Lesson 2.4: Switching Actor Behavior at Run-time with BecomeStacked and UnbecomeStacked

In this lesson we're going to learn about one of the really cool things actors can do: change their behavior at run-time!

Key Concepts / Background

Let's start with a real-world scenario in which you'd want the ability to change an actor's behavior.

Real-World Scenario: Authentication

Imagine you're building a simple chat system using Akka.NET actors, and here's what your UserActor looks like - this is the actor that is responsible for all communication to and from a specific human user.

public class UserActor : ReceiveActor {
	private readonly string _userId;
	private readonly string _chatRoomId;

	public UserActor(string userId, string chatRoomId) {
		_userId = userId;
		_chatRoomId = chatRoomId;
		Receive<IncomingMessage>(inc => inc.ChatRoomId == _chatRoomId,
			inc => {
				// print message for user
			});
		Receive<OutgoingMessage>(inc => inc.ChatRoomId == _chatRoomId,
			inc => {
				// send message to chatroom
			});
	}
}

So we have basic chat working - yay! But… right now there's nothing to guarantee that this user is who they say they are. This system needs some authentication.

How could we rewrite this actor to handle these same types of chat messages differently when:

• The user is authenticating
• The user is authenticated, or
• The user couldn't authenticate?

Simple: we can use switchable actor behaviors to do this!

What is switchable behavior?

One of the core attributes of an actor in the Actor Model is that an actor can change its behavior between messages that it processes.

This capability allows you to do all sorts of cool stuff, like build Finite State Machines or change how your actors handle messages based on other messages they've received.

Switchable behavior is one of the most powerful and fundamental capabilities of any true actor system. It's one of the key features enabling actor reusability, and helping you to do a massive amount of work with a very small code footprint.

How does switchable behavior work?

The Behavior Stack

Akka.NET actors have the concept of a "behavior stack". Whichever method sits at the top of the behavior stack defines the actor's current behavior. Currently, that behavior is Authenticating():

Use Become and BecomeStacked to adopt new behavior

Whenever we call BecomeStacked, we tell the ReceiveActor to push a new behavior onto the stack. This new behavior dictates which Receive methods will be used to process any messages delivered to an actor.

Here's what happens to the behavior stack when our example actor becomes Authenticated via BecomeStacked:

NOTE: If you're following along using the eBook / .ePub, you won't see the animation. Click here to see it.

NOTE: Become will delete the old behavior off of the stack - so the stack will never have more than one behavior in it at a time.

Use BecomeStacked if you want to push behavior onto the stack, and UnbecomeStacked if you want to revert to a previous behavior. Most users only ever need to use Become.

Use UnbecomeStacked to revert to old behavior

To make an actor revert to the previous behavior in the behavior stack, all we have to do is call UnbecomeStacked.

Whenever we call UnbecomeStacked, we pop our current behavior off of the stack and replace it with the previous behavior from before (again, this new behavior will dictate which Receive methods are used to handle incoming messages).

Here's what happens to the behavior stack when our example actor UnbecomeStackeds:

NOTE: If you're following along using the eBook / .ePub, you won't see the animation. Click here to see it.

What is the API to change behaviors?

The API to change behaviors is very simple:

• Become - Replaces the current receive loop with the specified one. Eliminates the behavior stack.
• BecomeStacked - Adds the specified method to the top of the behavior stack, while maintaining the previous ones below it;
• UnbecomeStacked - Reverts to the previous receive method from the stack (only works with BecomeStacked).

The difference is that BecomeStacked preserves the old behavior, so you can just call UnbecomeStacked to go back to the previous behavior. The preference of one over the other depends on your needs. You can call BecomeStacked as many times as you need, and you can call UnbecomeStacked as many times as you called BecomeStacked. Additional calls to UnbecomeStacked won't do anything if the current behavior is the only behavior in the stack.

Isn't it problematic for actors to change behaviors?

No, actually it's safe and is a feature that gives your ActorSystem a ton of flexibility and code reuse.

Here are some common questions about switchable behavior:

When is the new behavior applied?

We can safely switch actor message-processing behavior because Akka.NET actors only process one message at a time. The new message processing behavior won't be applied until the next message arrives.

Isn't it bad that Become blows away the behavior stack?

No, not really. This is the way it's most commonly used, by far. Explicitly switching from one behavior to another is the most common approach used for switching behavior. Simple, explicit switches also make it much easier to read and reason about your code.

If you find you actually need to take advantage of the behavior stack—and a simple, explicit Become(YourNewBehavior) won't work for the situation—the behavior stack is available to you.

In this lesson, we use BecomeStacked and UnbecomeStacked to demonstrate them. Usually we just use Become.

How deep can the behavior stack go?

The stack can go really deep, but it's not unlimited.

Also, each time your actor restarts, the behavior stack is cleared and the actor starts from the initial behavior you've coded.

What happens if you call UnbecomeStacked and with nothing left in the behavior stack?

