Unit test HttpClient with Polly

Question

I'm looking to unit test a HttpClient that has a Polly RetryPolicy and I'm trying to work out how to control what the HTTP response will be.

I have used a HttpMessageHandler on the client and then override the Send Async and this works great but when I add a Polly Retry Policy I have to create an instance of HTTP Client using the IServiceCollection and cant create a HttpMessageHandler for the client. I have tried using the .AddHttpMessageHandler() but this then blocks the Poll Retry Policy and it only fires off once.

This is how I set up my HTTP client in my test

IServiceCollection services = new ServiceCollection();

const string TestClient = "TestClient";
 
services.AddHttpClient(name: TestClient)
         .AddHttpMessageHandler()
         .SetHandlerLifetime(TimeSpan.FromMinutes(5))
         .AddPolicyHandler(KYA_GroupService.ProductMessage.ProductMessageHandler.GetRetryPolicy());

HttpClient configuredClient =
                services
                    .BuildServiceProvider()
                    .GetRequiredService<IHttpClientFactory>()
                    .CreateClient(TestClient);

public static IAsyncPolicy<HttpResponseMessage> GetRetryPolicy()
{
    return HttpPolicyExtensions
            .HandleTransientHttpError()
            .WaitAndRetryAsync(6,
                retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)),
                onRetryAsync: OnRetryAsync);
}

private async static Task OnRetryAsync(DelegateResult<HttpResponseMessage> outcome, TimeSpan timespan, int retryCount, Context context)
{
    //Log result
}

This will then fire the request when I call _httpClient.SendAsync(httpRequestMessage) but it actualy create a Http call to address and I need to intercept this some how and return a controlled response.

I would like to test that the policy is used to retry the request if the request fails and completes when it is a complete response.

The main restriction I have is I can't use Moq on MSTest.

Answer 1

You don't want your HttpClient to be issuing real HTTP requests as part of a unit test - that would be an integration test. To avoid making real requests you need to provide a custom HttpMessageHandler. You've stipulated in your post that you don't want to use a mocking framework, so rather than mocking HttpMessageHandler you could provide a stub.

With heavy influence from this comment on an issue on Polly's GitHub page, I've adjusted your example to call a stubbed HttpMessageHandler which throws a 500 the first time it's called, and then returns a 200 on subsequent requests.

The test asserts that the retry handler is called, and that when execution steps past the call to HttpClient.SendAsync the resulting response has a status code of 200:

public class HttpClient_Polly_Test
{
    const string TestClient = "TestClient";
    private bool _isRetryCalled;

    [Fact]
    public async Task Given_A_Retry_Policy_Has_Been_Registered_For_A_HttpClient_When_The_HttpRequest_Fails_Then_The_Request_Is_Retried()
    {
        // Arrange 
        IServiceCollection services = new ServiceCollection();
        _isRetryCalled = false;

        services.AddHttpClient(TestClient)
            .AddPolicyHandler(GetRetryPolicy())
            .AddHttpMessageHandler(() => new StubDelegatingHandler());

        HttpClient configuredClient =
            services
                .BuildServiceProvider()
                .GetRequiredService<IHttpClientFactory>()
                .CreateClient(TestClient);

        // Act
        var result = await configuredClient.GetAsync("https://www.stackoverflow.com");

        // Assert
        Assert.True(_isRetryCalled);
        Assert.Equal(HttpStatusCode.OK, result.StatusCode);
    }

    public IAsyncPolicy<HttpResponseMessage> GetRetryPolicy()
    {
        return HttpPolicyExtensions.HandleTransientHttpError()
            .WaitAndRetryAsync(
                6,
                retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)),
                onRetryAsync: OnRetryAsync);
    }

    private async Task OnRetryAsync(DelegateResult<HttpResponseMessage> outcome, TimeSpan timespan, int retryCount, Context context)
    {
        //Log result
        _isRetryCalled = true;
    }
}

public class StubDelegatingHandler : DelegatingHandler
{
    private int _count = 0;

    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,
        CancellationToken cancellationToken)
    {
        if (_count == 0)
        {
            _count++;
            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.InternalServerError));
        }

        return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK));
    }
}

Answer 2

The answer above was very helpful in getting me on the right track. However I wanted to test that Policies had been added to a typed http client. This client is defined at application startup. So the challenge was how to add a stub delegating handler after the handlers specified in the typed client definition and it had been added to the Services collection.

