算法：一致哈希及C++实现

在负载均衡中，我们需要在服务器列表中选择一个服务器，那么，我们该怎么选呢？

 

我们当然可以轮询。但是轮询却不能够实现回话保持。那么我们该怎么办呢？下面介绍一种算法，一致哈希。

 

一致哈希理解起来不困难，我们把服务器节点放在一个0~2^32上的圆环里，并把客户端的地址hash，选择大于hash值的最小节点。这样来实现负载均衡。同时也保证了回话保持。如图

如果，两个服务器的hash相近，那么后边的节点就很难接收到请求。为此，我们引入了虚拟节点。虚拟节点很好理解，就是一个服务器在圆环上部署多个节点，这样，只要节点分布的均匀，请求的分发可以很好的照顾到每一个服务器 
如图

最后直接上代码,代码能够直接在vs2012运行

#include<iostream>using namespace std;#include<stdlib h="">#include<map>#include<vector>#include<time h="">/*hash方法，使用murmurhash*/class CHashFun{public:long getHashVal(int addr){return MurMurHash((const void*)&addr,sizeof(addr));}private:unsigned int MurMurHash(const void *key, int len) { const unsigned int m = 0x5bd1e995; const int r = 24; const int seed = 97; unsigned int h = seed ^ len; // Mix 4 bytes at a time into the hash const unsigned char *data = (const unsigned char *)key; while(len >= 4) { unsigned int k = *(unsigned int *)data; k *= m; k ^= k >> r; k *= m; h *= m; h ^= k; data += 4; len -= 4; } // Handle the last few bytes of the input array switch(len) { case 3: h ^= data[2] << 16; case 2: h ^= data[1] << 8; case 1: h ^= data[0]; h *= m; }; // Do a few final mixes of the hash to ensure the last few // bytes are well-incorporated. h ^= h >> 13; h *= m; h ^= h >> 15; return h; }};/*实体结点*/class CNode_s{public:CNode_s():sernum(0),vNodeCount(0){};CNode_s(int pSernum , int pVNodeCount):sernum(pSernum),vNodeCount(pVNodeCount){};/*获取结点标示*/int getsernum(){return this->sernum;}int getVNodeCount(){return this->vNodeCount;}void addhashvector(int rhash){vhash.push_back(rhash);}int getandpop(){int val = vhash.back();vhash.pop_back();return val;}int empvhash(){return vhash.empty();}private:int sernum;/*结点标示串*/int vNodeCount; /*虚拟结点数目*/vector<int> vhash;};/*虚拟结点*/class CVirtualNode_s{public:CVirtualNode_s(int hash = -1);CVirtualNode_s(CNode_s * pNode):node(pNode){};/*获取虚拟结点所指向的实体结点*/CNode_s * getNode_s(){return node;}/*设置虚拟结点hash值*/void setHash(long pHash){this->hash = pHash;}/*获取虚拟结点hash值*/long getHash(){return this->hash;}private:long hash; CNode_s * node; /*虚拟结点所指向的实体结点*/};class CConHash{public:CConHash(){};/*增加结点 , 0代表成功 , -1代表失败*/int addNode_s(CNode_s * pNode){int res = 0;if(pNode==NULL) return res;int hash = 0;char str[1000]={"0"};char num[10] = {"0"};CVirtualNode_s *vnode;int vpnode = pNode->getVNodeCount();for(int i=0;i<vpnode;i++);map<int,CVirtualnode;>::iterator it = nodemap.find(hash);while(it!=nodemap.end()){ ++it;if(it == nodemap.end()) it = nodemap.begin();if(it->first - hash == 1){//中间没有间隔hash = it->first + 2; }else{//选择两者的中间位置hash += ( it->first>hash? it->first-hash:0xffffffff-hash+it->first )/2;}it = nodemap.find(hash);}vnode->setHash(hash);nodemap.insert(pair<int cvirtualnode_s="">(hash,*vnode));pNode->addhashvector(hash);this->vNodes++;}res = 1;cout<<"安装完成"<<endl return="" res="" 1="" 0="" int="" delnode_s="" cnode_s="" pnode="" int="" res="0;" if="" pnode="=" null="" return="" res="" int="" hash="0;" int="" vncount="pNode-">getVNodeCount();for(int i=0;i<vncount i="" hash="pNode-">getandpop();map<int cvirtualnode_s="">::iterator it = nodemap.find(hash);if(it!=nodemap.end()){nodemap.erase(it);}}res = 1;return res;}/*查找实体结点*/CNode_s * matchNode_s(int object){if(object==0||this->vNodes==0) return NULL;int arr[12] = {0};int hashkey = this->func.getHashVal(object);map<int cvirtualnode_s="">::iterator it = nodemap.lower_bound(hashkey);if(it!=nodemap.end()){return it->second.getNode_s();}return NULL;}int getVNodes(){return vNodes;}private:/*Hash函数*/CHashFun func;/*虚拟结点总个数*/int vNodes;/*放虚拟节点*/map<int cvirtualnode_s=""> nodemap;};void main(){srand((unsigned)time(NULL));CConHash *conhash = new CConHash;CNode_s * node1 = new CNode_s(1,2);CNode_s * node2 = new CNode_s(1,50);CNode_s * node3 = new CNode_s(3,50);CNode_s * node4 = new CNode_s(4,50);conhash->addNode_s(node1);conhash->addNode_s(node2);conhash->addNode_s(node3);conhash->addNode_s(node4);int ans1,ans2,ans3,ans4;ans1=ans2=ans3=ans4 = 0;int randkey;CNode_s *ser;for(int i=0;i<60 i="" randkey="rand();" ser="conhash-">matchNode_s(randkey);if(ser!=NULL){cout<<"Cli:"<<randkey<<"------>"<<ser->getsernum()<<endl if="" ser-="">getsernum() == 1) ans1++;if(ser->getsernum() == 2) ans2++;if(ser->getsernum() == 3) ans3++;if(ser->getsernum() == 4) ans4++;}}cout<<endl;cout<<"Cli1:"<<ans1<<endl;cout<<"Cli2:"<<ans2<<endl;cout<<"Cli3:"<<ans3<<endl;cout<<"Cli4:"<<ans4<<endl;cout<<endl;}</endl></ser-></60></int></int></int></vncount></endl></int></int></vpnode></int></time></vector></map></stdlib></iostream>