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Merge pull request #137 from jboner/wip-1435-doc-scala-actors-patriknw

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Updated documentation of Actors Scala. See #1435
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patriknw 2011-12-07 08:11:06 -08:00
commit 738857ccc2
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373
akka-docs/scala/actors.rst
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@ -28,17 +28,12 @@ its syntax from Erlang.
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
: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.
@ -46,21 +41,22 @@ along with the implementation of how the messages should be processed.
Here is an example:
.. includecode:: code/ActorDocSpec.scala
:include: imports,my-actor
: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.
the example above. Otherwise an ``UnhandledMessageException`` will be
thrown and the actor is restarted when an unknown message is received.
Creating Actors
---------------
.. includecode:: code/ActorDocSpec.scala#creating-actorOf
.. includecode:: code/ActorDocSpec.scala
:include: imports2,system-actorOf
The call to ``actorOf`` returns an instance of ``ActorRef``. This is a handle to
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
@ -68,6 +64,15 @@ 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/ActorDocSpec.scala#context-actorOf
Actors are automatically started asynchronously when created.
Creating Actors with non-default constructor
--------------------------------------------
@ -81,23 +86,21 @@ Here is an example:
.. includecode:: code/ActorDocSpec.scala#creating-constructor
Creating Actors with Props
--------------------------
``Props`` is a configuration object to specify additional things for the actor to
be created, such as the ``MessageDispatcher``.
.. includecode:: code/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::
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 =>
actorOf(new Actor {
def receive = {
case DoIt(msg) =>
val replyMsg = doSomeDangerousWork(msg)
self.reply(replyMsg)
self.stop()
}
def doSomeDangerousWork(msg: Message) = { ... }
}).start() ! m
}
.. includecode:: code/ActorDocSpec.scala#anonymous-actor
.. warning::
@ -108,41 +111,38 @@ When spawning actors for specific sub-tasks from within an actor, it may be conv
code will be scheduled concurrently to the enclosing actor. Unfortunately
there is not yet a way to detect these illegal accesses at compile time.
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
}
Actor Internal API
==================
Actor API
=========
The :class:`Actor` trait defines only one abstract method, the above mentioned
:meth:`receive`. In addition, it offers two convenience methods
:meth:`become`/:meth:`unbecome` for modifying the hotswap behavior stack as
described in :ref:`Actor.HotSwap` and the :obj:`self` reference to this actor’s
:class:`ActorRef` object. If the current actor behavior does not match a
received message, :meth:`unhandled` is called, which by default throws an
:class:`UnhandledMessageException`.
: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 throws an :class:`UnhandledMessageException`.
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:`context` exposes contextual information for the actor and the current message, such as:
* factory method to create child actors (:meth:`actorOf`)
* system that the actor belongs to
* parent supervisor
* supervised children
* 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/ActorDocSpec.scala#import-context
The remaining visible methods are user-overridable life-cycle hooks which are
described in the following::
def preStart() {}
def preRestart(cause: Throwable, message: Option[Any]) {}
def postRestart(cause: Throwable) {}
def preRestart(reason: Throwable, message: Option[Any]) { postStop() }
def postRestart(reason: Throwable) { preStart() }
def postStop() {}
The implementations shown above are the defaults provided by the :class:`Actor`
@ -156,7 +156,7 @@ Right after starting the actor, its :meth:`preStart` method is invoked.
::
override def preStart {
override def preStart() {
// registering with other actors
someService ! Register(self)
}
@ -165,9 +165,9 @@ Right after starting the actor, its :meth:`preStart` method is invoked.
Restart Hooks
-------------
A supervised actor, i.e. one which is linked to another actor with a fault
handling strategy, will be restarted in case an exception is thrown while
processing a message. This restart involves four of the hooks mentioned above:
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
@ -175,17 +175,18 @@ processing a message. This restart involves four of the hooks mentioned above:
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.
2. The initial factory from the ``Actor.actorOf`` call is used
By default it calls :meth:`postStop`.
2. The initial factory from the ``actorOf`` call is used
to produce the fresh instance.
3. The new actor’s :meth:`preStart` method is invoked, just as in the normal
start-up case.
4. The new actor’s :meth:`postRestart` method is called with the exception
which caused the restart.
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 and the hotswap stack are unaffected by the restart, so processing of
messages will resume after the :meth:`postRestart` hook returns. Any message
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.
