Retrying HttpClient Unsuccessful Requests

ASP.NET Core 2.1 Answer

ASP.NET Core 2.1 added support for Polly directly. Here UnreliableEndpointCallerService is a class which accepts a HttpClient in its constructor. Failed requests will retry with an exponential back-off so that the next retry takes place in an exponentially longer time after the previous one:

services
    .AddHttpClient<UnreliableEndpointCallerService>()
    .AddTransientHttpErrorPolicy(
        x => x.WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(3, retryAttempt)));

Also, consider reading my blog post "Optimally Configuring HttpClientFactory".

Other Platforms Answer

This implementation uses Polly to retry with an exponential back-off so that the next retry takes place in an exponentially longer time after the previous one. It also retries if a HttpRequestException or TaskCanceledException is thrown due to a timeout. Polly is much easier to use than Topaz.

public class HttpRetryMessageHandler : DelegatingHandler
{
    public HttpRetryMessageHandler(HttpClientHandler handler) : base(handler) {}

    protected override Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request,
        CancellationToken cancellationToken) =>
        Policy
            .Handle<HttpRequestException>()
            .Or<TaskCanceledException>()
            .OrResult<HttpResponseMessage>(x => !x.IsSuccessStatusCode)
            .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(3, retryAttempt)))
            .ExecuteAsync(() => base.SendAsync(request, cancellationToken));
}

using (var client = new HttpClient(new HttpRetryMessageHandler(new HttpClientHandler())))
{
    var result = await client.GetAsync("http://example.com");
}

Instead of implementing retry functionality that wraps the HttpClient, consider constructing the HttpClient with a HttpMessageHandler that performs the retry logic internally. For example:

public class RetryHandler : DelegatingHandler
{
    // Strongly consider limiting the number of retries - "retry forever" is
    // probably not the most user friendly way you could respond to "the
    // network cable got pulled out."
    private const int MaxRetries = 3;

    public RetryHandler(HttpMessageHandler innerHandler)
        : base(innerHandler)
    { }

    protected override async Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request,
        CancellationToken cancellationToken)
    {
        HttpResponseMessage response = null;
        for (int i = 0; i < MaxRetries; i++)
        {
            response = await base.SendAsync(request, cancellationToken);
            if (response.IsSuccessStatusCode) {
                return response;
            }
        }

        return response;
    }
}

public class BusinessLogic
{
    public void FetchSomeThingsSynchronously()
    {
        // ...

        // Consider abstracting this construction to a factory or IoC container
        using (var client = new HttpClient(new RetryHandler(new HttpClientHandler())))
        {
            myResult = client.PostAsync(yourUri, yourHttpContent).Result;
        }

        // ...
    }
}