写在前面

前面两篇文章透过源码角度，理解了HttpClientFactory的内部实现，当我们在项目中使用时，总会涉及以下几个问题：

• HttpClient超时处理以及重试机制
• HttpClient熔断器模式的实现
• HttpClient日志记录与追踪链

接下来我们将从使用角度对上述问题作出说明。

详细介绍

以下代码参考了MSDN，因为代码里展示的GitHub接口确实可以调通，省的我再写一个接口出来测试了。

HttpClient超时处理和重试机制

在此之前，我们需要了解一下Polly这个库，Polly是一款基于.NET的弹性及瞬间错误处理库, 它允许开发人员以顺畅及线程安全的方式执行重试(Retry)，断路器(Circuit)，超时(Timeout)，隔板隔离(Bulkhead Isolation)及后背策略(Fallback)。

以下代码描述了在.NET Core 3.0中如何使用超时机制。

   1:  Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(10))

那么如何将其注册到对应的HttpClient实例呢，有很多种方式：

• 通过AddPolicyHandler注册

1:  services.AddHttpClient("github", c =>

   2:              {

   3:                  c.BaseAddress = new Uri("https://api.github.com/");

  4:   

   5:                  c.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json");

   6:                  c.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample");

   7:              }).AddPolicyHandler(Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(10)))；

• 声明Policy注册对象，并将超时策略对象添加进去

 1:  var registry = services.AddPolicyRegistry();

   2:  var timeout = Policy.TimeoutAsync<HttpResponseMessage>(TimeSpan.FromSeconds(10));

 3:  registry.Add("regular", timeout);

调用方式

   1:   services.AddHttpClient("github", c =>

  2:              {

   3:                  c.BaseAddress = new Uri("https://api.github.com/");

   4:   

   5:                  c.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json"); // GitHub API versioning

   6:                  c.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample"); // GitHub requires a user-agent

   7:              }).AddPolicyHandlerFromRegistry("regular")

Polly重试也很简单

  1:  var policyRegistry = services.AddPolicyRegistry();

   2:   

   3:  policyRegistry.Add("MyHttpRetry",HttpPolicyExtensions.HandleTransientHttpError().WaitAndRetryAsync(3,retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt))));

这里的重试设置是在第一次调用失败后，还会有三次机会继续重试，每个请求的时间间隔是指数级延迟。

重试功能除了可以使用Polly实现外，还可以使用DelegatingHandler，DelegatingHandler继承自HttpMessageHandler，用于”处理请求、响应回复“，本质上就是一组HttpMessageHandler的有序组合，可以视为是一个“双向管道”。

此处主要展示DelegatingHandler的使用方式，在实际使用中，仍然建议使用Polly重试。

  1:  private class RetryHandler : DelegatingHandler

 2:  {

  3:      public int RetryCount { get; set; } = 5;

 4:   

   5:      protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)

   6:      {

   7:          for (var i = 0; i < RetryCount; i++)

  8:          {

   9:              try

  10:              {

  11:                  return await base.SendAsync(request, cancellationToken);

  12:              }

 13:              catch (HttpRequestException) when (i == RetryCount - 1)

14:              {

  15:                  throw;

  16:              }

 17:              catch (HttpRequestException)

 18:              {

 19:                  // 五十毫秒后重试

  20:                  await Task.Delay(TimeSpan.FromMilliseconds(50));

  21:              }

  22:          }

23:      }

 24:  }

注册方式如下：

  1:  services.AddHttpClient("github", c =>

 2:  {

  3:      c.BaseAddress = new Uri("https://api.github.com/");

   4:   

   5:      c.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json"); // GitHub API versioning

  6:      c.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample"); // GitHub requires a user-agent

 7:  })

8:  .AddHttpMessageHandler(() => new RetryHandler());

HttpClient熔断器模式的实现

如果非常了解Polly库的使用，那么熔断器模式的实现也会非常简单，

   1:  var policyRegistry = services.AddPolicyRegistry();

   2:   

   3:  policyRegistry.Add("MyCircuitBreaker",HttpPolicyExtensions.HandleTransientHttpError().CircuitBreakerAsync(handledEventsAllowedBeforeBreaking: 10,durationOfBreak: TimeSpan.FromSeconds(30)));

这里的熔断器设置规则是在连续10次请求失败后，会暂停30秒。这个地方可以写个扩展方法注册到IServiceCollection中。

HttpClient日志记录与追踪链

日志记录这块与追踪链，我们一般会通过request.Header实现，而在微服务中，十分关注相关调用方的信息及其获取，一般的做法是通过增加请求Id的方式来确定请求及其相关日志信息。

实现思路是增加一个DelegatingHandler实例，用以记录相关的日志以及请求链路

   1:      public class TraceEntryHandler : DelegatingHandler

 2:      {

 3:          private TraceEntry TraceEntry { get; set; }

   4:   

   5:          public TraceEntryHandler(TraceEntry traceEntry)

   6:          {

 7:              this.TraceEntry = traceEntry;

   8:          }

   9:   

 10:         protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)

  11:          {

12:              request.Headers.TryAddWithoutValidation("X-TRACE-CID", this.TraceEntry.ClientId);

 13:              request.Headers.TryAddWithoutValidation("X-TRACE-RID", this.TraceEntry.RequestId);

14:              request.Headers.TryAddWithoutValidation("X-TRACE-SID", this.TraceEntry.SessionId);

 15:              if (this.TraceEntry.SourceIP.IsNullOrEmpty())

16:              {

17:                  request.Headers.TryAddWithoutValidation("X-TRACE-IP", this.TraceEntry.SourceIP);

 18:              }

19:   

  20:              if (this.TraceEntry.SourceUserAgent.IsNullOrEmpty())

  21:              {

  22:                  request.Headers.TryAddWithoutValidation("X-TRACE-UA", this.TraceEntry.SourceUserAgent);

 23:              }

 24:   

 25:              if (this.TraceEntry.UserId.IsNullOrEmpty())

  26:              {

 27:                  request.Headers.TryAddWithoutValidation("X-TRACE-UID", this.TraceEntry.UserId);

 28:              }

  29:   

  30:              return base.SendAsync(request, cancellationToken);

 31:          }

  32:      }

我在查找相关资料的时候，发现有个老外使用CorrelationId组件实现，作为一种实现方式，我决定要展示一下，供大家选择：

   1:  public class CorrelationIdDelegatingHandler : DelegatingHandler

   2:  {

   3:      private readonly ICorrelationContextAccessor correlationContextAccessor;

 4:      private readonly IOptions<CorrelationIdOptions> options;

   5:   

  6:      public CorrelationIdDelegatingHandler(

   7:          ICorrelationContextAccessor correlationContextAccessor,

   8:          IOptions<CorrelationIdOptions> options)

   9:      {

  10:          this.correlationContextAccessor = correlationContextAccessor;

  11:          this.options = options;

  12:      }

  13:   

14:      protected override Task<HttpResponseMessage> SendAsync(

15:          HttpRequestMessage request,

16:          CancellationToken cancellationToken)

 17:      {

  18:          if (!request.Headers.Contains(this.options.Value.Header))

 19:          {

  20:              request.Headers.Add(this.options.Value.Header, correlationContextAccessor.CorrelationContext.CorrelationId);

  21:          }

  22:   

 23:          // Else the header has already been added due to a retry.

  24:   

25:          return base.SendAsync(request, cancellationToken);

  26:      }

  27:  }

参考链接：

https://docs.microsoft.com/zh-cn/aspnet/core/fundamentals/http-requests?view=aspnetcore-3.0

https://rehansaeed.com/optimally-configuring-asp-net-core-httpclientfactory/