@ -194,16 +195,15 @@ 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. sending
messages would fail with an :class:`IllegalActorStateException`.
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
==================
An actor is identified by its address. If no address is associated with an actor
then a unique identifier is used instead. The address of an actor can be
accessed using ``self.address``.
FIXME Actor Path documentation
Messages and immutability
@ -236,20 +236,9 @@ 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.
immediately. Also know as ``tell``.
* ``?`` sends a message asynchronously and returns a :class:`Future`
representing a possible reply.
.. note::
There used to be two more “bang” methods, which are now removed in Akka 2.0:
* ``!!`` was similar to the current ``(actor ? msg).as[T]``; deprecation
followed from the change of timeout handling described below.
* ``!!![T]`` was similar to the current ``(actor ? msg).mapTo[T]``, with the
same change in the handling of :class:`Future`’s timeout as for ``!!``, but
additionally the old method could defer possible type cast problems into
seemingly unrelated parts of the code base.
representing a possible reply. Also know as ``ask``.
Message ordering is guaranteed on a per-sender basis.
@ -265,13 +254,11 @@ message. This gives the best concurrency and scalability characteristics.
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 ``channel: UntypedChannel`` member field. The target actor can use this
to reply to the original sender, e.g. by using the ``self.reply(message: Any)``
method.
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 there will be no implicit
sender passed along with the message and you will get an
IllegalActorStateException when calling ``self.reply(...)``.
If invoked from an instance that is **not** an Actor the sender will be
:obj:`deadLetters` actor reference by default.
Send-And-Receive-Future
-----------------------
@ -284,25 +271,49 @@ will return a :class:`Future`:
val future = actor ? "hello"
The receiving actor should reply to this message, which will complete the
future with the reply message as value; if the actor throws an exception while
processing the invocation, this exception will also complete the future. If the
actor does not complete the future, it will expire after the timeout period,
which is taken from one of the following three locations in order of
precedence:
future with the reply message as value; ``sender ! result``.
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.
.. code-block:: scala
try {
operation()
} 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,
which is taken from one of the following locations in order of precedence:
#. explicitly given timeout as in ``actor.?("hello")(timeout = 12 millis)``
#. implicit argument of type :class:`Actor.Timeout`, e.g.
#. implicit argument of type :class:`akka.actor.Timeout`, e.g.
::
import akka.actor.Timeout
import akka.util.duration._
implicit val timeout = Actor.Timeout(12 millis)
implicit val timeout = Timeout(12 millis)
val future = actor ? "hello"
#. default timeout from ``akka.conf``
See :ref:`futures-scala` for more information on how to await or query a
future.
.. warning::
When using future callbacks, such as ``onComplete``, ``onResult``, and ``onTimeout``,
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.
Send-And-Receive-Eventually
---------------------------
@ -321,17 +332,7 @@ type, it will throw the exception or a :class:`ClassCastException` (if you want
to get :obj:`None` in the latter case, use :meth:`Future.asSilently[T]`). In
case of a timeout, :obj:`None` is returned.
.. code-block:: scala
(actor ? msg).as[String] match {
case Some(answer) => ...
case None => ...
}
val resultOption = (actor ? msg).as[String]
if (resultOption.isDefined) ... else ...
for (x <- (actor ? msg).as[Int]) yield { 2 * x }
.. includecode:: code/ActorDocSpec.scala#using-ask
Forward message
---------------
@ -363,25 +364,15 @@ This method should return a ``PartialFunction``, e.g. a ‘match/case’ clause
which the message can be matched against the different case clauses using Scala
pattern matching. Here is an example:
.. code-block:: scala
.. includecode:: code/ActorDocSpec.scala
:include: imports1,my-actor
class MyActor extends Actor {
def receive = {
case "Hello" =>
log.info("Received 'Hello'")
case _ =>
throw new RuntimeException("unknown message")
}
}
.. _Actor.Reply:
Reply to messages
=================
Reply using the sender
----------------------
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 ! Message``. You can also store the ActorRef
@ -403,51 +394,28 @@ 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
context.receiveTimeout = Some(30000L) // 30 seconds
def receive = {
case "Hello" =>
log.info("Received 'Hello'")
case ReceiveTimeout =>
throw new RuntimeException("received timeout")
}
.. includecode:: code/ActorDocSpec.scala#receive-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 created & started by invoking the ``actorOf`` method.
.. code-block:: scala
val actor = actorOf[MyActor]
actor
When you create 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.
