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
从源码中（62--82行代码）可以发现，计算hashcode的算法共三种：
（1）HashingAlgorithm.Native:
即使用.NET本身的hash算法，速度快，但与其他client可能不兼容，例如需要和java、ruby的client共享缓存的情况；
（2）HashingAlgorithm.OldCompatibleHash:
可以与其他客户端兼容，但速度慢；
（3）HashingAlgorithm.NewCompatibleHash:
可以与其他客户端兼容，据称速度快。
进一步分析发现，Memcached.ClientLibrary默认计算缓存key的hashcode的方式就是HashingAlgorithm.Native，而HashingAlgorithm.Native计算hashcode的算法为“hv
= key.GetHashCode()”，即用了.net类库string类型自带的GetHashCode()方法。
Bug就要浮现出来了，根据微软（http://msdn.microsoft.com/zh-cn/library/system.object.gethashcode.aspx）对GetHashCode的解释：the
.NET Framework does not guarantee the default implementation of the GetHashCode method, and the value this method
returns may differ between .NET Framework versions and platforms, such as 32-bit
and 64-bit platforms。string类型的GetHashCode()函数并不能保证不同平台同一个字符串返回的hash值相同，这样问题就出来了，对于不同服务器的同一缓存key来说，产生的hashcode可能不同，同一key对应的数据可能缓存到了不同的MemCache服务器上，数据的一致性无法保证，清除缓存的代码也可能失效。
// 64位 4.0
[__DynamicallyInvokable, ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail), SecuritySafeCritical]
public unsafe override int GetHashCode()
{
if (HashHelpers.s_UseRandomizedStringHashing)
{
return string.InternalMarvin32HashString(this, this.Length, 0L);
}
IntPtr arg_25_0;
IntPtr expr_1C = arg_25_0 = this;
if (expr_1C != 0)
{
arg_25_0 = (IntPtr)((int)expr_1C + RuntimeHelpers.OffsetToStringData);
}
char* ptr = arg_25_0;
int num = 5381;
int num2 = num;
char* ptr2 = ptr;
int num3;
while ((num3 = (int)(*(ushort*)ptr2)) != 0)
{
num = ((num << 5) + num ^ num3);
num3 = (int)(*(ushort*)(ptr2 + (IntPtr)2 / 2));
if (num3 == 0)
{
break;
}
num2 = ((num2 << 5) + num2 ^ num3);
ptr2 += (IntPtr)4 / 2;
}
return num + num2 * 1566083941;
}// 64位 2.0
// string
[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
public unsafe override int GetHashCode()
{
IntPtr arg_0F_0;
IntPtr expr_06 = arg_0F_0 = this;
if (expr_06 != 0)
{
arg_0F_0 = (IntPtr)((int)expr_06 + RuntimeHelpers.OffsetToStringData);
}
char* ptr = arg_0F_0;
int num = 5381;
int num2 = num;
char* ptr2 = ptr;
int num3;
while ((num3 = (int)(*(ushort*)ptr2)) != 0)
{
num = ((num << 5) + num ^ num3);
num3 = (int)(*(ushort*)(ptr2 + (IntPtr)2 / 2));
if (num3 == 0)
{
break;
}
num2 = ((num2 << 5) + num2 ^ num3);
ptr2 += (IntPtr)4 / 2;
}
return num + num2 * 1566083941;
}//32位 4.0
[__DynamicallyInvokable, ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail), SecuritySafeCritical]
public unsafe override int GetHashCode()
{
if (HashHelpers.s_UseRandomizedStringHashing)
{
return string.InternalMarvin32HashString(this, this.Length, 0L);
}
IntPtr arg_25_0;
IntPtr expr_1C = arg_25_0 = this;
if (expr_1C != 0)
{
arg_25_0 = (IntPtr)((int)expr_1C + RuntimeHelpers.OffsetToStringData);
}
char* ptr = arg_25_0;
int num = 352654597;
int num2 = num;
int* ptr2 = (int*)ptr;
int i;
for (i = this.Length; i > 2; i -= 4)
{
num = ((num << 5) + num + (num >> 27) ^ *ptr2);
num2 = ((num2 << 5) + num2 + (num2 >> 27) ^ ptr2[(IntPtr)4 / 4]);
ptr2 += (IntPtr)8 / 4;
}
if (i > 0)
{
num = ((num << 5) + num + (num >> 27) ^ *ptr2);
}
return num + num2 * 1566083941;
}
GetHashCode几种版本的实现代码
解决问题的方法就是不要用MemCache默认的hash算法，实现方式有两种：
（1）初始化MemCache服务器的时候，指定为MemCahce自带其它的hash算法，代码为“this.pool.HashingAlgorithm
= HashingAlgorithm.OldCompatibleHash;”。
（2）自定义hash算法，调用set()、get()、delete()等方式时传递hash值，这几个方法有参数传递hashcode的重载。
参考资料：分析Memcached客户端如何把缓存数据分布到多个服务器上(转)、memcached client - memcacheddotnet (Memcached.ClientLibrary)
1.1.5、memcache分布式实现、Object.GetHashCode 方法、关于 HashCode做key的可能性。