Nothing - UnbecomeStacked is a safe method and won't do anything if the current behavior is the only behavior in the stack.

Back to the real-world example

Okay, now that you understand switchable behavior, let's return to our real-world scenario and see how it is used. Recall that we need to add authentication to our chat system actor.

So, how could we rewrite this actor to handle chat messages differently when:

• The user is authenticating
• The user is authenticated, or
• The user couldn't authenticate?

Here's one way we can implement switchable message behavior in our UserActor to handle basic authentication:

public class UserActor : ReceiveActor {
	private readonly string _userId;
	private readonly string _chatRoomId;

	public UserActor(string userId, string chatRoomId) {
		_userId = userId;
		_chatRoomId = chatRoomId;

		// start with the Authenticating behavior
		Authenticating();
	}

	protected override void PreStart() {
		// start the authentication process for this user
		Context.ActorSelection("/user/authenticator/")
			.Tell(new AuthenticatePlease(_userId));
	}

	private void Authenticating() {
		Receive<AuthenticationSuccess>(auth => {
			Become(Authenticated); //switch behavior to Authenticated
		});
		Receive<AuthenticationFailure>(auth => {
			Become(Unauthenticated); //switch behavior to Unauthenticated
		});
		Receive<IncomingMessage>(inc => inc.ChatRoomId == _chatRoomId,
			inc => {
				// can't accept message yet - not auth'd
			});
		Receive<OutgoingMessage>(inc => inc.ChatRoomId == _chatRoomId,
			inc => {
				// can't send message yet - not auth'd
			});
	}

	private void Unauthenticated() {
		//switch to Authenticating
		Receive<RetryAuthentication>(retry => Become(Authenticating));
		Receive<IncomingMessage>(inc => inc.ChatRoomId == _chatRoomId,
			inc => {
				// have to reject message - auth failed
			});
		Receive<OutgoingMessage>(inc => inc.ChatRoomId == _chatRoomId,
			inc => {
				// have to reject message - auth failed
			});
	}

	private void Authenticated() {
		Receive<IncomingMessage>(inc => inc.ChatRoomId == _chatRoomId,
			inc => {
				// print message for user
			});
		Receive<OutgoingMessage>(inc => inc.ChatRoomId == _chatRoomId,
			inc => {
				// send message to chatroom
			});
	}
}

Whoa! What's all this stuff? Let's review it.

First, we took the Receive<T> handlers defined on our ReceiveActor and moved them into three separate methods. Each of these methods represents a state that will control how the actor processes messages:

• Authenticating(): this behavior is used to process messages when the user is attempting to authenticate (initial behavior).
• Authenticated(): this behavior is used to process messages when the authentication operation is successful; and,
• Unauthenticated(): this behavior is used to process messages when the authentication operation fails.

We called Authenticating() from the constructor, so our actor began in the Authenticating() state.

This means that only the Receive<T> handlers defined in the Authenticating() method will be used to process messages (initially).

However, if we receive a message of type AuthenticationSuccess or AuthenticationFailure, we use the Become method (docs) to switch behaviors to either Authenticated or Unauthenticated, respectively.

Can I switch behaviors in an UntypedActor?

Yes, but the syntax is a little different inside an UntypedActor. To switch behaviors in an UntypedActor, you have to access BecomeStacked and UnbecomeStacked via the ActorContext, instead of calling them directly.

These are the API calls inside an UntypedActor:

• Context.Become(Receive rec) - changes behavior without preserving the previous behavior on the stack;
• Context.BecomeStacked(Receive rec) - pushes a new behavior on the stack or
• Context.UnbecomeStacked() - pops the current behavior and switches to the previous (if applicable.)

The first argument to Context.Become is a Receive delegate, which is really any method with the following signature:

void MethodName(object someParameterName);

This delegate is just used to represent another method in the actor that receives a message and represents the new behavior state.

Here's an example (OtherBehavior is the Receive delegate):

public class MyActor : UntypedActor {
	protected override void OnReceive(object message) {
		if(message is SwitchMe) {
			// preserve the previous behavior on the stack
			Context.BecomeStacked(OtherBehavior);
		}
	}

	// OtherBehavior is a Receive delegate
	private void OtherBehavior(object message) {
		if(message is SwitchMeBack) {
			// switch back to previous behavior on the stack
			Context.UnbecomeStacked();
		}
	}
}

Aside from those syntactical differences, behavior switching works exactly the same way across both UntypedActor and ReceiveActor.

Now, let's put behavior switching to work for us!

Exercise

In this lesson we're going to add the ability to pause and resume live updates to the ChartingActor via switchable actor behaviors.

Phase 1 - Add a New Pause / Resume Button to Main.cs

This is the last button you'll have to add, we promise.

Go to the [Design] view of Main.cs and add a new button with the following text: PAUSE ||

Go to the Properties window in Visual Studio and change the name of this button to btnPauseResume.

Double click on the btnPauseResume to add a click handler to Main.cs.

private void btnPauseResume_Click(object sender, EventArgs e)
{

}

We'll fill this click handler in shortly.

