.NET Core HttpClient+Consul实现服务发现

简介

　　随着.NET Core的不断发展与成熟，基于.NET Core实现微服务的解决方案也越来越多。这其中必然需要注册中心，Consul成为了.NET Core实现服务注册与发现的首选。类似的解决方案还有很多比如Netflix Eureka，也有关于结合.NET Core的案例比如比较知名的就是SteeltoeOSS.Discovery 这里就不过多的介绍了，有兴趣的小伙伴可以自己在网上查阅资料。接下来我们讲解如何实现HttpClient结合Consul实现服务发现。

入门

　　关于Consul如何入门，相信很多小伙伴已经非常熟练了，这里就不再赘述。如果还有不熟悉的小伙伴请查阅Edison Zhou的 https://www.cnblogs.com/edisonchou/p/9124985.html 写的非常详细。

初步实现

如何将发现的地址结合到HttpClient相信很多小伙伴首先想到的就是如下方法

public string LookupService(string serviceName)
{
    using (ConsulClient consulClient = new ConsulClient(config=>config.Address=new Uri("http://localhost:8500/")))
    {
        var services = _consulClient.Catalog.Service(serviceName).Result.Response;
        if (services != null && services.Any())
        {
            //模拟负载均衡算法(随机获取一个地址)
            int index = r.Next(services.Count());
            var service = services.ElementAt(index);
            return $"{service.ServiceAddress}:{service.ServicePort}";
         }
         return null;
    }
}

然后调用的时候采用类似的方法

public async Task<Person> GetPerson(int personId)
{
    using (HttpClient client = new HttpClient())
    {
       var response = await client.GetAsync($"http://{LookupService("PersonService")}/Person/GetPerson?personId={personId}");
       var jsonResult = await response.Content.ReadAsStringAsync();
       return jsonResult.FromJson<Person>();
    }
}

或者封装了一层HttpHelper里面封装了针对Get Post的调用方法。

借助HttpMessageHandler

　　上面的方式实现确实是实现了,但是老是感觉差点意思，如果我在HttpHelper里多封装几个方法，那岂不是每个方法里面都得调用一次LookupService方法，总感觉不够优雅。难道没有更好的办法了吗？ Of course，有的小伙伴可能发现了HttpClient构造函数有几个重载方法

/// <summary>Initializes a new instance of the <see cref="T:System.Net.Http.HttpClient" /> class using a <see cref="T:System.Net.Http.HttpClientHandler" /> that is disposed when this instance is disposed.</summary>
public HttpClient ()
    : base (null);

/// <summary>Initializes a new instance of the <see cref="T:System.Net.Http.HttpClient" /> class with the specified handler. The handler is disposed when this instance is disposed.</summary>
/// <param name="handler">The HTTP handler stack to use for sending requests.</param>
/// <exception cref="T:System.ArgumentNullException">The <paramref name="handler" /> is <see langword="null" />.</exception>
public HttpClient (HttpMessageHandler handler)
    : base (null);

/// <summary>Initializes a new instance of the <see cref="T:System.Net.Http.HttpClient" /> class with the provided handler, and specifies whether that handler should be disposed when this instance is disposed.</summary>
/// <param name="handler">The <see cref="T:System.Net.Http.HttpMessageHandler" /> responsible for processing the HTTP response messages.</param>
/// <param name="disposeHandler">
///   <see langword="true" /> if the inner handler should be disposed of by HttpClient.Dispose; <see langword="false" /> if you intend to reuse the inner handler.</param>
/// <exception cref="T:System.ArgumentNullException">The <paramref name="handler" /> is <see langword="null" />.</exception>
public HttpClient (HttpMessageHandler handler, bool disposeHandler)
    : base (null);

其中我们看到了HttpMessageHandler这是处理HttpClient消息的拦截器类，通过它可以获取和设置HttpClient的请求和返回内容。 具体实现如下

/// <summary>A base type for HTTP message handlers.</summary>
public abstract class HttpMessageHandler : IDisposable
{
    /// <summary>Releases the unmanaged resources and disposes of the managed resources used by the <see cref="T:System.Net.Http.HttpMessageHandler" />.</summary>
    public void Dispose ();

