Routers
You are viewing the documentation for the new actor APIs, to view the Akka Classic documentation, see Classic Routing.
Dependency
The Akka dependencies are available from Akka’s library repository. To access them there, you need to configure the URL for this repository.
To use Akka Actor Typed, you must add the following dependency in your project:
- sbt
val AkkaVersion = "2.10.2+2-bc9758d0-SNAPSHOT" libraryDependencies += "com.typesafe.akka" %% "akka-actor-typed" % AkkaVersion
- Maven
- Gradle
Introduction
In some cases it is useful to distribute messages of the same type over a set of actors, so that messages can be processed in parallel - a single actor will only process one message at a time.
The router itself is a behavior that is spawned into a running actor that will then forward any message sent to it to one final recipient out of the set of routees.
There are two kinds of routers included in Akka Typed - the pool router and the group router.
Pool Router
The pool router is created with a routee Behavior
and spawns a number of children with that behavior which it will then forward messages to.
If a child is stopped the pool router removes it from its set of routees. When the last child stops the router itself stops. To make a resilient router that deals with failures the routee Behavior
must be supervised.
As actor children are always local the routees are never spread across a cluster with a pool router.
Let’s first introduce the routee:
- Scala
-
source
object Worker { sealed trait Command case class DoLog(text: String) extends Command def apply(): Behavior[Command] = Behaviors.setup { context => context.log.info("Starting worker") Behaviors.receiveMessage { case DoLog(text) => context.log.info("Got message {}", text) Behaviors.same } } }
- Java
After having defined the routee, we can now concentrate on configuring the router itself. Note again the the router is an Actor in itself:
- Scala
-
source
import akka.actor.testkit.typed.scaladsl.{ LogCapturing, ScalaTestWithActorTestKit } import akka.actor.typed.{ ActorRef, Behavior, SupervisorStrategy } import akka.actor.typed.receptionist.{ Receptionist, ServiceKey } import akka.actor.typed.scaladsl.{ Behaviors, Routers } // This would be defined within your actor object Behaviors.setup[Unit] { ctx => val pool = Routers.pool(poolSize = 4) { // make sure the workers are restarted if they fail Behaviors.supervise(Worker()).onFailure[Exception](SupervisorStrategy.restart) } val router = ctx.spawn(pool, "worker-pool") (0 to 10).foreach { n => router ! Worker.DoLog(s"msg $n") } val poolWithBroadcast = pool.withBroadcastPredicate(_.isInstanceOf[DoBroadcastLog]) val routerWithBroadcast = ctx.spawn(poolWithBroadcast, "pool-with-broadcast") //this will be sent to all 4 routees routerWithBroadcast ! DoBroadcastLog("msg") Behaviors.empty }
- Java
Configuring Dispatchers
Since the router itself is spawned as an actor the dispatcher used for it can be configured directly in the call to spawn
. The routees, however, are spawned by the router. Therefore, the PoolRouter
has a property to configure the Props
of its routees:
- Scala
-
source
// make sure workers use the default blocking IO dispatcher val blockingPool = pool.withRouteeProps(routeeProps = DispatcherSelector.blocking()) // spawn head router using the same executor as the parent val blockingRouter = ctx.spawn(blockingPool, "blocking-pool", DispatcherSelector.sameAsParent())
- Java
Broadcasting a message to all routees
Pool routers can be configured to identify messages intended to be broad-casted to all routees. Therefore, the PoolRouter
has a property to configure its broadcastPredicate
:
- Scala
-
source
val poolWithBroadcast = pool.withBroadcastPredicate(_.isInstanceOf[DoBroadcastLog]) val routerWithBroadcast = ctx.spawn(poolWithBroadcast, "pool-with-broadcast") //this will be sent to all 4 routees routerWithBroadcast ! DoBroadcastLog("msg") Behaviors.empty
- Java
Group Router
The group router is created with a ServiceKey
and uses the receptionist (see Receptionist) to discover available actors for that key and routes messages to one of the currently known registered actors for a key.
Since the receptionist is used this means the group router is cluster-aware out of the box. The router sends messages to registered actors on any node in the cluster that is reachable. If no reachable actor exists the router will fallback and route messages to actors on nodes marked as unreachable.
That the receptionist is used also means that the set of routees is eventually consistent, and that immediately when the group router is started the set of routees it knows about is empty, until it has seen a listing from the receptionist it stashes incoming messages and forwards them as soon as it gets a listing from the receptionist.
When the router has received a listing from the receptionist and the set of registered actors is empty the router will drop them (publish them to the event stream as akka.actor.Dropped
).
- Scala
-
source
val serviceKey = ServiceKey[Worker.Command]("log-worker") Behaviors.setup[Unit] { ctx => // this would likely happen elsewhere - if we create it locally we // can just as well use a pool val worker = ctx.spawn(Worker(), "worker") ctx.system.receptionist ! Receptionist.Register(serviceKey, worker) val group = Routers.group(serviceKey) val router = ctx.spawn(group, "worker-group") // the group router will stash messages until it sees the first listing of registered // services from the receptionist, so it is safe to send messages right away (0 to 10).foreach { n => router ! Worker.DoLog(s"msg $n") } Behaviors.empty }
- Java
Routing strategies
There are three different strategies for selecting which routee a message is forwarded to that can be selected from the router before spawning it:
Round Robin
Rotates over the set of routees making sure that if there are n
routees, then for n
messages sent through the router, each actor is forwarded one message.
Round robin gives fair routing where every available routee gets the same amount of messages as long as the set of routees stays relatively stable, but may be unfair if the set of routees changes a lot.
This is the default for pool routers as the pool of routees is expected to remain the same.
An optional parameter preferLocalRoutees
can be used for this strategy. Routers will only use routees located in local actor system if preferLocalRoutees
is true and local routees do exist. The default value for this parameter is false.
Random
Randomly selects a routee when a message is sent through the router.
This is the default for group routers as the group of routees is expected to change as nodes join and leave the cluster.
An optional parameter preferLocalRoutees
can be used for this strategy. Routers will only use routees located in local actor system if preferLocalRoutees
is true and local routees do exist. The default value for this parameter is false.
Consistent Hashing
Uses consistent hashing to select a routee based on the sent message. This article gives good insight into how consistent hashing is implemented.
Currently you have to define hashMapping of the router to map incoming messages to their consistent hash key. This makes the decision transparent for the sender.
Consistent hashing delivers messages with the same hash to the same routee as long as the set of routees stays the same. When the set of routees changes, consistent hashing tries to make sure, but does not guarantee, that messages with the same hash are routed to the same routee.
- Scala
-
source
val router = spawn(Routers.group(Proxy.RegisteringKey).withConsistentHashingRouting(10, Proxy.mapping)) router ! Proxy.Message("123", "Text1") router ! Proxy.Message("123", "Text2") router ! Proxy.Message("zh3", "Text3") router ! Proxy.Message("zh3", "Text4") // the hash is calculated over the Proxy.Message first parameter obtained through the Proxy.mapping function
- Java
See also Akka Cluster Sharding which provides stable routing and rebalancing of the routee actors.
Routers and performance
Note that if the routees are sharing a resource, the resource will determine if increasing the number of actors will actually give higher throughput or faster answers. For example if the routees are CPU bound actors it will not give better performance to create more routees than there are threads to execute the actors.
Since the router itself is an actor and has a mailbox this means that messages are routed sequentially to the routees where it can be processed in parallel (depending on the available threads in the dispatcher). In a high throughput use cases the sequential routing could become a bottle neck. Akka Typed does not provide an optimized tool for this.