Routing - Version 2.5-SNAPSHOT


Messages can be sent via a router to efficiently route them to destination actors, known as its routees. A Router can be used inside or outside of an actor, and you can manage the routees yourselves or use a self contained router actor with configuration capabilities.

Different routing strategies can be used, according to your application's needs. Akka comes with several useful routing strategies right out of the box. But, as you will see in this chapter, it is also possible to create your own.

A Simple Router

The following example illustrates how to use a Router and manage the routees from within an actor.

import akka.routing.{ ActorRefRoutee, RoundRobinRoutingLogic, Router }

class Master extends Actor {
  var router = {
    val routees = Vector.fill(5) {
      val r = context.actorOf(Props[Worker])
      context watch r
    Router(RoundRobinRoutingLogic(), routees)

  def receive = {
    case w: Work =>
      router.route(w, sender())
    case Terminated(a) =>
      router = router.removeRoutee(a)
      val r = context.actorOf(Props[Worker])
      context watch r
      router = router.addRoutee(r)

We create a Router and specify that it should use RoundRobinRoutingLogic when routing the messages to the routees.

The routing logic shipped with Akka are:

  • akka.routing.RoundRobinRoutingLogic
  • akka.routing.RandomRoutingLogic
  • akka.routing.SmallestMailboxRoutingLogic
  • akka.routing.BroadcastRoutingLogic
  • akka.routing.ScatterGatherFirstCompletedRoutingLogic
  • akka.routing.TailChoppingRoutingLogic
  • akka.routing.ConsistentHashingRoutingLogic

We create the routees as ordinary child actors wrapped in ActorRefRoutee. We watch the routees to be able to replace them if they are terminated.

Sending messages via the router is done with the route method, as is done for the Work messages in the example above.

The Router is immutable and the RoutingLogic is thread safe; meaning that they can also be used outside of actors.


In general, any message sent to a router will be sent onwards to its routees, but there is one exception. The special Broadcast Messages will send to all of a router's routees. However, do not use Broadcast Messages when you use BalancingPool for routees as described in Specially Handled Messages.

A Router Actor

A router can also be created as a self contained actor that manages the routees itself and loads routing logic and other settings from configuration.

This type of router actor comes in two distinct flavors:

  • Pool - The router creates routees as child actors and removes them from the router if they terminate.
  • Group - The routee actors are created externally to the router and the router sends messages to the specified path using actor selection, without watching for termination.

The settings for a router actor can be defined in configuration or programmatically. In order to make an actor to make use of an externally configurable router the FromConfig props wrapper must be used to denote that the actor accepts routing settings from configuration. This is in contrast with Remote Deployment where such marker props is not necessary. If the props of an actor is NOT wrapped in FromConfig it will ignore the router section of the deployment configuration.

You send messages to the routees via the router actor in the same way as for ordinary actors, i.e. via its ActorRef. The router actor forwards messages onto its routees without changing the original sender. When a routee replies to a routed message, the reply will be sent to the original sender, not to the router actor.


In general, any message sent to a router will be sent onwards to its routees, but there are a few exceptions. These are documented in the Specially Handled Messages section below.


The following code and configuration snippets show how to create a round-robin router that forwards messages to five Worker routees. The routees will be created as the router's children. {
  /parent/router1 {
    router = round-robin-pool
    nr-of-instances = 5
val router1: ActorRef =
  context.actorOf(FromConfig.props(Props[Worker]), "router1")

Here is the same example, but with the router configuration provided programmatically instead of from configuration.

val router2: ActorRef =
  context.actorOf(RoundRobinPool(5).props(Props[Worker]), "router2")

Remote Deployed Routees

In addition to being able to create local actors as routees, you can instruct the router to deploy its created children on a set of remote hosts. Routees will be deployed in round-robin fashion. In order to deploy routees remotely, wrap the router configuration in a RemoteRouterConfig, attaching the remote addresses of the nodes to deploy to. Remote deployment requires the akka-remote module to be included in the classpath.

import{ Address, AddressFromURIString }
import akka.remote.routing.RemoteRouterConfig
val addresses = Seq(
  Address("akka.tcp", "remotesys", "otherhost", 1234),
val routerRemote = system.actorOf(
  RemoteRouterConfig(RoundRobinPool(5), addresses).props(Props[Echo]))


By default, when a routee sends a message, it will implicitly set itself as the sender.

sender() ! x // replies will go to this actor

However, it is often useful for routees to set the router as a sender. For example, you might want to set the router as the sender if you want to hide the details of the routees behind the router. The following code snippet shows how to set the parent router as sender.

sender().tell("reply", context.parent) // replies will go back to parent
sender().!("reply")(context.parent) // alternative syntax (beware of the parens!)


