Hello world example with actors

HelloWorldActors is a simple program to get you started using the Coyote actors programming model.

What you will need

To run the Hello World Actors example, you will need to:

Build the sample

Build the coyote-samples repo by running the following command:

powershell -f build.ps1

Run the HelloWorldActors application

Now you can run the HelloWorldActors application:


Press the ENTER key to terminate the program when it is done. Note that a bug has been inserted into the code intentionally and will be caught by a Coyote Assert. The program should display 1 to 5 greetings like Hello World! or Good Morning. The intentional bug is hard to reproduce when you run the program manually.

The typical “normal” run will look like this:

press ENTER to terminate...
Requesting 5 greetings
Received greeting: Good Morning
Received greeting: Hello World!
Received greeting: Hello World!
Received greeting: Hello World!
Received greeting: Good Morning

When the error is caught, an extra line of output is written saying:

Exception 'Microsoft.Coyote.AssertionFailureException' was thrown in 0 (action '1'):
Too many greetings returned!

Finding the bug using Coyote test tool

See Using Coyote for information on how to find the coyote test tool and setup your environment to use it.

Enter the following from the command line:

coyote test ./bin/netcoreapp3.1/HelloWorldActors.dll --iterations 30

The result is:

. Testing .\bin\netcoreapp3.1\HelloWorldActors.dll
Starting TestingProcessScheduler in process 16432
... Created '1' testing task.
... Task 0 is using 'random' strategy (seed:308255541).
..... Iteration #1
..... Iteration #2
... Task 0 found a bug.
... Emitting task 0 traces:
..... Writing .\bin\netcoreapp3.1\Output\HelloWorldActors.exe\CoyoteOutput\HelloWorldActors_0_2.txt
..... Writing .\bin\netcoreapp3.1\Output\HelloWorldActors.exe\CoyoteOutput\HelloWorldActors_0_2.schedule
... Elapsed 0.0906639 sec.
... Testing statistics:
..... Found 1 bug.
... Scheduling statistics:
..... Explored 2 schedules: 2 fair and 0 unfair.
..... Found 50.00% buggy schedules.
..... Number of scheduling points in fair terminating schedules: 23 (min), 30 (avg), 37 (max).
... Elapsed 0.1819877 sec.
. Done

The Coyote tester has found the bug very quickly—something that takes much longer if testing manually. When using Coyote for testing more complex, real-world systems the time savings can be huge! This will give you the ability to find and fix even the most difficult to reproduce concurrency bugs before pushing to production.

To learn more about testing with Coyote read Testing Coyote programs end-to-end and reproducing bugs.

The Greeter

Please read Programming model: asynchronous actors to gain a basic understanding of the Actors programming model.

The core of an Actor based Coyote program, is, well, an Actor class! The following Actor named Greeter is able to receive a RequestGreetingEvent to which it responds with a GreetingEvent:

[OnEventDoAction(typeof(RequestGreetingEvent), nameof(HandleGreeting))]
public class Greeter : Actor
    /// This method is called when this actor receives a RequestGreetingEvent.
    private void HandleGreeting(Event e)
        if (e is RequestGreetingEvent ge)
            string greeting = this.RandomBoolean() ? "Hello World!" : "Good Morning";
            this.SendEvent(ge.Caller, new GreetingEvent(greeting));
            if (this.RandomBoolean(10))
                // bug: a 1 in 10 chance of sending too many greetings.
                this.SendEvent(ge.Caller, new GreetingEvent(greeting));

Notice the OnEventDoAction custom attribute decorating the class. This tells the Coyote runtime that this class expects to receive RequestGreetingEvents. These events will be queued in the inbox for this Actor until the right time to dequeue them and process them. The custom attribute tells the Coyote runtime to call the HandleGreeting method. The Coyote runtime will also report an error if a different type of event is sent to the Greeter.

