about Extensions
  extensions = []

  # Toggles whether threads created by this ActorSystem should be daemons or not
  daemonic = off

  # JVM shutdown, System.exit(-1), in case of a fatal error,
  # such as OutOfMemoryError
  jvm-exit-on-fatal-error = on

  actor {

    # FQCN of the ActorRefProvider to be used; the below is the built-in default,
    # another one is akka.remote.RemoteActorRefProvider in the akka-remote bundle.
    provider = "akka.actor.LocalActorRefProvider"

    # The guardian "/user" will use this class to obtain its supervisorStrategy.
    # It needs to be a subclass of akka.actor.SupervisorStrategyConfigurator.
    # In addition to the default there is akka.actor.StoppingSupervisorStrategy.
    guardian-supervisor-strategy = "akka.actor.DefaultSupervisorStrategy"

    # Timeout for ActorSystem.actorOf
    creation-timeout = 20s

    # Serializes and deserializes (non-primitive) messages to ensure immutability,
    # this is only intended for testing.
    serialize-messages = off

    # Serializes and deserializes creators (in Props) to ensure that they can be
    # sent over the network, this is only intended for testing. Purely local deployments
    # as marked with deploy.scope == LocalScope are exempt from verification.
    serialize-creators = off

    # Timeout for send operations to top-level actors which are in the process
    # of being started. This is only relevant if using a bounded mailbox or the
    # CallingThreadDispatcher for a top-level actor.
    unstarted-push-timeout = 10s

    typed {
      # Default timeout for typed actor methods with non-void return type
      timeout = 5s
    }
    
    # Mapping between ´deployment.router' short names to fully qualified class names
    router.type-mapping {
      from-code = "akka.routing.NoRouter"
      round-robin-pool = "akka.routing.RoundRobinPool"
      round-robin-group = "akka.routing.RoundRobinGroup"
      random-pool = "akka.routing.RandomPool"
      random-group = "akka.routing.RandomGroup"
      balancing-pool = "akka.routing.BalancingPool"
      smallest-mailbox-pool = "akka.routing.SmallestMailboxPool"
      broadcast-pool = "akka.routing.BroadcastPool"
      broadcast-group = "akka.routing.BroadcastGroup"
      scatter-gather-pool = "akka.routing.ScatterGatherFirstCompletedPool"
      scatter-gather-group = "akka.routing.ScatterGatherFirstCompletedGroup"
      tail-chopping-pool = "akka.routing.TailChoppingPool"
      tail-chopping-group = "akka.routing.TailChoppingGroup"
      consistent-hashing-pool = "akka.routing.ConsistentHashingPool"
      consistent-hashing-group = "akka.routing.ConsistentHashingGroup"
    }

    deployment {

      # deployment id pattern - on the format: /parent/child etc.
      default {
      
        # The id of the dispatcher to use for this actor.
        # If undefined or empty the dispatcher specified in code
        # (Props.withDispatcher) is used, or default-dispatcher if not
        # specified at all.
        dispatcher = ""

        # The id of the mailbox to use for this actor.
        # If undefined or empty the default mailbox of the configured dispatcher
        # is used or if there is no mailbox configuration the mailbox specified
        # in code (Props.withMailbox) is used.
        # If there is a mailbox defined in the configured dispatcher then that
        # overrides this setting.
        mailbox = ""

        # routing (load-balance) scheme to use
        # - available: "from-code", "round-robin", "random", "smallest-mailbox",
        #              "scatter-gather", "broadcast"
        # - or:        Fully qualified class name of the router class.
        #              The class must extend akka.routing.CustomRouterConfig and
        #              have a public constructor with com.typesafe.config.Config
        #              and optional akka.actor.DynamicAccess parameter.
        # - default is "from-code";
        # Whether or not an actor is transformed to a Router is decided in code
        # only (Props.withRouter). The type of router can be overridden in the
        # configuration; specifying "from-code" means that the values specified
        # in the code shall be used.
        # In case of routing, the actors to be routed to can be specified
        # in several ways:
        # - nr-of-instances: will create that many children
        # - routees.paths: will route messages to these paths using ActorSelection,
        #   i.e. will not create children
        # - resizer: dynamically resizable number of routees as specified in
        #   resizer below
        router = "from-code"

