My exploration of Actor model started with Akka.NET framework - a .NET port of JVM-based Akka. Actor programming model made a lot of sense to me, but once I started playing with it, some questions arose. Most of those questions were related to the following definition:

An actor is a container for State, Behavior, a Mailbox, Children and a Supervisor Strategy.

So, based on the Akka.NET Bootcamp course I understood that an Actor

  • knows what kind of messages it can accept
  • does some processing of each message
  • holds some state which is changed during message processing
  • potentially changes its behavior based on the current state
  • creates and stores references to child actors
  • obtains references to other actors
  • sends messages to children and other actors

While it's nice that the framework enables us to develop for different aspects of actor behavior, it might also be dangerous in case you do all the things in one place. Ball of spaghetti mud was where I ended up during my first attempt. My actors were doing all the things from the above list and the code got messy very quick. So, the following questions popped up in my head:

How do I avoid mixing several concerns in one piece of code?

How do I make the code easily testable?

How do I minimize the usage of mutable state?

How do I avoid boilerplate code when it's not needed?

Functional Actors

I am now developing the actor-based application in F#, the functional first language. Functions are easy to reason about, reusable and testable. But the actors are usually defined in terms of objects and classes. F# supports classes but that's not the path that I'm willing to go.

How do we make actors out of functions? Well, most of the time actors don't need all the features of the framework. In this case we can define the required actor behavior in terms of a minimal function and then use creational patterns to spawn actor instances out of it.

Let's look at some common patterns that I identified. For each pattern, I will define

  • an example of a core function which implements the business logic
  • a generic function to create actors with behavior of a core function
  • an example of actor instantiation using the two functions above

Message Sink

Stateless Message Sink is the simplest type of actor.

Message Sink actor

It receives a message and executes some action on it. The action is not related to any other actors, and there is no state, so the processing of each message is always the same. Obviously, it's related to some kind of side effects: logging the message, saving the data to the external storage and so on.

So, we don't need the majority of actor features in this case. The whole actor processing could be represented by a function of type 'a -> unit. Here is an example of a core function:

let print msg =
  printn "Message received: %A" msg

So how do we make an actor out of this function? Well, it's already implemented as actorOf helper function in Akka.NET F# extensions:

let actorOfSink (f : 'a -> unit) = actorOf f

And here is how we spawn an actor instance:

let printActorRef = 
  actorOfSink print 
  |> spawn system "print-actor"

printActorRef <! 3
// "Message received: 3" is printed

That's the simplicity that I'm searching for. Let's look at a slightly more complex example.


Converter

Stateless Converter maps the incoming message into another message and sends it to another predefined actor.

Converter actor

The core of this actor is a classic function with one input and one output parameter (type 'a - 'b):

let square msg =
  msg * msg

The actor function is similar to the one of Message Sink, but it also accepts a reference to the output actor and knows how to send messages to it:

let actorOfConvert f outputRef =
  actorOf2 (fun _ msg -> outputRef <! f msg)

Here is how we spawn an instance of a Converter using our print-actor as the output:

let squareActorRef = 
  actorOfConvert square printActorRef 
  |> spawn system "square-actor"

squareActorRef <! 3
// "Message received: 9" is printed

Both actor patterns had no notion of state so far. Let's see how we can treat the statefulness in a functional way.


Stateful Sink

Let's get back to our Message Sink actor with side-effects, and make it dependent on its internal state. The state is affected by the incoming messages and is preserved until the next message comes in.

Stateful Sink actor

Does not look very functional, right? But this impression is wrong in fact. We can represent the state before a message came in - as an extra input parameter, and the state after the message got processed - as an output parameter. We start with an initial state and the output of the first message becomes the input state of the second message:

Stateful Sink functional actor

Here is an example of a function which prints out the index of a message together with the message contents:

let printIndex index msg =
  printn "Message [%i] received: %A" index msg
  index + 1

For the actor implementation we need a recursive function so we can't use actorOf2 anymore. Actor workflow is a bit more lines but still very simple:

let actorOfStatefulSink f initialState (mailbox : Actor<'a>) =

  let rec imp lastState =
    actor {
      let! msg = mailbox.Receive()
      let newState = f msg lastState
      return! imp newState
    }

  imp initialState

And here is a usage example:

let printIndexActorRef = 
  actorOfSink printIndex 1
  |> spawn system "print-ix-actor"

printActorRef <! 3
// "Message [1] received: 3" is printed

printActorRef <! 4
// "Message [2] received: 4" is printed

Stateful Converter

By now, the core function of the Stateful Converter actor should be a no-brainer for you. The actor would have two input parameters and two outputs (in a tuple). One of the outputs is a message and goes to another actor, the other output becomes an input for the next actor:

