Actors (Scala)

Actors (Scala)

The Actor Model provides a higher level of abstraction for writing concurrent and distributed systems. It alleviates the developer from having to deal with explicit locking and thread management, making it easier to write correct concurrent and parallel systems. Actors were defined in the 1973 paper by Carl Hewitt but have been popularized by the Erlang language, and used for example at Ericsson with great success to build highly concurrent and reliable telecom systems.

The API of Akka’s Actors is similar to Scala Actors which has borrowed some of its syntax from Erlang.

Creating Actors

Since Akka enforces parental supervision every actor is supervised and (potentially) the supervisor of its children; it is advisable that you familiarize yourself with Actor Systems and Supervision and Monitoring and it may also help to read Summary: actorOf vs. actorFor.

Defining an Actor class

Actor classes are implemented by extending the Actor class and implementing the receive method. The receive method should define a series of case statements (which has the type PartialFunction[Any, Unit]) that defines which messages your Actor can handle, using standard Scala pattern matching, along with the implementation of how the messages should be processed.

Here is an example:

import akka.actor.Actor
import akka.actor.Props
import akka.event.Logging

class MyActor extends Actor {
  val log = Logging(context.system, this)
  def receive = {
    case "test"  log.info("received test")
    case _       log.info("received unknown message")
  }
}

Please note that the Akka Actor receive message loop is exhaustive, which is different compared to Erlang and Scala Actors. This means that you need to provide a pattern match for all messages that it can accept and if you want to be able to handle unknown messages then you need to have a default case as in the example above. Otherwise an akka.actor.UnhandledMessage(message, sender, recipient) will be published to the ActorSystem‘s EventStream.

Creating Actors with default constructor

object Main extends App {
  val system = ActorSystem("MySystem")
  val myActor = system.actorOf(Props[MyActor], name = "myactor")

The call to actorOf returns an instance of ActorRef. This is a handle to the Actor instance which you can use to interact with the Actor. The ActorRef is immutable and has a one to one relationship with the Actor it represents. The ActorRef is also serializable and network-aware. This means that you can serialize it, send it over the wire and use it on a remote host and it will still be representing the same Actor on the original node, across the network.

In the above example the actor was created from the system. It is also possible to create actors from other actors with the actor context. The difference is how the supervisor hierarchy is arranged. When using the context the current actor will be supervisor of the created child actor. When using the system it will be a top level actor, that is supervised by the system (internal guardian actor).

class FirstActor extends Actor {
  val myActor = context.actorOf(Props[MyActor], name = "myactor")

The name parameter is optional, but you should preferably name your actors, since that is used in log messages and for identifying actors. The name must not be empty or start with $. If the given name is already in use by another child to the same parent actor an InvalidActorNameException is thrown.

Actors are automatically started asynchronously when created. When you create the Actor then it will automatically call the preStart callback method on the Actor trait. This is an excellent place to add initialization code for the actor.

override def preStart() = {
  ... // initialization code
}

Creating Actors with non-default constructor

If your Actor has a constructor that takes parameters then you can’t create it using actorOf(Props[TYPE]). Instead you can use a variant of actorOf that takes a call-by-name block in which you can create the Actor in any way you like.

Here is an example:

// allows passing in arguments to the MyActor constructor
val myActor = system.actorOf(Props(new MyActor("...")), name = "myactor")

Props

Props is a configuration class to specify options for the creation of actors. Here are some examples on how to create a Props instance.

import akka.actor.Props
val props1 = Props()
val props2 = Props[MyActor]
val props3 = Props(new MyActor)
val props4 = Props(
  creator = { ()  new MyActor },
  dispatcher = "my-dispatcher")
val props5 = props1.withCreator(new MyActor)
val props6 = props5.withDispatcher("my-dispatcher")

Creating Actors with Props

Actors are created by passing in a Props instance into the actorOf factory method.

import akka.actor.Props
val myActor = system.actorOf(Props[MyActor].withDispatcher("my-dispatcher"), name = "myactor2")

Creating Actors using anonymous classes

When spawning actors for specific sub-tasks from within an actor, it may be convenient to include the code to be executed directly in place, using an anonymous class.

def receive = {
  case m: DoIt 
    context.actorOf(Props(new Actor {
      def receive = {
        case DoIt(msg) 
          val replyMsg = doSomeDangerousWork(msg)
          sender ! replyMsg
          context.stop(self)
      }
      def doSomeDangerousWork(msg: ImmutableMessage): String = { "done" }
    })) forward m
}

Warning

In this case you need to carefully avoid closing over the containing actor’s reference, i.e. do not call methods on the enclosing actor from within the anonymous Actor class. This would break the actor encapsulation and may introduce synchronization bugs and race conditions because the other actor’s code will be scheduled concurrently to the enclosing actor. Unfortunately there is not yet a way to detect these illegal accesses at compile time. See also: Actors and shared mutable state

Actor API

The Actor trait defines only one abstract method, the above mentioned receive, which implements the behavior of the actor.

