pekko/akka-docs/scala/actors.rst

Actors
======

Module stability: **SOLID**

The `Actor Model <http://en.wikipedia.org/wiki/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.

The Akka 0.9 release introduced a new concept; ActorRef, which requires some refactoring. If you are new to Akka just read along, but if you have used Akka 0.6.x, 0.7.x and 0.8.x then you might be helped by the :doc:`0.8.x => 0.9.x migration guide <migration-guide-0.8.x-0.9.x>`

Creating Actors
---------------

Actors can be created either by:

* Extending the Actor class and implementing the receive method.
* Create an anonymous actor using one of the actor methods.

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:

.. code-block:: scala

  class MyActor extends Actor {
    def receive = {
      case "test" => EventHandler.info(this, "received test")
      case _ => EventHandler.info(this, "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.

Creating Actors
^^^^^^^^^^^^^^^

.. code-block:: scala

  val myActor = Actor.actorOf[MyActor]
  myActor.start()

Normally you would want to import the ``actorOf`` method like this:

.. code-block:: scala

  import akka.actor.Actor._

  val myActor = actorOf[MyActor]

To avoid prefixing it with ``Actor`` every time you use it.

You can also start it in the same statement:

.. code-block:: scala

  val myActor = actorOf[MyActor].start()

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.

Creating Actors with non-default constructor
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If your Actor has a constructor that takes parameters then you can't create it using ``actorOf[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:

.. code-block:: scala

  val a = actorOf(new MyActor(..)).start() // allows passing in arguments into the MyActor constructor

Running a block of code asynchronously
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Here we create a light-weight actor-based thread, that can be used to spawn off a task. Code blocks spawned up like this are always implicitly started, shut down and made eligible for garbage collection. The actor that is created "under the hood" is not reachable from the outside and there is no way of sending messages to it. It being an actor is only an implementation detail. It will only run the block in an event-based thread and exit once the block has run to completion.

.. code-block:: scala

  spawn {
    ... // do stuff
  }

Identifying Actors
------------------

Each Actor has two fields:

* ``self.uuid``
* ``self.id``

The difference is that the ``uuid`` is generated by the runtime, guaranteed to be unique and can't be modified. While the ``id`` is modifiable by the user, and defaults to the Actor class name. You can retrieve Actors by both UUID and ID using the ``ActorRegistry``, see the section further down for details.

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:

.. code-block:: scala

  // 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 “bang” methods.

* ! means “fire-and-forget”, e.g. send a message asynchronously and return immediately.
* !! means “send-and-reply-eventually”, e.g. send a message asynchronously and wait for a reply through aFuture. Here you can specify a timeout. Using timeouts is very important. If no timeout is specified then the actor’s default timeout (set by the this.timeout variable in the actor) is used. This method returns an ``Option[Any]`` which will be either ``Some(result)`` if returning successfully or None if the call timed out.
* !!! sends a message asynchronously and returns a ``Future``.

You can check if an Actor can handle a specific message by invoking the ``isDefinedAt`` method:

.. code-block:: scala

  if (actor.isDefinedAt(message)) actor ! message
  else ...

Fire-forget
^^^^^^^^^^^

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

.. code-block:: scala

  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: Option[AnyRef]`` member field. He can use this to reply to the original sender or use the ``reply(message: Any)`` method.

If invoked from an instance that is **not** an Actor there will be no implicit sender passed along the message and you will get an IllegalStateException if you call ``self.reply(..)``.

Send-And-Receive-Eventually
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Using ``!!`` will send a message to the receiving Actor asynchronously but it will wait for a reply on a ``Future``, blocking the sender Actor until either:

* A reply is received, or
* The Future times out

You can pass an explicit time-out to the ``!!`` method and if none is specified then the default time-out defined in the sender Actor will be used.

The ``!!`` method returns an ``Option[Any]`` which will be either ``Some(result)`` if returning successfully, or ``None`` if the call timed out.
Here are some examples:

.. code-block:: scala

  val resultOption = actor !! ("Hello", 1000)
  if (resultOption.isDefined) ... // handle reply
  else ... // handle timeout

  val result: Option[String] = actor !! "Hello"
  resultOption match {
    case Some(reply) => ... // handle reply
    case None =>        ... // handle timeout
  }

  val result = (actor !! "Hello").getOrElse(throw new RuntimeException("TIMEOUT"))

  (actor !! "Hello").foreach(result => ...) // handle result

Send-And-Receive-Future
^^^^^^^^^^^^^^^^^^^^^^^

Using ``!!!`` will send a message to the receiving Actor asynchronously and will return a 'Future':

.. code-block:: scala

  val future = actor !!! "Hello"

See `Futures <futures-scala>`_ for more information.

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.