Actors are stopped by invoking the ``stop`` method of the ``ActorRef``.
The actual termination of the actor is performed asynchronously, i.e.
``stop`` may return before the actor is stopped.
.. code-block:: scala
actor.stop()
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.
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.
@ -462,7 +430,9 @@ PoisonPill
==========
You can also send an actor the ``akka.actor.PoisonPill`` message, which will
stop the actor when the message is processed.
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.
If the sender is a ``Future`` (e.g. the message is sent with ``?``), the
``Future`` will be completed with an
@ -481,7 +451,7 @@ Akka supports hotswapping the Actor’s message loop (e.g. its implementation) a
runtime. There are two ways you can do that:
* Send a ``HotSwap`` message to the Actor.
* Invoke the ``become`` method from within the Actor.
* Invoke the ``context.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
@ -493,30 +463,11 @@ pushed and popped.
To hotswap the Actor body using the ``HotSwap`` message:
.. code-block:: scala
actor ! HotSwap( context => {
case message => context reply "hotswapped body"
})
.. includecode:: code/ActorDocSpec.scala#hot-swap-message
To hotswap the Actor using ``become``:
.. code-block:: scala
def angry: Receive = {
case "foo" => context reply "I am already angry?"
case "bar" => become(happy)
}
def happy: Receive = {
case "bar" => context reply "I am already happy :-)"
case "foo" => become(angry)
}
def receive = {
case "foo" => become(angry)
case "bar" => become(happy)
}
.. includecode:: code/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
@ -526,35 +477,12 @@ example of it is in example where it is used to implement a Finite State Machine
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]
swap ! Swap // prints Hi
swap ! Swap // prints Ho
swap ! Swap // prints Hi
swap ! Swap // prints Ho
swap ! Swap // prints Hi
swap ! Swap // prints Ho
.. includecode:: code/ActorDocSpec.scala#swapper
Encoding Scala Actors nested receives without accidentally leaking memory
-------------------------------------------------------------------------
See this `Unnested receive example <https://gist.github.com/797035>`_.
See this `Unnested receive example <http://github.com/jboner/akka/blob/master/akka/akka-docs/scala/code/UnnestedReceives.scala>`_.
Downgrade
@ -579,7 +507,7 @@ Revert the Actor body using the ``unbecome`` method:
.. code-block:: scala
def receive: Receive = {
def receive = {
case "revert" => unbecome()
}
@ -625,16 +553,11 @@ messages on that mailbox, will be there as well.
What happens to the actor
-------------------------
If an exception is thrown and the actor is supervised, the actor object itself
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).
If the actor is _not_ supervised, but its lifeCycle is set to Permanent
(default), it will just keep on processing messages as if nothing had happened.
If the actor is _not_ supervised, but its lifeCycle is set to Temporary, it will
be stopped immediately.
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
@ -644,32 +567,4 @@ 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)
.. includecode:: code/ActorDocSpec.scala#receive-orElse

197
akka-docs/scala/code/ActorDocSpec.scala
View file

@ -1,16 +1,19 @@
package akka.docs.stm
package akka.docs.actor
//#imports1
import akka.actor.Actor
import akka.event.Logging
//#imports1
//#imports2
import akka.actor.ActorSystem
//#imports2
import org.scalatest.{ BeforeAndAfterAll, WordSpec }
import org.scalatest.matchers.MustMatchers
import akka.testkit._
import akka.util.duration._
//#imports
import akka.actor.Actor
import akka.event.Logging
//#imports
//#my-actor
class MyActor extends Actor {
val log = Logging(context.system, this)
@ -21,12 +24,110 @@ class MyActor extends Actor {
}
//#my-actor
case class DoIt(msg: Message)
case class Message(s: String)
//#context-actorOf
class FirstActor extends Actor {
val myActor = context.actorOf[MyActor]
//#context-actorOf
//#anonymous-actor
def receive = {
case m: DoIt ⇒
context.actorOf(new Actor {
def receive = {
case DoIt(msg) ⇒
val replyMsg = doSomeDangerousWork(msg)
sender ! replyMsg
self.stop()
}
def doSomeDangerousWork(msg: Message): String = { "done" }
}) ! m
case replyMsg: String ⇒ sender ! replyMsg
}
//#anonymous-actor
}