I was able to leverage IHttpMessageHandlerBuilderFilter.Configure and added my stub handler as the last handler in the chain.

public sealed class HttpClientInterceptionFilter : IHttpMessageHandlerBuilderFilter
{
    HandlerConfig handlerconfig { get; set; }

    public HttpClientInterceptionFilter(HandlerConfig calls)
    {
        handlerconfig = calls;
    }
    /// <inheritdoc/>
    public Action<HttpMessageHandlerBuilder> Configure(Action<HttpMessageHandlerBuilder> next)
    {
        return (builder) =>
        {
            // Run any actions the application has configured for itself
            next(builder);

            // Add the interceptor as the last message handler
            builder.AdditionalHandlers.Add(new StubDelegatingHandler(handlerconfig));
        };
    }
}

Register this class with the DI container in your unit test:

services.AddTransient<IHttpMessageHandlerBuilderFilter>(n => new HttpClientInterceptionFilter(handlerConfig));

I needed pass in some parameters to the stub handler and to get data out of it and back to my unit test. I used this class to do so:

public class HandlerConfig
{
    public int CallCount { get; set; }
    public DateTime[] CallTimes { get; set; }
    public int BackOffSeconds { get; set; }
    public ErrorTypeEnum ErrorType { get; set; }
}

public enum ErrorTypeEnum
{
    Transient,
    TooManyRequests
}

My stub handler generates transient and too many request responses:

public class StubDelegatingHandler : DelegatingHandler
{
    private HandlerConfig _config;
    HttpStatusCode[] TransientErrors = new HttpStatusCode[] { HttpStatusCode.RequestTimeout, HttpStatusCode.InternalServerError, HttpStatusCode.OK };

    public StubDelegatingHandler(HandlerConfig config)
    {
        _config = config;
    }
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,
        CancellationToken cancellationToken)
    {
        _config.CallTimes[_config.CallCount] = DateTime.Now;

        if (_config.ErrorType == ErrorTypeEnum.Transient)
        {              
            var response = new HttpResponseMessage(TransientErrors[_config.CallCount]);
            _config.CallCount++;
            return Task.FromResult(response);
        }

        HttpResponseMessage response429;
        if (_config.CallCount < 2)
        {
            //generate 429 errors
            response429 = new HttpResponseMessage(HttpStatusCode.TooManyRequests);
            response429.Headers.Date = DateTime.UtcNow;

            DateTimeOffset dateTimeOffSet = DateTimeOffset.UtcNow.Add(new TimeSpan(0, 0, 5));
            long resetDateTime = dateTimeOffSet.ToUnixTimeSeconds();
            response429.Headers.Add("x-rate-limit-reset", resetDateTime.ToString());
        }
        else
        {
            response429 = new HttpResponseMessage(HttpStatusCode.OK);
        }

        _config.CallCount++;

        return Task.FromResult(response429);

    }
}

And finally the unit test:

[TestMethod]
public async Task Given_A_429_Retry_Policy_Has_Been_Registered_For_A_HttpClient_When_429_Errors_Occur_Then_The_Request_Is_Retried()
    {
        // Arrange 
        IServiceCollection services = new ServiceCollection();

        var handlerConfig = new HandlerConfig { ErrorType = ErrorTypeEnum.TooManyRequests, BackOffSeconds = 5, CallTimes = new System.DateTime[RetryCount] };

        // this registers a stub message handler that returns the desired error codes
        services.AddTransient<IHttpMessageHandlerBuilderFilter>(n => new HttpClientInterceptionFilter(handlerConfig));

        services.ConfigureAPIClient();  //this is an extension method that adds a typed client to the services collection

        HttpClient configuredClient =
            services
                .BuildServiceProvider()
                .GetRequiredService<IHttpClientFactory>()
               .CreateClient("APIClient");  //Note this must be the same name used in ConfigureAPIClient

        //  Act
        var result = await configuredClient.GetAsync("https://localhost/test");

        //   Assert
        Assert.AreEqual(3, handlerConfig.CallCount, "Expected number of  calls made");
        Assert.AreEqual(HttpStatusCode.OK, result.StatusCode, "Verfiy status code");

        var actualWaitTime = handlerConfig.CallTimes[1] - handlerConfig.CallTimes[0];
        var expectedWaitTime = handlerConfig.BackOffSeconds + 1;  //ConfigureAPIClient adds one second to give a little buffer
        Assert.AreEqual(expectedWaitTime, actualWaitTime.Seconds);           
    }
}