MemCache分布式缓存的一个bug

       Memcached分布式缓存策略不是由服务器端至支持的，多台服务器之间并不知道彼此的存在。分布式的实现是由客户端代码（Memcached.ClientLibrary）通过缓存key-server映射来实现的，基本原理就是对缓存key求hash值，用hash值对服务器数量进行模运算，该key值被分配到模运算结果为索引的那台server上。

       Memcached.ClientLibrary对缓存key计算hashcode的核心算法如下：

  1 /// <summary>
  2 /// Returns appropriate SockIO object given
  3 /// string cache key and optional hashcode.
  4 /// 
  5 /// Trys to get SockIO from pool.  Fails over
  6 /// to additional pools in event of server failure.
  7 /// </summary>
  8 /// <param name="key">hashcode for cache key</param>
  9 /// <param name="hashCode">if not null, then the int hashcode to use</param>
 10 /// <returns>SockIO obj connected to server</returns>
 11 public SockIO GetSock(string key, object hashCode)
 12 {
 13     string hashCodeString = "<null>";
 14     if(hashCode != null)
 15         hashCodeString = hashCode.ToString();
 16 
 17     if(Log.IsDebugEnabled)
 18     {
 19         Log.Debug(GetLocalizedString("cache socket pick").Replace("$$Key$$", key).Replace("$$HashCode$$", hashCodeString));
 20     }
 21 
 22     if (key == null || key.Length == 0)
 23     {
 24         if(Log.IsDebugEnabled)
 25         {
 26             Log.Debug(GetLocalizedString("null key"));
 27         }
 28         return null;
 29     }
 30 
 31     if(!_initialized)
 32     {
 33         if(Log.IsErrorEnabled)
 34         {
 35             Log.Error(GetLocalizedString("get socket from uninitialized pool"));
 36         }
 37         return null;
 38     }
 39 
 40     // if no servers return null
 41     if(_buckets.Count == 0)
 42         return null;
 43 
 44     // if only one server, return it
 45     if(_buckets.Count == 1)
 46         return GetConnection((string)_buckets[0]);
 47 
 48     int tries = 0;
 49 
 50     // generate hashcode
 51     int hv;
 52     if(hashCode != null)
 53     {
 54         hv = (int)hashCode;
 55     }
 56     else
 57     {
 58 
 59         // NATIVE_HASH = 0
 60         // OLD_COMPAT_HASH = 1
 61         // NEW_COMPAT_HASH = 2
 62         switch(_hashingAlgorithm)
 63         {
 64             case HashingAlgorithm.Native:
 65                 hv = key.GetHashCode();
 66                 break;
 67 
 68             case HashingAlgorithm.OldCompatibleHash:
 69                 hv = OriginalHashingAlgorithm(key);
 70                 break;
 71 
 72             case HashingAlgorithm.NewCompatibleHash:
 73                 hv = NewHashingAlgorithm(key);
 74                 break;
 75 
 76             default:
 77                 // use the native hash as a default
 78                 hv = key.GetHashCode();
 79                 _hashingAlgorithm = HashingAlgorithm.Native;
 80                 break;
 81         }
 82     }
 83 
 84     // keep trying different servers until we find one
 85     while(tries++ <= _buckets.Count)
 86     {
 87         // get bucket using hashcode 
 88         // get one from factory
 89         int bucket = hv % _buckets.Count;
 90         if(bucket < 0)
 91             bucket += _buckets.Count;
 92 
 93         SockIO sock = GetConnection((string)_buckets[bucket]);
 94 
 95         if(Log.IsDebugEnabled)
 96         {
 97             Log.Debug(GetLocalizedString("cache choose").Replace("$$Bucket$$", _buckets[bucket].ToString()).Replace("$$Key$$", key));
 98         }
 99 
100         if(sock != null)
101             return sock;
102 
103         // if we do not want to failover, then bail here
104         if(!_failover)
105             return null;
106 
107         // if we failed to get a socket from this server
108         // then we try again by adding an incrementer to the
109         // current key and then rehashing 
110         switch(_hashingAlgorithm)
111         {
112             case HashingAlgorithm.Native:
113                 hv += ((string)("" + tries + key)).GetHashCode();
114                 break;
115 
116             case HashingAlgorithm.OldCompatibleHash:
117                 hv += OriginalHashingAlgorithm("" + tries + key);
118                 break;
119 
120             case HashingAlgorithm.NewCompatibleHash:
121                 hv += NewHashingAlgorithm("" + tries + key);
122                 break;
123 
124             default:
125                 // use the native hash as a default
126                 hv += ((string)("" + tries + key)).GetHashCode();
127                 _hashingAlgorithm = HashingAlgorithm.Native;
128                 break;
129         }
130     }
131 
132     return null;
133 }

根据缓存key得到服务器的核心代码