Phase 2 - Add Switchable Behavior to ChartingActor

We're going to add some dynamic behavior to the ChartingActor - but first we need to do a little cleanup.

First, add a using reference for the Windows Forms namespace at the top of Actors/ChartingActor.cs.

// Actors/ChartingActor.cs

using System.Windows.Forms;

Next we need to declare a new message type inside the Messages region of ChartingActor.

// Actors/ChartingActor.cs - add inside the Messages region
/// <summary>
/// Toggles the pausing between charts
/// </summary>
public class TogglePause { }

Next, add the following field declaration just above the ChartingActor constructor declarations:

// Actors/ChartingActor.cs - just above ChartingActor's constructors

private readonly Button _pauseButton;

Move all of the Receive<T> declarations from ChartingActor's main constructor into a new method called Charting().

// Actors/ChartingActor.cs - just after ChartingActor's constructors
private void Charting()
{
    Receive<InitializeChart>(ic => HandleInitialize(ic));
    Receive<AddSeries>(addSeries => HandleAddSeries(addSeries));
    Receive<RemoveSeries>(removeSeries => HandleRemoveSeries(removeSeries));
    Receive<Metric>(metric => HandleMetrics(metric));

	//new receive handler for the TogglePause message type
    Receive<TogglePause>(pause =>
    {
        SetPauseButtonText(true);
        BecomeStacked(Paused);
    });
}

Add a new method called HandleMetricsPaused to the ChartingActor's Individual Message Type Handlers region.

// Actors/ChartingActor.cs - inside Individual Message Type Handlers region
private void HandleMetricsPaused(Metric metric)
{
    if (!string.IsNullOrEmpty(metric.Series) 
        && _seriesIndex.ContainsKey(metric.Series))
    {
        var series = _seriesIndex[metric.Series];
        // set the Y value to zero when we're paused
        series.Points.AddXY(xPosCounter++, 0.0d);
        while (series.Points.Count > MaxPoints) series.Points.RemoveAt(0);
        SetChartBoundaries();
    }
}

Define a new method called SetPauseButtonText at the very bottom of the ChartingActor class:

// Actors/ChartingActor.cs - add to the very bottom of the ChartingActor class
private void SetPauseButtonText(bool paused)
    {
        _pauseButton.Text = string.Format("{0}", !paused ? "PAUSE ||" : "RESUME ->");
    }

Add a new method called Paused just after the Charting method inside ChartingActor:

// Actors/ChartingActor.cs - just after the Charting method
private void Paused()
{
    Receive<Metric>(metric => HandleMetricsPaused(metric));
    Receive<TogglePause>(pause =>
    {
        SetPauseButtonText(false);
        UnbecomeStacked();
    });
}

And finally, let's replace both of ChartingActor's constructors:

public ChartingActor(Chart chart, Button pauseButton) :
    this(chart, new Dictionary<string, Series>(), pauseButton)
{
}

public ChartingActor(Chart chart, Dictionary<string, Series> seriesIndex,
    Button pauseButton)
{
    _chart = chart;
    _seriesIndex = seriesIndex;
    _pauseButton = pauseButton;
    Charting();
}

Phase 3 - Update the Main_Load and Pause / Resume Click Handler in Main.cs

Since we changed the constructor arguments for ChartingActor in Phase 2, we need to fix this inside our Main_Load event handler.

// Main.cs - Main_Load event handler
_chartActor = Program.ChartActors.ActorOf(Props.Create(() => 
    new ChartingActor(sysChart, btnPauseResume)), "charting");

And finally, we need to update our btnPauseResume click event handler to have it tell the ChartingActor to pause or resume live updates:

//Main.cs - btnPauseResume click handler
private void btnPauseResume_Click(object sender, EventArgs e)
{
    _chartActor.Tell(new ChartingActor.TogglePause());
}

Once you're done

Build and run SystemCharting.sln and you should see the following:

NOTE: If you're following along using the eBook / .ePub, you won't see the animation. Click here to see it.

Compare your code to the code in the /Completed/ folder to compare your final output to what the instructors produced.

Great job!

YEAAAAAAAAAAAAAH! We have a live updating chart that we can pause over time!

Here is a high-level overview of our working system at this point:

But wait a minute!

What happens if I toggle a chart on or off when the ChartingActor is in a paused state?

NOTE: If you're following along using the eBook / .ePub, you won't see the animation. Click here to see it.

DOH!!!!!! It doesn't work!

This is exactly the problem we're going to solve in the next lesson, using a message Stash to defer processing of messages until we're ready.

Let's move onto Lesson 5 - Using Stash to Defer Processing of Messages.

Any questions?

Come ask any questions you have, big or small, in this ongoing Bootcamp chat with the Petabridge & Akka.NET teams.

Problems with the code?

If there is a problem with the code running, or something else that needs to be fixed in this lesson, please create an issue and we'll get right on it. This will benefit everyone going through Bootcamp.