Polly HandleTransientHttpError 未捕获 HttpRequestException

Question

我在 Startup.ConfigureServices 方法中为我的 HttpClient 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 [Information] 行，这些行显示在我的问题末尾的日志中。

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

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。

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

HandleTransientHttpError() 是一个 Polly 扩展，在它的 cmets 中声明：

配置要处理的条件是： • 网络故障（如 System.Net.Http.HttpRequestException）

我的httpclient使用是这样的：

using (HttpResponseMessage response = await _httpClient.SendAsync(request)) 
{
    response.EnsureSuccessStatusCode();

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

捕获以下日志：

[Information] System.Net.Http.HttpClient.ResilientClient.LogicalHandler: Start processing HTTP request GET https://api.au.... obfuscated
[Information] System.Net.Http.HttpClient.ResilientClient.CustomClientHandler: Sending HTTP request GET https://api.au..... obfuscated
[Information] System.Net.Http.HttpClient.ResilientClient.CustomClientHandler: Received HTTP response after 2421.8895ms - 200
[Information] System.Net.Http.HttpClient.ResilientClient.LogicalHandler: End processing HTTP request after 2422.1636ms - OK
    
Unknown error responding to request: HttpRequestException:
System.Net.Http.HttpRequestException: Error while copying content to a stream. ---> System.IO.IOException: The server returned an invalid or unrecognized response.

at System.Net.Http.HttpConnection.FillAsync()
at System.Net.Http.HttpConnection.ChunkedEncodingReadStream.CopyToAsyncCore(Stream destination, CancellationToken cancellationToken)
at System.Net.Http.HttpConnection.HttpConnectionResponseContent.SerializeToStreamAsync(Stream stream, TransportContext context, CancellationToken cancellationToken)
at System.Net.Http.HttpContent.LoadIntoBufferAsyncCore(Task serializeToStreamTask, MemoryStream tempBuffer)
--- End of inner exception stack trace ---
at System.Net.Http.HttpContent.LoadIntoBufferAsyncCore(Task serializeToStreamTask, MemoryStream tempBuffer)
at System.Net.Http.HttpClient.FinishSendAsyncBuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)
at nd_activity_service.Controllers.ActivityController.GetND(String url) in /codebuild/output/src251819872/src/src/nd-activity-service/Controllers/ActivityController.cs:line 561

Http 调用成功，我可以看到它返回 200 - OK。但随后抛出 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 200 - OK。那么它是如何在此之外抛出异常的呢？

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

此错误似乎是暂时的。这是一个预定的作业，在下次调用时工作。

Answer 1

您的政策是针对HttpClient 而非HttpResponseMessage 定义的。

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

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

---

因为您的异常 StackTrace 如下所示：

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。

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

这将不会触发重试，因为您已在 HttpClient 上定义了策略。

---

如果您想在 response.EnsureSuccessStatusCode() 抛出 HRE 或 response.Content.ReadAsStringAsync() 抛出 HRE 时也重试，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。

让我告诉你如何做到这一点。

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索要寄存器，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;
}

我已经使用httpstat.us 来模拟 428 响应。