分布式唯一id(数据库主键):snowflake算法

简介

项目中大家喜欢用java自带的UUID工具类去生成唯一的数据库主键，这样做的原因是简单。但mysql的innodb存储引擎的主键是聚簇索引（一种索引类型，数据与索引数据放在一起），既然数据和索引数据放在一起，那么在数据插入或者更新的时候，那么在数据插入或者更新的时候，我们需要找到要插入的位置，再把数据写到特定的位置上，这就产生了随机的 IO，而innodb存储引擎采用的是B+树的结果存储索引，一旦发生了页分裂，就不可避免会做数据的移动，也会极大地损耗写入性能,但是如果使用mysql的自增，则在分库分表下无法保证主键id的唯一性，因此我们采用twitter的snowflake算法来生成分布式的唯一id。

snowflake算法

snowflake是Twitter开源的分布式ID生成算法，结果是一个long型的ID。其核心思想是：使用41bit作为毫秒数，10bit作为机器的ID（5个bit是数据中心，5个bit的机器ID），12bit作为毫秒内的流水号（意味着每个节点在每毫秒可以产生 4096 个 ID），最后还有一个符号位，永远是0。41为的时间戳大概可以生成69年((2^41)/1000/60/60/24/365),10bit的工作机器id可以支持1024(2^10)台机器，序列号支持1毫秒产生4096(2^12)个自增序列id

算法java实现

Twitter在github上提供了scala的实现版本,以下版本是根据scala转换成java的版本

package com.liu;
public class IdWorker{

    private long workerId;
    private long datacenterId;
    private long sequence;

    //开始时间,比如我想从2000-1-1开始生成日期就填入2000-1-1的时间
    private long twepoch = 1288834974657L;
    /**
     *
     * @param workerId 机器id(占用5比特，最大为31)
     * @param datacenterId 机房id（占用5比特）
     * @param sequence (12位序列号起始编号)
     * long twepoch 参数开始时间,比如我想从2000-1-1开始生成日期就填入2000-1-1的时间
     */
    public IdWorker(long workerId, long datacenterId, long sequence){
        // sanity check for workerId
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0",maxWorkerId));
        }
        if (datacenterId > maxDatacenterId || datacenterId < 0) {
            throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0",maxDatacenterId));
        }
        System.out.printf("worker starting. timestamp left shift %d, datacenter id bits %d, worker id bits %d, sequence bits %d, workerid %d",
                timestampLeftShift, datacenterIdBits, workerIdBits, sequenceBits, workerId);

        this.workerId = workerId;
        this.datacenterId = datacenterId;
        this.sequence = sequence;
    }


    private long workerIdBits = 5L;
    private long datacenterIdBits = 5L;
    private long maxWorkerId = -1L ^ (-1L << workerIdBits);
    private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
    private long sequenceBits = 12L;

    private long workerIdShift = sequenceBits;
    private long datacenterIdShift = sequenceBits + workerIdBits;
    private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
    private long sequenceMask = -1L ^ (-1L << sequenceBits);

    private long lastTimestamp = -1L;

    public long getWorkerId(){
        return workerId;
    }

    public long getDatacenterId(){
        return datacenterId;
    }

    public long getTimestamp(){
        return System.currentTimeMillis();
    }

    public synchronized long nextId() {
        long timestamp = timeGen();

        if (timestamp < lastTimestamp) {
            System.err.printf("clock is moving backwards.  Rejecting requests until %d.", lastTimestamp);
            throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds",
                    lastTimestamp - timestamp));
        }

        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0;
        }

        lastTimestamp = timestamp;
        return ((timestamp - twepoch) << timestampLeftShift) |
                (datacenterId << datacenterIdShift) |
                (workerId << workerIdShift) |
                sequence;
    }

    private long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    private long timeGen(){
        return System.currentTimeMillis();
    }

    //---------------测试---------------
    public static void main(String[] args) {
        IdWorker worker = new IdWorker(3,1,1);
        for (int i = 0; i < 30; i++) {
            System.out.println(worker.nextId());
        }
    }

}