.. code-block:: scala

  actor.forward(message)

Receive messages
----------------

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

.. code-block:: scala

  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:

.. code-block:: scala

  class MyActor extends Actor {
    def receive = {
      case "Hello" =>
        log.info("Received 'Hello'")

      case _ =>
        throw new RuntimeException("unknown message")
    }
  }

Actor internal API
------------------

The Actor trait contains almost no member fields or methods to invoke, you just use the Actor trait to implement the:

#. ``receive`` message handler
#. life-cycle callbacks:

  #. preStart
  #. postStop
  #. preRestart
  #. postRestart

The ``Actor`` trait has one single member field (apart from the ``log`` field from the mixed in ``Logging`` trait):

.. code-block:: scala

  val self: ActorRef

This ``self`` field holds a reference to its ``ActorRef`` and it is this reference you want to access the Actor's API. Here, for example, you find methods to reply to messages, send yourself messages, define timeouts, fault tolerance etc., start and stop etc.

However, for convenience you can import these functions and fields like below, which will allow you do drop the ``self`` prefix:

.. code-block:: scala

  class MyActor extends Actor {
    import self._
    id = ...
    dispatcher = ...
    start
    ...
  }

But in this documentation we will always prefix the calls with ``self`` for clarity.

Let's start by looking how we can reply to messages in a convenient way using this ``ActorRef`` API.

Reply to messages
-----------------

Reply using the reply and reply\_? methods
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If you want to send a message back to the original sender of the message you just received then you can use the ``reply(..)`` method.

.. code-block:: scala

  case request =>
    val result = process(request)
    self.reply(result)

In this case the ``result`` will be send back to the Actor that sent the ``request``.

The ``reply`` method throws an ``IllegalStateException`` if unable to determine what to reply to, e.g. the sender is not an actor. You can also use the more forgiving ``reply_?`` method which returns ``true`` if reply was sent, and ``false`` if unable to determine what to reply to.

.. code-block:: scala

  case request =>
    val result = process(request)
    if (self.reply_?(result)) ...// success
    else ... // handle failure

Reply using the sender reference
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If the sender is an Actor then its reference will be implicitly passed along together with the message and will end up in the ``sender: Option[ActorRef]`` member field in the ``ActorRef``. This means that you can use this field to send a message back to the sender.

.. code-block:: scala

  // receiver code
  case request =>
    val result = process(request)
    self.sender.get ! result

It's important to know that ``sender.get`` will throw an exception if the ``sender`` is not defined, e.g. the ``Option`` is ``None``. You can check if it is defined by invoking the ``sender.isDefined`` method, but a more elegant solution is to use ``foreach`` which will only be executed if the sender is defined in the ``sender`` member ``Option`` field. If it is not, then the operation in the ``foreach`` method is ignored.

.. code-block:: scala

  // receiver code
  case request =>
    val result = process(request)
    self.sender.foreach(_ ! result)

The same pattern holds for using the ``senderFuture`` in the section below.

Reply using the sender future
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If a message was sent with the ``!!`` or ``!!!`` methods, which both implements request-reply semantics using Future's, then you either have the option of replying using the ``reply`` method as above. This method will then resolve the Future. But you can also get a reference to the Future directly and resolve it yourself or if you would like to store it away to resolve it later, or pass it on to some other Actor to resolve it.

The reference to the Future resides in the ``senderFuture: Option[CompletableFuture[_]]`` member field in the ``ActorRef`` class.

Here is an example of how it can be used:

.. code-block:: scala

  case request =>
    try {
      val result = process(request)
      self.senderFuture.foreach(_.completeWithResult(result))
    } catch {
      case e =>
        senderFuture.foreach(_.completeWithException(this, e))
    }

Reply using the channel
^^^^^^^^^^^^^^^^^^^^^^^

If you want to have a handle to an object to whom you can reply to the message, you can use the ``Channel`` abstraction.
Simply call ``self.channel`` and then you can forward that to others, store it away or otherwise until you want to reply, which you do by ``Channel ! response``:

.. code-block:: scala

  case request =>
      val result = process(request)
      self.channel ! result

.. code-block:: scala

  case request =>
      friend forward self.channel

Summary of reply semantics and options
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

* ``self.reply(...)`` can be used to reply to an ``Actor`` or a ``Future``.
* ``self.sender`` is a reference to the ``Actor`` you can reply to, if it exists
* ``self.senderFuture`` is a reference to the ``Future`` you can reply to, if it exists
* ``self.channel`` is a reference providing an abstraction to either ``self.sender`` or ``self.senderFuture`` if one is set, providing a single reference to store and reply to (the reference equivalent to the ``reply(...)`` method).
* ``self.sender`` and ``self.senderFuture`` will never be set at the same time, as there can only be one reference to accept a reply.