    /// <summary>Releases the unmanaged resources used by the <see cref="T:System.Net.Http.HttpMessageHandler" /> and optionally disposes of the managed resources.</summary>
    /// <param name="disposing">
    ///   <see langword="true" /> to release both managed and unmanaged resources; <see langword="false" /> to releases only unmanaged resources.</param>
    protected virtual void Dispose (bool disposing);

    /// <summary>Send an HTTP request as an asynchronous operation.</summary>
    /// <param name="request">The HTTP request message to send.</param>
    /// <param name="cancellationToken">The cancellation token to cancel operation.</param>
    /// <returns>The task object representing the asynchronous operation.</returns>
    /// <exception cref="T:System.ArgumentNullException">The <paramref name="request" /> was <see langword="null" />.</exception>
    protected internal abstract Task<HttpResponseMessage> SendAsync (HttpRequestMessage request, CancellationToken cancellationToken);
}

　　这个一个抽象类，通过继承这个抽象类，实现SendAsync抽象方法可以处理请求消息和返回消息。我们就从这里入手了，我们使用是DelegatingHandler这也是一个抽象类，这个抽象类继承自HttpMessageHandler是系统自带的一个抽象类，其实常用的还有一个叫HttpClientHandler,这个类也是继承自HttpMessageHandler，且提供了一些设置和获取操作输出和输出的具体实现。这里我们选用实现DelegatingHandler抽象类，至于为什么下篇文章会做详解，自定义一个ConsulDiscoveryDelegatingHandler实现类具体代码如下

public class ConsulDiscoveryDelegatingHandler : DelegatingHandler
{
     protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
     {
          var current = request.RequestUri;
          try
          {
              //调用的服务地址里的域名(主机名)传入发现的服务名称即可
              request.RequestUri = new Uri($"{current.Scheme}://{LookupService(current.Host)}/{current.PathAndQuery}");
              return await base.SendAsync(request, cancellationToken).ConfigureAwait(false);
          }
          catch (Exception e)
          {
              throw;
          }
          finally
          {
              request.RequestUri = current;
          }
      }

      private string LookupService(string serviceName)
      {
          using (ConsulClient consulClient = new ConsulClient(config=>config.Address=new Uri("http://localhost:8500/")))
          {
              var services = _consulClient.Catalog.Service(serviceName).Result.Response;
              if (services != null && services.Any())
              {
                    //模拟负载均衡算法(随机获取一个地址)
                    int index = r.Next(services.Count());
                    var service = services.ElementAt(index);
                    return $"{service.ServiceAddress}:{service.ServicePort}");
               }
               return null;
          }
      }
}

这样的话就大致实现了一个基于consul 发现的ConsulDiscoveryDelegatingHandler，具体的使用方式如下

public async Task<Person> GetPerson(int personId)
{
    using (HttpClient client = new HttpClient(new ConsulDiscoveryDelegatingHandler()))
    {
       //调用时的域名(主机名)传入服务发现的名称即可
       var response = await client.GetAsync($"http://PersonService/Person/GetPerson?personId={personId}");
       var jsonResult = await response.Content.ReadAsStringAsync();
       return jsonResult.FromJson<Person>();
    }
}

到这里为止，关于HttpClient结合Consul实现服务发现的具体实现大致就差不多了，本身还存在很多不足，比如没有结合自带的IOC，没有做连接异常处理等等。但是能给大家提供一些思路或者帮助本人就已经满足了。

最后

到这里还并没有结束，相信很多小伙伴都知道HttpClient存在不足，就是Dispose的时候，套接字本身会延迟释放，会导致端口号占用的问题，高并发情况下会导致端口号用尽，导致服务器拒绝服务。虽然可以通过单例或者设置系统句柄释放时间解决这个问题，但是还是会存在一定的问题。庆幸的是微软也意识到了这个问题，从.NET Core 2.1版本开始推出了HttpClientFactory 通过池化技术管理HttpClient实例能很好的解决这个问题。在以后的版本里我们去访问网络请求都会使用HttpClientFactory。下一篇文章，我将会通过分析HttpClientFactory源码的方式，一步步探讨如何使用更优雅的方式实现HttpClientFactory+Consul实现服务发现。

本系列未完待续。。。