Configuration

Configuration

Akka uses the Typesafe Config Library, which might also be a good choice for the configuration of your own application or library built with or without Akka. This library is implemented in Java with no external dependencies; you should have a look at its documentation (in particular about ConfigFactory), which is only summarized in the following.

Where configuration is read from

All configuration for Akka is held within instances of ActorSystem, or put differently, as viewed from the outside, ActorSystem is the only consumer of configuration information. While constructing an actor system, you can either pass in a Config object or not, where the second case is equivalent to passing ConfigFactory.load() (with the right class loader). This means roughly that the default is to parse all application.conf, application.json and application.properties found at the root of the class path—please refer to the aforementioned documentation for details. The actor system then merges in all reference.conf resources found at the root of the class path to form the fallback configuration, i.e. it internally uses

appConfig.withFallback(ConfigFactory.defaultReference(classLoader))

The philosophy is that code never contains default values, but instead relies upon their presence in the reference.conf supplied with the library in question.

Highest precedence is given to overrides given as system properties, see the HOCON specification (near the bottom). Also noteworthy is that the application configuration—which defaults to application—may be overridden using the config.resource property (there are more, please refer to the Config docs).

Note

If you are writing an Akka application, keep you configuration in application.conf at the root of the class path. If you are writing an Akka-based library, keep its configuration in reference.conf at the root of the JAR file.

When using JarJar, OneJar, Assembly or any jar-bundler

Warning

Akka’s configuration approach relies heavily on the notion of every module/jar having its own reference.conf file, all of these will be discovered by the configuration and loaded. Unfortunately this also means that if you put merge multiple jars into the same jar, you need to merge all the reference.confs as well. Otherwise all defaults will be lost and Akka will not function.

How to structure your configuration

Given that ConfigFactory.load() merges all resources with matching name from the whole class path, it is easiest to utilize that functionality and differenciate actor systems within the hierarchy of the configuration:

myapp1 {
  akka.loglevel = WARNING
  my.own.setting = 43
}
myapp2 {
  akka.loglevel = ERROR
  app2.setting = "appname"
}
my.own.setting = 42
my.other.setting = "hello"
val config = ConfigFactory.load()
val app1 = ActorSystem("MyApp1", config.getConfig("myapp1").withFallback(config))
val app2 = ActorSystem("MyApp2", config.getConfig("myapp2").withOnlyPath("akka").withFallback(config))

These two samples demonstrate different variations of the “lift-a-subtree” trick: in the first case, the configuration accessible from within the actor system is this

akka.loglevel = WARNING
my.own.setting = 43
my.other.setting = "hello"
// plus myapp1 and myapp2 subtrees

while in the second one, only the “akka” subtree is lifted, with the following result:

akka.loglevel = ERROR
my.own.setting = 42
my.other.setting = "hello"
// plus myapp1 and myapp2 subtrees

Note

The configuration library is really powerful, explaining all features exceeds the scope affordable here. In particular not covered are how to include other configuration files within other files (see a small example at Including files) and copying parts of the configuration tree by way of path substitutions.

You may also specify and parse the configuration programmatically in other ways when instantiating the ActorSystem.

import akka.actor.ActorSystem
import com.typesafe.config.ConfigFactory
    val customConf = ConfigFactory.parseString("""
      akka.actor.deployment {
        /my-service {
          router = round-robin
          nr-of-instances = 3
        }
      }
      """)
    // ConfigFactory.load sandwiches customConfig between default reference
    // config and default overrides, and then resolves it.
    val system = ActorSystem("MySystem", ConfigFactory.load(customConf))

Listing of the Reference Configuration

Each Akka module has a reference configuration file with the default values.

akka-actor

####################################
# Akka Actor Reference Config File #
####################################

# This is the reference config file that contains all the default settings.
# Make your edits/overrides in your application.conf.

akka {
  # Akka version, checked against the runtime version of Akka.
  version = "2.0.1"

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

  # Event handlers to register at boot time (Logging$DefaultLogger logs to STDOUT)
  event-handlers = ["akka.event.Logging$DefaultLogger"]

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

  # Log level for the very basic logger activated during AkkaApplication startup
  # Options: 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

  # 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 the 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 {

    provider = "akka.actor.LocalActorRefProvider"

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

    # frequency with which stopping actors are prodded in case they had to be
    # removed from their parents
    reaper-interval = 5s