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.

.. code-block:: scala

  self.receiveTimeout = Some(30000L) // 30 seconds

  def receive = {
    case "Hello" =>
      log.info("Received 'Hello'")
    case ReceiveTimeout =>
        throw new RuntimeException("received timeout")
  }

This mechanism also work for hotswapped receive functions. Every time a ``HotSwap`` is sent, the receive timeout is reset and rescheduled.

Starting actors
---------------

Actors are started by invoking the ``start`` method.

.. code-block:: scala

  val actor = actorOf[MyActor]
  actor.start()

You can create and start the ``Actor`` in a oneliner like this:

.. code-block:: scala

  val actor = actorOf[MyActor].start()

When you start the ``Actor`` then it will automatically call the ``def preStart`` callback method on the ``Actor`` trait. This is an excellent place to add initialization code for the actor.

.. code-block:: scala

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

Stopping actors
---------------

Actors are stopped by invoking the ``stop`` method.

.. code-block:: scala

  actor.stop()

When stop is called then a call to the ``def postStop`` callback method will take place. The ``Actor`` can use this callback to implement shutdown behavior.

.. code-block:: scala

  override def postStop = {
    ... // clean up resources
  }

You can shut down all Actors in the system by invoking:

.. code-block:: scala

  Actor.registry.shutdownAll()


PoisonPill
----------

You can also send an actor the ``akka.actor.PoisonPill`` message, which will stop the actor when the message is processed.

If the sender is a ``Future`` (e.g. the message is sent with ``!!`` or ``!!!``), the ``Future`` will be completed with an ``akka.actor.ActorKilledException("PoisonPill")``.

HotSwap
-------

Upgrade
^^^^^^^

Akka supports hotswapping the Actor’s message loop (e.g. its implementation) at runtime. There are two ways you can do that:

* Send a ``HotSwap`` message to the Actor.
* Invoke the ``become`` method from within the Actor.

Both of these takes a ``ActorRef => PartialFunction[Any, Unit]`` that implements the new message handler. The hotswapped code is kept in a Stack which can be pushed and popped.

To hotswap the Actor body using the ``HotSwap`` message:

.. code-block:: scala

  actor ! HotSwap( self => {
    case message => self.reply("hotswapped body")
  })

Using the ``HotSwap`` message for hotswapping has its limitations. You can not replace it with any code that uses the Actor's ``self`` reference. If you need to do that the the ``become`` method is better.

To hotswap the Actor using ``become``:

.. code-block:: scala

  def angry: Receive = {
    case "foo" => self reply "I am already angry!!!"
    case "bar" => become(happy)
  }

  def happy: Receive = {
    case "bar" => self reply "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 <http://github.com/jboner/akka/blob/master/akka-samples/akka-sample-fsm/src/main/scala/DiningHakkersOnBecome.scala>`_

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

.. code-block:: scala

  case object Swap
  class Swapper extends Actor {
   def receive = {
     case Swap =>
       println("Hi")
       become {
         case Swap =>
           println("Ho")
           unbecome() // resets the latest 'become' (just for fun)
       }
   }
  }

  val swap = actorOf[Swapper].start()

  swap ! Swap // prints Hi
  swap ! Swap // prints Ho
  swap ! Swap // prints Hi
  swap ! Swap // prints Ho
  swap ! Swap // prints Hi
  swap ! Swap // prints Ho

Encoding Scala Actors nested receives without accidentally leaking memory: `UnnestedReceive <https://gist.github.com/797035>`_
------------------------------------------------------------------------------------------------------------------------------

Downgrade
^^^^^^^^^

Since the hotswapped code is pushed to a Stack you can downgrade the code as well. There are two ways you can do that:

* Send the Actor a ``RevertHotswap`` message
* Invoke the ``unbecome`` method from within the Actor.

Both of these will pop the Stack and replace the Actor's implementation with the ``PartialFunction[Any, Unit]`` that is at the top of the Stack.

Revert the Actor body using the ``RevertHotSwap`` message:

.. code-block:: scala

  actor ! RevertHotSwap

Revert the Actor body using the ``unbecome`` method:

.. code-block:: scala

  def receive: Receive = {
    case "revert" => 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:

.. code-block:: scala

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

Actor life-cycle
----------------

The actor has a well-defined non-circular life-cycle.

::

  NEW (newly created actor) - can't receive messages (yet)
      => STARTED (when 'start' is invoked) - can receive messages
          => SHUT DOWN (when 'exit' or 'stop' is invoked) - can't do anything

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.

In generic base Actor:

.. code-block:: scala

  import akka.actor.Actor.Receive
  
  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
  }

In subclassing Actor:

.. code-block:: scala

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