Routees that are created by a pool router will be created as the router's children. The router is therefore also the children's supervisor.

The supervision strategy of the router actor can be configured with the supervisorStrategy property of the Pool. If no configuration is provided, routers default to a strategy of “always escalate”. This means that errors are passed up to the router's supervisor for handling. The router's supervisor will decide what to do about any errors.

Note the router's supervisor will treat the error as an error with the router itself. Therefore a directive to stop or restart will cause the router itself to stop or restart. The router, in turn, will cause its children to stop and restart.

It should be mentioned that the router's restart behavior has been overridden so that a restart, while still re-creating the children, will still preserve the same number of actors in the pool.

This means that if you have not specified supervisorStrategy of the router or its parent a failure in a routee will escalate to the parent of the router, which will by default restart the router, which will restart all routees (it uses Escalate and does not stop routees during restart). The reason is to make the default behave such that adding withRouter to a child’s definition does not change the supervision strategy applied to the child. This might be an inefficiency that you can avoid by specifying the strategy when defining the router.

Setting the strategy is easily done:

val escalator = OneForOneStrategy() {
  case e  testActor ! e; SupervisorStrategy.Escalate
val router = system.actorOf(RoundRobinPool(1, supervisorStrategy = escalator).props(
  routeeProps = Props[TestActor]))


If the child of a pool router terminates, the pool router will not automatically spawn a new child. In the event that all children of a pool router have terminated the router will terminate itself unless it is a dynamic router, e.g. using a resizer.


Sometimes, rather than having the router actor create its routees, it is desirable to create routees separately and provide them to the router for its use. You can do this by passing an paths of the routees to the router's configuration. Messages will be sent with ActorSelection to these paths.

The example below shows how to create a router by providing it with the path strings of three routee actors. {
  /parent/router3 {
    router = round-robin-group
    routees.paths = ["/user/workers/w1", "/user/workers/w2", "/user/workers/w3"]
val router3: ActorRef =
  context.actorOf(FromConfig.props(), "router3")

Here is the same example, but with the router configuration provided programmatically instead of from configuration.

val router4: ActorRef =
  context.actorOf(RoundRobinGroup(paths).props(), "router4")

The routee actors are created externally from the router:

system.actorOf(Props[Workers], "workers")
class Workers extends Actor {
  context.actorOf(Props[Worker], name = "w1")
  context.actorOf(Props[Worker], name = "w2")
  context.actorOf(Props[Worker], name = "w3")
  // ...

The paths may contain protocol and address information for actors running on remote hosts. Remoting requires the akka-remote module to be included in the classpath. {
  /parent/remoteGroup {
    router = round-robin-group
    routees.paths = [

Router usage

In this section we will describe how to create the different types of router actors.

The router actors in this section are created from within a top level actor named parent. Note that deployment paths in the configuration starts with /parent/ followed by the name of the router actor.

system.actorOf(Props[Parent], "parent")

RoundRobinPool and RoundRobinGroup

Routes in a round-robin fashion to its routees.

RoundRobinPool defined in configuration: {
  /parent/router1 {
    router = round-robin-pool
    nr-of-instances = 5
val router1: ActorRef =
  context.actorOf(FromConfig.props(Props[Worker]), "router1")

RoundRobinPool defined in code:

val router2: ActorRef =
  context.actorOf(RoundRobinPool(5).props(Props[Worker]), "router2")

RoundRobinGroup defined in configuration: {
  /parent/router3 {
    router = round-robin-group
    routees.paths = ["/user/workers/w1", "/user/workers/w2", "/user/workers/w3"]
val router3: ActorRef =
  context.actorOf(FromConfig.props(), "router3")

RoundRobinGroup defined in code:

val paths = List("/user/workers/w1", "/user/workers/w2", "/user/workers/w3")
val router4: ActorRef =
  context.actorOf(RoundRobinGroup(paths).props(), "router4")

RandomPool and RandomGroup

This router type selects one of its routees randomly for each message.