    # 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.
    serialize-creators = off

    typed {
      # Default timeout for typed actor methods with non-void return type
      timeout = 5s
    }

    deployment {

      # deployment id pattern - on the format: /parent/child etc.
      default {

        # 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 router class must extend akka.routing.CustomRouterConfig and and have constructor
        #                with com.typesafe.config.Config 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 look the paths up using actorFor and route to
        #   them, 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 non-direct 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

        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 = []
        }

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

          # 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 to evaluate if 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

          # When the resizer reduce the capacity the abandoned routee actors are stopped
          # with PoisonPill after this delay. The reason for the delay is to give concurrent
          # messages a chance to be placed in mailbox before sending PoisonPill.
          # Use 0s to skip delay.
          stop-delay = 1s

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

    default-dispatcher {
      # Must be one of the following
      # Dispatcher, (BalancingDispatcher, only valid when all actors using it are of
      # the same type), PinnedDispatcher, or a FQCN to a class inheriting
      # MessageDispatcherConfigurator with a constructor with
      # com.typesafe.config.Config parameter and akka.dispatch.DispatcherPrerequisites
      # parameters.
      # PinnedDispatcher must be used toghether with executor=thread-pool-executor.
      type = "Dispatcher"

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

      # This will be used if you have set "executor = "fork-join-executor""
      fork-join-executor {
        # Min number of threads to cap factor-based parallelism number to
        parallelism-min = 8

        # Parallelism (threads) ... ceil(available processors * factor)
        parallelism-factor = 3.0

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

      # This will be used if you have set "executor = "thread-pool-executor""
      thread-pool-executor {
        # Keep alive time for threads
        keep-alive-time = 60s

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

        # No of core threads ... ceil(available processors * factor)
        core-pool-size-factor = 3.0

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

        # Hint: max-pool-size is only used for bounded task queues
        # minimum number of threads to cap factor-based max number to
        max-pool-size-min = 8

        # Max no of threads ... ceil(available processors * factor)
        max-pool-size-factor  = 3.0

        # Max number of threads to cap factor-based max 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

      # If negative (or zero) then an unbounded mailbox is used (default)
      # If positive then a bounded mailbox is used and the capacity is set using the
      # property
      # NOTE: setting a mailbox to 'blocking' can be a bit dangerous, could lead to
      # deadlock, use with care
      # The following mailbox-push-timeout-time is only used for type=Dispatcher and
      # only if mailbox-capacity > 0
      mailbox-capacity = -1

      # Specifies the timeout to add a new message to a mailbox that is full -
      # negative number means infinite timeout. It is only used for type=Dispatcher
      # and only if mailbox-capacity > 0
      mailbox-push-timeout-time = 10s

      # FQCN of the MailboxType, if not specified the default bounded or unbounded
      # mailbox is used. The Class of the FQCN must have a constructor with
      # (akka.actor.ActorSystem.Settings, com.typesafe.config.Config) parameters.
      mailbox-type = ""

      # 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

      # 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
    }

    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 and the like)
      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
    }

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

    # 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 {
      "java.io.Serializable" = java
    }
  }

  # 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!
  scheduler {
    # The HashedWheelTimer (HWT) implementation from Netty is used as the default scheduler
    # in the system.
    # HWT does not execute the scheduled tasks on exact time.
    # It will, on every tick, check if there are any tasks behind the schedule and execute them.
    # 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.
    # For more information see: http://www.jboss.org/netty/
    tick-duration = 100ms
    ticks-per-wheel = 512
  }
}

akka-remote

#####################################
# Akka Remote Reference Config File #
#####################################

# This is the reference config file that contains all the default settings.
# Make your edits/overrides in your application.conf.

# comments about akka.actor settings left out where they are already in akka-
# actor.jar, because otherwise they would be repeated in config rendering.

akka {

  actor {

    serializers {
      proto = "akka.serialization.ProtobufSerializer"
    }


    serialization-bindings {
      # Since com.google.protobuf.Message does not extend Serializable but GeneratedMessage
      # does, need to use the more specific one here in order to avoid ambiguity
      "com.google.protobuf.GeneratedMessage" = proto
    }

    deployment {

      default {

        # if this is set to a valid remote address, the named actor will be deployed
        # at that node e.g. "akka://sys@host:port"
        remote = ""

        target {

          # A list of hostnames and ports for instantiating the children of a
          # non-direct router
          #   The format should be on "akka://sys@host:port", where:
          #    - sys is the remote actor system name
          #    - hostname can be either hostname or IP address the remote actor
          #      should connect to
          #    - port should be the port for the remote server on the other node
          # The number of actor instances to be spawned is still taken from the
          # nr-of-instances setting as for local routers; the instances will be
          # distributed round-robin among the given nodes.
          nodes = []

