Polly HandleTransientHttpError 未捕获 HttpRequestException

Question

我在 Startup.ConfigureServices 方法中在 HttpClient 上创建了重试策略。另请注意，默认情况下，asp.net core 2.1 为 HttpClient 进行的每个调用记录 4 行 [信息] 行，这些行显示在我的问题末尾的日志中。

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services.AddHttpClient("ResilientClient")\n            .AddPolicyHandler(\n                Policy.WrapAsync(\n                    PollyRetryPolicies.TransientErrorRetryPolicy(),\n                    Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(60))));\n

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该策略定义如下。请注意，我将重试尝试写入日志，因此我会知道是否调用了重试策略。

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public static IAsyncPolicy < HttpResponseMessage > TransientErrorRetryPolicy() {\n    return HttpPolicyExtensions\n        .HandleTransientHttpError()\n        .Or < TimeoutRejectedException > ()\n        .WaitAndRetryAsync(sleepDurations: ExponentialBackoffPolicy.DecorrelatedJitter(3, SEED_DELAY, MAX_DELAY),\n            onRetry: (message, timespan, attempt, context) => {\n                context.GetLogger() ? .LogInformation($ "Retrying request to {message?.Result?.RequestMessage?.RequestUri} in {timespan.TotalSeconds} seconds. Retry attempt {attempt}.");\n    });\n}\n

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HandleTransientHttpError() 是一个 Polly 扩展，它在注释中指出：

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配置要处理的条件是：\n\xe2\x80\xa2 网络故障（如 System.Net.Http.HttpRequestException）

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我的httpclient用法是这样的：

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using (HttpResponseMessage response = await _httpClient.SendAsync(request)) \n{\n    response.EnsureSuccessStatusCode();\n\n    try \n    {\n        string result = await response.Content.ReadAsStringAsync();\n        if (result == null || result.Trim().Length == 0) {\n            result = "[]";\n        }\n        return JArray.Parse(result);\n    } catch (Exception ex) {\n        _logger.LogInformation($ "Failed to read response from {url}. {ex.GetType()}:{ex.Message}");\n        throw new ActivityException($ "Failed to read response from {url}.", ex);\n    }\n}\n

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捕获以下日志：

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[Information] System.Net.Http.HttpClient.ResilientClient.LogicalHandler: Start processing HTTP request GET https://api.au.... obfuscated\n[Information] System.Net.Http.HttpClient.ResilientClient.CustomClientHandler: Sending HTTP request GET https://api.au..... obfuscated\n[Information] System.Net.Http.HttpClient.ResilientClient.CustomClientHandler: Received HTTP response after 2421.8895ms - 200\n[Information] System.Net.Http.HttpClient.ResilientClient.LogicalHandler: End processing HTTP request after 2422.1636ms - OK\n    \nUnknown error responding to request: HttpRequestException:\nSystem.Net.Http.HttpRequestException: Error while copying content to a stream. ---> System.IO.IOException: The server returned an invalid or unrecognized response.\n\nat System.Net.Http.HttpConnection.FillAsync()\nat System.Net.Http.HttpConnection.ChunkedEncodingReadStream.CopyToAsyncCore(Stream destination, CancellationToken cancellationToken)\nat System.Net.Http.HttpConnection.HttpConnectionResponseContent.SerializeToStreamAsync(Stream stream, TransportContext context, CancellationToken cancellationToken)\nat System.Net.Http.HttpContent.LoadIntoBufferAsyncCore(Task serializeToStreamTask, MemoryStream tempBuffer)\n--- End of inner exception stack trace ---\nat System.Net.Http.HttpContent.LoadIntoBufferAsyncCore(Task serializeToStreamTask, MemoryStream tempBuffer)\nat System.Net.Http.HttpClient.FinishSendAsyncBuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)\nat nd_activity_service.Controllers.ActivityController.GetND(String url) in /codebuild/output/src251819872/src/src/nd-activity-service/Controllers/ActivityController.cs:line 561\n

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Http 调用成功，我可以看到它返回 200 - OK。但随后会抛出 HttpRequestException。我假设该策略没有被调用，因为 HttpClient 消息管道已经解析，我们可以看到它返回了 200 - OK。那么它是如何抛出异常的呢？

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我该如何处理？围绕专门处理 HttpRequestExceptions 的方法包装另一个策略？

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此错误似乎是暂时的。这是一项预定的作业，并在下次调用时开始工作。

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Answer 1

您的政策是针对而HttpClient不是针对HttpResponseMessage。

因此，即使您收到例如 428，也response.EnsureSuccessStatusCode()不会触发重试。

HandleTransientHttpError如果您从下游系统收到 408 或 5XX 状态代码，将触发重试。当SendAsync抛出HttpRequestException

因为你的异常 StackTrace 看起来像这样：

System.Net.Http.HttpRequestException：将内容复制到流时出错。

System.IO.IOException：服务器返回无效或无法识别的响应。

HttpContent这就是为什么我有根据的猜测是，当您尝试读取响应正文 ( ) 时，类会抛出此异常ReadAsStringAsync。

这不会触发重试，因为您已在 HttpClient 上定义了策略。

如果您想在这些情况下重试，或者当抛出response.EnsureSuccessStatusCode()HRE 或当执行时response.Content.ReadAsStringAsync()，那么您必须将整个 http 通信和响应处理逻辑包装到重试策略中。

让我向您展示如何做到这一点。

首先使用PolicyRegistry而不是AddPolicyHandler：

//services.AddHttpClient("ResilientClient")
//    .AddPolicyHandler(
//        Policy.WrapAsync(
//            TransientErrorRetryPolicy(),
//            Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(60))));

services.AddHttpClient("ResilientClient");
var registry = services.AddPolicyRegistry();
registry.Add("retry", Policy.WrapAsync(
            TransientErrorRetryPolicy(),
            Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(60))));

然后向DI询问寄存器，例如：

private readonly IHttpClientFactory factory;
private readonly IReadOnlyPolicyRegistry<string> registry;

public TestController(IHttpClientFactory factory, IReadOnlyPolicyRegistry<string> registry)
{
    this.factory = factory;
    this.registry = registry;
}

最后检索组合策略并执行 http 调用：

var retryPolicy = registry.Get<IAsyncPolicy<HttpResponseMessage>>("retry");
await retryPolicy.ExecuteAsync(async () => await IssueRequest());

private async Task<HttpResponseMessage> IssueRequest()
{
    var _httpClient = factory.CreateClient("ResilientClient");
    HttpResponseMessage response = await _httpClient.GetAsync("http://httpstat.us/428");

    response.EnsureSuccessStatusCode();
    return response;
}

我使用httpstat.us来模拟 428 响应。