Polly策略无法使用“ AddPolicyHandler”运行

Question

我有一个应用程序，它请求通过身份验证的服务，因此需要传递access_token。

我的想法是在access_token过期时使用Polly重试。

我正在.NET Core 3.1应用程序中使用Refit（v5.1.67）和Polly（v7.2.1）。

服务注册如下：

services.AddTransient<ExampleDelegatingHandler>();

IAsyncPolicy<HttpResponseMessage> retryPolicy = Policy<HttpResponseMessage>
    .Handle<ApiException>()
    .RetryAsync(1, (response, retryCount) =>
    {
        System.Diagnostics.Debug.WriteLine($"Polly Retry => Count: {retryCount}");
    });

services.AddRefitClient<TwitterApi>()
    .ConfigureHttpClient(c =>
    {
        c.BaseAddress = new Uri("https://api.twitter.com/");
    })
    .AddHttpMessageHandler<ExampleDelegatingHandler>()
    .AddPolicyHandler((sp, req) =>
    {
        //this policy does not works, because the exception is not catched on 
        //"Microsoft.Extensions.Http.PolicyHttpMessageHandler" (DelegatingHandler)
        return retryPolicy;
    });

public interface TwitterApi
{
    [Get("/2/users")]
    Task<string> GetUsers();
}

public class ExampleDelegatingHandler : DelegatingHandler
{
    protected async override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        try
        {
            return await base.SendAsync(request, cancellationToken);
        }
        catch (Exception)
        {
            //Why do not catch the exception?
            throw;
        }
    }
}

重试策略不起作用！

分析问题后，我意识到HttpClient的DelegatingHandler中没有捕获异常。由于AddPolicyHandler语句正在生成DelegatingHandler（PolicyHttpMessageHandler）来执行策略，并且没有捕获异常，因此该策略从不执行。我意识到问题仅出现在可以发送请求的异步方案中。在同步方案中，它可以工作（例如：超时）。

为什么未在DelegatingHandler内捕获异常？

我正在附上一个模拟Twitter呼叫的示例项目。

https://www.dropbox.com/s/q1797rq1pbjvcls/ConsoleApp2.zip?dl=0

外部参考：

https://github.com/reactiveui/refit#using-httpclientfactory

https://www.hanselman.com/blog/UsingASPNETCore21sHttpClientFactoryWithRefitsRESTLibrary.aspx

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/http-requests?view=aspnetcore-3.1

Answer 1

TL; DR：AddPolicyHandler和AddHttpMessageHandler的顺序确实很重要。

---

我已经用HttpClient重新创建了问题（因此没有重新安装）。

键入HttpClient进行测试

public interface ITestClient
{
    Task<string> Get();
}

public class TestClient: ITestClient
{
    private readonly HttpClient client;
    public TestClient(HttpClient client)
    {
        this.client = client;
    }
    public async Task<string> Get()
    {
        var resp = await client.GetAsync("http://not-existing.site");
        return "Finished";
    }
}

测试控制器

[ApiController]
[Route("[controller]")]
public class TestController : ControllerBase
{
    private readonly ITestClient client;

    public TestController(ITestClient client)
    {
        this.client = client;
    }

    [HttpGet]
    public async Task<string> Get()
    {
        return await client.Get();
    }
}

用于测试的DelegateHandler

public class TestHandler: DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        try
        {
            return await base.SendAsync(request, cancellationToken);
        }
        catch (System.Exception ex)
        {
            _ = ex;
            throw;
        }
    }
}

---

订购＃1-处理程序，策略

启动

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();
    services.AddTransient<TestHandler>();
    services.AddHttpClient<ITestClient, TestClient>()
        .AddHttpMessageHandler<TestHandler>() //Handler first
        .AddPolicyHandler(RetryPolicy()); //Policy second
}

private IAsyncPolicy<HttpResponseMessage> RetryPolicy()
    => Policy<HttpResponseMessage>
    .Handle<HttpRequestException>()
    .RetryAsync(1, (resp, count) =>
    {
        Console.WriteLine(resp.Exception);
    });

执行顺序

1. TestController的{​​{1}}
2. Get的{​​{1}}
3. TestClient的{​​{1}}的{​​{1}}
4. Get的{​​{1}}
5. TestHandler的{​​{1}}的{​​{1}}
6. SendAsync的{​​{1}}失败，try（内部：RetryPolicy）

因此，此处不会触发重试策略。

订购＃2-策略，处理程序

启动

onRetry

执行顺序

1. TestHandler的{​​{1}}
2. SendAsync的{​​{1}}
3. catch的{​​{1}}的{​​{1}}
4. TestController的{​​{1}}的{​​{1}}
5. Get的{​​{1}}
6. HttpRequestException的{​​{1}}的{​​{1}}
7. SocketException的{​​{1}}的{​​{1}}
8. public void ConfigureServices(IServiceCollection services) { services.AddControllers(); services.AddTransient<TestHandler>(); services.AddHttpClient<ITestClient, TestClient>() .AddPolicyHandler(RetryPolicy()) //Policy first .AddHttpMessageHandler<TestHandler>(); //Handler second } private IAsyncPolicy<HttpResponseMessage> RetryPolicy() => Policy<HttpResponseMessage> .Handle<HttpRequestException>() .RetryAsync(1, (resp, count) => { Console.WriteLine(resp.Exception); }); 的{​​{1}}失败，出现TestController（内部：Get）

因此，此处的重试策略已被触发。