Stateful Converter actor

Here is a function which squares the messaged number, then calculates the running total and sends it forward:

let squareAndSum sum msg =
  let result = sum + msg*msg
  (result, result)

In this particular case the output message and state are equal, but they don't have to be. Here is the actor implementation:

let actorOfStatefulConvert f initialState outputRef (mailbox : Actor<'a>) =

  let rec imp lastState =
    actor {
      let! msg = mailbox.Receive()
      let (result, newState) = f msg lastState
      outputRef <! result
      return! imp newState
    }

  imp initialState

And a usage example:

let squareAndSumActorRef = 
  actorOfConvert square 0 printIndexActorRef 
  |> spawn system "square-sum-actor"

squareAndSumActorRef <! 3
// "Message [1] received: 9" is printed

squareAndSumActorRef <! 4
// "Message [2] received: 25" is printed

Converter-Supervisor

In the previous patterns the Converter actors were sending messages to predefined actor references which were not managed (or supervised in Akka terms) by those actors. Now, let's say that the actor needs to create a child to send converted messages to it afterwards:

Supervised Child actor

We can treat such child reference as the state and instantiate it when the first message comes in. (We can't spawn it before the first message because the mailbox object is required.) The message goes to the actor reference that we store in the state, something like this:

Supervised Child as State

Here is the generic actor implementation:

let actorOfConvertToChild f spawnChild (mailbox : Actor<'a>) =

  let rec imp state =
    actor {
      let newstate =
        match state with
        | Some s -> s
        | None -> spawnChild mailbox

      let! msg = mailbox.Receive()
      newstate <! f msg
      return! imp (Some newstate)
    }

  imp None

The only difference is that we accept a spawnChild function instead of pre-baked actor reference. Here is the first calculator example refactored to Print actor being a child of Square actor.

let squareWithChildRef = 
  actorOfConvertToChild print (spawnChild square "print-actor")
  |> spawn system "square-with-child-actor"

Notice that the square and print functions have exactly the same signatures and implementations as we used before, and the concern of actor hierarchy is completely separated from the business logic of the actors.

This hierarchy is handy whenever you need multiple instances of one actor type (f-actor from the picture) and corresponding instances of another actor type (g-actor):

Multiple Parents and Children actor


Router-Supervisor

Routers are the kind of actors which forward each incoming message to one or more downstream actors. In this example the downstream actors are supervised by the Router itself. So, the Router-Supervisor can have multiple children and send the result of message processing to one or more of them:

Router actor

To keep the spirit of functional actors, we represent the router logic with a function of type 'a -> seq<string * 'b>, where 'a is the type of incoming messages, 'b is the type of outgoing messages, and string represents the identifier of the actor to get the message. Here is a sample implementation:

let routeSensorData msg = 
  msg |> Seq.map (fun x -> ("sensor-actor-" + x.SensorId, x.Temperature))

Based on the incoming metadata (sensor identifier) the actor forwards its temperature to corresponding sensor-specific actor.

Here is the implementation of the generic actor function:

let actorOfRouteToChildren f spawnChild (mailbox : Actor<'a>) =

  let getActor id = 
    let actorRef = mailbox.Context.Child(id)
    if actorRef.IsNobody() then
      spawnChild id mailbox
    else 
      actorRef

  let rec imp () =
    actor {
      let! msg = mailbox.Receive()
      f msg |> Seq.iter (fun (id, x) -> (getActor id) <! x) 
      return! imp ()
    }

  imp ()

And a usage example:

let sensorRouterRef = 
  actorOfRouteToChildren routeSensorData (spawnChild square)
  |> spawn system "route-sensor-actor"

Note that spawnChild does not accept the child ID anymore because it's being controlled by the router itself.


Conclusion

With the patterns that we have so far we should be able to build quite powerful hierarchies like the one shown below:

Actor use case

There might be many other scenarios and types of actors that would make sense in your use case. I'm just showing the basic patterns, but more importantly the way of reasoning about the code. Don't let the multitude of actor aspects push you into the world of poorly structured code.