最近在研究"一致性HASH算法"（Consistent Hashing），用于解决memcached集群中当服务器出现增减变动时对散列值的影响。后来 在JAVAEYE上的一篇文章中，找到了其中的 KetamaHash 算法的JAVA实现（一种基于虚拟结点的HASH算法），于是为了加深理解，对照 JAVA版本，用C#重写了一个。放到这里，如果大家感兴趣的话， 可以下载测试一下，如果发现写法有问题请及时告之我，以便我及时修正。
下面是对Ketama的介绍：
Ketama is an implementation of a consistent hashing algorithm, meaning you can add or remove servers from the memcached pool without causing a complete remap of all keys.
Here’s how it works:
* Take your list of servers (eg: 1.2.3.4:11211, 5.6.7.8:11211, 9.8.7.6:11211)
* Hash each server string to several (100-200) unsigned ints
* Conceptually, these numbers are placed on a circle called the continuum. (imagine a clock face that goes from 0 to 2^32)
* Each number links to the server it was hashed from, so servers appear at several points on the continuum, by each of the numbers they hashed to.
* To map a key->server, hash your key to a single unsigned int, and find the next biggest number on the continuum. The server linked to that number is the correct server for that key.
* If you hash your key to a value near 2^32 and there are no points on the continuum greater than your hash, return the first server in the continuum.
If you then add or remove a server from the list, only a small proportion of keys end up mapping to different servers.
Here’s how it works:
* Take your list of servers (eg: 1.2.3.4:11211, 5.6.7.8:11211, 9.8.7.6:11211)
* Hash each server string to several (100-200) unsigned ints
* Conceptually, these numbers are placed on a circle called the continuum. (imagine a clock face that goes from 0 to 2^32)
* Each number links to the server it was hashed from, so servers appear at several points on the continuum, by each of the numbers they hashed to.
* To map a key->server, hash your key to a single unsigned int, and find the next biggest number on the continuum. The server linked to that number is the correct server for that key.
* If you hash your key to a value near 2^32 and there are no points on the continuum greater than your hash, return the first server in the continuum.
If you then add or remove a server from the list, only a small proportion of keys end up mapping to different servers.