前言

　　.NetCore2.1新推出HttpClientFactory工厂类， 替代了早期的HttpClient，并新增了弹性Http调用机制 (集成Policy组件)。

替换的初衷还是简单说下：

①  using(var client= new HttpClient()) 调用Dispose()方法，并不会很快释放底层Socket连接，同时新建Socket需要时间，这在高并发场景下Socket耗尽。 传送门

②  由于①很多人会想到用单例或静态类构建HttpClient实例，但是这里还有一个坑，HttpClient会忽略DNS的变化。 传送门

HttpClientFactory 以一种模块化、可命名、弹性可预期的方式重建了HttpClient的使用方式。HttpClientFactory以依赖注入的方式集成到.NETCore 框架：

 HttpClientFactory典型用法

使用时从IHttpClientFactory工厂创建所需HttpClient实例，发起业务端请求。

观察Info级别日志：

19/12/04 11:06:46 [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[18}].[]Start processing HTTP request GET http://localhost:5000/v1/eqid/b827a9400004132a000000065dc26470 19/12/04 11:06:46 [Info].[System.Net.Http.HttpClient.bce-request.ClientHandler].[18}].[]Sending HTTP request GET http://localhost:5000/v1/eqid/b827a9400004132a000000065dc26470 19/12/04 11:06:46 [Info].[System.Net.Http.HttpClient.bce-request.ClientHandler].[34}].[]Received HTTP response after 174.5088ms - OK 19/12/04 11:06:46 [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[34}].[]End processing HTTP request after 211.1478ms - OK

一切都是那么自然、优雅。

头脑风暴

　　观察上面单次请求的日志，由外层LogicHandler和内层ClientHandler 日志头组成。这样的日志可以想象到有2个问题：

① 在高并发使用HttpClient，日志条数众多，没有类似TraceId 这样的机制定位 某次HttpClient调用的完整日志。

②  若是微服务/ 分布式调用，可能还有 将本次HttpClient调用日志与后置api日志 结合分析的需求， 这个日志也支持不了。

因此本文打算重新构建 HttpClientFactory日志：给某次请求的全部日志设置TraceId

结合我给出的典型用法来看IHttpClientFactory组件原理：

示例中System.Net.Http.HttpClient.bce-request.LogicalHandler，System.Net.Http.HttpClient.bce-request.ClientHandler 日志头即是来自LoggingScopeHttpMessageHandler ，LoggingHttpMessageHandler 两个处理器。

给出手绘的UML类图：

本次要扩展的入口便是IHttpMessageHandlerFilter接口，核心是自定义DelegatingHandler日志处理器。

编程实践

如以上分析，使用aspNetCore2.2需要做如下扩展：

P1

实现IHttpMessageHandlerFilter接口，移除接口中默认的2个日志处理器

public class TraceIdLoggingMessageHandlerFilter : IHttpMessageHandlerBuilderFilter    {        private readonly ILoggerFactory _loggerFactory;

        public TraceIdLoggingMessageHandlerFilter(ILoggerFactory loggerFactory)        {            _loggerFactory = loggerFactory ?? throw new ArgumentNullException(nameof(loggerFactory));        }

        public Action<HttpMessageHandlerBuilder> Configure(Action<HttpMessageHandlerBuilder> next)        {            if (next == null)            {                throw new ArgumentNullException(nameof(next));            }

            return (builder) =>            {                // Run other configuration first, we want to decorate.                next(builder);

                var outerLogger =_loggerFactory.CreateLogger($"System.Net.Http.HttpClient.{builder.Name}.LogicalHandler");                builder.AdditionalHandlers.Clear();                builder.AdditionalHandlers.Insert(0,new CustomLoggingScopeHttpMessageHandler(outerLogger));            };        }    }

P2

实现带有TraceId能力的HttpClient 日志处理器，并加入到IHttpMessageHandlerFilter接口实现类

public class CustomLoggingScopeHttpMessageHandler : DelegatingHandler    {        private readonly ILogger _logger;

        public CustomLoggingScopeHttpMessageHandler(ILogger logger)        {            _logger = logger ?? throw new ArgumentNullException(nameof(logger));        }

        protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,            CancellationToken cancellationToken)        {            if (request == null)            {                throw new ArgumentNullException(nameof(request));            }

            using (Log.BeginRequestPipelineScope(_logger, request))            {                Log.RequestPipelineStart(_logger, request);                var response = await base.SendAsync(request, cancellationToken);                Log.RequestPipelineEnd(_logger, response);