        # number of children to create in case of a router;
        # this setting is ignored if routees.paths is given
        nr-of-instances = 1

        # within is the timeout used for routers containing future calls
        within = 5 seconds

        # number of virtual nodes per node for consistent-hashing router
        virtual-nodes-factor = 10

        tail-chopping-router {
          # interval is duration between sending message to next routee
          interval = 10 milliseconds
        }

        routees {
          # Alternatively to giving nr-of-instances you can specify the full
          # paths of those actors which should be routed to. This setting takes
          # precedence over nr-of-instances
          paths = []
        }
        
        # To use a dedicated dispatcher for the routees of the pool you can
        # define the dispatcher configuration inline with the property name 
        # 'pool-dispatcher' in the deployment section of the router.
        # For example:
        # pool-dispatcher {
        #   fork-join-executor.parallelism-min = 5
        #   fork-join-executor.parallelism-max = 5
        # }

        # Routers with dynamically resizable number of routees; this feature is
        # enabled by including (parts of) this section in the deployment
        resizer {
        
          enabled = off

          # The fewest number of routees the router should ever have.
          lower-bound = 1

          # The most number of routees the router should ever have.
          # Must be greater than or equal to lower-bound.
          upper-bound = 10

          # Threshold used to evaluate if a routee is considered to be busy
          # (under pressure). Implementation depends on this value (default is 1).
          # 0:   number of routees currently processing a message.
          # 1:   number of routees currently processing a message has
          #      some messages in mailbox.
          # > 1: number of routees with at least the configured pressure-threshold
          #      messages in their mailbox. Note that estimating mailbox size of
          #      default UnboundedMailbox is O(N) operation.
          pressure-threshold = 1

          # Percentage to increase capacity whenever all routees are busy.
          # For example, 0.2 would increase 20% (rounded up), i.e. if current
          # capacity is 6 it will request an increase of 2 more routees.
          rampup-rate = 0.2

          # Minimum fraction of busy routees before backing off.
          # For example, if this is 0.3, then we'll remove some routees only when
          # less than 30% of routees are busy, i.e. if current capacity is 10 and
          # 3 are busy then the capacity is unchanged, but if 2 or less are busy
          # the capacity is decreased.
          # Use 0.0 or negative to avoid removal of routees.
          backoff-threshold = 0.3

          # Fraction of routees to be removed when the resizer reaches the
          # backoffThreshold.
          # For example, 0.1 would decrease 10% (rounded up), i.e. if current
          # capacity is 9 it will request an decrease of 1 routee.
          backoff-rate = 0.1

          # Number of messages between resize operation.
          # Use 1 to resize before each message.
          messages-per-resize = 10
        }

        # Routers with dynamically resizable number of routees based on
        # performance metrics.
        # This feature is enabled by including (parts of) this section in
        # the deployment, cannot be enabled together with default resizer.
        optimal-size-exploring-resizer {

          enabled = off

          # The fewest number of routees the router should ever have.
          lower-bound = 1

          # The most number of routees the router should ever have.
          # Must be greater than or equal to lower-bound.
          upper-bound = 10

          # probability of doing a ramping down when all routees are busy
          # during exploration.
          chance-of-ramping-down-when-full = 0.2

          # Interval between each resize attempt
          action-interval = 5s

          # If the routees have not been fully utilized (i.e. all routees busy)
          # for such length, the resizer will downsize the pool.
          downsize-after-underutilized-for = 72h

          # Duration exploration, the ratio between the largest step size and
          # current pool size. E.g. if the current pool size is 50, and the
          # explore-step-size is 0.1, the maximum pool size change during
          # exploration will be +- 5
          explore-step-size = 0.1

          # Probabily of doing an exploration v.s. optmization.
          chance-of-exploration = 0.4

          # When downsizing after a long streak of underutilization, the resizer
          # will downsize the pool to the highest utiliziation multiplied by a
          # a downsize rasio. This downsize ratio determines the new pools size
          # in comparison to the highest utilization.
          # E.g. if the highest utilization is 10, and the down size ratio
          # is 0.8, the pool will be downsized to 8
          downsize-ratio = 0.8

