如何解决当从 Web API 项目调用时,Polly 在 .NET Framework 4.6.1 上无限期地等待线程
.NET 版本:.NET Framework 4.6.1
波莉版本:7.2.2
在 .NET Framework 4.6.1 上,当使用 Web API 项目时,Polly 将无限期等待正在运行请求的线程,从而导致永远不会对调用它的客户端进行响应。从控制台应用程序调用相同的方法就可以正常工作。
这是使用 Visual Studio 'ASP.NET Web 应用程序 (.NET Framework)' 中新创建的解决方案测试的。 我也在 .NET 5 中尝试了相同的代码,但此问题不存在,仅在 .NET Framework 4.6.1 上发生。
重现问题的代码:
PolicyContainer.cs:
public class PolicyContainer
{
public IAsyncPolicy<HttpResponseMessage> CircutBreakerPolicy { get; set; }
public PolicyContainer()
{
SetCircutBreakerPolicy();
}
private void SetCircutBreakerPolicy()
{
//////////////////////////////////////////////////////////////////////////////////////
// normally these values would be set by a config file,hardcoded for this example. //
//////////////////////////////////////////////////////////////////////////////////////
// 0.5 means 50% of requests must fail before the circut breaks
double failureThreshold = 0.5;
// 60 means only the most recent 60 seconds are considered for breaking the circut
double samplingDuration = 60;
// 10 means at least this many calls must pass through the circut within the samplingDuration before breaking the circut
int minimumThroughput = 10;
// 60 means the circut will be broken for 60 seconds after the threshold is met
double durationOfBreak = 60;
CircutBreakerPolicy = Policy.HandleResult<HttpResponseMessage>(result => !result.IsSuccessstatusCode)
.AdvancedCircuitBreakerAsync(failureThreshold,TimeSpan.FromSeconds(samplingDuration),minimumThroughput,TimeSpan.FromSeconds(durationOfBreak),OnBreak,OnReset,OnHalfOpen);
}
private void OnBreak(DelegateResult<HttpResponseMessage> response,TimeSpan timespan,Context context)
{
Console.WriteLine("Circut broken");
}
private void OnReset(Context context)
{
Console.WriteLine("Circut Reset");
}
private void OnHalfopen()
{
Console.WriteLine("Circut Half-Open");
}
}
PollyTestRequest.cs:
public class PollyTestRequest
{
///////////////////////////////////////////////////////////////////////////////////////////////////////////
// If set to true the Web API will never return a response,though any other type of project works fine. //
///////////////////////////////////////////////////////////////////////////////////////////////////////////
private const bool USE_POLLY = true;
public static async Task<HttpResponseMessage> Send()
{
HttpClient httpClient = new HttpClient();
PolicyContainer policyContainer = new PolicyContainer();
HttpResponseMessage response;
if (USE_POLLY)
{
// Does not work in a Web API application.
// I stepped through the decompiled code this calls and it will arrive at a "public static bool Wait(object obj,int millisecondsTimeout,bool exitContext)" method.
// Inside this method there is a call to "ObjWait(exitContext,millisecondsTimeout,obj)",however the debugger will not decompile this method so the debugging session will stop if you try to step into it.
// The 'millisecondsTimeout' variable passed here will be "-1" and the 'exitContext' will be "null". I believe that this is what is hanging the thread indefinitely.
// Its very strange though,calling this from a Console app,it will work fine,but from a Web API application it will hang indefinitely.
response = await policyContainer.CircutBreakerPolicy.ExecuteAsync(
async token => await httpClient.PostAsync(new Uri("http://example.com"),new StringContent(""),token),CancellationToken.None
);
}
else
{
// Works perfectly fine in both Web API and Console Apps
response = await httpClient.PostAsync(new Uri("http://example.com"),new StringContent("")).ConfigureAwait(false);
}
return response;
}
}
TestController.cs:
[Route("[controller]")]
public class TestController : ApiController
{
[HttpGet]
[Route("testRoute")]
public IHttpActionResult TestGetRoute()
{
var response = PollyTestRequest.Send().Result;
if (response.IsSuccessstatusCode)
{
return Ok();
}
else
{
return new StatusCodeResult(HttpStatusCode.InternalServerError,this);
}
}
}
解决方法
这是您的错误:
var response = PollyTestRequest.Send().Result;
Don't block on async code;在像 ASP.NET(pre-Core)这样的情况下,它可能会导致死锁。
正确的解决方法是使用 async
all the way:
public async Task<IHttpActionResult> TestGetRoute()
{
var response = await PollyTestRequest.Send();
...
}
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