AKKA.NET and DeathWatchNotification - akka.net

Question

I get the following message when a ProductActor tries to Tell a ValidatorActor to validate the message. Although I see this message, I get the expected result.
I did not try to send a message from ProductActor to itself. Why still I get the following message?
[INFO][5/17/2015 8:06:03 AM][Thread 0012][akka://catalogSystem/user/productActor] Message DeathWatchNotification from akka://catalogSystem/user/productActor to akka://catalogSystem/user/productActor was not delivered. 1 dead letters encountered.
--UPDATE--
The two actors are given below:
public class ProductActor : UntypedActor
{
protected override void OnReceive(object message)
{
if (message is ReportableStatusChanged)
{
_reportableState = ((ReportableStatusChanged) message).ReportableState;
}
else
{
if (message is RetrieveProductState)
{
var state = new ProductState()
{
ReportableState = _reportableState
};
Sender.Tell(state);
}
else
{
Context.ActorSelection("akka://ProductSystem/user/ProductActor/validator").Tell(message);
}
}
}
protected override void PreStart()
{
Context.ActorOf(Props.Create(() => new ProductValidatorActor()), "validator");
base.PreStart();
}
private IReportableState _reportableState;
}
public class ProductValidatorActor : UntypedActor
{
protected override void OnReceive(object message)
{
if (message is ChangeReportableStatus)
{
Sender.Tell(new ReportableStatusChanged(ReportableStates.ReportableState));
}
}
}
This is the test to check the status:
class ChangeReportableStatusTest
{
public void Do()
{
var system = ActorSystem.Create("catalogSystem");
var ProductActor = system.ActorOf(Props.Create<ProductActor>(), "productActor");
ProductActor.Tell(new ChangeReportableStatus(true));
Thread.Sleep(50);
var state = ProductActor.Ask<ProductState>(new RetrieveProductState());
Console.WriteLine("Reportable State: " + (state.Result.ReportableState == ReportableStates.ReportableState ? "TRUE" : "FALSE"));
system.Shutdown();
system.AwaitTermination();
Console.WriteLine("Please press any key to terminate.");
Console.ReadKey();
}
}

Answer 1

You're getting a dead letters notification, which means that the message you're trying to send is not deliverable. The actor you're trying to send a message to may be dead, or it may have never existed. In this case, it appears to be the latter.
I noticed that the name of the ActorSystem that your ProductActor lives within is different in your error message (catalogSystem) vs your code (ProductSystem).
With your ActorSelection, you're sending a message to an actor path in the wrong ActorSystem, to an actor path where no actor exists. Hence the DeadLetters notice. Assuming ProductActor is created as a top-level actor in the catalogSystem, the path you're trying to send to is correct (/user/ProductActor/validator), but the actor system name is not (should be catalogSystem but here it is ProductSystem).
How to fix it
So how do you fix it? Two options:
Use the correct path in your ActorSelection like so: Context.ActorSelection("akka://catalogSystem/user/ProductActor/validator").Tell(message);. While this works, it's the wrong answer.
Since you create the ProductValidatorActor as a child of the ProductActor, just store the IActorRef of the child in the parent, and send messages to it directly. This is the approach I recommend. In this particular case, you don't need an ActorSelection at all.
It works now, but what can we learn here?
There are two lessons to take from this.
Lesson 1: don't use an ActorSelection when don't need it
Generally, you should be Telling messages to IActorRefs, not to ActorSelections. With an IActorRef, you know that the actor has existed at some point in time in the past. This is a guarantee of the Akka framework, that all IActorRefs have existed at some point, even if the actor is now dead.
With an ActorSelection, you have no such guarantee. It's kind of like UDP—you're just firing messages at an address with no idea if anyone is listening.
This brings up the question of "so when should I use an ActorSelection?" The guideline I follow is to use an ActorSelection when:
I need to take advantage of wildcard selection in actor paths for some reason.
I need to send an initial message to an actor on a remote actor system, so I don't actually have a handle to it yet (and don't have a guarantee that it ever ex
Lesson 2: don't fat finger actor paths in your actor code
If you need to use ActorSelections, put the paths in a shared class and then have all your other actors reference that class. Something like this:
using Akka.Actor;
namespace ProductActors
{
/// <summary>
/// Static helper class used to define paths to fixed-name actors
/// (helps eliminate errors when using <see cref="ActorSelection"/>)
/// </summary>
public static class ActorPaths
{
public static readonly ActorMetaData ProductValidatorActor = new ActorMetaData("validator", "akka://ProductActors/user/validator");
public static readonly ActorMetaData ProductCoordinatorActor = new ActorMetaData("coordinator", "akka://ProductActors/user/commander/coordinator");
}
/// <summary>
/// Meta-data class
/// </summary>
public class ActorMetaData
{
public ActorMetaData(string name, string path)
{
Name = name;
Path = path;
}
public string Name { get; private set; }
public string Path { get; private set; }
}
}
... which can then be referenced like so:
Context.ActorSelection(ActorPaths.ProductValidatorActor.Path).Tell(message);