If the current actor behavior does not match a received message, unhandled is called, which by default publishes an akka.actor.UnhandledMessage(message, sender, recipient) on the actor system’s event stream.

In addition, it offers:

  • self reference to the ActorRef of the actor
  • sender reference sender Actor of the last received message, typically used as described in Reply to messages
  • supervisorStrategy user overridable definition the strategy to use for supervising child actors
  • context exposes contextual information for the actor and the current message, such as:
    • factory methods to create child actors (actorOf)
    • system that the actor belongs to
    • parent supervisor
    • supervised children
    • lifecycle monitoring
    • hotswap behavior stack as described in Become/Unbecome

You can import the members in the context to avoid prefixing access with context.

class FirstActor extends Actor {
  import context._
  val myActor = actorOf(Props[MyActor], name = "myactor")
  def receive = {
    case x  myActor ! x
  }
}

The remaining visible methods are user-overridable life-cycle hooks which are described in the following:

def preStart() {}
def preRestart(reason: Throwable, message: Option[Any]) {
  context.children foreach (context.stop(_))
  postStop()
}
def postRestart(reason: Throwable) { preStart() }
def postStop() {}

The implementations shown above are the defaults provided by the Actor trait.

Lifecycle Monitoring aka DeathWatch

In order to be notified when another actor terminates (i.e. stops permanently, not temporary failure and restart), an actor may register itself for reception of the Terminated message dispatched by the other actor upon termination (see Stopping Actors). This service is provided by the DeathWatch component of the actor system.

Registering a monitor is easy:

import akka.actor.{ Actor, Props, Terminated }

class WatchActor extends Actor {
  val child = context.actorOf(Props.empty, "child")
  context.watch(child) // <-- this is the only call needed for registration
  var lastSender = system.deadLetters

  def receive = {
    case "kill"               context.stop(child); lastSender = sender
    case Terminated(`child`)  lastSender ! "finished"
  }
}

It should be noted that the Terminated message is generated independent of the order in which registration and termination occur. Registering multiple times does not necessarily lead to multiple messages being generated, but there is no guarantee that only exactly one such message is received: if termination of the watched actor has generated and queued the message, and another registration is done before this message has been processed, then a second message will be queued, because registering for monitoring of an already terminated actor leads to the immediate generation of the Terminated message.

It is also possible to deregister from watching another actor’s liveliness using context.unwatch(target), but obviously this cannot guarantee non-reception of the Terminated message because that may already have been queued.

Start Hook

Right after starting the actor, its preStart method is invoked.

override def preStart() {
  // registering with other actors
  someService ! Register(self)
}

Restart Hooks

All actors are supervised, i.e. linked to another actor with a fault handling strategy. Actors will be restarted in case an exception is thrown while processing a message. This restart involves the hooks mentioned above:

  1. The old actor is informed by calling preRestart with the exception which caused the restart and the message which triggered that exception; the latter may be None if the restart was not caused by processing a message, e.g. when a supervisor does not trap the exception and is restarted in turn by its supervisor. This method is the best place for cleaning up, preparing hand-over to the fresh actor instance, etc. By default it stops all children and calls postStop.
  2. The initial factory from the actorOf call is used to produce the fresh instance.
  3. The new actor’s postRestart method is invoked with the exception which caused the restart. By default the preStart is called, just as in the normal start-up case.

An actor restart replaces only the actual actor object; the contents of the mailbox is unaffected by the restart, so processing of messages will resume after the postRestart hook returns. The message that triggered the exception will not be received again. Any message sent to an actor while it is being restarted will be queued to its mailbox as usual.

Stop Hook

After stopping an actor, its postStop hook is called, which may be used e.g. for deregistering this actor from other services. This hook is guaranteed to run after message queuing has been disabled for this actor, i.e. messages sent to a stopped actor will be redirected to the deadLetters of the ActorSystem.