RandomPool defined in configuration: {
  /parent/router5 {
    router = random-pool
    nr-of-instances = 5
val router5: ActorRef =
  context.actorOf(FromConfig.props(Props[Worker]), "router5")

RandomPool defined in code:

val router6: ActorRef =
  context.actorOf(RandomPool(5).props(Props[Worker]), "router6")

RandomGroup defined in configuration: {
  /parent/router7 {
    router = random-group
    routees.paths = ["/user/workers/w1", "/user/workers/w2", "/user/workers/w3"]
val router7: ActorRef =
  context.actorOf(FromConfig.props(), "router7")

RandomGroup defined in code:

val paths = List("/user/workers/w1", "/user/workers/w2", "/user/workers/w3")
val router8: ActorRef =
  context.actorOf(RandomGroup(paths).props(), "router8")


A Router that will try to redistribute work from busy routees to idle routees. All routees share the same mailbox.


The BalancingPool has the property that its routees do not have truly distinct identity: they have different names, but talking to them will not end up at the right actor in most cases. Therefore you cannot use it for workflows that require state to be kept within the routee, you would in this case have to include the whole state in the messages.

With a SmallestMailboxPool you can have a vertically scaling service that can interact in a stateful fashion with other services in the back-end before replying to the original client. The other advantage is that it does not place a restriction on the message queue implementation as BalancingPool does.


Do not use Broadcast Messages when you use BalancingPool for routers. as described in Specially Handled Messages,

BalancingPool defined in configuration: {
  /parent/router9 {
    router = balancing-pool
    nr-of-instances = 5
val router9: ActorRef =
  context.actorOf(FromConfig.props(Props[Worker]), "router9")

BalancingPool defined in code:

val router10: ActorRef =
  context.actorOf(BalancingPool(5).props(Props[Worker]), "router10")

Addition configuration for the balancing dispatcher, which is used by the pool, can be configured in the pool-dispatcher section of the router deployment configuration. {
  /parent/router9b {
    router = balancing-pool
    nr-of-instances = 5
    pool-dispatcher {
      attempt-teamwork = off

The BalancingPool automatically uses a special BalancingDispatcher for its routees - disregarding any dispatcher that is set on the routee Props object. This is needed in order to implement the balancing semantics via sharing the same mailbox by all the routees.

While it is not possible to change the dispatcher used by the routees, it is possible to fine tune the used executor. By default the fork-join-dispatcher is used and can be configured as explained in Dispatchers. In situations where the routees are expected to perform blocking operations it may be useful to replace it with a thread-pool-executor hinting the number of allocated threads explicitly: {
  /parent/router10b {
    router = balancing-pool
    nr-of-instances = 5
    pool-dispatcher {
      executor = "thread-pool-executor"

      # allocate exactly 5 threads for this pool
      thread-pool-executor {
        core-pool-size-min = 5
        core-pool-size-max = 5

It is also possible to change the mailbox used by the balancing dispatcher for scenarios where the default unbounded mailbox is not well suited. An example of such a scenario could arise whether there exists the need to manage priority for each message. You can then implement a priority mailbox and configure your dispatcher: {
  /parent/router10c {
    router = balancing-pool
    nr-of-instances = 5
    pool-dispatcher {
      mailbox = myapp.myprioritymailbox


Bear in mind that BalancingDispatcher requires a message queue that must be thread-safe for multiple concurrent consumers. So it is mandatory for the message queue backing a custom mailbox for this kind of dispatcher to implement akka.dispatch.MultipleConsumerSemantics. See details on how to implement your custom mailbox in Mailboxes.

There is no Group variant of the BalancingPool.


A Router that tries to send to the non-suspended child routee with fewest messages in mailbox. The selection is done in this order:

  • pick any idle routee (not processing message) with empty mailbox
  • pick any routee with empty mailbox
  • pick routee with fewest pending messages in mailbox
  • pick any remote routee, remote actors are consider lowest priority, since their mailbox size is unknown

SmallestMailboxPool defined in configuration: {
  /parent/router11 {
    router = smallest-mailbox-pool
    nr-of-instances = 5
val router11: ActorRef =
  context.actorOf(FromConfig.props(Props[Worker]), "router11")

SmallestMailboxPool defined in code:

val router12: ActorRef =
  context.actorOf(SmallestMailboxPool(5).props(Props[Worker]), "router12")

There is no Group variant of the SmallestMailboxPool because the size of the mailbox and the internal dispatching state of the actor is not practically available from the paths of the routees.