          # When optimizing, the resizer only considers the sizes adjacent to the
          # current size. This number indicates how many adjacent sizes to consider.
          optimization-range = 16

          # The weight of the latest metric over old metrics when collecting
          # performance metrics.
          # E.g. if the last processing speed is 10 millis per message at pool
          # size 5, and if the new processing speed collected is 6 millis per
          # message at pool size 5. Given a weight of 0.3, the metrics
          # representing pool size 5 will be 6 * 0.3 + 10 * 0.7, i.e. 8.8 millis
          # Obviously, this number should be between 0 and 1.
          weight-of-latest-metric = 0.5
        }
      }

      /IO-DNS/inet-address {
        mailbox = "unbounded"
        router = "consistent-hashing-pool"
        nr-of-instances = 4
      }
    }

    default-dispatcher {
      # Must be one of the following
      # Dispatcher, PinnedDispatcher, or a FQCN to a class inheriting
      # MessageDispatcherConfigurator with a public constructor with
      # both com.typesafe.config.Config parameter and
      # akka.dispatch.DispatcherPrerequisites parameters.
      # PinnedDispatcher must be used together with executor=thread-pool-executor.
      type = "Dispatcher"

      # Which kind of ExecutorService to use for this dispatcher
      # Valid options:
      #  - "default-executor" requires a "default-executor" section
      #  - "fork-join-executor" requires a "fork-join-executor" section
      #  - "thread-pool-executor" requires a "thread-pool-executor" section
      #  - A FQCN of a class extending ExecutorServiceConfigurator
      executor = "default-executor"

      # This will be used if you have set "executor = "default-executor"".
      # If an ActorSystem is created with a given ExecutionContext, this
      # ExecutionContext will be used as the default executor for all
      # dispatchers in the ActorSystem configured with
      # executor = "default-executor". Note that "default-executor"
      # is the default value for executor, and therefore used if not
      # specified otherwise. If no ExecutionContext is given,
      # the executor configured in "fallback" will be used.
      default-executor {
        fallback = "fork-join-executor"
      }

      # This will be used if you have set "executor = "fork-join-executor""
      # Underlying thread pool implementation is scala.concurrent.forkjoin.ForkJoinPool
      fork-join-executor {
        # Min number of threads to cap factor-based parallelism number to
        parallelism-min = 8

        # 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

        # Max number of threads to cap factor-based parallelism number to
        parallelism-max = 64

        # Setting to "FIFO" to use queue like peeking mode which "poll" or "LIFO" to use stack
        # like peeking mode which "pop".
        task-peeking-mode = "FIFO"
      }

      # This will be used if you have set "executor = "thread-pool-executor""
      # Underlying thread pool implementation is java.util.concurrent.ThreadPoolExecutor
      thread-pool-executor {
        # Keep alive time for threads
        keep-alive-time = 60s
        
        # Define a fixed thread pool size with this property. The corePoolSize
        # and the maximumPoolSize of the ThreadPoolExecutor will be set to this
        # value, if it is defined. Then the other pool-size properties will not
        # be used. 
        # 
        # Valid values are: `off` or a positive integer.
        fixed-pool-size = off

        # Min number of threads to cap factor-based corePoolSize number to
        core-pool-size-min = 8

        # The core-pool-size-factor is used to determine corePoolSize of the 
        # ThreadPoolExecutor using the following formula: 
        # ceil(available processors * factor).
        # Resulting size is then bounded by the core-pool-size-min and
        # core-pool-size-max values.
        core-pool-size-factor = 3.0

        # Max number of threads to cap factor-based corePoolSize number to
        core-pool-size-max = 64

        # Minimum number of threads to cap factor-based maximumPoolSize number to
        max-pool-size-min = 8

        # The max-pool-size-factor is used to determine maximumPoolSize of the 
        # ThreadPoolExecutor using the following formula:
        # ceil(available processors * factor)
        # The maximumPoolSize will not be less than corePoolSize.
        # It is only used if using a bounded task queue.
        max-pool-size-factor  = 3.0

