Getting Started with Actors

You need to know only a few quite self-explanatory functions to get started with concurrent actor programming:

API functionBrief description
spawncreate an actor with a behavior and get a Link to it,
send, receivesend and receive messages over an actor Link,
request, callrequest or call something from an actor (behavior),
castcast something to an actor (behavior),
become, become!change the behavior of an actor,
query, update!query or update an actor's state.

When we introduce those functions, please follow along in your Julia REPL and don't hesitate to try things out.

Create an actor

The basic mechanism to create a new actor is the spawn function. You have to import it explicitly:

julia> using Actors, .Threads
julia> import Actors: spawn, newLink

You spawn an actor with a behavior. A behavior is a callable Julia object (a function, functor …) and some parameters to it. We start our first actor with a parameterless threadid behavior, which returns the thread number, where it resides:

julia> myactor = spawn(threadid)Link{Channel{Any}}(Channel{Any}(32), 1, :default)

That returned an actor Link. Now - as we will discover right away - our actor is responsive and waits for our messages.

A Link is the actor's mail address and its only representation. You use it to send messages to the actor or to use other messaging functions like info. If you call it, you trigger its behavior and get a result back:

julia> info(myactor)         # get some info from itActor    default
Behavior threadid
Pid      1, Thread 1
Task     @0x00007f7075b2cd30
Ident    x-lutub-likuf-suhub
julia> call(myactor) # call its behavior function1

Now what happens if you send your actor a message which it cannot understand?

julia> send(myactor, :boom)  # cause the actor to fail(:boom,)
julia> info(myactor)Actors.Timeout()

We caused our actor to fail. Ouch!

Actor behavior

Now let's write our own behavior function! We want our actor to execute a given function f on parameters args... and to send the result back to a given address addr:

julia> calc(addr::Link, f::Function, args...) = send(addr, f(args...))calc (generic function with 1 method)

With newLink we create a link, where we can receive the actor's answer. We spawn a new actor with our calc behavior function and give it that newly created link as acquaintance parameter:

julia> me = newLink()Link{Channel{Any}}(Channel{Any}(32), 1, :local)
julia> myactor = spawn(calc, me)Link{Channel{Any}}(Channel{Any}(32), 1, :default)

Now our actor holds the me link as acquaintance. Its behavior is the calc function together with the me link.

send and receive Messages

We need to send it only the rest of the parameters to cause it to execute its behavior and to send the result back. If we send it a + and some values, it will add them and send the result to the given me link:

julia> send(myactor, +, 1, 2, 3, 4)(+, 1, 2, 3, 4)
julia> receive(me)10

Now we did asynchronous communication with our actor. After sending it something, we could have done other work and then received the result later.

Change behavior

If we want our actor to do only multiplication, we can change its behavior with become! to calc with new acquaintances:

julia> become!(myactor, calc, me, *);

The actor's new behavior is calc with two acquaintances me and *, and thus it does multiplication only. As before we send it the communication parameters to multiply and receive the result:

julia> send(myactor, 1,2,3,4)(1, 2, 3, 4)
julia> receive(me)24

An actor can change also its own behavior with become inside its behavior function. Instead become! is a call from the outside of an actor. A behavior change is effective for the next message an actor receives.

A communication failure

What happens if a communication fails? Let's try that out: We cause our actor to fail, so it will not send back anything. Then we try to receive an answer:

julia> send(myactor, +, 5, "x")(+, 5, "x")
julia> receive(me)Actors.Timeout()

After some seconds we got a Timeout().

The actor protocol

We don't give up with it and start it again, but now we want it to be an adding machine with an offset of 1000:

julia> myactor = spawn(+, 1000)Link{Channel{Any}}(Channel{Any}(32), 1, :default)

Our actor now has no acquaintance of me, neither has its behavior any send instruction. If we send it something, it will only add that to 1000 but not respond anything. The message protocol allows us to communicate with actors even if their behaviors don't send anything. Actors understand messaging patterns.

The request function is a wrapper for synchronous communication. It will create a link internally and send it with the communication parameters as a Call (or another given message type) to the actor. That sends a Response back to the received link, and request then delivers the response:

julia> request(myactor, 1,2,3)1006

This is synchronous communication since request blocks until it receives the result (or a Timeout()).

call, cast and query

Then there are more user API functions.

We can do asynchronous communication with our actor if we use call with the me link. This sends the given link to the actor and it responds to it. Then we can receive the result asynchronously:

julia> call(myactor, me, 1000)Actors.Call((1000,), Link{Channel{Any}}(Channel{Any}(32), 1, :local))
julia> receive(me).y2000

If we don't provide a return link to call, it will use request and deliver the result synchronously:

julia> call(myactor, 2000)3000

Another possibility to communicate asynchronously with an actor is to cast it parameters and then to query the result afterwards.

julia> cast(myactor, 3000)Actors.Cast((3000,))
julia> query(myactor, :res)4000

But this will work fine only if between those two calls to myactor there is not another actor communicating with it. If for example our actor has mutual variables as acquaintances, we can use cast to set parameters or to do anything else where we don't need a response.

More Control: exec, update!, exit!

With exec we can tell an actor to execute any function and to deliver the result:

julia> exec(myactor, broadcast, cos, pi .* (-2:2))5-element Vector{Float64}:

Actors have internal state variables, which we normally don't need to work with. We can update! those variables. Let's update! the actor's current acquaintance parameter (1000) to 500. Then it adds to 500:

julia> update!(myactor, Args(500))Actors.Update(:arg, Args{Tuple{Int64}, Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}}((500,), Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}}()))
julia> call(myactor, 500)1000

But we could have achieved the same with become!(myactor, +, 500).

Finally we exit! our actor since we are finished with this introduction:

julia> exit!(myactor)Actors.Exit{Symbol, Nothing, Nothing}(:normal, nothing, nothing, nothing)
julia> send(myactor, 500)(500,)

Trying to send it something now throws an exception.

Actor Systems

For this introduction we have worked with one actor. But there is no point in doing that. Actors come in systems. They can assume different and arbitrary behaviors, create other actors and then compose into systems by communicating via messages. Thus they can represent arbitrary concurrent systems.

Let's build a minimal actor system. It consists of two actors, a greeter actor for composing a greeting and a sayhello actor for communication:

julia> greet(greeting, msg) = greeting*", "*msg*"!" # a greetings server behaviorgreet (generic function with 1 method)
julia> hello(greeter, to) = request(greeter, to) # a greetings client behaviorhello (generic function with 1 method)
julia> greeter = spawn(greet, "Hello") # start the server with a greet stringLink{Channel{Any}}(Channel{Any}(32), 1, :default)
julia> sayhello = spawn(hello, greeter) # start the client with a link to the serverLink{Channel{Any}}(Channel{Any}(32), 1, :default)
julia> request(sayhello, "World") # request the client"Hello, World!"
julia> request(sayhello, "Kermit")"Hello, Kermit!"

We could have done that without actors. But, come on, add a third one, saying hello asynchronously. Then you need a forth one to channel the access of the hello actors to the console …