问Polly HandleTransientHttpError没有捕捉到HttpRequestException
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Stack Overflow用户

提问于 2021-10-27 22:20:46

回答 1查看 2.5K关注 0票数 3

我在HttpClient方法中为我的Startup.ConfigureServices创建了一个重试策略。还请注意，默认情况下，asp.net Core2.1为HttpClient发出的每个调用记录4行信息行，这些信息行显示在我问题末尾的日志中。

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，因此我将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，它在注释中声明：

配置为处理的条件是：·网络故障(如System.Net.Http.HttpRequestException)

我的httpclient用法如下：

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);
    }
}

捕获了以下日志：

[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。但随后抛出了HttpRequestException。我假设没有调用策略，因为HttpClient消息管道已经解析，因为我们可以看到它返回200-OK。那么，它是如何抛出异常的呢？

我该怎么处理呢？围绕专门处理HttpRequestExceptions的方法包装另一个策略？

这个错误似乎是暂时的。这是一个计划好的作业，下次调用时可以工作。

dotnet-httpclient

asp.net-core-2.1

polly

retry-logic

回答 1

Stack Overflow用户

回答已采纳

发布于 2021-10-27 23:41:26

您的策略是针对HttpClient而不是针对HttpResponseMessage定义的。

因此，即使您收到例如428，response.EnsureSuccessStatusCode()也将而不是触发器重试。

如果从下游系统接收到408或5XX状态代码，则HandleTransientHttpError将触发重试。当SendAsync抛出HttpRequestException时

因为您的异常StackTrace如下所示：

System.Net.Http.HttpRequestException:将内容复制到流中时出错。 System.IO.IOException:服务器返回无效或无法识别的响应。

这就是为什么我的猜测是，这个异常是由HttpContent类抛出的，而您试图读取响应体(ReadAsStringAsync)。

这将不会触发重试，因为您已经在HttpClient上定义了策略。

如果您想在这些情况下进行重试，那么当response.EnsureSuccessStatusCode()抛出HRE或response.Content.ReadAsStringAsync()抛出HRE时，您必须将整个http通信和响应处理逻辑封装到重试策略中。

我来教你怎么做。

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请求注册，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;
}