        # Max number of threads to cap factor-based maximumPoolSize number to
        max-pool-size-max = 64

        # Specifies the bounded capacity of the task queue (< 1 == unbounded)
        task-queue-size = -1

        # Specifies which type of task queue will be used, can be "array" or
        # "linked" (default)
        task-queue-type = "linked"

        # Allow core threads to time out
        allow-core-timeout = on
      }

      # How long time the dispatcher will wait for new actors until it shuts down
      shutdown-timeout = 1s

      # Throughput defines the number of messages that are processed in a batch
      # before the thread is returned to the pool. Set to 1 for as fair as possible.
      throughput = 5

      # Throughput deadline for Dispatcher, set to 0 or negative for no deadline
      throughput-deadline-time = 0ms

      # For BalancingDispatcher: If the balancing dispatcher should attempt to
      # schedule idle actors using the same dispatcher when a message comes in,
      # and the dispatchers ExecutorService is not fully busy already.
      attempt-teamwork = on

      # If this dispatcher requires a specific type of mailbox, specify the
      # fully-qualified class name here; the actually created mailbox will
      # be a subtype of this type. The empty string signifies no requirement.
      mailbox-requirement = ""
    }

    default-mailbox {
      # FQCN of the MailboxType. The Class of the FQCN must have a public
      # constructor with
      # (akka.actor.ActorSystem.Settings, com.typesafe.config.Config) parameters.
      mailbox-type = "akka.dispatch.UnboundedMailbox"

      # If the mailbox is bounded then it uses this setting to determine its
      # capacity. The provided value must be positive.
      # NOTICE:
      # Up to version 2.1 the mailbox type was determined based on this setting;
      # this is no longer the case, the type must explicitly be a bounded mailbox.
      mailbox-capacity = 1000

      # If the mailbox is bounded then this is the timeout for enqueueing
      # in case the mailbox is full. Negative values signify infinite
      # timeout, which should be avoided as it bears the risk of dead-lock.
      mailbox-push-timeout-time = 10s

      # For Actor with Stash: The default capacity of the stash.
      # If negative (or zero) then an unbounded stash is used (default)
      # If positive then a bounded stash is used and the capacity is set using
      # the property
      stash-capacity = -1
    }

    mailbox {
      # Mapping between message queue semantics and mailbox configurations.
      # Used by akka.dispatch.RequiresMessageQueue[T] to enforce different
      # mailbox types on actors.
      # If your Actor implements RequiresMessageQueue[T], then when you create
      # an instance of that actor its mailbox type will be decided by looking
      # up a mailbox configuration via T in this mapping
      requirements {
        "akka.dispatch.UnboundedMessageQueueSemantics" =
          akka.actor.mailbox.unbounded-queue-based
        "akka.dispatch.BoundedMessageQueueSemantics" =
          akka.actor.mailbox.bounded-queue-based
        "akka.dispatch.DequeBasedMessageQueueSemantics" =
          akka.actor.mailbox.unbounded-deque-based
        "akka.dispatch.UnboundedDequeBasedMessageQueueSemantics" =
          akka.actor.mailbox.unbounded-deque-based
        "akka.dispatch.BoundedDequeBasedMessageQueueSemantics" =
          akka.actor.mailbox.bounded-deque-based
        "akka.dispatch.MultipleConsumerSemantics" =
          akka.actor.mailbox.unbounded-queue-based
        "akka.dispatch.ControlAwareMessageQueueSemantics" =
          akka.actor.mailbox.unbounded-control-aware-queue-based
        "akka.dispatch.UnboundedControlAwareMessageQueueSemantics" =
          akka.actor.mailbox.unbounded-control-aware-queue-based
        "akka.dispatch.BoundedControlAwareMessageQueueSemantics" =
          akka.actor.mailbox.bounded-control-aware-queue-based
        "akka.event.LoggerMessageQueueSemantics" =
          akka.actor.mailbox.logger-queue
      }

      unbounded-queue-based {
        # FQCN of the MailboxType, The Class of the FQCN must have a public
        # constructor with (akka.actor.ActorSystem.Settings,
        # com.typesafe.config.Config) parameters.
        mailbox-type = "akka.dispatch.UnboundedMailbox"
      }