BroadcastPool and BroadcastGroup

A broadcast router forwards the message it receives to all its routees.

BroadcastPool defined in configuration: {
  /parent/router13 {
    router = broadcast-pool
    nr-of-instances = 5
val router13: ActorRef =
  context.actorOf(FromConfig.props(Props[Worker]), "router13")

BroadcastPool defined in code:

val router14: ActorRef =
  context.actorOf(BroadcastPool(5).props(Props[Worker]), "router14")

BroadcastGroup defined in configuration: {
  /parent/router15 {
    router = broadcast-group
    routees.paths = ["/user/workers/w1", "/user/workers/w2", "/user/workers/w3"]
val router15: ActorRef =
  context.actorOf(FromConfig.props(), "router15")

BroadcastGroup defined in code:

val paths = List("/user/workers/w1", "/user/workers/w2", "/user/workers/w3")
val router16: ActorRef =
  context.actorOf(BroadcastGroup(paths).props(), "router16")


Broadcast routers always broadcast every message to their routees. If you do not want to broadcast every message, then you can use a non-broadcasting router and use Broadcast Messages as needed.

ScatterGatherFirstCompletedPool and ScatterGatherFirstCompletedGroup

The ScatterGatherFirstCompletedRouter will send the message on to all its routees. It then waits for first reply it gets back. This result will be sent back to original sender. Other replies are discarded.

It is expecting at least one reply within a configured duration, otherwise it will reply with akka.pattern.AskTimeoutException in a

ScatterGatherFirstCompletedPool defined in configuration: {
  /parent/router17 {
    router = scatter-gather-pool
    nr-of-instances = 5
    within = 10 seconds
val router17: ActorRef =
  context.actorOf(FromConfig.props(Props[Worker]), "router17")

ScatterGatherFirstCompletedPool defined in code:

val router18: ActorRef =
  context.actorOf(ScatterGatherFirstCompletedPool(5, within = 10.seconds).
    props(Props[Worker]), "router18")

ScatterGatherFirstCompletedGroup defined in configuration: {
  /parent/router19 {
    router = scatter-gather-group
    routees.paths = ["/user/workers/w1", "/user/workers/w2", "/user/workers/w3"]
    within = 10 seconds
val router19: ActorRef =
  context.actorOf(FromConfig.props(), "router19")

ScatterGatherFirstCompletedGroup defined in code:

val paths = List("/user/workers/w1", "/user/workers/w2", "/user/workers/w3")
val router20: ActorRef =
    within = 10.seconds).props(), "router20")

TailChoppingPool and TailChoppingGroup

The TailChoppingRouter will first send the message to one, randomly picked, routee and then after a small delay to a second routee (picked randomly from the remaining routees) and so on. It waits for first reply it gets back and forwards it back to original sender. Other replies are discarded.

The goal of this router is to decrease latency by performing redundant queries to multiple routees, assuming that one of the other actors may still be faster to respond than the initial one.

This optimisation was described nicely in a blog post by Peter Bailis: Doing redundant work to speed up distributed queries.

TailChoppingPool defined in configuration: {
  /parent/router21 {
    router = tail-chopping-pool
    nr-of-instances = 5
    within = 10 seconds
    tail-chopping-router.interval = 20 milliseconds
val router21: ActorRef =
  context.actorOf(FromConfig.props(Props[Worker]), "router21")

TailChoppingPool defined in code:

val router22: ActorRef =
  context.actorOf(TailChoppingPool(5, within = 10.seconds, interval = 20.millis).
    props(Props[Worker]), "router22")

TailChoppingGroup defined in configuration: {
  /parent/router23 {
    router = tail-chopping-group
    routees.paths = ["/user/workers/w1", "/user/workers/w2", "/user/workers/w3"]
    within = 10 seconds
    tail-chopping-router.interval = 20 milliseconds
val router23: ActorRef =
  context.actorOf(FromConfig.props(), "router23")

TailChoppingGroup defined in code:

val paths = List("/user/workers/w1", "/user/workers/w2", "/user/workers/w3")
val router24: ActorRef =
    within = 10.seconds, interval = 20.millis).props(), "router24")

ConsistentHashingPool and ConsistentHashingGroup

The ConsistentHashingPool uses consistent hashing to select a routee based on the sent message. This article gives good insight into how consistent hashing is implemented.

