一致性hash实现

一致性哈希算法在1997年由麻省理工学院提出的一种分布式哈希（DHT）实现算法，设计目标是为了解决因特网中的热点(Hot spot)问题，初衷和CARP十分类似。一致性哈希修正了CARP使用的简 单哈希算法带来的问题，使得分布式哈希（DHT）可以在P2P环境中真正得到应用。

具体代码实现：

#include <iostream>
#include <map>
#include <vector>
#include <cstdio>
#include <cstdint>
using namespace std;
namespace CRC
{
    static uint32_t crcTable[256] = {
        0x0, 0x77073096, 0xee0e612c, 0x990951ba, 0x76dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
        0xedb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988, 0x9b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
        0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
        0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
        0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
        0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
        0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
        0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924, 0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
        0x76dc4190, 0x1db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x6b6b51f, 0x9fbfe4a5, 0xe8b8d433,
        0x7807c9a2, 0xf00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x86d3d2d, 0x91646c97, 0xe6635c01,
        0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
        0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
        0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
        0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
        0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
        0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
        0xedb88320, 0x9abfb3b6, 0x3b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x4db2615, 0x73dc1683,
        0xe3630b12, 0x94643b84, 0xd6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0xa00ae27, 0x7d079eb1,
        0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
        0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
        0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
        0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
        0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236, 0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
        0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
        0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x26d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x5005713,
        0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0xcb61b38, 0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0xbdbdf21,
        0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
        0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
        0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
        0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
        0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
        0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d,
    };

    static uint32_t CRC32(char* c, int len)
    {
        uint32_t crc;
        char *e = c + len;

        crc = 0xFFFFFFFF;
        while (c < e)
        {
            crc = ((crc >> 8) & 0x00FFFFFF) ^ crcTable[(crc^ *c) & 0xFF];
            ++c;
        }
        return (crc ^ 0xFFFFFFFF);
    }

    static uint32_t CRC32(const std::string& str)
    {
        return CRC32((char*)str.data(), (int)str.length());
    }

}

//保存server类型信息
class Server{
    public:
        Server() {}
        Server(string ip, int port, string name):m_ip(ip),
            m_port(port),m_name(name){}
        virtual ~Server(){}
        std::string getName() const { return m_name;} 
    private:
        std::string m_ip;
        int m_port;
        std::string m_name;
};

//进行hash运算
class Hasher{
    public:
        int hashValue(const std::string& name)
        {
            return CRC::CRC32(name);
        }
};

class VirtualNode{
    public:
        VirtualNode(){} 
        VirtualNode(Server& server, int num): m_server(server), m_num(num){}
        virtual ~VirtualNode(){}
        std::string getName() const
        {
            string name = m_server.getName();
            return name+"-"+to_string(m_num);
        }
        Server getRealServer(){ return m_server;}
    private:
        int m_num; //号码
        Server m_server;
};


class VirtualNodes{
    typedef std::map<int, VirtualNode> mServerType;
    public:
        void Insert(Server& server)
        {
            for(int i=0;i<m_counts;++i)  //将真实节点虚拟成多个虚拟节点，然后hash保存
            {
                VirtualNode node(server, i);
                Insert(node);
            }
        }
        void Insert(const VirtualNode& node)
        {
            int key = m_hash.hashValue(node.getName());
            auto iter = m_servers.find(key);
            if(iter == m_servers.end())
            {
                m_servers.insert(make_pair(key, node));    
            }
        }
        //获取提供服务的服务器
        const void getServer(const std::string& name, VirtualNode& node)
        {
            int key = m_hash.hashValue(name);
            auto iter = m_servers.lower_bound(key);
            if(iter == m_servers.end())
            {
                iter = m_servers.begin();
            }
            node = iter->second;
        }

    private:
        int m_counts{500};  //默认每个真实节点可以虚拟500个
        mServerType m_servers; //保存server信息
        Hasher m_hash;
};



int main(int argc, char const *argv[])
{
    // int ret = CRC::CRC32("jjjjj");
    // cout << ret << endl;

    Server server1("127.0.0.1", 111, "server1");    
    Server server2("127.0.0.2", 222, "server2");    
    Server server3("127.0.0.3", 333, "server3");    
    Server server4("127.0.0.4", 444, "server4");    
    Server server5("127.0.0.5", 555, "server5");    
    
    VirtualNodes nodes;
    nodes.Insert(server1);
    nodes.Insert(server2);
    nodes.Insert(server3);
    nodes.Insert(server4);
    nodes.Insert(server5);

    string req = "pic1";
    VirtualNode virNode;
    nodes.getServer(req, virNode);
    cout << virNode.getName() << endl;

    string req1 = "pic100";
    VirtualNode virNode1;
    nodes.getServer(req1, virNode1);
    cout << virNode1.getName() << endl;

    string req2 = "pic1000";
    VirtualNode virNode2;
    nodes.getServer(req2, virNode2);
    cout << virNode2.getName() << endl;
    return 0;
}

一致性hash原理具体参考：https://zhuanlan.zhihu.com/p/34985026