      bounded-queue-based {
        # FQCN of the MailboxType, The Class of the FQCN must have a public
        # constructor with (akka.actor.ActorSystem.Settings,
        # com.typesafe.config.Config) parameters.
        mailbox-type = "akka.dispatch.BoundedMailbox"
      }

      unbounded-deque-based {
        # FQCN of the MailboxType, The Class of the FQCN must have a public
        # constructor with (akka.actor.ActorSystem.Settings,
        # com.typesafe.config.Config) parameters.
        mailbox-type = "akka.dispatch.UnboundedDequeBasedMailbox"
      }

      bounded-deque-based {
        # FQCN of the MailboxType, The Class of the FQCN must have a public
        # constructor with (akka.actor.ActorSystem.Settings,
        # com.typesafe.config.Config) parameters.
        mailbox-type = "akka.dispatch.BoundedDequeBasedMailbox"
      }

      unbounded-control-aware-queue-based {
        # FQCN of the MailboxType, The Class of the FQCN must have a public
        # constructor with (akka.actor.ActorSystem.Settings,
        # com.typesafe.config.Config) parameters.
        mailbox-type = "akka.dispatch.UnboundedControlAwareMailbox"
      }

      bounded-control-aware-queue-based {
        # FQCN of the MailboxType, The Class of the FQCN must have a public
        # constructor with (akka.actor.ActorSystem.Settings,
        # com.typesafe.config.Config) parameters.
        mailbox-type = "akka.dispatch.BoundedControlAwareMailbox"
      }
      
      # The LoggerMailbox will drain all messages in the mailbox
      # when the system is shutdown and deliver them to the StandardOutLogger.
      # Do not change this unless you know what you are doing.
      logger-queue {
        mailbox-type = "akka.event.LoggerMailboxType"
      }
    }

    debug {
      # enable function of Actor.loggable(), which is to log any received message
      # at DEBUG level, see the “Testing Actor Systems” section of the Akka
      # Documentation at http://akka.io/docs
      receive = off

      # enable DEBUG logging of all AutoReceiveMessages (Kill, PoisonPill et.c.)
      autoreceive = off

      # enable DEBUG logging of actor lifecycle changes
      lifecycle = off

      # enable DEBUG logging of all LoggingFSMs for events, transitions and timers
      fsm = off

      # enable DEBUG logging of subscription changes on the eventStream
      event-stream = off

      # enable DEBUG logging of unhandled messages
      unhandled = off

      # enable WARN logging of misconfigured routers
      router-misconfiguration = off
    }

    # Entries for pluggable serializers and their bindings.
    serializers {
      java = "akka.serialization.JavaSerializer"
      bytes = "akka.serialization.ByteArraySerializer"
    }

    # Class to Serializer binding. You only need to specify the name of an
    # interface or abstract base class of the messages. In case of ambiguity it
    # is using the most specific configured class, or giving a warning and
    # choosing the “first” one.
    #
    # To disable one of the default serializers, assign its class to "none", like
    # "java.io.Serializable" = none
    serialization-bindings {
      "[B" = bytes
      "java.io.Serializable" = java
    }

    # Log warnings when the default Java serialization is used to serialize messages.
    # The default serializer uses Java serialization which is not very performant and should not
    # be used in production environments unless you don't care about performance. In that case
    # you can turn this off.
    warn-about-java-serializer-usage = on

    # Configuration namespace of serialization identifiers.
    # Each serializer implementation must have an entry in the following format:
    # `akka.actor.serialization-identifiers."FQCN" = ID`
    # where `FQCN` is fully qualified class name of the serializer implementation
    # and `ID` is globally unique serializer identifier number.
    # Identifier values from 0 to 16 are reserved for Akka internal usage.
    serialization-identifiers {
      "akka.serialization.JavaSerializer" = 1
      "akka.serialization.ByteArraySerializer" = 4  
    }

    # Configuration items which are used by the akka.actor.ActorDSL._ methods
    dsl {
      # Maximum queue size of the actor created by newInbox(); this protects
      # against faulty programs which use select() and consistently miss messages
      inbox-size = 1000