There is 3 ways to define what data to use for the consistent hash key.

  • You can define hashMapping of the router to map incoming messages to their consistent hash key. This makes the decision transparent for the sender.
  • The messages may implement akka.routing.ConsistentHashingRouter.ConsistentHashable. The key is part of the message and it's convenient to define it together with the message definition.
  • The messages can be wrapped in a akka.routing.ConsistentHashingRouter.ConsistentHashableEnvelope to define what data to use for the consistent hash key. The sender knows the key to use.

These ways to define the consistent hash key can be use together and at the same time for one router. The hashMapping is tried first.

Code example:

import akka.routing.ConsistentHashingRouter.ConsistentHashable

class Cache extends Actor {
  var cache = Map.empty[String, String]

  def receive = {
    case Entry(key, value) => cache += (key -> value)
    case Get(key)          => sender() ! cache.get(key)
    case Evict(key)        => cache -= key

final case class Evict(key: String)

final case class Get(key: String) extends ConsistentHashable {
  override def consistentHashKey: Any = key

final case class Entry(key: String, value: String)
import akka.routing.ConsistentHashingPool
import akka.routing.ConsistentHashingRouter.ConsistentHashMapping
import akka.routing.ConsistentHashingRouter.ConsistentHashableEnvelope

def hashMapping: ConsistentHashMapping = {
  case Evict(key) => key

val cache: ActorRef =
  context.actorOf(ConsistentHashingPool(10, hashMapping = hashMapping).
    props(Props[Cache]), name = "cache")

cache ! ConsistentHashableEnvelope(
  message = Entry("hello", "HELLO"), hashKey = "hello")
cache ! ConsistentHashableEnvelope(
  message = Entry("hi", "HI"), hashKey = "hi")

cache ! Get("hello")

cache ! Get("hi")

cache ! Evict("hi")
cache ! Get("hi")

In the above example you see that the Get message implements ConsistentHashable itself, while the Entry message is wrapped in a ConsistentHashableEnvelope. The Evict message is handled by the hashMapping partial function.

ConsistentHashingPool defined in configuration: {
  /parent/router25 {
    router = consistent-hashing-pool
    nr-of-instances = 5
    virtual-nodes-factor = 10
val router25: ActorRef =
  context.actorOf(FromConfig.props(Props[Worker]), "router25")

ConsistentHashingPool defined in code:

val router26: ActorRef =

ConsistentHashingGroup defined in configuration: {
  /parent/router27 {
    router = consistent-hashing-group
    routees.paths = ["/user/workers/w1", "/user/workers/w2", "/user/workers/w3"]
    virtual-nodes-factor = 10
val router27: ActorRef =
  context.actorOf(FromConfig.props(), "router27")

ConsistentHashingGroup defined in code:

val paths = List("/user/workers/w1", "/user/workers/w2", "/user/workers/w3")
val router28: ActorRef =
  context.actorOf(ConsistentHashingGroup(paths).props(), "router28")

virtual-nodes-factor is the number of virtual nodes per routee that is used in the consistent hash node ring to make the distribution more uniform.

Specially Handled Messages

Most messages sent to router actors will be forwarded according to the routers' routing logic. However there are a few types of messages that have special behavior.

Note that these special messages, except for the Broadcast message, are only handled by self contained router actors and not by the akka.routing.Router component described in A Simple Router.

Broadcast Messages

A Broadcast message can be used to send a message to all of a router's routees. When a router receives a Broadcast message, it will broadcast that message's payload to all routees, no matter how that router would normally route its messages.

The example below shows how you would use a Broadcast message to send a very important message to every routee of a router.

import akka.routing.Broadcast
router ! Broadcast("Watch out for Davy Jones' locker")

In this example the router receives the Broadcast message, extracts its payload ("Watch out for Davy Jones' locker"), and then sends the payload on to all of the router's routees. It is up to each routee actor to handle the received payload message.


Do not use Broadcast Messages when you use BalancingPool for routers. Routees on BalancingPool shares the same mailbox instance, thus some routees can possibly get the broadcast message multiple times, while other routees get no broadcast message.

