解剖HttpClientFactory，自由扩展HttpMessageHandler

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。

现在的HttpClientFactory以依赖注入的方式集成到.NETCore 框架：

// 截取自Startup.cs文件服务配置部分
public void ConfigureServices(IServiceCollection services)
{
            services.AddHttpClient("bce-request", x =>
                   x.BaseAddress = new Uri(Configuration.GetSection("BCE").GetValue<string>("BaseUrl")))
                .ConfigurePrimaryHttpMessageHandler(_ => new BceAuthClientHandler()
               {
                   AccessKey = Configuration.GetSection("BCE").GetValue<string>("AccessKey"),
                   SerectAccessKey = Configuration.GetSection("BCE").GetValue<string>("SecretAccessKey"),
                   AllowAutoRedirect = true,
                   UseDefaultCredentials = true
               })
               .SetHandlerLifetime(TimeSpan.FromHours(12))
               .AddPolicyHandler(GetRetryPolicy(3));
}

static IAsyncPolicy<HttpResponseMessage> GetRetryPolicy(int  retry)
{
      var retryPolicy = HttpPolicyExtensions
                .HandleTransientHttpError()
                .OrResult(msg => msg.StatusCode == System.Net.HttpStatusCode.NotFound)
                .WaitAndRetryAsync(retry, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));
            return retryPolicy;
}

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。

以下是利用NLog观察到的文件日志：

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

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。

因此本文打算重新自定义HttpClientFactory日志处理器(给请求的全部日志设置TraceId)，实际上CustomLoggingHttpMessageHandler只是一个引子，掌握如何扩展才是关键。

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

+ https://github.com/dotnet/extensions/blob/master/src/HttpClientFactory/Http/src/Logging/LoggingHttpMessageHandlerBuilderFilter.cs

编程实践

如以上分析，

P1 实现 IHttpMessageHandlerFilter接口，在接口中移除默认的两个日志处理器

    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;
                if (request.Headers.TryGetValues("X-Correlation-ID", out var values))
                    correlationId = values.First();
                else
                   {correlationId = Guid.NewGuid().ToString()； 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。

值得提醒的是：

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 现在这个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，本次为解决HttpClientFactory日志无追踪机制的探索，思考 + 实践 + UML制图。

实现CustomLoggingScopeHttpMessageHandler只是扩展HttpClientFactory能力的一个引子，如何扩展HttpClientFactory能力才是关键，希望能给大家一些启发。

--------------------------------------------------------------2019.12.06 更新------------------------

实际上HttpClientFactory内原生LoggingHandler是支持LoggingScope, 在Console 输出如下：

info: System.Net.Http.HttpClient.bce-request.LogicalHandler[100]
      => ConnectionId:0HLRQ6DAF0JKV => RequestId:0HLRQ6DAF0JKV:00000004 RequestPath:/eqid/f53990dc0002adf0000000045de9c421 => EqidManager.Controllers.DebugController.ResolveEqid (EqidManager) => HTTP GET http://localhost:5000/v1/eqid/f53990dc0002adf0000000045de9c421
      Start processing HTTP request GET http://localhost:5000/v1/eqid/f53990dc0002adf0000000045de9c421
info: System.Net.Http.HttpClient.bce-request.ClientHandler[100]
      => ConnectionId:0HLRQ6DAF0JKV => RequestId:0HLRQ6DAF0JKV:00000004 RequestPath:/eqid/f53990dc0002adf0000000045de9c421 => EqidManager.Controllers.DebugController.ResolveEqid (EqidManager) => HTTP GET http://localhost:5000/v1/eqid/f53990dc0002adf0000000045de9c421
      Sending HTTP request GET http://localhost:5000/v1/eqid/f53990dc0002adf0000000045de9c421

info: System.Net.Http.HttpClient.bce-request.ClientHandler[101]
      => ConnectionId:0HLRQ6DAF0JKV => RequestId:0HLRQ6DAF0JKV:00000004 RequestPath:/eqid/f53990dc0002adf0000000045de9c421 => EqidManager.Controllers.DebugController.ResolveEqid (EqidManager) => HTTP GET http://localhost:5000/v1/eqid/f53990dc0002adf0000000045de9c421
      Received HTTP response after 195.1112ms - OK
info: System.Net.Http.HttpClient.bce-request.LogicalHandler[101]
      => ConnectionId:0HLRQ6DAF0JKV => RequestId:0HLRQ6DAF0JKV:00000004 RequestPath:/eqid/f53990dc0002adf0000000045de9c421 => EqidManager.Controllers.DebugController.ResolveEqid (EqidManager) => HTTP GET http://localhost:5000/v1/eqid/f53990dc0002adf0000000045de9c421
      End processing HTTP request after 232.4906ms - OK

Scope需要LoggingProvider 支持，而我们使用的NLog不支持scope，所以最上面的nlog 文件日志没有输出Scope。

这就引出了本文的目的，所以本文通过解构HttpClientFactory的HttpMessageHandler，为请求响应添加TraceId，当然你也可以根据HttpClient业务加入其它HttpMessageHandler