HttpClient超时时处理TaskCanceledException

Question

我尝试使用

Microsoft.Extensions.Http.Polly

策略处理程序在

TaskCanceledException

或 http 瞬时错误时重试，但它无法正常工作。

我编写了下面的最小 API 只是为了测试：

程序.cs

using Polly;
using Polly.Extensions.Http;
using Polly.Retry;

WebApplicationBuilder builder = WebApplication.CreateBuilder(args);

AsyncRetryPolicy<HttpResponseMessage>? policy = HttpPolicyExtensions.HandleTransientHttpError()
    .Or<TaskCanceledException>()
    .WaitAndRetryAsync(5, _ => TimeSpan.FromSeconds(5));

builder.Services.AddHttpClient("test", client =>
    {
        client.BaseAddress = new Uri("http://localhost:5000");
        client.Timeout = TimeSpan.FromSeconds(2);
    })
    .AddPolicyHandler(policy);

WebApplication app = builder.Build();

app.MapPost("/status/{code}",
    async (string code, IHttpClientFactory httpClientFactory, CancellationToken cancellationToken) =>
    {
        HttpClient client = httpClientFactory.CreateClient("test");
        HttpResponseMessage response = await client.GetAsync($"/status/{code}", cancellationToken);

        response.EnsureSuccessStatusCode();

        return response.Content.ReadAsStringAsync(cancellationToken);
    });

app.MapPost("/timeout/{timeout:int}",
    async (int timeout, IHttpClientFactory httpClientFactory, CancellationToken cancellationToken) =>
    {
        HttpClient client = httpClientFactory.CreateClient("test");
        HttpResponseMessage response = await client.GetAsync($"/timeout/{timeout}", cancellationToken);

        response.EnsureSuccessStatusCode();

        return response.Content.ReadAsStringAsync(cancellationToken);
    });

#region Simulated API

app.MapGet("/status/{code:int}", (int code) => Results.StatusCode(code));

app.MapGet("/timeout/{timeout:int}", async (int timeout) =>
{
    await Task.Delay(TimeSpan.FromSeconds(timeout));
    return Results.Ok();
});

#endregion

app.Run();

调用

POST http://localhost:5000/timeout/10

后，它会引发此异常，而无需进一步尝试：

Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
An unhandled exception has occurred while executing the request.
System.Threading.Tasks.TaskCanceledException: The request was canceled due to the configured HttpClient.Timeout of 2 seconds elapsing.
---> System.TimeoutException: A task was canceled.
---> System.Threading.Tasks.TaskCanceledException: A task was canceled.
 at Polly.Retry.AsyncRetryEngine.ImplementationAsync[TResult](Func`3 action, Context context, CancellationToken cancellationToken, ExceptionPredicates shouldRetryExceptionPredicates, ResultPredicates`1 shouldRetryResultPredicates, Func`5 onRetryAsync, Int32 permittedRetryCount, IEnumerable`1 sleepDurationsEnumerable, Func`4 sleepDurationProvider, Boolean continueOnCapturedContext)
 at Polly.AsyncPolicy`1.ExecuteAsync(Func`3 action, Context context, CancellationToken cancellationToken, Boolean continueOnCapturedContext)       
 at Microsoft.Extensions.Http.PolicyHttpMessageHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
 at Microsoft.Extensions.Http.Logging.LoggingScopeHttpMessageHandler.<>c__DisplayClass5_0.<<SendCoreAsync>g__Core|0>d.MoveNext()
--- End of stack trace from previous location ---
 at System.Net.Http.HttpClient.<SendAsync>g__Core|83_0(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationTokenSource cts, Boolean disposeCts, CancellationTokenSource pendingRequestsCts, CancellationToken originalCancellationToken)
 --- End of inner exception stack trace ---
 --- End of inner exception stack trace ---
 at System.Net.Http.HttpClient.HandleFailure(Exception e, Boolean telemetryStarted, HttpResponseMessage response, CancellationTokenSource cts, CancellationToken cancellationToken, CancellationTokenSource pendingRequestsCts)
 at System.Net.Http.HttpClient.<SendAsync>g__Core|83_0(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationTokenSource cts, Boolean disposeCts, CancellationTokenSource pendingRequestsCts, CancellationToken originalCancellationToken)
 at Program.<>c.<<<Main>$>b__0_3>d.MoveNext() in Z:\Repositories\POCs\PollyRetryTest\PollyRetryTest\Program.cs:line 35
--- End of stack trace from previous location ---
 at Microsoft.AspNetCore.Http.RequestDelegateFactory.<ExecuteTaskOfT>g__ExecuteAwaited|129_0[T](Task`1 task, HttpContext httpContext, JsonTypeInfo`1 jsonTypeInfo)
 at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

