nodejs雪花ID算法（SnowflakeID）

前言

项目中常使用的三种id类型，分别是自增id、uuid、雪花id，这三种各有优劣。本篇主要实现nodejs中snowflake算法的代码。

一、Snowflake实现

这里需要加入big-integer的模块，下载npm install --save big-integer

var Snowflake = (function() {
            function Snowflake(_workerId, _dataCenterId, _sequence) {
                this.twepoch = 1702032780;//初始时间，设置为0，只能到2100年，尽可能设置当前时间，可以延长时间
                this.workerIdBits = 5; //工作机器码位数
                this.dataCenterIdBits = 5;//数据中心码位数
                this.maxWrokerId = -1 ^ (-1 << this.workerIdBits); // 31位
                this.maxDataCenterId = -1 ^ (-1 << this.dataCenterIdBits); //31位
                this.sequenceBits = 12;//序号位数
                this.workerIdShift = this.sequenceBits; // 12位
                this.dataCenterIdShift = this.sequenceBits + this.workerIdBits; //17位
                this.timestampLeftShift = this.sequenceBits + this.workerIdBits + this.dataCenterIdBits; // 22位
                this.sequenceMask = -1 ^ (-1 << this.sequenceBits); //4095
                this.lastTimestamp = -1;
                //设置默认值
                this.workerId = 1;
                this.dataCenterId = 1;
                this.sequence = 0;
　　　　　　　　　　//
                if (this.workerId > this.maxWrokerId || this.workerId < 0) {
                    throw new Error('config.worker_id must max than 0 and small than maxWrokerId-[' + this.maxWrokerId + ']');
                }
                if (this.dataCenterId > this.maxDataCenterId || this.dataCenterId < 0) {
                    throw new Error('config.data_center_id must max than 0 and small than maxDataCenterId-[' + this.maxDataCenterId + ']');
                }
                this.workerId = _workerId;
                this.dataCenterId = _dataCenterId;
                this.sequence = _sequence;
            }
　　　　　　　　//获取下一个时间戳
            Snowflake.prototype.tilNextMillis = function(lastTimestamp) {
                var timestamp = this.timeGen();
                while (timestamp <= lastTimestamp) {
                    timestamp = this.timeGen();
                }
                return timestamp;
            };
　　　　　　 //获取时间戳
            Snowflake.prototype.timeGen = function() {
                //new Date().getTime() === Date.now()
                return Math.floor(Date.now()/1000); //这里获取秒级别主要是生成53位内的id数
            };
　　　　　　//生成id
            Snowflake.prototype.nextId = function() {
                var timestamp = this.timeGen();
                if (timestamp < this.lastTimestamp) {
                    throw new Error('Clock moved backwards. Refusing to generate id for ' +
                        (this.lastTimestamp - timestamp));
                }
                if (this.lastTimestamp === timestamp) {
                    this.sequence = (this.sequence + 1) & this.sequenceMask;
                    if (this.sequence === 0) {
                        timestamp = this.tilNextMillis(this.lastTimestamp);
                    }
                } else {
                    this.sequence = 0;
                }
                this.lastTimestamp = timestamp;
                var shiftNum = (this.dataCenterId << this.dataCenterIdShift) |
                    (this.workerId << this.workerIdShift) |
                    this.sequence; // dataCenterId:1,workerId:1,sequence:0  shiftNum:135168
                var nfirst = new bigInt(String(timestamp - this.twepoch), 10);
                nfirst = nfirst.shiftLeft(this.timestampLeftShift);
                var nnextId = nfirst.or(new bigInt(String(shiftNum), 10)).toString(10);
                return nnextId;
            };
            return Snowflake;
        }());

二、一些问题

　　若正常的snowflakeID需要64位支持，但是在nodejs中这样需要字符串或者bigInt进行存储。这里可以尝试使用其他方式将snowflakeID降低到53位

    1.将时间戳改为秒级别，2^10刚好是1024，这样产生id不是很多。
    2.将10位工作编号舍去，刚好53位（这个适合单机模式）　　
    3.12位序列号+10位机器号适当减少都可以，满足一定条件

参考

http://hk.javashuo.com/article/p-moygzecn-bd.html    #Snowflake(雪花算法)的JavaScript实现