        }
      }
    }
  }

  remote {

    # Which implementation of akka.remote.RemoteTransport to use
    # default is a TCP-based remote transport based on Netty
    transport = "akka.remote.netty.NettyRemoteTransport"

    # Enable untrusted mode for full security of server managed actors, allows
    # untrusted clients to connect.
    untrusted-mode = off

    # Timeout for ACK of cluster operations, like checking actor out etc.
    remote-daemon-ack-timeout = 30s

    # If this is "on", Akka will log all inbound messages at DEBUG level, if off then they are not logged
    log-received-messages = off

    # If this is "on", Akka will log all outbound messages at DEBUG level, if off then they are not logged
    log-sent-messages = off

    # Each property is annotated with (I) or (O) or (I&O), where I stands for “inbound” and O for “outbound” connections.
    # The NettyRemoteTransport always starts the server role to allow inbound connections, and it starts
    # active client connections whenever sending to a destination which is not yet connected; if configured
    # it reuses inbound connections for replies, which is called a passive client connection (i.e. from server
    # to client).
    netty {

      # (O) In case of increased latency / overflow how long should we wait (blocking the sender)
      # until we deem the send to be cancelled?
      # 0 means "never backoff", any positive number will indicate time to block at most.
      backoff-timeout = 0ms

      # (I&O) Generate your own with '$AKKA_HOME/scripts/generate_config_with_secure_cookie.sh'
      # or using 'akka.util.Crypt.generateSecureCookie'
      secure-cookie = ""

      # (I) Should the remote server require that its peers share the same secure-cookie
      # (defined in the 'remote' section)?
      require-cookie = off

      # (I) Reuse inbound connections for outbound messages
      use-passive-connections = on

      # (I) The hostname or ip to bind the remoting to,
      # InetAddress.getLocalHost.getHostAddress is used if empty
      hostname = ""

      # (I) The default remote server port clients should connect to.
      # Default is 2552 (AKKA), use 0 if you want a random available port
      port = 2552

      # (O) The address of a local network interface (IP Address) to bind to when creating
      # outbound connections. Set to "" or "auto" for automatic selection of local address.
      outbound-local-address = "auto"

      # (I&O) Increase this if you want to be able to send messages with large payloads
      message-frame-size = 1 MiB

      # (O) Timeout duration
      connection-timeout = 120s

      # (I) Sets the size of the connection backlog
      backlog = 4096

      # (I) Length in akka.time-unit how long core threads will be kept alive if idling
      execution-pool-keepalive = 60s

      # (I) Size of the core pool of the remote execution unit
      execution-pool-size = 4

      # (I) Maximum channel size, 0 for off
      max-channel-memory-size = 0b

      # (I) Maximum total size of all channels, 0 for off
      max-total-memory-size = 0b

      # (O) Time between reconnect attempts for active clients
      reconnect-delay = 5s

      # (O) Read inactivity period (lowest resolution is seconds)
      # after which active client connection is shutdown;
      # will be re-established in case of new communication requests.
      # A value of 0 will turn this feature off
      read-timeout = 0s

      # (O) Write inactivity period (lowest resolution is seconds)
      # after which a heartbeat is sent across the wire.
      # A value of 0 will turn this feature off
      write-timeout = 10s

      # (O) Inactivity period of both reads and writes (lowest resolution is seconds)
      # after which active client connection is shutdown;
      # will be re-established in case of new communication requests
      # A value of 0 will turn this feature off
      all-timeout = 0s

      # (O) Maximum time window that a client should try to reconnect for
      reconnection-time-window = 600s
    }

    # The dispatcher used for the system actor "network-event-sender"
    network-event-sender-dispatcher {
      executor = thread-pool-executor
      type = PinnedDispatcher
    }
  }
}

akka-testkit

######################################
# Akka Testkit Reference Config File #
######################################

# This is the reference config file that contains all the default settings.
# Make your edits/overrides in your application.conf.

akka {
  test {
    # factor by which to scale timeouts during tests, e.g. to account for shared
    # build system load
    timefactor =  1.0

    # duration of EventFilter.intercept waits after the block is finished until
    # all required messages are received
    filter-leeway = 3s

