实现：
1 Hash环：将节点的Hash值映射到一个Hash环中。每一个Key顺时针第一个找到的节点。就是这个Key被路由到的机器
2 "虚拟节点"：将节点虚拟成多个"虚拟节点"分布在Hash环上，使得分布更均匀。扩缩容影响较小

代码实例：

/** @ 一致性Hash模拟測试* @ 结论：模拟4台机器扩容1台。遍历Key[0,999983]- 一致性Hash需移动181161个Key，约占18%(1/5左右，符合预期效果)- 取模Hash需移动799984个Key，约占80%* @ 2014.05.30 */#include <stdint.h>
#include <iostream>
#include <string.h>
#include <sstream>
#include <map>
#include <vector>
using namespace std;#define HASH_MOD (999983) template <class T>
string ToStr(const T &t)
{stringstream stream;stream << t;return stream.str();
}uint32_t APHash(string &sKey)
{char *key = (char*)sKey.c_str();unsigned int hash = 0;for (int i=0; *key; i++){if ((i & 1) == 0) {hash ^= ((hash<<7)^(*key++)^(hash>>3));} else {hash ^= (~((hash<<11)^(*key++)^(hash>>5)));}}return hash%HASH_MOD;
}class CMyConHash
{
public:/* 加入一台机器(IP) */void AddIp(const string &sIp) {// 每一个IP分配128个虚拟节点，原因：结合APHash实验结果分布较均匀for (int i = 0; i < 128; i ++){string sCode = sIp + ToStr(i) + "#Hash";uint32_t uVirKey = APHash(sCode);mapVirKey2Ip[uVirKey] = sIp;mapIp2VirKey[sIp].push_back(uVirKey);}}/* 删除一台机器(IP) */void DelIp(const string &sIp) {if (mapIp2VirKey.count(sIp) == 0) { cout << "DelIp Err: mapIp2VirKey Don`t Has Ip=" << sIp << endl;return;}vector<uint32_t> vecVirKey = mapIp2VirKey[sIp];for (int i = 0; i < vecVirKey.size(); i ++){uint32_t uVirKey = vecVirKey[i];if (mapVirKey2Ip[uVirKey] == sIp) {// 得推断下。有可能2个IP虚拟节点相等后覆盖了mapVirKey2Ip.erase(uVirKey);}}mapIp2VirKey.erase(sIp);}/* 路由：给每一个Key找到负责的机器(IP) */int FindIp(uint32_t uKey, string &sIp){if (mapVirKey2Ip.size() == 0) {cout << "FindIp Err: mapVirKey2Ip.size() == 0" << endl;return -1;}bool bFind = false;uint32_t uVirKey;map<uint32_t, string>::iterator iter;// 遍历std::map是按Key大小顺序输出(差别std::tr1::unordered_map)for(iter = mapVirKey2Ip.begin(); iter != mapVirKey2Ip.end(); iter ++){uVirKey = iter->first;if (uVirKey > uKey%HASH_MOD) {sIp = iter->second; bFind = true;break;}}if (!bFind) {// 找不到比Key小的虚拟节点，故使用最小的虚拟节点(环)iter = mapVirKey2Ip.begin();uVirKey = iter->first;sIp = iter->second; }//cout << "FindIp Suc:" << uKey%HASH_MOD << "=>" << uVirKey << "," << sIp << endl;return 0;}/* 打印各个IP负责的Key区域大小。影响因素：1 Hash函数 2 虚拟节点个数 *//* 4台机器的情况，相对还是较均匀：Ip=202.168.14.241,Cnt=251649Ip=202.168.14.242,Cnt=257902Ip=202.168.14.243,Cnt=245945Ip=202.168.14.244,Cnt=235516 */void EchoIpState(){map<string, uint32_t> mapIpCnt; map<uint32_t, string>::iterator iter = mapVirKey2Ip.end(); iter --;uint32_t uPreKey = iter->first;string sPreIp = iter->second;do {iter --;uint32_t uVirKey = iter->first;string sIp = iter->second;if (mapIpCnt.count(sPreIp) == 0) {mapIpCnt[sPreIp] = uPreKey-uVirKey;} else {mapIpCnt[sPreIp] += uPreKey-uVirKey;} uPreKey = uVirKey;sPreIp = sIp;} while (iter != mapVirKey2Ip.begin()); cout << "Ip Size=" << mapIpCnt.size() << endl;map<string, uint32_t>::iterator iter1;for(iter1 = mapIpCnt.begin(); iter1 != mapIpCnt.end(); iter1 ++){cout << "Ip=" << iter1->first << ",Cnt=" << iter1->second << endl;}}
private:map< uint32_t, string > mapVirKey2Ip;map< string, vector<uint32_t> > mapIp2VirKey;
};class CMyModHash
{
public: void AddIp(const string &sIp) {vecIpList.push_back(sIp);}void FindIp(uint32_t uKey, string &sIp){sIp = vecIpList[uKey%vecIpList.size()];}void EchoIpState(){cout << "Ip Cnt=" << vecIpList.size() << endl;}
private: vector<string> vecIpList;
};int main()
{CMyConHash oMyHash;// CMyModHash oMyHash;// 模拟初始化4台机器oMyHash.AddIp("202.168.14.241"); oMyHash.AddIp("202.168.14.242"); oMyHash.AddIp("202.168.14.243"); oMyHash.AddIp("202.168.14.244"); oMyHash.EchoIpState();// 保存下各个Key路由的机器 string sIp, arrKeyIp[HASH_MOD];for (uint32_t key = 0; key < HASH_MOD; key ++){oMyHash.FindIp(key, sIp);arrKeyIp[key] = sIp;}// 模拟加入1台机器oMyHash.AddIp("202.168.14.245"); oMyHash.EchoIpState();// 推断多少Key相应数据须要移动机器uint32_t uCnt = 0;for (uint32_t key = 0; key < HASH_MOD; key ++){oMyHash.FindIp(key, sIp);if (arrKeyIp[key] != sIp) {uCnt ++;}}cout << "Key Sum=" << HASH_MOD << " , Need To Move:" << uCnt << endl;return 0;
}