      # Default timeout to assume for operations like Inbox.receive et al
      default-timeout = 5s
    }
  }

  # Used to set the behavior of the scheduler.
  # Changing the default values may change the system behavior drastically so make
  # sure you know what you're doing! See the Scheduler section of the Akka
  # Documentation for more details.
  scheduler {
    # The LightArrayRevolverScheduler is used as the default scheduler in the
    # system. It does not execute the scheduled tasks on exact time, but on every
    # tick, it will run everything that is (over)due. You can increase or decrease
    # the accuracy of the execution timing by specifying smaller or larger tick
    # duration. If you are scheduling a lot of tasks you should consider increasing
    # the ticks per wheel.
    # Note that it might take up to 1 tick to stop the Timer, so setting the
    # tick-duration to a high value will make shutting down the actor system
    # take longer.
    tick-duration = 10ms

    # The timer uses a circular wheel of buckets to store the timer tasks.
    # This should be set such that the majority of scheduled timeouts (for high
    # scheduling frequency) will be shorter than one rotation of the wheel
    # (ticks-per-wheel * ticks-duration)
    # THIS MUST BE A POWER OF TWO!
    ticks-per-wheel = 512

    # This setting selects the timer implementation which shall be loaded at
    # system start-up.
    # The class given here must implement the akka.actor.Scheduler interface
    # and offer a public constructor which takes three arguments:
    #  1) com.typesafe.config.Config
    #  2) akka.event.LoggingAdapter
    #  3) java.util.concurrent.ThreadFactory
    implementation = akka.actor.LightArrayRevolverScheduler

    # When shutting down the scheduler, there will typically be a thread which
    # needs to be stopped, and this timeout determines how long to wait for
    # that to happen. In case of timeout the shutdown of the actor system will
    # proceed without running possibly still enqueued tasks.
    shutdown-timeout = 5s
  }

  io {

    # By default the select loops run on dedicated threads, hence using a
    # PinnedDispatcher
    pinned-dispatcher {
      type = "PinnedDispatcher"
      executor = "thread-pool-executor"
      thread-pool-executor.allow-core-timeout = off
    }

    tcp {

      # The number of selectors to stripe the served channels over; each of
      # these will use one select loop on the selector-dispatcher.
      nr-of-selectors = 1

      # Maximum number of open channels supported by this TCP module; there is
      # no intrinsic general limit, this setting is meant to enable DoS
      # protection by limiting the number of concurrently connected clients.
      # Also note that this is a "soft" limit; in certain cases the implementation
      # will accept a few connections more or a few less than the number configured
      # here. Must be an integer > 0 or "unlimited".
      max-channels = 256000

      # When trying to assign a new connection to a selector and the chosen
      # selector is at full capacity, retry selector choosing and assignment
      # this many times before giving up
      selector-association-retries = 10

      # The maximum number of connection that are accepted in one go,
      # higher numbers decrease latency, lower numbers increase fairness on
      # the worker-dispatcher
      batch-accept-limit = 10

      # The number of bytes per direct buffer in the pool used to read or write
      # network data from the kernel.
      direct-buffer-size = 128 KiB

      # The maximal number of direct buffers kept in the direct buffer pool for
      # reuse.
      direct-buffer-pool-limit = 1000

      # The duration a connection actor waits for a `Register` message from
      # its commander before aborting the connection.
      register-timeout = 5s

      # The maximum number of bytes delivered by a `Received` message. Before
      # more data is read from the network the connection actor will try to
      # do other work.
      # The purpose of this setting is to impose a smaller limit than the 
      # configured receive buffer size. When using value 'unlimited' it will
      # try to read all from the receive buffer.
      max-received-message-size = unlimited

      # Enable fine grained logging of what goes on inside the implementation.
      # Be aware that this may log more than once per message sent to the actors
      # of the tcp implementation.
      trace-logging = off

      # Fully qualified config path which holds the dispatcher configuration
      # to be used for running the select() calls in the selectors
      selector-dispatcher = "akka.io.pinned-dispatcher"

      # Fully qualified config path which holds the dispatcher configuration
      # for the read/write worker actors
      worker-dispatcher = "akka.actor.default-dispatcher"

      # Fully qualified config path which holds the dispatcher configuration
      # for the selector management actors
      management-dispatcher = "akka.actor.default-dispatcher"

