Polly政策无法使用"AddPolicyHandler"运行. [英] Polly policy not working using "AddPolicyHandler"

问题描述

我有一个请求认证服务的应用程序，需要传递 access_token .

I have an application that makes a request for an authenticated service, where it is necessary to pass the access_token.

我的想法是，如果 access_token 过期，请使用Polly重试.

My idea is to use Polly to retry if the access_token is expired.

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

I'm using Refit (v5.1.67) and Polly (v7.2.1) in a .NET Core 3.1 application.

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，并且未捕获异常，因此该策略永远不会执行.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).

Analyzing the problem, I realized that the exception is not being caught inside the HttpClient's DelegatingHandler. Since the AddPolicyHandler statement is generating a DelegatingHandler (PolicyHttpMessageHandler) to execute the policy and the exception is not caught there, the policy never executes. I realized that the problem only occurs in asynchronous scenarios, where the request can be sent. In synchronous scenarios it works (example: timeout).

为什么未在 DelegatingHandler 中捕获异常???

Why the exception is not caught inside DelegatingHandler??

我正在附上一个模拟Twitter呼叫的示例项目.

I am attaching an example project simulating a Twitter call.

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/UsingASPNETCoreCore21sHttpClientFactoryWithRefitsRESTLibrary.aspx

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

推荐答案

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

TL;DR: The ordering of AddPolicyHandler and AddHttpMessageHandler does matter.

我已经用 HttpClient 重新创建了问题(因此没有Refit).

I've recreated the problem with HttpClient (so without Refit).

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 Get
2. TestClient 的 Get
3. TestHandler 的 SendAsync 的 try
4. RetryPolicy 的 onRetry
5. TestHandler 的 SendAsync 的 catch
6. TestController 的 Get 失败，并出现 HttpRequestException (内部: SocketException )

1. TestController's Get
2. TestClient's Get
3. TestHandler's SendAsync's try
4. RetryPolicy's onRetry
5. TestHandler's SendAsync's catch
6. TestController's Get fails with HttpRequestException (inner: SocketException)

因此，此处的重试策略不会触发.

So, here the retry policy does not fired.

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);
    });

执行顺序

1. TestController 的 Get
2. TestClient 的 Get
3. TestHandler 的 SendAsync 的 try
4. TestHandler 的 SendAsync 的 catch
5. RetryPolicy 的 onRetry
6. TestHandler 的 SendAsync 的 try
7. TestHandler 的 SendAsync 的 catch
8. TestController 的 Get 失败，并出现 HttpRequestException (内部: SocketException )

1. TestController's Get
2. TestClient's Get
3. TestHandler's SendAsync's try
4. TestHandler's SendAsync's catch
5. RetryPolicy's onRetry
6. TestHandler's SendAsync's try
7. TestHandler's SendAsync's catch
8. TestController's Get fails with HttpRequestException (inner: SocketException)

因此，此处的重试策略已被触发.

So, here the retry policy has been fired.

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