PoisonPill Messages

A PoisonPill message has special handling for all actors, including for routers. When any actor receives a PoisonPill message, that actor will be stopped. See the PoisonPill documentation for details.

router ! PoisonPill

For a router, which normally passes on messages to routees, it is important to realise that PoisonPill messages are processed by the router only. PoisonPill messages sent to a router will not be sent on to routees.

However, a PoisonPill message sent to a router may still affect its routees, because it will stop the router and when the router stops it also stops its children. Stopping children is normal actor behavior. The router will stop routees that it has created as children. Each child will process its current message and then stop. This may lead to some messages being unprocessed. See the documentation on Stopping actors for more information.

If you wish to stop a router and its routees, but you would like the routees to first process all the messages currently in their mailboxes, then you should not send a PoisonPill message to the router. Instead you should wrap a PoisonPill message inside a Broadcast message so that each routee will receive the PoisonPill message. Note that this will stop all routees, even if the routees aren't children of the router, i.e. even routees programmatically provided to the router.

import akka.routing.Broadcast
router ! Broadcast(PoisonPill)

With the code shown above, each routee will receive a PoisonPill message. Each routee will continue to process its messages as normal, eventually processing the PoisonPill. This will cause the routee to stop. After all routees have stopped the router will itself be stopped automatically unless it is a dynamic router, e.g. using a resizer.


Brendan W McAdams' excellent blog post Distributing Akka Workloads - And Shutting Down Afterwards discusses in more detail how PoisonPill messages can be used to shut down routers and routees.

Kill Messages

Kill messages are another type of message that has special handling. See Killing an Actor for general information about how actors handle Kill messages.

When a Kill message is sent to a router the router processes the message internally, and does not send it on to its routees. The router will throw an ActorKilledException and fail. It will then be either resumed, restarted or terminated, depending how it is supervised.

Routees that are children of the router will also be suspended, and will be affected by the supervision directive that is applied to the router. Routees that are not the routers children, i.e. those that were created externally to the router, will not be affected.

router ! Kill

As with the PoisonPill message, there is a distinction between killing a router, which indirectly kills its children (who happen to be routees), and killing routees directly (some of whom may not be children.) To kill routees directly the router should be sent a Kill message wrapped in a Broadcast message.

import akka.routing.Broadcast
router ! Broadcast(Kill)

Management Messages

  • Sending akka.routing.GetRoutees to a router actor will make it send back its currently used routees in a akka.routing.Routees message.
  • Sending akka.routing.AddRoutee to a router actor will add that routee to its collection of routees.
  • Sending akka.routing.RemoveRoutee to a router actor will remove that routee to its collection of routees.
  • Sending akka.routing.AdjustPoolSize to a pool router actor will add or remove that number of routees to its collection of routees.

These management messages may be handled after other messages, so if you send AddRoutee immediately followed by an ordinary message you are not guaranteed that the routees have been changed when the ordinary message is routed. If you need to know when the change has been applied you can send AddRoutee followed by GetRoutees and when you receive the Routees reply you know that the preceding change has been applied.

Dynamically Resizable Pool

Most pools can be used with a fixed number of routees or with a resize strategy to adjust the number of routees dynamically.

There are two types of resizers: the default Resizer and the OptimalSizeExploringResizer.

Default Resizer

The default resizer ramps up and down pool size based on pressure, measured by the percentage of busy routees in the pool. It ramps up pool size if the pressure is higher than a certain threshold and backs off if the pressure is lower than certain threshold. Both thresholds are configurable.

Pool with default resizer defined in configuration: {
  /parent/router29 {
    router = round-robin-pool
    resizer {
      lower-bound = 2
      upper-bound = 15
      messages-per-resize = 100
val router29: ActorRef =
  context.actorOf(FromConfig.props(Props[Worker]), "router29")

Several more configuration options are available and described in section of the reference Configuration.

Pool with resizer defined in code:

val resizer = DefaultResizer(lowerBound = 2, upperBound = 15)
val router30: ActorRef =
    RoundRobinPool(5, Some(resizer)).props(Props[Worker]),

It is also worth pointing out that if you define the ``router`` in the configuration file then this value will be used instead of any programmatically sent parameters.

Optimal Size Exploring Resizer

The OptimalSizeExploringResizer resizes the pool to an optimal size that provides the most message throughput.