    # duration to wait in expectMsg and friends outside of within() block by default
    single-expect-default = 3s

    # The timeout that is added as an implicit by DefaultTimeout trait
    default-timeout = 5s

    calling-thread-dispatcher {
      type = akka.testkit.CallingThreadDispatcherConfigurator
    }
  }
}

akka-transactor

#########################################
# Akka Transactor Reference Config File #
#########################################

# This is the reference config file that contains all the default settings.
# Make your edits/overrides in your application.conf.

akka {
  transactor {
    # The timeout used for coordinated transactions across actors
    coordinated-timeout = 5s
  }
}

akka-agent

####################################
# Akka Agent Reference Config File #
####################################

# This is the reference config file that contains all the default settings.
# Make your edits/overrides in your application.conf.

akka {
  agent {

    # The dispatcher used for agent-send-off actor
    send-off-dispatcher {
      executor = thread-pool-executor
      type = PinnedDispatcher
    }

    # The dispatcher used for agent-alter-off actor
    alter-off-dispatcher {
      executor = thread-pool-executor
      type = PinnedDispatcher
    }
  }
}

akka-zeromq

#####################################
# Akka ZeroMQ Reference Config File #
#####################################

# This is the reference config file that contains all the default settings.
# Make your edits/overrides in your application.conf.

akka {

  zeromq {

    # The default timeout for a poll on the actual zeromq socket.
    poll-timeout = 100ms

    # Timeout for creating a new socket
    new-socket-timeout = 5s

    socket-dispatcher {
      # A zeromq socket needs to be pinned to the thread that created it.
      # Changing this value results in weird errors and race conditions within zeromq
      executor = thread-pool-executor
      type = "PinnedDispatcher"
    }
  }
}

akka-beanstalk-mailbox

##################################################
# Akka Beanstalk Mailboxes Reference Config File #
##################################################

# This is the reference config file that contains all the default settings.
# Make your edits/overrides in your application.conf.
#
# for more information see <https://github.com/kr/beanstalkd/blob/v1.3/doc/protocol.txt>

akka {
  actor {
    mailbox {
      beanstalk {
        # hostname to connect to
        hostname = "127.0.0.1"
        
        # port to connect to
        port = 11300
        
        # wait period in case of a connection failure before reconnect
        reconnect-window = 5s
        
        # integer number of seconds to wait before putting the job in
        # the ready queue. The job will be in the "delayed" state during this time.
        message-submit-delay = 0s
        
        # time to run -- is an integer number of seconds to allow a worker
        # to run this job. This time is counted from the moment a worker reserves
        # this job. If the worker does not delete, release, or bury the job within
        # <ttr> seconds, the job will time out and the server will release the job.
        message-time-to-live = 120s
      }
    }
  }

}

akka-file-mailbox

#############################################
# Akka File Mailboxes Reference Config File #
#############################################

# This is the reference config file that contains all the default settings.
# Make your edits/overrides in your application.conf.
#
# For more information see <https://github.com/robey/kestrel/>

akka {
  actor {
    mailbox {
      file-based {
        # directory below which this queue resides
        directory-path = "./_mb"
        
        # attempting to add an item after the queue reaches this size (in items) will fail.
        max-items = 2147483647
        
        # attempting to add an item after the queue reaches this size (in bytes) will fail.
        max-size = 2147483647 bytes
        
        # attempting to add an item larger than this size (in bytes) will fail.
        max-item-size = 2147483647 bytes
        
        # maximum expiration time for this queue (seconds).
        max-age = 0s
        
        # maximum journal size before the journal should be rotated.
        max-journal-size = 16 MiB
        
        # maximum size of a queue before it drops into read-behind mode.
        max-memory-size = 128 MiB
        
        # maximum overflow (multiplier) of a journal file before we re-create it.
        max-journal-overflow = 10
        
        # absolute maximum size of a journal file until we rebuild it, no matter what.
        max-journal-size-absolute = 9223372036854775807 bytes
        
        # whether to drop older items (instead of newer) when the queue is full
        discard-old-when-full = on  
        
        # whether to keep a journal file at all
        keep-journal = on  
        
        # whether to sync the journal after each transaction
        sync-journal = off  
      }
    }
  }
}

akka-mongo-mailbox

################################################
# Akka MongoDB Mailboxes Reference Config File #
################################################

# This is the reference config file that contains all the default settings.
# Make your edits/overrides in your application.conf.