      # Fully qualified config path which holds the dispatcher configuration
      # on which file IO tasks are scheduled
      file-io-dispatcher = "akka.actor.default-dispatcher"

      # The maximum number of bytes (or "unlimited") to transfer in one batch
      # when using `WriteFile` command which uses `FileChannel.transferTo` to
      # pipe files to a TCP socket. On some OS like Linux `FileChannel.transferTo`
      # may block for a long time when network IO is faster than file IO.
      # Decreasing the value may improve fairness while increasing may improve
      # throughput.
      file-io-transferTo-limit = 512 KiB

      # The number of times to retry the `finishConnect` call after being notified about
      # OP_CONNECT. Retries are needed if the OP_CONNECT notification doesn't imply that
      # `finishConnect` will succeed, which is the case on Android.
      finish-connect-retries = 5

      # On Windows connection aborts are not reliably detected unless an OP_READ is
      # registered on the selector _after_ the connection has been reset. This
      # workaround enables an OP_CONNECT which forces the abort to be visible on Windows.
      # Enabling this setting on other platforms than Windows will cause various failures
      # and undefined behavior.
      # Possible values of this key are on, off and auto where auto will enable the
      # workaround if Windows is detected automatically.
      windows-connection-abort-workaround-enabled = off
    }

    udp {

      # The number of selectors to stripe the served channels over; each of
      # these will use one select loop on the selector-dispatcher.
      nr-of-selectors = 1

      # Maximum number of open channels supported by this UDP module Generally
      # UDP does not require a large number of channels, therefore it is
      # recommended to keep this setting low.
      max-channels = 4096

      # The select loop can be used in two modes:
      # - setting "infinite" will select without a timeout, hogging a thread
      # - setting a positive timeout will do a bounded select call,
      #   enabling sharing of a single thread between multiple selectors
      #   (in this case you will have to use a different configuration for the
      #   selector-dispatcher, e.g. using "type=Dispatcher" with size 1)
      # - setting it to zero means polling, i.e. calling selectNow()
      select-timeout = infinite

      # When trying to assign a new connection to a selector and the chosen
      # selector is at full capacity, retry selector choosing and assignment
      # this many times before giving up
      selector-association-retries = 10

      # The maximum number of datagrams that are read in one go,
      # higher numbers decrease latency, lower numbers increase fairness on
      # the worker-dispatcher
      receive-throughput = 3

      # The number of bytes per direct buffer in the pool used to read or write
      # network data from the kernel.
      direct-buffer-size = 128 KiB

      # The maximal number of direct buffers kept in the direct buffer pool for
      # reuse.
      direct-buffer-pool-limit = 1000

      # Enable fine grained logging of what goes on inside the implementation.
      # Be aware that this may log more than once per message sent to the actors
      # of the tcp implementation.
      trace-logging = off

      # Fully qualified config path which holds the dispatcher configuration
      # to be used for running the select() calls in the selectors
      selector-dispatcher = "akka.io.pinned-dispatcher"

      # Fully qualified config path which holds the dispatcher configuration
      # for the read/write worker actors
      worker-dispatcher = "akka.actor.default-dispatcher"

      # Fully qualified config path which holds the dispatcher configuration
      # for the selector management actors
      management-dispatcher = "akka.actor.default-dispatcher"
    }

    udp-connected {

      # The number of selectors to stripe the served channels over; each of
      # these will use one select loop on the selector-dispatcher.
      nr-of-selectors = 1

      # Maximum number of open channels supported by this UDP module Generally
      # UDP does not require a large number of channels, therefore it is
      # recommended to keep this setting low.
      max-channels = 4096

      # The select loop can be used in two modes:
      # - setting "infinite" will select without a timeout, hogging a thread
      # - setting a positive timeout will do a bounded select call,
      #   enabling sharing of a single thread between multiple selectors
      #   (in this case you will have to use a different configuration for the
      #   selector-dispatcher, e.g. using "type=Dispatcher" with size 1)
      # - setting it to zero means polling, i.e. calling selectNow()
      select-timeout = infinite

      # When trying to assign a new connection to a selector and the chosen
      # selector is at full capacity, retry selector choosing and assignment
      # this many times before giving up
      selector-association-retries = 10

