Today I want to talk about one of the Akka.NET features, I think deserves a lot more attention - Akka.IO. What it gives you, is the ability to connect your actors directly to OS socket layer. This way you could handle any TCP/UDP-based protocol directly in your actor model. Just imagine the possibilities :)

But lets start from the basics. What would you say for simple telnet listener? We'll create a simple TCP server, which will listen for incoming connections and print input on the console.

To do so, we'll need two types of actors:

  1. Listener which will bind itself to the given endpoint and accept incoming connections.
  2. Connection handler, which will be delegated to serve particular client, once it connects to our server.

Nice thing here is, that whole TCP model uses the same ordinary actors, you can create by yourself. The whole process is pretty simple:

  • Create your TCP listener actor.
  • Bind it to some endpoint.
  • Handle connection event by creating connection handler actor and registering it for an incoming connection.
  • Inside connection handler, handle all received data, print it or make it stop, once connection gets closed.

First part is pretty simple. To bind actor to specific endpoint you can use so called TCP manager, which is part of Akka.NET extension. It's just an actor and you can get it by calling Context.System.Tcp() method. Then just send it a new Tcp.Bind(Self, endpoint) message if you want to register current actor as a listener for defined endpoint. Everything is message-based.

Second part is making our listener aware of incoming connections. They come in the form of Tcp.Connected messages. Once you receive them, Sender value associated with message is instance of actor reference being an abstraction over provided connection. What's great here is that it honors all attributes of actors, including location transparency. This means, we can handle incoming connections by actors materialized on different machines out of the box!

In this case we'll simply create a new actor and register it for this connection by sending new Tcp.Register(connectionHandler) message. To make this all clear and visible, you can see the whole actor definition below:

public class TelnetListener : ReceiveActor  
{
    public TelnetListener(EndPoint endpoint)
    {
        Context.System.Tcp().Tell(new Tcp.Bind(Self, endpoint));
        Receive<Tcp.Connected>(connected =>
        {
            var connection = Sender;
            var connectionHandler = Context.ActorOf(Props.Create(() => new TelnetHandler(connection)));
            connection.Tell(new Tcp.Register(connectionHandler));
        });
    }
}

Once you've got this ready, it's time for our connection handler actor. As we gonna provide it a reference to our connection - which as you may remember, is also an actor - we also want to link it's life time directly to the connection itself. In this context, having actor working on dead connection is unwanted. Instead we gonna watch for the connection using Context.Watch(connection) and stop current actor once it'll receive either Terminated or Tcp.ConnectionClosed message.

Then all that is left, is to handle incoming bytes from socket, transform them to string and display on the console. They come in form of Tcp.Received messages. The whole definition can be seen below:

public class TelnetHandler : ReceiveActor  
{
    public TelnetHandler(IActorRef connection)
    {
        Context.Watch(connection);
        Receive<Tcp.Received>(received =>
        {
            var text = Encoding.UTF8.GetString(received.Data.ToArray()).Trim();
            Console.Write(text);
        });
        Receive<Tcp.ConnectionClosed>(closed => Context.Stop(Self));
        Receive<Terminated>(terminated => Context.Stop(Self));
    }
}

What I think, is worth mentioning here is Data property of Tcp.Received message. It's not an usual byte array. It's instance of ByteString type instead. One of the problems with byte arrays, is that they may be heavily used in socket communication, causing a lot of allocations and deallocations, increasing pressure on .NET garbage collector. To avoid this we can use ByteStrings, which are fragments of preallocated buffer. Once they are no longer necessary, they are send back to buffer to be reused later. This way we can avoid unnecessary garbage collection cycles.

To test whole example, simply initialize TelnetListener actor under any free port, and connect to it using telnet <ip> <port> from your terminal. After that you should be able to see any text, you're writing in command line on the application console.