之前我们已经基于ARSoft.Tools.Net简单实现了DNS解析模块的功能，但是当性能要求升高时，每一次爬取都要进行DNS请求，甚至很有可能一段时间内每次请求的都是相同的地址，频繁的DNS请求就会成为性能瓶颈，所以我们要通过缓存机制将DNS解析结果缓存下来，降低DNS解析操作，提升系统性能。

如此，我们基于之前封装的MemoryCacheHelper类对DnsResolver类进行改造：

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Net;
using ARSoft.Tools.Net;
using ARSoft.Tools.Net.Dns;
using Mem = Crawler.Common.MemoryCacheHelper;

namespace Crawler.Protocol
{
   public class DnsResolver
   {
       public TimeSpan TimeSpan { get; set; }
       public string Url { get; set; }
       public ARecord Record { get; set; }
       public string DnsServer { get; set; }
       public int TimeOut { get; set; }
       public ReturnCode ReturnCode { get; set; }
       public bool IsSuccess { get; private set; }
       public TimeSpan TimeToLive { get; set; }
       public DnsResolver(string url, string dnsServer = "223.5.5.5", int timeOut = 10000)
       {
           Url = url;
           DnsServer = dnsServer;
           TimeOut = timeOut;
           IsSuccess = false;
           if (Mem.Contains(url))
               Fill(Mem.Get<DnsResolver>(url));
           else
               Dig();
       }

       private void Fill(DnsResolver resolver)
       {
           TimeSpan = resolver.TimeSpan;
           Url = resolver.Url;
           Record = resolver.Record;
           DnsServer = resolver.DnsServer;
           TimeOut = resolver.TimeOut;
           ReturnCode = resolver.ReturnCode;
           IsSuccess = resolver.IsSuccess;
       }

       public void Dig()
       {
           //初始化DnsClient，第一个参数为DNS服务器的IP，第二个参数为超时时间
           var dnsClient = new DnsClient(IPAddress.Parse(DnsServer), TimeOut);
           var s = new Stopwatch();
           s.Start();
           //解析域名。将域名请求发送至DNS服务器解析，参数为需要解析的域名
           var dnsMessage = dnsClient.Resolve(DomainName.Parse(Url));
           s.Stop();
           TimeSpan = s.Elapsed;
           //若返回结果为空，或者存在错误，则该请求失败。
           if (dnsMessage == null || (dnsMessage.ReturnCode != ReturnCode.NoError && dnsMessage.ReturnCode != ReturnCode.NxDomain))
               IsSuccess = false;
           //循环遍历返回结果，将返回的IPV4记录添加到结果集List中。
           if (dnsMessage != null)
           {
               if (dnsMessage.AnswerRecords.Count > 0)
               {
                   Record = dnsMessage.AnswerRecords[0] as ARecord;
                   if (Record != null)
                   {
                       IsSuccess = true;
                       TimeToLive=new TimeSpan(0,0,Record.TimeToLive);
                       Mem.Add(Url, this, TimeToLive);
                   }
               }
           }
           if (dnsMessage != null) ReturnCode = dnsMessage.ReturnCode;
       }
   }
}

这样,每次做完DNS解析后，会根据域名的TTL将解析结果缓存下来，下次查询时可以直接调用缓存，提高系统性能。