The HandleGreeting method is called with an Event parameter, and you know in this case the event will be of type RequestGreetingEvent. This event is defined in Events.cs like this:

internal class RequestGreetingEvent : Event
    public readonly ActorId Caller;

    public RequestGreetingEvent(ActorId caller)
        this.Caller = caller;

The inbox is crucial to the Actor model

You can think of an Event declaration as a type of interface definition for Actors. Events really define the interface of an Actor because the caller will never get to see the Greeter class, so the caller can’t just call HandleGreeting directly. The caller only gets to see an ActorId, which is like a Coyote runtime handle to the actor.

There is a really important reason for this.

Events are queued in an inbox managed by the Actor base class. This serializes the incoming events so that at any given time, only one event is being handled in the Greeter. This greatly simplifies concurrent programming. For example, there is no need to use lock to protect against data race conditions, and therefore no need to worry about deadlocks. Each Actor instance can run in a separate thread, so all actors in the system can be hugely parallel, but at the same time the processing inside an Actor is incredibly simple. So you get the best of both worlds, code that scales but is also easy to write. An Actor receives messages, and can create other Actor objects, and can send events.

Notice the Greeter gets an ActorId from the RequestGreetingEvent and uses that to send a greeting back to the caller using SendEvent:

this.SendEvent(ge.Caller, new GreetingEvent(greeting));

You can also see the bug which has been injected deliberately, namely, it randomly returns more than one GreetingEvent.

A TestActor

To test this Greeter you will need to setup a TestActor which is done in Program.cs. The TestActor creates the Greeter and sends between 1 and 5 RequestGreetingEvents. The TestActor is declared in a way that tells the Coyote runtime it is expecting to receive GreetingEvents as follows:

 [OnEventDoAction(typeof(GreetingEvent), nameof(HandleGreeting))]

To make this a good test, the TestActor keeps track of how many greetings were sent and how many were received, and it writes an Assert to ensure it doesn’t receive too many as follows:

private void HandleGreeting(Event e)
    // this is perfectly thread safe, because all message handling in actors is
    // serialized within the Actor class.
    string greeting = ((GreetingEvent)e).Greeting;
    Console.WriteLine("Received greeting: {0}", greeting);
    this.Assert(this.Count >= 0, "Too many greetings returned!");

Lastly, to test the TestActor you need to fire up the Coyote runtime which is done in the HostProgram Main entry point as follows:

public static void Main()
    var config = Configuration.Create();
    IActorRuntime runtime = RuntimeFactory.Create(config);

    runtime.OnFailure += OnRuntimeFailure;

    Console.WriteLine("press ENTER to terminate...");

Notice that Coyote actors run in parallel, so you have to stop the program from terminating prematurely, which can be done with a Console.ReadLine call. In order for this program to also be testable using the coyote test tool, you need to declare a test method as follows:

public static void Execute(IActorRuntime runtime)

The coyote test tool will call this method. But it will not call your Main entry point. So this Execute method needs to be standalone. It cannot depend on anything except statically initialized variables. There is no way to get command line arguments from coyote test through to this method.

This Execute method is very simple, it just creates the TestActor. In Coyote once an actor is created it lives forever until it is halted. Note that Coyote programs can run forever, the coyote test tool has ways of interrupting and restarting this Execute method based on --iterations and --max-steps arguments provided to the test tool.

So now you know what happened when you ran the following command line:

coyote test ./bin/netcoreapp3.1/HelloWorldActors.exe --iterations 30

A special coyote TestingEngine was created, it invoked the Execute method 30 times, and during those executions the test engine took over all concurrent activity and the non-determinism (like RandomInteger) to ensure the program covered lots of async non-deterministic choices, and recorded all this in a way that when a bug was found, it is able to reproduce that bug. See also Using Coyote for information on how to replay a test that found a bug using coyote replay.


In this tutorial you learned:

  1. How to create an Actor and use SendEvent to send messages to it.
  2. How an Actor can handle events using OnEventDoAction.
  3. The importance of the inbox to simplify parallel programming.
  4. How to run the HelloWorldActors sample from the command-line.
  5. How to use Assert in Coyote code.
  6. How to write a [Test] method for use in coyote test runs.
  7. How to run that test using the coyote test tool.