//#system-actorOf
object Main extends App {
val system = ActorSystem("MySystem")
val myActor = system.actorOf[FirstActor]
//#system-actorOf
}
//#swapper
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[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
}
//#swapper
//#receive-orElse
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
}
class SpecificActor extends GenericActor {
def specificMessageHandler = {
case event: MyMsg ⇒ printf("specific: %s\n", event.subject)
}
}
case class MyMsg(subject: String)
//#receive-orElse
class ActorDocSpec extends AkkaSpec(Map("akka.loglevel" -> "INFO")) {
"import context" in {
//#import-context
class FirstActor extends Actor {
import context._
val myActor = actorOf[MyActor]
def receive = {
case x ⇒ myActor ! x
}
}
//#import-context
val first = system.actorOf(new FirstActor)
first.stop()
}
"creating actor with AkkaSpec.actorOf" in {
//#creating-actorOf
val myActor = system.actorOf[MyActor]
//#creating-actorOf
// testing the actor
@ -62,4 +163,82 @@ class ActorDocSpec extends AkkaSpec(Map("akka.loglevel" -> "INFO")) {
myActor.stop()
}
"creating actor with Props" in {
//#creating-props
import akka.actor.Props
val dispatcher = system.dispatcherFactory.fromConfig("my-dispatcher")
val myActor = system.actorOf(Props[MyActor].withDispatcher(dispatcher), name = "myactor")
//#creating-props
myActor.stop()
}
"using ask" in {
//#using-ask
class MyActor extends Actor {
def receive = {
case x: String ⇒ sender ! x.toUpperCase
case n: Int ⇒ sender ! (n + 1)
}
}
val myActor = system.actorOf(new MyActor)
implicit val timeout = system.settings.ActorTimeout
val future = myActor ? "hello"
future.as[String] match {
case Some(answer) ⇒ //...
case None ⇒ //...
}
val result: Option[Int] = for (x ← (myActor ? 3).as[Int]) yield { 2 * x }
//#using-ask
myActor.stop()
}
"using receiveTimeout" in {
//#receive-timeout
import akka.actor.ReceiveTimeout
import akka.util.duration._
class MyActor extends Actor {
context.receiveTimeout = Some(30 seconds)
def receive = {
case "Hello" ⇒ //...
case ReceiveTimeout ⇒ throw new RuntimeException("received timeout")
}
}
//#receive-timeout
}
"using hot-swap" in {
//#hot-swap-actor
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)
}
}
//#hot-swap-actor
val actor = system.actorOf(new MyActor)
//#hot-swap-message
import akka.actor.HotSwap
actor ! HotSwap(context ⇒ {
case message ⇒ context.sender ! "hotswapped body"
})
//#hot-swap-message
}
}

48
akka-docs/scala/code/UnnestedReceives.scala Normal file
View file

@ -0,0 +1,48 @@
package akka.docs.actor
import akka.actor._
import akka.actor.Actor._
import scala.collection.mutable.ListBuffer
/**
* Requirements are as follows:
* The first thing the actor needs to do, is to subscribe to a channel of events,
* Then it must replay (process) all "old" events
* Then it has to wait for a GoAhead signal to begin processing the new events
* It mustn't "miss" events that happen between catching up with the old events and getting the GoAhead signal
*/
class UnnestedReceives extends Actor {
import context.become
//If you need to store sender/senderFuture you can change it to ListBuffer[(Any, Channel)]
val queue = new ListBuffer[Any]()
//This message processes a message/event
def process(msg: Any): Unit = println("processing: " + msg)
//This method subscribes the actor to the event bus
def subscribe() {} //Your external stuff
//This method retrieves all prior messages/events
def allOldMessages() = List()
override def preStart {
//We override preStart to be sure that the first message the actor gets is
//'Replay, that message will start to be processed _after_ the actor is started
self ! 'Replay
//Then we subscribe to the stream of messages/events
subscribe()
}
def receive = {
case 'Replay ⇒ //Our first message should be a 'Replay message, all others are invalid
allOldMessages() foreach process //Process all old messages/events
become { //Switch behavior to look for the GoAhead signal
case 'GoAhead ⇒ //When we get the GoAhead signal we process all our buffered messages/events
queue foreach process
queue.clear
become { //Then we change behaviour to process incoming messages/events as they arrive
case msg ⇒ process(msg)
}
case msg ⇒ //While we haven't gotten the GoAhead signal, buffer all incoming messages
queue += msg //Here you have full control, you can handle overflow etc
}
}
}