Identifying Actors

As described in Actor References, Paths and Addresses, each actor has a unique logical path, which is obtained by following the chain of actors from child to parent until reaching the root of the actor system, and it has a physical path, which may differ if the supervision chain includes any remote supervisors. These paths are used by the system to look up actors, e.g. when a remote message is received and the recipient is searched, but they are also useful more directly: actors may look up other actors by specifying absolute or relative paths—logical or physical—and receive back an ActorRef with the result:

context.actorFor("/user/serviceA/aggregator") // will look up this absolute path
context.actorFor("../joe")                    // will look up sibling beneath same supervisor

The supplied path is parsed as a java.net.URI, which basically means that it is split on / into path elements. If the path starts with /, it is absolute and the look-up starts at the root guardian (which is the parent of "/user"); otherwise it starts at the current actor. If a path element equals .., the look-up will take a step “up” towards the supervisor of the currently traversed actor, otherwise it will step “down” to the named child. It should be noted that the .. in actor paths here always means the logical structure, i.e. the supervisor.

If the path being looked up does not exist, a special actor reference is returned which behaves like the actor system’s dead letter queue but retains its identity (i.e. the path which was looked up).

Remote actor addresses may also be looked up, if remoting is enabled:

context.actorFor("akka://app@otherhost:1234/user/serviceB")

These look-ups return a (possibly remote) actor reference immediately, so you will have to send to it and await a reply in order to verify that serviceB is actually reachable and running. An example demonstrating actor look-up is given in Remote Lookup.

Messages and immutability

IMPORTANT: Messages can be any kind of object but have to be immutable. Scala can’t enforce immutability (yet) so this has to be by convention. Primitives like String, Int, Boolean are always immutable. Apart from these the recommended approach is to use Scala case classes which are immutable (if you don’t explicitly expose the state) and works great with pattern matching at the receiver side.

Here is an example:

// define the case class
case class Register(user: User)

// create a new case class message
val message = Register(user)

Other good messages types are scala.Tuple2, scala.List, scala.Map which are all immutable and great for pattern matching.

Send messages

Messages are sent to an Actor through one of the following methods.

  • ! means “fire-and-forget”, e.g. send a message asynchronously and return immediately. Also known as tell.
  • ? sends a message asynchronously and returns a Future representing a possible reply. Also known as ask.

Message ordering is guaranteed on a per-sender basis.

Tell: Fire-forget

This is the preferred way of sending messages. No blocking waiting for a message. This gives the best concurrency and scalability characteristics.

actor ! "hello"

If invoked from within an Actor, then the sending actor reference will be implicitly passed along with the message and available to the receiving Actor in its sender: ActorRef member field. The target actor can use this to reply to the original sender, by using sender ! replyMsg.

If invoked from an instance that is not an Actor the sender will be deadLetters actor reference by default.

Ask: Send-And-Receive-Future

The ask pattern involves actors as well as futures, hence it is offered as a use pattern rather than a method on ActorRef:

import akka.pattern.{ ask, pipe }

case class Result(x: Int, s: String, d: Double)
case object Request

implicit val timeout = Timeout(5 seconds) // needed for `?` below

val f: Future[Result] =
  for {
    x  ask(actorA, Request).mapTo[Int] // call pattern directly
    s  actorB ask Request mapTo manifest[String] // call by implicit conversion
    d  actorC ? Request mapTo manifest[Double] // call by symbolic name
  } yield Result(x, s, d)

f pipeTo actorD // .. or ..
pipe(f) to actorD

This example demonstrates ask together with the pipeTo pattern on futures, because this is likely to be a common combination. Please note that all of the above is completely non-blocking and asynchronous: ask produces a Future, three of which are composed into a new future using the for-comprehension and then pipeTo installs an onComplete-handler on the future to effect the submission of the aggregated Result to another actor.

Using ask will send a message to the receiving Actor as with tell, and the receiving actor must reply with sender ! reply in order to complete the returned Future with a value. The ask operation involves creating an internal actor for handling this reply, which needs to have a timeout after which it is destroyed in order not to leak resources; see more below.

To complete the future with an exception you need send a Failure message to the sender. This is not done automatically when an actor throws an exception while processing a message.

try {
  val result = operation()
  sender ! result
} catch {
  case e: Exception 
    sender ! akka.actor.Status.Failure(e)
    throw e
}

If the actor does not complete the future, it will expire after the timeout period, completing it with an AskTimeoutException. The timeout is taken from one of the following locations in order of precedence:

  1. explicitly given timeout as in:
import akka.util.duration._
import akka.pattern.ask
val future = myActor.ask("hello")(5 seconds)
  1. implicit argument of type akka.util.Timeout, e.g.
import akka.util.duration._
import akka.util.Timeout
import akka.pattern.ask
implicit val timeout = Timeout(5 seconds)
val future = myActor ? "hello"

See Futures (Scala) for more information on how to await or query a future.

