Presently the behaviour of Actors.jl is changing quite rapidly so this documentation may not be up to date.

Background

The Actor Model

Informally, an Actor is some arbitrary state (e.g. a Julia struct), an inbox which contains received messages and some behaviours (e.g. Julia methods) which are activated when messages are taken from the inbox.

Each actor has at least one address which can be used to send it messages. Messages are some arbitrary value (e.g. 1 or struct Foo end).

Messages are delivered in an arbitrary order and are buffered. That is the inbox (a Julia Channel in this case) can store multiple messages. This makes Actors asynchronous as they can continue to receive messages before finishing processing the current one.

While processing a message an Actor may update its state, thus changing its behaviour. It may send more messages, create new actors or do nothing in response to a message.

Note

Often when people refer to the Actor Model, they really mean Erlang's implementation of it. However there is also formal definition of the Actor model set forth in Actors. This library takes inspiration from everywhere.

Messages Vs. Methods

Julia's multi-methods (which we exploit heavily in Actors.jl) are quite similar to messages except in three very important regards.

  1. Messages are asynchronous
  2. Messages can be ignored
  3. Messages never return a value

Sending messages is similar to calling a multi-method with @spawn (similar to @async, but really happens in parallel) without fetching the task's result.

struct Foo end
struct Bar end

behaviour(::Foo) = println("Dispatched Foo")
behaviour(::Bar) = println("Dispatched Bar")

send(msg) = Threads.@spawn behaviour(msg)

foo_task = send(Foo())
bar_task = send(Bar())

sleep(1)

Dispatched Foo
Dispatched Bar

The only way we know that foo_task and bar_task succeeded and what order they were processed in is if look at the side effects. In this case the side effects are printing to stdout.

With actual messages we would confirm some action by sending another message in return.

Stopwatch tutorial

To begin with we will create an actor system with an actor which can function as a stop watch.

Setting the Stage and starting the Play

At the root of an independent "actor system" is the Stage. This the first Actors.Actor and it manages the system. Like any other actor you can pass it messages and it can pass them back.

Stage bootstraps the system and then passes a message (Genesis!) to the Play actor to get things started. The Play actor is defined by you, the user. The Play actor can be of any type, even an Int.

Note

This is a lie, the type is actually Actor{Int, Any}; Int is the type of the actor's state. However it is sometimes to convenient to refer to the state as the actor itself.

using Actors
# This allows us to add new Actors.hear methods without writing Actors.hear
import Actors: hear

function hear(s::Scene{Int}, ::Genesis!)
	@say_info s "My state is $(my(s))"

	leave!(s)
end

play!(0)
Info Int64@4 Main.##ex-#424 none:3: My state is 0

OK, this is a bit silly, lets do it again with a dedicated play type.

using Actors
import Actors: hear

mutable struct StopwatchPlay
	i::Int
end

function hear(s::Scene{StopwatchPlay}, ::Genesis!)
	my(s).i = 1

	@say_info s "My state is $(my(s).i)"

	leave!(s)
end

play!(StopwatchPlay(0))
Info Main.##ex-#425.StopwatchPlay@4 Main.##ex-#425 none:5: My state is 1

So, first we define our play actor, then we define a message handler for Genesis! (by defining a new hear method) and then we start the actor system with play!.

The Stage is passed StopwatchPlay which it turns into an Actors.Actor and it then sends that actor Genesis!. Looking at hear(s::Scene{StopwatchPlay}, ...) we can see the special Scene variable which has our play actor as the type parameter. In any other framework this would be called the 'context'.

The majority of the Actors.jl's API takes s::Scene as the first argument. This allows us to get commonly needed information about the actor system and current actor. It is recommended to always call this variable s, so that you have the option of using unhygenic macros which implicitly use this information.

The my accessor method allows us to get or set the actor state.

The @say_info macro sends a message to an automatically created actor which logs messages to stdout. Finally we send Leave! to the Stage actor which then propagates this to all other actors and shutsdown the actor system.

Now let's actually create a stopwatch...

using Actors
import Actors: hear

struct Start! end
struct Stop! end
struct Status!
	re::Id
end

struct StopwatchPlay end

mutable struct Watch
	start::Union{UInt64, Nothing}
	stop::Union{UInt64, Nothing}
end

hear(s::Scene{Watch}, ::Start!) = my(s).start = time_ns()
hear(s::Scene{Watch}, ::Stop!) = my(s).stop = time_ns()
hear(s::Scene{Watch}, msg::Status!) = let w = my(s)
	say(s, msg.re, w.stop - w.start)
end

function hear(s::Scene{StopwatchPlay}, ::Genesis!)
	watch = invite!(s, Watch(nothing, nothing))
	say(s, watch, Start!())
	say(s, watch, Stop!())
	time = ask(s, watch, Status!(me(s)), UInt64)

	@say_info s "It took $(time)ns to process Start and Stop"

	leave!(s)
end

play!(StopwatchPlay())
Info Main.##ex-#426.StopwatchPlay@4 Main.##ex-#426 none:8: It took 10453084ns to process Start and Stop

Much new stuff has been added here; firstly the Start!, Stop! and Status! message types. These are just plain Julia types, but by convention a ! is added at the end to distiguish dedicated message types from everything else.

The start and stop messages don't contain any data, their type just decides which hear method is called. Which I hope displays the power of Julia's multi-methods.

Note

You may override Actors.listen! instead to process messages. This allows you to avoid the multiple dispatch on hear.

The status message contains a return address, which by convention is given the name re. Messages often don't evoke a response or, if they do, it is not directed at the originator of a message, so we must specify the address a response is sent to explicitly.

Next, a definition for a new actor has been added (or rather a new Actors.Actor's state). This is Watch which contains the start and stop times. Smaller implementation(s) could be achieved by removing the stop time and merging Status! with Stop!.

Then there are the message handlers, see that we use the Id from Status!.re in the status handler to send the time back to the requestor (or some other arbitrary actor for that matter).

Finally there are a couple of new functions being used in the Genesis! handler. The first is invite! which allows us to create a new actor. It returns the new actor's address (the type is called Id to avoid typing and anger pedants) which we can use to send it messages.

After starting and stopping the message we use ask to get the value of the recorded time.

Warning

There are some pretty big problems with this implementation of a stopwatch. Importantly, there is no guaranteed order for message delivery. In practice the messages are unlikely to get switched around in a local system, but you can't rely on this. So the messages should contain some sequence information if this is important. Of course it is also fairly pointless creating a stopwatch with nanosecond precision which is using message passing, but never mind that.