On Akka .NET Actor Creation

Abstract: this article is a bit of a rant about how actors are created in Akka .NET, and suggests a way of making it just a little bit more manageable. This is written based on my limited knowledge of Akka .NET, and I will be more than happy to stand corrected on the matters I write about.

Actors Need Props

Creating an ActorSystem and actors in Akka .NET is one of the most basic necessities, and it is something you’ll be doing all the time. Once you have an ActorSystem, you can create actors using an IActorRefFactory implementation (i.e. the ActorSystem itself, or an actor’s Context). This requires you to use Props.

            using (var actorSystem = ActorSystem.Create("MyActorSystem"))
                var props = Props.Create<MyActor>();
                var actor = actorSystem.ActorOf(props);


What the hell are Props?

This is explained by Unit 1 Lesson 3 of the Petabridge Akka .NET Bootcamp. Basically, the answer is something like: it’s a recipe for creating actors, but don’t worry about it for now; we’ll use it later.

As a matter of fact, it’s needed mainly for advanced scenarios such as remote deployment and clustering. Most of the time as you’re learning to use Akka .NET, you don’t really care about them.

Creating Props

There are three ways to create Props, all involving some manner of a call to Props.Create().

The first way is to give it a Type.

var props = Props.Create(typeof(MyActor));

This is discouraged by Akka .NET, because it has no type safety and in fact lets you do stupid things like this:


The second way is to use a generic form:

var props = Props.Create<MyActor>();

While this is encouraged in the bootcamp, I personally discourage this. This is because while it gives you type safety over the actor type, it doesn’t give you any guarantees with the parameters:


The third way is to pass in a factory method. This is a great way to create Props because it’s the only one that lets you pass dependencies into the actor’s constructor in a type-safe manner (particularly important to use constructor injection if you’re thinking of writing tests against your actors).

var props = Props.Create(() => new MyActor());

Making It Better

In reality, most of the time I don’t care about Props. So why do I have to constantly bother about them? Actually, if we take a closer look at the third way of creating Props, we can wrap them into oblivion:


See that generic Expression over there? That’s what we need to avoid all this madness. Based on that, we can create a generic method to take care of everything:

    public static class IActorRefFactoryExtensionscs
        /// <summary>
        /// Creates an actor, creating props based on the provided
        /// actor factory method.
        /// </summary>
        /// <typeparam name="T">The actor type.</typeparam>
        /// <param name="actorRefFactory">ActorSystem or actor Context.</param>
        /// <param name="actorFactory">Actor factory method.</param>
        public static IActorRef CreateActor<T>(this IActorRefFactory actorRefFactory,
            Expression<Func<T>> actorFactory) where T : ActorBase
            var props = Props.Create(actorFactory);
            var actor = actorRefFactory.ActorOf(props);
            return actor;

To create an actor directly from the ActorSystem, we now only need to do this:

var actor = actorSystem.CreateActor(() => new MyActor());

…and to do it from inside an actor, it’s just as easy:

var actor = Context.CreateActor(() => new MyActor());

Weird Conventions

This is where the rant begins.

Apart from all the weirdness associated with having to deal with Props in the first place, Unit 1 Lesson 3 of the Petabridge Akka .NET Bootcamp has this little gem that makes my day brighter every time I read it:

How do I make Props?

“Before we tell you how to make Props, let me tell you what NOT to do.

DO NOT TRY TO MAKE PROPS BY CALLING new Props(...). Similar to trying to make an actor by calling new MyActorClass(), this is fighting the framework and not letting Akka’s ActorSystem do its work under the hood to provide safe guarantees about actor restarts and lifecycle management.”

Here’s a similar gem from Unit 1 Lesson 1:

NOTE: When creating Props, ActorSystem, or ActorRef you will very rarely see the new keyword. These objects must be created through the factory methods built into Akka.NET. If you’re using new you might be making a mistake.”

Wait, what? These guys are telling us to call static Create() methods rather than using constructors. These are the same people who told us that using async/await in actors is bad (which has since been corrected). I don’t know, but I bet if you ask anyone who has done OOP before, they’ll tell you that if there’s a mistake, it’s in Akka .NET’s design.