我做错了什么？我必须覆盖主处理程序吗？

Answer 1

TL;DR：HttpClient 的超时用作全局超时（涵盖所有重试尝试。

让我们看看这里有什么：

您长时间运行操作（10秒）
您的重试策略最多可进行 5 次重试，每次尝试之间有 5 秒的延迟
HttpClient 上有 2 秒的“全局”超时
- 所以，这不是每个请求超时

让我们看看会发生什么：

您针对长时间运行的操作发出请求
HttpClient 超时
重试政策生效
- 如果您为
```
onRetry
```
  /
```
onRetryAsync
```
  添加日志记录，您可以看到它仅触发一次
然后进入睡眠状态5秒
HttpClient 的
```
GetAsync
```
调用会抛出 TCE

解决方案

使用超时策略来定义每个请求的超时时间

IAsyncPolicy<HttpResponseMessage> timeout = Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(2));

IAsyncPolicy<HttpResponseMessage> retry = HttpPolicyExtensions.HandleTransientHttpError()
    .Or<TaskCanceledException>()
    .Or<TimeoutRejectedException>()
    .WaitAndRetryAsync(5, _ => TimeSpan.FromSeconds(5), 
        onRetry:(_, __) => Console.WriteLine("RETRIED"));

builder.Services.AddHttpClient("test", client =>
    {
        client.BaseAddress = new Uri("http://localhost:5000");
        client.Timeout = TimeSpan.FromSeconds(20);
    })
    .AddPolicyHandler(Policy.WrapAsync(retry, timeout));

```
timeout
```
将根据请求超时（2秒）使用
```
retry
```
具有超时策略意识 (
```
Or<TimeoutRejectedException>()
```
)
- 我添加了一些日志记录以查看重试启动了多少次
我已将全局 HttpClient 超时从 2 秒增加到 20 秒
为了协作，我结合了
```
timeout
```
和
```
retry
```
政策

您应该在日志中看到类似的内容

RETRIED
...
RETRIED
...
RETRIED
...
TaskCancelledException thrown

[00-02s]：初始请求抛出
```
TimeoutRejectedException
```
@2s：打印
```
RETRIED
```
[02-07s]：波莉正在睡觉
[07-09s]：第一次重试尝试抛出
```
TimeoutRejectedException
```
@9s：打印
```
RETRIED
```
[09-14s]：波莉正在睡觉
[14-16s]：第二次重试尝试抛出
```
TimeoutRejectedException
```
@16s：打印
```
RETRIED
```
[16-21s]：波莉正在睡觉
@20s：
```
HttpClient
```
抛出
```
TaskCancelledException
```

注意

请注意，您也可以使用超时策略来实现全局超时

IAsyncPolicy<HttpResponseMessage> globalTimeout = Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(20));

IAsyncPolicy<HttpResponseMessage> localTimeout = Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(2));

IAsyncPolicy<HttpResponseMessage> retry = HttpPolicyExtensions.HandleTransientHttpError()
    .Or<TimeoutRejectedException>()
    .WaitAndRetryAsync(5, _ => TimeSpan.FromSeconds(5), 
        onRetry:(_, __) => Console.WriteLine("RETRIED"));

builder.Services.AddHttpClient("test", client =>
    {
        client.BaseAddress = new Uri("http://localhost:5000");
    })
    .AddPolicyHandler(Policy.WrapAsync(globalTimeout, retry, localTimeout));

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