      # The maximum number of datagrams that are read in one go,
      # higher numbers decrease latency, lower numbers increase fairness on
      # the worker-dispatcher
      receive-throughput = 3

      # The number of bytes per direct buffer in the pool used to read or write
      # network data from the kernel.
      direct-buffer-size = 128 KiB

      # The maximal number of direct buffers kept in the direct buffer pool for
      # reuse.
      direct-buffer-pool-limit = 1000
      
      # Enable fine grained logging of what goes on inside the implementation.
      # Be aware that this may log more than once per message sent to the actors
      # of the tcp implementation.
      trace-logging = off

      # Fully qualified config path which holds the dispatcher configuration
      # to be used for running the select() calls in the selectors
      selector-dispatcher = "akka.io.pinned-dispatcher"

      # Fully qualified config path which holds the dispatcher configuration
      # for the read/write worker actors
      worker-dispatcher = "akka.actor.default-dispatcher"

      # Fully qualified config path which holds the dispatcher configuration
      # for the selector management actors
      management-dispatcher = "akka.actor.default-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"

      # Name of the subconfig at path akka.io.dns, see inet-address below
      resolver = "inet-address"

      inet-address {
        # Must implement akka.io.DnsProvider
        provider-object = "akka.io.InetAddressDnsProvider"

        # These TTLs are set to default java 6 values
        positive-ttl = 30s
        negative-ttl = 10s

        # How often to sweep out expired cache entries.
        # Note that this interval has nothing to do with TTLs
        cache-cleanup-interval = 120s
      }
    }
  }

}

AkkaStreamsFlatMapPerf.java

import java.util.concurrent.*;

import org.openjdk.jmh.annotations.*;
import org.openjdk.jmh.infra.Blackhole;
import org.reactivestreams.*;

import com.typesafe.config.*;

import akka.actor.ActorSystem;
import akka.stream.ActorMaterializer;
import akka.stream.javadsl.*;

@BenchmarkMode(Mode.Throughput)
@Warmup(iterations = 5)
@Measurement(iterations = 5, time = 2, timeUnit = TimeUnit.SECONDS)
@OutputTimeUnit(TimeUnit.SECONDS)
@Fork(value = 1)
@State(Scope.Thread)
public class AkkaStreamsFlatMapPerf {
    @Param({ "1", "1000", "1000000" })
    public int times;

    @Param({"1", "2", "16"})
    public int maxConcurrent;
    
    ActorSystem actorSystem;

    ActorMaterializer materializer;

    Publisher<Integer> akRangeFlatMapJust;

    @Setup
    public void setup() throws Exception {
        Config cfg = ConfigFactory.parseResources(AkkaStreamsFlatMapPerf.class, "/akka-streams.conf");
        actorSystem = ActorSystem.create("sys", cfg);

        materializer = ActorMaterializer.create(actorSystem);

        akRangeFlatMapJust = s -> 
        Source.range(1, times)
        .flatMapMerge(maxConcurrent, v -> Source.single(v))
        .runWith(Sink.asPublisher(AsPublisher.WITHOUT_FANOUT), materializer)
        .subscribe(s)
        ;
    }
    
    @TearDown
    public void teardown() {
        actorSystem.terminate();
    }
    
    @Benchmark
    public void rangeFlatMapJust_akka(Blackhole bh) throws InterruptedException {
        LatchedSubscriber<Integer> lo = new LatchedSubscriber<>(bh);
        akRangeFlatMapJust.subscribe(lo);
        
        if (times == 1) {
            while (lo.latch.getCount() != 0);
        } else {
            lo.latch.await();
        }
    }
    
    public class LatchedSubscriber<T> implements Subscriber<T> {

        public CountDownLatch latch = new CountDownLatch(1);
        private final Blackhole bh;

        public LatchedSubscriber(Blackhole bh) {
            this.bh = bh;
        }

        @Override
        public void onSubscribe(Subscription s) {
            s.request(Long.MAX_VALUE);
        }
        
        @Override
        public void onComplete() {
            latch.countDown();
        }

        @Override
        public void onError(Throwable e) {
            latch.countDown();
        }

        @Override
        public void onNext(T t) {
            bh.consume(t);
        }

    }

}