But to really top it all, check out the following comment from Aaron Standard (Petabridge CTO and Akka .NET co-founder) on the Reddit thread about one of his articles (emphasis mine):

Orleans is a piss-poor implementation of the Actor model and breaks a lot of the conventions that make it powerful, aside from having an overall hideous set of programming conventions. But because MSFT released it, people will flock to it like lemmings.

“We’re going to keep working on Akka.NET because there’s a community supporting it now and because we believe in TypeSafe’s vision for actor systems.”

A case of the pot calling the kettle black? Or quoting Confucius, “Don’t complain about the snow on your neighbor’s roof when your own doorstep is unclean.”

In any case, my goal is not to start a flame war but to understand why the Akka .NET API (and codebase) is such a mess. If you look at the source code for ActorSystem, for instance, it does not do anything so particularly complicated that would justify banning constructor calls. In fact, the call to ActorSystem.Create() ends up here:

        private static ActorSystem CreateAndStartSystem(string name, Config withFallback)
            var system = new ActorSystemImpl(name, withFallback);
            return system;

In fact, although you shouldn’t do this, this code works just as well as what we had before:

            using (var actorSystem = new ActorSystemImpl("MyActorSystem"))

                var actor = actorSystem.CreateActor(() => new MyActor());


Why is this even public such that I can call it?


The API provided by Akka .NET, particular in ActorSystem and actor creation, is very strange indeed. We are discouraged to do something as trivial a calling a constructor, and have to deal with Props even though we won’t need them most of the time. It is hard to speculate on why this API was written this way without having the developers provide insight on it.

At the very least, aside from pointing out these obvious flaws, this article aims to suggest best practices on how to go about creating Props, and to provide an extension method to hide the existence of Props for the majority of cases where using them directly isn’t really necessary.

Using ReceiveTimeout in Akka .NET

In Akka .NET, you can opt to receive a special message when an actor is idle (i.e. does not receive any message) for a certain period of time. This message is the ReceiveTimeout, and is particularly useful to clean up idle actors when using the Entity Per Child pattern (although various other scenarios exist where this might be used).

The source code for this article is available at the Gigi Labs BitBucket repository.

Let’s consider an ActorSystem with a single actor. The ActorSystem sends a message to this actor every few seconds, using the built-in Akka .NET Scheduler:

            using (var actorSystem = ActorSystem.Create("MyActorSystem"))
                var actor = actorSystem.ActorOf(
                    Props.Create<SomeActor>(), "SomeActor");

                    initialDelay: TimeSpan.FromSeconds(1),
                    interval: TimeSpan.FromSeconds(5),
                    receiver: actor,
                    message: "Hello!",
                    sender: ActorRefs.NoSender


In the actor itself, we call SetReceiveTimeout() in order to be able to receive a ReceiveTimeout message after the specified idle period elapses. Then, we handle the ReceiveTimeout message just like we would handle any other message.

    public class SomeActor : ReceiveActor
        public SomeActor()
            this.Receive<string>(Handle, null);
            this.Receive<ReceiveTimeout>(Handle, null);

            var timeout = TimeSpan.FromSeconds(3);

        private void Handle(string msg)

        private void Handle(ReceiveTimeout msg)
            Console.WriteLine("Timeout received!");

If we run the application with the code we have so far, we get this:


Although we are handling the ReceiveTimeout message, we aren’t really doing anything to react to it. As the ReceiveTimeout documentation states, the ReceiveTimeout will keep firing after periods of inactivity.

To disable the ReceiveTimeout altogether, just call SetReceiveTimeout() again, passing in null:


You will often want to kill the actor altogether. In particular when using the Entity Per Child pattern, you will want to kill off actors that aren’t in use to keep your memory footprint low; and since you’ll have a parent actor taking care of routing messages and creating child actors as needed, any new message to that actor will cause it to be recreated.

There are different ways to kill an actor, so just pick one and use it in your ReceiveTimeout handler:

        private void Handle(ReceiveTimeout msg)
            Console.WriteLine("Timeout received!");

            Context.Stop(Context.Self); // stop the actor

Since we’re killing the actor now, any subsequent messages become undeliverable: