.. _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. .. _Actor Model: http://en.wikipedia.org/wiki/Actor_model 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 :ref:`actor-systems` and :ref:`supervision` and it may also help to read :ref:`actorOf-vs-actorFor`. Defining an Actor class ----------------------- Actor classes are implemented by extending the Actor class and implementing the :meth:`receive` method. The :meth:`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: .. includecode:: code/docs/actor/ActorDocSpec.scala :include: imports1,my-actor 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 ---------------------------------------- .. includecode:: code/docs/actor/ActorDocSpec.scala :include: imports2,system-actorOf The call to :meth:`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). .. includecode:: code/docs/actor/ActorDocSpec.scala#context-actorOf 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. .. code-block:: scala 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: .. includecode:: code/docs/actor/ActorDocSpec.scala#creating-constructor .. warning:: You might be tempted at times to offer an ``Actor`` factory which always returns the same instance, e.g. by using a ``lazy val`` or an ``object ... extends Actor``. This is not supported, as it goes against the meaning of an actor restart, which is described here: :ref:`supervision-restart`. 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. .. includecode:: code/docs/actor/ActorDocSpec.scala#creating-props-config Creating Actors with Props -------------------------- Actors are created by passing in a ``Props`` instance into the ``actorOf`` factory method. .. includecode:: code/docs/actor/ActorDocSpec.scala#creating-props 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. .. includecode:: code/docs/actor/ActorDocSpec.scala#anonymous-actor .. 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: :ref:`jmm-shared-state` Actor API ========= The :class:`Actor` trait defines only one abstract method, the above mentioned :meth:`receive`, which implements the behavior of the actor. If the current actor behavior does not match a received message, :meth:`unhandled` is called, which by default publishes an ``akka.actor.UnhandledMessage(message, sender, recipient)`` on the actor system’s event stream (set configuration item ``akka.event-handler-startup-timeout`` to ``true`` to have them converted into actual Debug messages) In addition, it offers: * :obj:`self` reference to the :class:`ActorRef` of the actor * :obj:`sender` reference sender Actor of the last received message, typically used as described in :ref:`Actor.Reply` * :obj:`supervisorStrategy` user overridable definition the strategy to use for supervising child actors * :obj:`context` exposes contextual information for the actor and the current message, such as: * factory methods to create child actors (:meth:`actorOf`) * system that the actor belongs to * parent supervisor * supervised children * lifecycle monitoring * hotswap behavior stack as described in :ref:`Actor.HotSwap` You can import the members in the :obj:`context` to avoid prefixing access with ``context.`` .. includecode:: code/docs/actor/ActorDocSpec.scala#import-context 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 :class:`Actor` trait. .. _deathwatch-scala: 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 :class:`Terminated` message dispatched by the other actor upon termination (see `Stopping Actors`_). This service is provided by the :class:`DeathWatch` component of the actor system. Registering a monitor is easy: .. includecode:: code/docs/actor/ActorDocSpec.scala#watch It should be noted that the :class:`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 :class:`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 :class:`Terminated` message because that may already have been queued. Start Hook ---------- Right after starting the actor, its :meth:`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 :meth:`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 :meth:`postStop`. 2. The initial factory from the ``actorOf`` call is used to produce the fresh instance. 3. The new actor’s :meth:`postRestart` method is invoked with the exception which caused the restart. By default the :meth:`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 :meth:`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 :meth:`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 :obj:`deadLetters` of the :obj:`ActorSystem`. Identifying Actors ================== As described in :ref:`addressing`, 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 :class:`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 :class:`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 :ref:`remote-lookup-sample-scala`. 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 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 :class:`Future` representing a possible reply. Also known as ``ask``. Message ordering is guaranteed on a per-sender basis. .. note:: There are performance implications of using ``ask`` since something needs to keep track of when it times out, there needs to be something that bridges a ``Promise`` into an ``ActorRef`` and it also needs to be reachable through remoting. So always prefer ``tell`` for performance, and only ``ask`` if you must. 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. .. 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: 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 :obj:`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 :class:`ActorRef`: .. includecode:: code/docs/actor/ActorDocSpec.scala#ask-pipeTo 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 :class:`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 :class:`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 :class:`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. .. includecode:: code/docs/actor/ActorDocSpec.scala#reply-exception If the actor does not complete the future, it will expire after the timeout period, completing it with an :class:`AskTimeoutException`. The timeout is taken from one of the following locations in order of precedence: 1. explicitly given timeout as in: .. includecode:: code/docs/actor/ActorDocSpec.scala#using-explicit-timeout 2. implicit argument of type :class:`akka.util.Timeout`, e.g. .. includecode:: code/docs/actor/ActorDocSpec.scala#using-implicit-timeout See :ref:`futures-scala` for more information on how to await or query a future. The ``onComplete``, ``onSuccess``, or ``onFailure`` 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: :ref:`jmm-shared-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. .. code-block:: scala myActor.forward(message) Receive messages ================ An Actor has to implement the ``receive`` method to receive messages: .. code-block:: scala 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: .. includecode:: code/docs/actor/ActorDocSpec.scala :include: imports1,my-actor .. _Actor.Reply: 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. .. code-block:: scala 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. .. includecode:: code/docs/actor/ActorDocSpec.scala#receive-timeout .. _stopping-actors-scala: Stopping actors =============== Actors are stopped by invoking the :meth:`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. :meth:`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 :obj:`deadLetters` of the :obj:`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 :meth:`postStop`, dumping mailbox, publishing :class:`Terminated` on the :ref:`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 :meth:`ActorSystem.shutdown()`, the system guardian actors will be stopped, and the aforementioned process will ensure proper termination of the whole system. The :meth:`postStop()` hook is invoked after an actor is fully stopped. This enables cleaning up of resources: .. code-block:: scala 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 :class:`InvalidActorNameException`. Instead, :meth:`watch()` the terminating actor and create its replacement in response to the :class:`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 ------------- :meth:`gracefulStop` is useful if you need to wait for termination or compose ordered termination of several actors: .. includecode:: code/docs/actor/ActorDocSpec.scala#gracefulStop When ``gracefulStop()`` returns successfully, the actor’s ``postStop()`` hook will have been executed: there exists a happens-before edge between the end of ``postStop()`` and the return of ``gracefulStop()``. .. warning:: Keep in mind that an actor stopping and its name being deregistered are separate events which happen asynchronously from each other. Therefore it may be that you will find the name still in use after ``gracefulStop()`` returned. In order to guarantee proper deregistration, only reuse names from within a supervisor you control and only in response to a :class:`Terminated` message, i.e. not for top-level actors. .. _Actor.HotSwap: 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``: .. includecode:: code/docs/actor/ActorDocSpec.scala#hot-swap-actor 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`_. .. _Dining Hakkers: http://github.com/akka/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: .. includecode:: code/docs/actor/ActorDocSpec.scala#swapper 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: .. code-block:: scala 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: .. code-block:: scala // 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 to 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. .. includecode:: code/docs/actor/ActorDocSpec.scala#receive-orElse Or: .. includecode:: code/docs/actor/ActorDocSpec.scala#receive-orElse2