Testing Your Code When Using Polly

Full source code here.

When developing an application with Polly you will also probably want to write some unit tests.

Here are the scenarios I test for –

1. How my code behaves when the policy throws an exception, such as TimeoutRejectionException, BulkheadRejectedException or BrokenCircuitException.
2. How my code behaves when a policy becomes active and changes the outcome of a call, such as when an unreliable request works because Polly performs a retry.
3. What my code should do if there was no policy in place.

I have a few classes to demonstrate these scenarios, BusinessLogic.cs and OtherBusinessLogic.cs are the classes under test. ErrorProneCode.cs is the unreliable class that I will mock and pass mocked policies into.

For the first case I use Moq to mock the error prone code so it returns an incorrect value.

[Fact]
public void Should_Return_999_When_TimeoutRejectedException_Thrown()
{
    //Arrange 
    Mock<IErrorProneCode> mockedErrorProneCode = new Mock<IErrorProneCode>();
    mockedErrorProneCode.Setup(e => e.GetSomeNumber()).Returns(0);

    Mock<ISyncPolicy> mockedPolicy = new Mock<ISyncPolicy>();
    mockedPolicy.Setup(p => p.Execute(It.IsAny<Func<int>>())).Throws(new TimeoutRejectedException("Mocked Timeout Exception"));

    IBusinessLogic businessLogic = new BusinessLogic(mockedPolicy.Object, mockedErrorProneCode.Object);

    //Act
    // if there is a TimeoutRejectedException in this CallSomeSlowBadCode it will return 999
    int num = businessLogic.CallSomeSlowBadCode();

    //Assert
    Assert.Equal(999, num);
}

In the next case I verify that the application has correctly used the retry policy method. I use a seeded random number generator that produces an known sequence to return values from the ErrorProneCode class.

  
[Fact]
public void Should_Return_Odd_When_Retry()
{
    //Arrange 
    Mock<IErrorProneCode> mockedErrorProneCode = new Mock<IErrorProneCode>();

    Random rnd = new Random(1); // rnd.Next(10) retruns 2, 1, 4 

    mockedErrorProneCode.Setup(e => e.GetSomeNumber()).Returns(() => rnd.Next(10));

    ISyncPolicy<int> policy = Policy.HandleResult<int>(i => i %2 !=1) // retry if the number is not odd.
        .Retry(1);
    OtherBusinessLogic otherBusinessLogic = new OtherBusinessLogic(policy, mockedErrorProneCode.Object);

    //Act
    int num = otherBusinessLogic.CallSomeCodeThatNeedsToBeRetried();

    //Assert
    Assert.Equal(1, num % 2);
}

Finally, I want to verify that my code will work if no Polly policy is in use. To do this, I pass in a NoOp policy.

[Fact]
public void Should_Return_Even_With_NoOp()
{
    //Arrange 
    Mock<IErrorProneCode> mockedErrorProneCode = new Mock<IErrorProneCode>();

    Random rnd = new Random(1); // rnd.Next(10) retruns 2, 1, 4 

    mockedErrorProneCode.Setup(e => e.GetSomeNumber()).Returns(() => rnd.Next(10));

    ISyncPolicy<int> policy = Policy.NoOp<int>();
    OtherBusinessLogic otherBusinessLogic = new OtherBusinessLogic(policy, mockedErrorProneCode.Object);

    //Act
    int num = otherBusinessLogic.CallSomeCodeThatNeedsToBeRetried();

    //Assert
    Assert.Equal(0, num % 2); //even number
}

Full source code here.

2 thoughts on “Testing Your Code When Using Polly

  1. This only works if you provide a policy externally to something that needs to do retries. I haven’t seen anyone build a repository or service this way.

    I mean I could be doing it wrong, however I seem to be following the getting started docs on how to use Polly. However, if I cannot use their brief doc about unit testing with Polly I clearly must be doing it wrong which makes me second guess their getting started docs or myself.

    If you have dynamic policies that are driven at runtime or that are encapsulated within repositories or services, or use a common class that simplifies it but that still encapsulates Polly.

    I haven’t seen how to mock away the use of Policy as it uses static methods on an abstract class to be the base for building policies. They return PolicyBuilder and that isn’t easy to mock away either.

    • Hi Rod,

      Let me first apologize that the zip file did not include all the source code. That is corrected now.

      “However, if I cannot use their brief doc about unit testing with Polly I clearly must be doing it wrong which makes me second guess their getting started docs or myself ” – can you explain what you mean here?

      Why do you need to mock the policy?

      Are you on the Polly Slack channel? Lots of live help there.

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