The onComplete, onResult, or onTimeout methods of the Future can be used to register a callback to get a notification when the Future completes. Gives you a way to avoid blocking.

Warning

When using future callbacks, such as onComplete, onSuccess, and onFailure, inside actors you need to carefully avoid closing over the containing actor’s reference, i.e. do not call methods or access mutable state on the enclosing actor from within the callback. This would break the actor encapsulation and may introduce synchronization bugs and race conditions because the callback will be scheduled concurrently to the enclosing actor. Unfortunately there is not yet a way to detect these illegal accesses at compile time. See also: Actors and shared mutable state

Forward message

You can forward a message from one actor to another. This means that the original sender address/reference is maintained even though the message is going through a ‘mediator’. This can be useful when writing actors that work as routers, load-balancers, replicators etc.

myActor.forward(message)

Receive messages

An Actor has to implement the receive method to receive messages:

protected def receive: PartialFunction[Any, Unit]

Note: Akka has an alias to the PartialFunction[Any, Unit] type called Receive (akka.actor.Actor.Receive), so you can use this type instead for clarity. But most often you don’t need to spell it out.

This method should return a PartialFunction, e.g. a ‘match/case’ clause in which the message can be matched against the different case clauses using Scala pattern matching. Here is an example:

import akka.actor.Actor
import akka.actor.Props
import akka.event.Logging

class MyActor extends Actor {
  val log = Logging(context.system, this)
  def receive = {
    case "test"  log.info("received test")
    case _       log.info("received unknown message")
  }
}

Reply to messages

If you want to have a handle for replying to a message, you can use sender, which gives you an ActorRef. You can reply by sending to that ActorRef with sender ! replyMsg. You can also store the ActorRef for replying later, or passing on to other actors. If there is no sender (a message was sent without an actor or future context) then the sender defaults to a ‘dead-letter’ actor ref.

case request =>
  val result = process(request)
  sender ! result       // will have dead-letter actor as default

Initial receive timeout

A timeout mechanism can be used to receive a message when no initial message is received within a certain time. To receive this timeout you have to set the receiveTimeout property and declare a case handing the ReceiveTimeout object.

import akka.actor.ReceiveTimeout
import akka.util.duration._
class MyActor extends Actor {
  context.setReceiveTimeout(30 milliseconds)
  def receive = {
    case "Hello"         //...
    case ReceiveTimeout  throw new RuntimeException("received timeout")
  }
}

Stopping actors

Actors are stopped by invoking the stop method of a ActorRefFactory, i.e. ActorContext or ActorSystem. Typically the context is used for stopping child actors and the system for stopping top level actors. The actual termination of the actor is performed asynchronously, i.e. stop may return before the actor is stopped.

Processing of the current message, if any, will continue before the actor is stopped, but additional messages in the mailbox will not be processed. By default these messages are sent to the deadLetters of the ActorSystem, but that depends on the mailbox implementation.

Termination of an actor proceeds in two steps: first the actor suspends its mailbox processing and sends a stop command to all its children, then it keeps processing the termination messages from its children until the last one is gone, finally terminating itself (invoking postStop, dumping mailbox, publishing Terminated on the DeathWatch, telling its supervisor). This procedure ensures that actor system sub-trees terminate in an orderly fashion, propagating the stop command to the leaves and collecting their confirmation back to the stopped supervisor. If one of the actors does not respond (i.e. processing a message for extended periods of time and therefore not receiving the stop command), this whole process will be stuck.

Upon ActorSystem.shutdown, the system guardian actors will be stopped, and the aforementioned process will ensure proper termination of the whole system.

The postStop hook is invoked after an actor is fully stopped. This enables cleaning up of resources:

override def postStop() = {
  // close some file or database connection
}

Note

Since stopping an actor is asynchronous, you cannot immediately reuse the name of the child you just stopped; this will result in an InvalidActorNameException. Instead, watch the terminating actor and create its replacement in response to the Terminated message which will eventually arrive.

PoisonPill

You can also send an actor the akka.actor.PoisonPill message, which will stop the actor when the message is processed. PoisonPill is enqueued as ordinary messages and will be handled after messages that were already queued in the mailbox.