akka {
  actor {
    mailbox {
      mongodb {

        # Any specified collection name will be used as a prefix for
        # collections that use durable mongo mailboxes.
        # Follow Mongo URI Spec - http://www.mongodb.org/display/DOCS/Connections
        uri = "mongodb://localhost/akka.mailbox"

        # Configurable timeouts for certain ops
        timeout {
          # time to wait for a read to succeed before timing out the future
          read = 3000ms
          # time to wait for a write to succeed before timing out the future
          write = 3000ms
        }
      }
    }
  }
}

akka-redis-mailbox

##############################################
# Akka Redis Mailboxes Reference Config File #
##############################################

# This is the reference config file that contains all the default settings.
# Make your edits/overrides in your application.conf.
#
# for more information see <http://redis.io/>

akka {
  actor {
    mailbox {
      redis {
        # hostname of where the redis queue resides
        hostname = "127.0.0.1"
        
        # port at which the redis queue resides
        port = 6379
      }
    }
  }
}

akka-zookeeper-mailbox

##################################################
# Akka ZooKepper Mailboxes Reference Config File #
##################################################

# This is the reference config file that contains all the default settings.
# Make your edits/overrides in your application.conf.
#
# For more information see <http://wiki.apache.org/hadoop/ZooKeeper>

akka {
  actor {
    mailbox {
      zookeeper {
        # host and port to connect to ZooKeeper
        server-addresses = "127.0.0.1:2181"
        
        # timeout after which an unreachable client is considered dead and its session is closed
        session-timeout = 60s
        
        # maximum wait period while connecting to ZooKeeper service
        connection-timeout = 60s
      }
    }
  }
}

Custom application.conf

A custom application.conf might look like this:

# In this file you can override any option defined in the reference files.
# Copy in parts of the reference files and modify as you please.

akka {

  # Event handlers to register at boot time (Logging$DefaultLogger logs to STDOUT)
  event-handlers = ["akka.event.slf4j.Slf4jEventHandler"]

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

  # Log level for the very basic logger activated during AkkaApplication startup
  # Options: ERROR, WARNING, INFO, DEBUG
  stdout-loglevel = DEBUG

  actor {
    default-dispatcher {
      # Throughput for default Dispatcher, set to 1 for as fair as possible
      throughput = 10
    }
  }

  remote {
    server {
      # The port clients should connect to. Default is 2552 (AKKA)
      port = 2562
    }
  }
}

Including files

Sometimes it can be useful to include another configuration file, for example if you have one application.conf with all environment independent settings and then override some settings for specific environments.

Specifying system property with -Dconfig.resource=/dev.conf will load the dev.conf file, which includes the application.conf

dev.conf:

include "application"

akka {
  loglevel = "DEBUG"
}

More advanced include and substitution mechanisms are explained in the HOCON specification.

Logging of Configuration

If the system or config property akka.log-config-on-start is set to on, then the complete configuration at INFO level when the actor system is started. This is useful when you are uncertain of what configuration is used.

If in doubt, you can also easily and nicely inspect configuration objects before or after using them to construct an actor system:

Welcome to Scala version 2.9.1.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_27).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import com.typesafe.config._
import com.typesafe.config._

scala> ConfigFactory.parseString("a.b=12")
res0: com.typesafe.config.Config = Config(SimpleConfigObject({"a" : {"b" : 12}}))

scala> res0.root.render
res1: java.lang.String =
{
    # String: 1
    "a" : {
        # String: 1
        "b" : 12
    }
}

The comments preceding every item give detailed information about the origin of the setting (file & line number) plus possible comments which were present, e.g. in the reference configuration. The settings as merged with the reference and parsed by the actor system can be displayed like this:

final ActorSystem system = ActorSystem.create();
println(system.settings());
// this is a shortcut for system.settings().config().root().render()

A Word About ClassLoaders

In several places of the configuration file it is possible to specify the fully-qualified class name of something to be instantiated by Akka. This is done using Java reflection, which in turn uses a ClassLoader. Getting the right one in challenging environments like application containers or OSGi bundles is not always trivial, the current approach of Akka is that each ActorSystem implementation stores the current thread’s context class loader (if available, otherwise just its own loader as in this.getClass.getClassLoader) and uses that for all reflective accesses. This implies that putting Akka on the boot class path will yield NullPointerException from strange places: this is simply not supported.

Application specific settings

The configuration can also be used for application specific settings. A good practice is to place those settings in an Extension, as described in:

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