This resizer works best when you expect the pool size to performance function to be a convex function. For example, when you have a CPU bound tasks, the optimal size is bound to the number of CPU cores. When your task is IO bound, the optimal size is bound to optimal number of concurrent connections to that IO service - e.g. a 4 node elastic search cluster may handle 4-8 concurrent requests at optimal speed.

It achieves this by keeping track of message throughput at each pool size and performing the following three resizing operations (one at a time) periodically:

  • Downsize if it hasn't seen all routees ever fully utilized for a period of time.
  • Explore to a random nearby pool size to try and collect throughput metrics.
  • Optimize to a nearby pool size with a better (than any other nearby sizes) throughput metrics.

When the pool is fully-utilized (i.e. all routees are busy), it randomly choose between exploring and optimizing. When the pool has not been fully-utilized for a period of time, it will downsize the pool to the last seen max utilization multiplied by a configurable ratio.

By constantly exploring and optimizing, the resizer will eventually walk to the optimal size and remain nearby. When the optimal size changes it will start walking towards the new one.

It keeps a performance log so it's stateful as well as having a larger memory footprint than the default Resizer. The memory usage is O(n) where n is the number of sizes you allow, i.e. upperBound - lowerBound.

Pool with OptimalSizeExploringResizer defined in configuration: {
  /parent/router31 {
    router = round-robin-pool
    optimal-size-exploring-resizer {
      enabled = on
      action-interval = 5s
      downsize-after-underutilized-for = 72h
val router31: ActorRef =
  context.actorOf(FromConfig.props(Props[Worker]), "router31")

Several more configuration options are available and described in section of the reference Configuration.


Resizing is triggered by sending messages to the actor pool, but it is not completed synchronously; instead a message is sent to the “head” RouterActor to perform the size change. Thus you cannot rely on resizing to instantaneously create new workers when all others are busy, because the message just sent will be queued to the mailbox of a busy actor. To remedy this, configure the pool to use a balancing dispatcher, see Configuring Dispatchers for more information.

How Routing is Designed within Akka

On the surface routers look like normal actors, but they are actually implemented differently. Routers are designed to be extremely efficient at receiving messages and passing them quickly on to routees.

A normal actor can be used for routing messages, but an actor's single-threaded processing can become a bottleneck. Routers can achieve much higher throughput with an optimization to the usual message-processing pipeline that allows concurrent routing. This is achieved by embedding routers' routing logic directly in their ActorRef rather than in the router actor. Messages sent to a router's ActorRef can be immediately routed to the routee, bypassing the single-threaded router actor entirely.

The cost to this is, of course, that the internals of routing code are more complicated than if routers were implemented with normal actors. Fortunately all of this complexity is invisible to consumers of the routing API. However, it is something to be aware of when implementing your own routers.

Custom Router

You can create your own router should you not find any of the ones provided by Akka sufficient for your needs. In order to roll your own router you have to fulfill certain criteria which are explained in this section.

Before creating your own router you should consider whether a normal actor with router-like behavior might do the job just as well as a full-blown router. As explained above, the primary benefit of routers over normal actors is their higher performance. But they are somewhat more complicated to write than normal actors. Therefore if lower maximum throughput is acceptable in your application you may wish to stick with traditional actors. This section, however, assumes that you wish to get maximum performance and so demonstrates how you can create your own router.

The router created in this example is replicating each message to a few destinations.

Start with the routing logic:

import scala.collection.immutable
import java.util.concurrent.ThreadLocalRandom
import akka.routing.RoundRobinRoutingLogic
import akka.routing.RoutingLogic
import akka.routing.Routee
import akka.routing.SeveralRoutees

class RedundancyRoutingLogic(nbrCopies: Int) extends RoutingLogic {
  val roundRobin = RoundRobinRoutingLogic()
  def select(message: Any, routees: immutable.IndexedSeq[Routee]): Routee = {
    val targets = (1 to nbrCopies).map(_ =>, routees))

select will be called for each message and in this example pick a few destinations by round-robin, by reusing the existing RoundRobinRoutingLogic and wrap the result in a SeveralRoutees instance. SeveralRoutees will send the message to all of the supplied routes.

The implementation of the routing logic must be thread safe, since it might be used outside of actors.