Graceful Stop

gracefulStop is useful if you need to wait for termination or compose ordered termination of several actors:

import akka.pattern.gracefulStop
import akka.dispatch.Await
import akka.actor.ActorTimeoutException

try {
  val stopped: Future[Boolean] = gracefulStop(actorRef, 5 seconds)(system)
  Await.result(stopped, 6 seconds)
  // the actor has been stopped
} catch {
  case e: ActorTimeoutException ⇒ // the actor wasn't stopped within 5 seconds
}

Become/Unbecome

Upgrade

Akka supports hotswapping the Actor’s message loop (e.g. its implementation) at runtime: Invoke the context.become method from within the Actor.

Become takes a PartialFunction[Any, Unit] that implements the new message handler. The hotswapped code is kept in a Stack which can be pushed and popped.

Warning

Please note that the actor will revert to its original behavior when restarted by its Supervisor.

To hotswap the Actor behavior using become:

class HotSwapActor extends Actor {
  import context._
  def angry: Receive = {
    case "foo"  sender ! "I am already angry?"
    case "bar"  become(happy)
  }

  def happy: Receive = {
    case "bar"  sender ! "I am already happy :-)"
    case "foo"  become(angry)
  }

  def receive = {
    case "foo"  become(angry)
    case "bar"  become(happy)
  }
}

The become method is useful for many different things, but a particular nice example of it is in example where it is used to implement a Finite State Machine (FSM): Dining Hakkers.

Here is another little cute example of become and unbecome in action:

case object Swap
class Swapper extends Actor {
  import context._
  val log = Logging(system, this)

  def receive = {
    case Swap 
      log.info("Hi")
      become {
        case Swap 
          log.info("Ho")
          unbecome() // resets the latest 'become' (just for fun)
      }
  }
}

object SwapperApp extends App {
  val system = ActorSystem("SwapperSystem")
  val swap = system.actorOf(Props[Swapper], name = "swapper")
  swap ! Swap // logs Hi
  swap ! Swap // logs Ho
  swap ! Swap // logs Hi
  swap ! Swap // logs Ho
  swap ! Swap // logs Hi
  swap ! Swap // logs Ho
}

Encoding Scala Actors nested receives without accidentally leaking memory

See this Unnested receive example.

Downgrade

Since the hotswapped code is pushed to a Stack you can downgrade the code as well, all you need to do is to: Invoke the context.unbecome method from within the Actor.

This will pop the Stack and replace the Actor’s implementation with the PartialFunction[Any, Unit] that is at the top of the Stack.

Here’s how you use the unbecome method:

def receive = {
  case "revert" => context.unbecome()
}

Killing an Actor

You can kill an actor by sending a Kill message. This will restart the actor through regular supervisor semantics.

Use it like this:

// kill the actor called 'victim'
victim ! Kill

Actors and exceptions

It can happen that while a message is being processed by an actor, that some kind of exception is thrown, e.g. a database exception.

What happens to the Message

If an exception is thrown while a message is being processed (so taken of his mailbox and handed over the the receive), then this message will be lost. It is important to understand that it is not put back on the mailbox. So if you want to retry processing of a message, you need to deal with it yourself by catching the exception and retry your flow. Make sure that you put a bound on the number of retries since you don’t want a system to livelock (so consuming a lot of cpu cycles without making progress).

What happens to the mailbox

If an exception is thrown while a message is being processed, nothing happens to the mailbox. If the actor is restarted, the same mailbox will be there. So all messages on that mailbox, will be there as well.

What happens to the actor

If an exception is thrown, the actor instance is discarded and a new instance is created. This new instance will now be used in the actor references to this actor (so this is done invisible to the developer). Note that this means that current state of the failing actor instance is lost if you don’t store and restore it in preRestart and postRestart callbacks.

Extending Actors using PartialFunction chaining

A bit advanced but very useful way of defining a base message handler and then extend that, either through inheritance or delegation, is to use PartialFunction.orElse chaining.

abstract class GenericActor extends Actor {
  // to be defined in subclassing actor
  def specificMessageHandler: Receive

  // generic message handler
  def genericMessageHandler: Receive = {
    case event ⇒ printf("generic: %s\n", event)
  }

  def receive = specificMessageHandler orElse genericMessageHandler
}

class SpecificActor extends GenericActor {
  def specificMessageHandler = {
    case event: MyMsg ⇒ printf("specific: %s\n", event.subject)
  }
}

case class MyMsg(subject: String)

Or:

trait ComposableActor extends Actor {
  private var receives: List[Receive] = List()
  protected def registerReceive(receive: Receive) {
    receives = receive :: receives
  }

  def receive = receives reduce { _ orElse _ }
}

class MyComposableActor extends ComposableActor {
  override def preStart() {
    registerReceive({
      case "foo"  /* Do something */
    })

    registerReceive({
      case "bar"  /* Do something */
    })
  }
}

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