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.

public class KetamaNodeLocator
{
//原文中的JAVA類TreeMap實現了Comparator方法，這里我圖省事，直接用了net下的SortedList，其中Comparer接口方法）
private SortedList<long, string> ketamaNodes = new SortedList<long, string>();
private HashAlgorithm hashAlg;
private int numReps = 160;

//此處參數與JAVA版中有區別，因為使用的靜態方法，所以不再傳遞HashAlgorithm alg參數
public KetamaNodeLocator(List<string> nodes, int nodeCopies)
{
ketamaNodes = new SortedList<long, string>();

numReps = nodeCopies;
//對所有節點，生成nCopies個虛擬結點
foreach (string node in nodes)
{
//每四個虛擬結點為一組
for (int i = 0; i < numReps / 4; i++)
{
//getKeyForNode方法為這組虛擬結點得到惟一名稱
byte[] digest = HashAlgorithm.computeMd5(node + i);
/** Md5是一個16字節長度的數組，將16字節的數組每四個字節一組，分別對應一個虛擬結點，這就是為什么上面把虛擬結點四個劃分一組的原因*/  
for (int h = 0; h < 4; h++)
{
long m = HashAlgorithm.hash(digest, h);
ketamaNodes[m] = node;
}
}
}
}

public string GetPrimary(string k)
{
byte[] digest = HashAlgorithm.computeMd5(k);
string rv = GetNodeForKey(HashAlgorithm.hash(digest, 0));
return rv;
}

string GetNodeForKey(long hash)
{
string rv;
long key = hash;
//如果找到這個節點，直接取節點，返回  
if (!ketamaNodes.ContainsKey(key))
{
//得到大于當前key的那個子Map，然后從中取出第一個key，就是大于且離它最近的那個key 說明詳見: //www.javaeye.com/topic/684087
var tailMap = from coll in ketamaNodes
where coll.Key > hash
select new { coll.Key };
if (tailMap == null || tailMap.Count() == 0)
key = ketamaNodes.FirstOrDefault().Key;
else
key = tailMap.FirstOrDefault().Key;
}
rv = ketamaNodes[key];
return rv;
}
}
 

public class HashAlgorithm
{
public static long hash(byte[] digest, int nTime)
{
long rv = ((long)(digest[3 + nTime * 4] & 0xFF) << 24)
| ((long)(digest[2 + nTime * 4] & 0xFF) << 16)
| ((long)(digest[1 + nTime * 4] & 0xFF) << 8)
| ((long)digest[0 + nTime * 4] & 0xFF);

return rv & 0xffffffffL; /* Truncate to 32-bits */
}

/**
* Get the md5 of the given key.
*/
public static byte[] computeMd5(string k)
{
MD5 md5 = new MD5CryptoServiceProvider();

byte[] keyBytes = md5.ComputeHash(Encoding.UTF8.GetBytes(k));
md5.Clear();
//md5.update(keyBytes);
//return md5.digest();
return keyBytes;
}
}