A unit test of the routing logic:

final case class TestRoutee(n: Int) extends Routee {
  override def send(message: Any, sender: ActorRef): Unit = ()

  val logic = new RedundancyRoutingLogic(nbrCopies = 3)

  val routees = for (n <- 1 to 7) yield TestRoutee(n)

  val r1 ="msg", routees)
  r1.asInstanceOf[SeveralRoutees].routees should be(
    Vector(TestRoutee(1), TestRoutee(2), TestRoutee(3)))

  val r2 ="msg", routees)
  r2.asInstanceOf[SeveralRoutees].routees should be(
    Vector(TestRoutee(4), TestRoutee(5), TestRoutee(6)))

  val r3 ="msg", routees)
  r3.asInstanceOf[SeveralRoutees].routees should be(
    Vector(TestRoutee(7), TestRoutee(1), TestRoutee(2)))

You could stop here and use the RedundancyRoutingLogic with a akka.routing.Router as described in A Simple Router.

Let us continue and make this into a self contained, configurable, router actor.

Create a class that extends Pool, Group or CustomRouterConfig. That class is a factory for the routing logic and holds the configuration for the router. Here we make it a Group.

import akka.dispatch.Dispatchers
import akka.routing.Group
import akka.routing.Router
import akka.japi.Util.immutableSeq
import com.typesafe.config.Config

final case class RedundancyGroup(routeePaths: immutable.Iterable[String], nbrCopies: Int) extends Group {

  def this(config: Config) = this(
    routeePaths = immutableSeq(config.getStringList("routees.paths")),
    nbrCopies = config.getInt("nbr-copies"))

  override def paths(system: ActorSystem): immutable.Iterable[String] = routeePaths

  override def createRouter(system: ActorSystem): Router =
    new Router(new RedundancyRoutingLogic(nbrCopies))

  override val routerDispatcher: String = Dispatchers.DefaultDispatcherId

This can be used exactly as the router actors provided by Akka.

for (n <- 1 to 10) system.actorOf(Props[Storage], "s" + n)

val paths = for (n <- 1 to 10) yield ("/user/s" + n)
val redundancy1: ActorRef =
    RedundancyGroup(paths, nbrCopies = 3).props(),
    name = "redundancy1")
redundancy1 ! "important"

Note that we added a constructor in RedundancyGroup that takes a Config parameter. That makes it possible to define it in configuration. {
  /redundancy2 {
    router = "docs.routing.RedundancyGroup"
    routees.paths = ["/user/s1", "/user/s2", "/user/s3"]
    nbr-copies = 5

Note the fully qualified class name in the router property. The router class must extend akka.routing.RouterConfig (Pool, Group or CustomRouterConfig) and have constructor with one com.typesafe.config.Config parameter. The deployment section of the configuration is passed to the constructor.

val redundancy2: ActorRef = system.actorOf(
  name = "redundancy2")
redundancy2 ! "very important"

Configuring Dispatchers

The dispatcher for created children of the pool will be taken from Props as described in Dispatchers.

To make it easy to define the dispatcher of the routees of the pool you can define the dispatcher inline in the deployment section of the config. {
  /poolWithDispatcher {
    router = random-pool
    nr-of-instances = 5
    pool-dispatcher {
      fork-join-executor.parallelism-min = 5
      fork-join-executor.parallelism-max = 5

That is the only thing you need to do enable a dedicated dispatcher for a pool.


If you use a group of actors and route to their paths, then they will still use the same dispatcher that was configured for them in their Props, it is not possible to change an actors dispatcher after it has been created.

The “head” router cannot always run on the same dispatcher, because it does not process the same type of messages, hence this special actor does not use the dispatcher configured in Props, but takes the routerDispatcher from the RouterConfig instead, which defaults to the actor system’s default dispatcher. All standard routers allow setting this property in their constructor or factory method, custom routers have to implement the method in a suitable way.

val router: ActorRef = system.actorOf(
  // “head” router actor will run on "router-dispatcher" dispatcher
  // Worker routees will run on "pool-dispatcher" dispatcher
  RandomPool(5, routerDispatcher = "router-dispatcher").props(Props[Worker]),
  name = "poolWithDispatcher")


It is not allowed to configure the routerDispatcher to be a akka.dispatch.BalancingDispatcherConfigurator since the messages meant for the special router actor cannot be processed by any other actor.