                return response;            }        }

        private static class Log        {            private static class EventIds            {                public static readonly EventId PipelineStart = new EventId(100, "RequestPipelineStart");                public static readonly EventId PipelineEnd = new EventId(101, "RequestPipelineEnd");            }

            private static readonly Func<ILogger, HttpMethod, Uri, string, IDisposable> _beginRequestPipelineScope =                LoggerMessage.DefineScope<HttpMethod, Uri, string>(                    "HTTP {HttpMethod} {Uri} {CorrelationId}");

            private static readonly Action<ILogger, HttpMethod, Uri, string, Exception> _requestPipelineStart =                LoggerMessage.Define<HttpMethod, Uri, string>(                    LogLevel.Information,                    EventIds.PipelineStart,                    "Start processing HTTP request {HttpMethod} {Uri} [Correlation: {CorrelationId}]");

            private static readonly Action<ILogger, HttpStatusCode,string,Exception> _requestPipelineEnd =                LoggerMessage.Define<HttpStatusCode,string>(                    LogLevel.Information,                    EventIds.PipelineEnd,                    "End processing HTTP request - {StatusCode}, [Correlation: {CorrelationId}]");

            public static IDisposable BeginRequestPipelineScope(ILogger logger, HttpRequestMessage request)            {                var correlationId = GetCorrelationIdFromRequest(request);                return _beginRequestPipelineScope(logger, request.Method, request.RequestUri, correlationId);            }

            public static void RequestPipelineStart(ILogger logger, HttpRequestMessage request)            {                var correlationId = GetCorrelationIdFromRequest(request);                _requestPipelineStart(logger, request.Method, request.RequestUri, correlationId, null);            }

            public static void RequestPipelineEnd(ILogger logger, HttpResponseMessage response)            {                var correlationId = GetCorrelationIdFromRequest(response.RequestMessage);                _requestPipelineEnd(logger, response.StatusCode, correlationId, null);            }

            private static string GetCorrelationIdFromRequest(HttpRequestMessage request)            {                string correlationId = Guid.NewGuid().ToString();                if (request.Headers.TryGetValues("X-Correlation-ID", out var values))                    correlationId = values.First();                else                    request.Headers.Add("X-Correlation-ID",correlationId);                return correlationId;            }        }    }

以上TraceId的实现思路，参考了我前一篇博文《被忽略的TraceId，可以用起来了》的思路，为每次HttpClient调用过程设定  全局唯一的GUID标记， 后置api服务可酌情修改以上代码处理。》其中写入日志的代码Copy自HttpClientFactory源代码。

P3

在DI框架中替换原有的IHttpMessageHandlerFilter实现

services.Replace(ServiceDescriptor.Singleton<IHttpMessageHandlerBuilderFilter, TraceIdLoggingMessageHandlerFilter>());

发起两次HttpClient请求， 输出的日志如下:

19/12/04 12:59:22 [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[17}].[]Start processing HTTP request GET http://localhost:5000/v1/eqid/ad78deef00444ed7000000035de704e8 [Correlation: 03de676d-680e-4a92-aef5-749bcc3ba499]19/12/04 12:59:22 [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[4}].[]End processing HTTP request - OK, [Correlation: 03de676d-680e-4a92-aef5-749bcc3ba499]19/12/04 12:59:48 [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[17}].[]Start processing HTTP request GET http://localhost:5000/v1/eqid/8ea0c3b66b60f0ff100000005de704fb [Correlation: 6f14393a-3a2b-45c4-a9b4-0b4ab874ef1d]19/12/04 12:59:48 [Info].[System.Net.Http.HttpClient.bce-request.LogicalHandler].[42}].[]End processing HTTP request - OK, [Correlation: 6f14393a-3a2b-45c4-a9b4-0b4ab874ef1d]

可以看到每次请求的开始和结束都带上了设定的GUID TraceId。

Tip

① 这个TraceId 可以使用你业务上独具一格的标记，这样在排查时， 能根据上游业务更好的追踪日志。

② 目前这个TraceId位于LogMessage，实际上可以为nlog自定义LogoutRenderer，将该TraceId放在显著位置，便于ETL等日志集成框架过滤。

That's All， 这是本次解决HttpClient日志无法追踪的思路和方案，思考+实践+UML制图，希望能给大家一些启发。

+ https://github.com/aspnet/HttpClientFactory/blob/master/src/Microsoft.Extensions.Http/Logging/LoggingHttpMessageHandlerBuilderFilter.cs

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