Integration Testing using Akka.NET's TestKit

In Getting Started with Akka.NET and Handling messages and state with Akka.NET we created a calculator in Akka.NET that was able to add and subtract numbers and return the answer. It also stores the last answer and can respond with it when asked.

In Unit Testing using Akka.NET’s TestKit we created some unit tests to verify internal state of the actor.

In this post we’ll switch to integration tests – verifying we get correct messages back.

Code for this post: https://github.com/HCanber/akka.net-blog-examples/tree/master/04-calculator-testkit-integration

All posts in this series: Tutorials for Akka.NET

Note! This post was written using Akka.NET 0.7.0 and might not work for later versions

Unit Testing Recap

Unit Testing using Akka.NET’s TestKit we created some unit tests to verify internal state of the actor. We used ActorOfAsTestActorRef<T>() which creates the actor in the test’s ActorSystem (which is exposed thru the property Sys inside tests) and returns a TestActorRef<T> which give us access to the underlycing actor instance using the property UnderlyingActor.

[Fact]
public void Answer_should_initially_be_0()
{           
    TestActorRef<CalculatorActor> calculatorRef = ActorOfAsTestActorRef<CalculatorActor>("calculator");
    CalculatorActor calculator = calculatorRef.UnderlyingActor;
    Assert.Equal(0, calculator.Answer);
}

[Fact]
public void After_adding_1_and_1_Answer_should_be_2()
{
    TestActorRef<CalculatorActor> calculatorRef = ActorOfAsTestActorRef<CalculatorActor>("calculator");
    calculatorRef.Tell(new Add(1,1));
    CalculatorActor calculator = calculatorRef.UnderlyingActor;
    Assert.Equal(2, calculator.Answer);
}

Remember that everything is synchronous when writing these kind of tests. It’s the use of ActorOfAsTestActorRef<T>() that makes it synchronous. A CallingThreadDispatcher is used for actors created using ActorOfAsTestActorRef<T>() so when we send it a message using Tell(message) it’s not dispatched on another thread, but instead immediately processed before Tell returns control back to our test.

Integration tests

When writing integration tests, we create and run the actor the way we normally do. This means multi threaded under full concurrency (actors are shielded by the Actor model – the actor only processes on message at a time).

The first test: GetLastAnswer should initially respond with Answer(0)

In our first unit test we verfied that the internal state answer is 0 initially. We can also verify this by sending the actor a GetLastAnswer message and verify that we get an Answer(0) back.

The skeleton for the test looks like this:

public class CalculatorIntegrationTests : TestKit
{
    [Fact]
    public void Answer_should_initially_be_0()
    {
        var calculator = ActorOf<CalculatorActor>("calculator");
        calculator.Tell(GetLastAnswer.Instance);

        //Somehow verify we get an Anser(0) back            
    } 
}

So how can we verify that the calculator responds with what we expect? We could use Ask but there is a better way using TestKit.

TestActor

When the TestKit’s ActorSystem is created it also creates a special actor called TestActor. This instance is used as an implicit sender, so when we sent a message to calculator like this:

calculator.Tell(GetLastAnswer.Instance);

It was actually sent as if we’d specified TestActor as the sender:

calculator.Tell(GetLastAnswer.Instance, TestActor);

So when calculator send the response it will send it to TestActor which in turn will put the message in a queue that we can test against.

To test that the queue contains the correct message, or another way of seeing it: that TestActor received the correct message we use ExpectMessage<T>() and then assert that the value is correct.

[Fact]
public void Answer_should_initially_be_0()
{
    var calculator = ActorOf<CalculatorActor>("calculator");
    calculator.Tell(GetLastAnswer.Instance);

    var answer = ExpectMsg<Answer>();
    Assert.Equal(0, answer.Value);
}

The last two lines can also be written like this:

ExpectMsg<Answer>(a => a.Value == 0);

Isn’t it asynchronous?

We send a message to calculator which will process the message on another thread. At the same time we test that we have received a response. How can we be sure that the calculator has responded before we execute ExpectMessage<Answer>()? Don’t we need to synchronize somehow? Or is ir running synchronously as with unit tests?

No, it really is asynchronous, BUT ExpectMsg will wait up to 3 seconds before it fails.

Verifying Add

Rewriting the second unit test as a integration test is really easy now that we have all building blocks.

    [Fact]
    public void After_adding_1_and_1_Answer_should_be_2()
    {
        var calculator = ActorOf<CalculatorActor>("calculator");

        calculator.Tell(new Add(1, 1));
        var answer = ExpectMsg<Answer>();
        Assert.Equal(2, answer.Value);
    }

Be aware that the tests run in a full ActorSystem and everything is asynchronous even though it might look like synchronous code.

Code

Code on GitHub: https://github.com/HCanber/akka.net-blog-examples/tree/master/04-calculator-testkit-integration

All posts in this series: Tutorials for Akka.NET