Twitter的分布式自增ID算法snowflake

Twitter-Snowflake算法产生的背景相当简单，为了满足Twitter每秒上万条消息的请求，每条消息都必须分配一条唯一的id，这些id还需要一些大致的顺序（方便客户端排序），并且在分布式系统中不同机器产生的id必须不同。

Snowflake算法核心

把时间戳，工作机器id，序列号组合在一起。

除了最高位bit标记为不可用以外，其余三组bit占位均可浮动，看具体的业务需求而定。默认情况下41bit的时间戳可以支持该算法使用到2082年，10bit的工作机器id可以支持1023台机器，序列号支持1毫秒产生4095个自增序列id。下文会具体分析。

Snowflake – 时间戳

这里时间戳的细度是毫秒级，具体代码如下，建议使用64位linux系统机器，因为有vdso，gettimeofday()在用户态就可以完成操作，减少了进入内核态的损耗。

uint64_t generateStamp()
{
    timeval tv;
    gettimeofday(&tv, 0);
    return (uint64_t)tv.tv_sec * 1000 + (uint64_t)tv.tv_usec / 1000;
}

默认情况下有41个bit可以供使用，那么一共有T（1llu << 41）毫秒供你使用分配，年份 = T / (3600 * 24 * 365 * 1000) = 69.7年。如果你只给时间戳分配39个bit使用，那么根据同样的算法最后年份 = 17.4年。

Snowflake – 工作机器id

严格意义上来说这个bit段的使用可以是进程级，机器级的话你可以使用MAC地址来唯一标示工作机器，工作进程级可以使用IP+Path来区分工作进程。如果工作机器比较少，可以使用配置文件来设置这个id是一个不错的选择，如果机器过多配置文件的维护是一个灾难性的事情。

这里的解决方案是需要一个工作id分配的进程，可以使用自己编写一个简单进程来记录分配id，或者利用Mysql auto_increment机制也可以达到效果。

工作进程与工作id分配器只是在工作进程启动的时候交互一次，然后工作进程可以自行将分配的id数据落文件，下一次启动直接读取文件里的id使用。

PS：这个工作机器id的bit段也可以进一步拆分，比如用前5个bit标记进程id，后5个bit标记线程id之类:D

Snowflake – 序列号

序列号就是一系列的自增id（多线程建议使用atomic），为了处理在同一毫秒内需要给多条消息分配id，若同一毫秒把序列号用完了，则“等待至下一毫秒”。

uint64_t waitNextMs(uint64_t lastStamp)
{
    uint64_t cur = 0;
    do {
        cur = generateStamp();
    } while (cur <= lastStamp);
    return cur;
}

总体来说，是一个很高效很方便的GUID产生算法，一个int64_t字段就可以胜任，不像现在主流128bit的GUID算法，即使无法保证严格的id序列性，但是对于特定的业务，比如用做游戏服务器端的GUID产生会很方便。另外，在多线程的环境下，序列号使用atomic可以在代码实现上有效减少锁的密度。

在分布式系统中，需要生成全局UID的场合还是比较多的，twitter的snowflake解决了这种需求，实现也还是很简单的，除去配置信息，核心代码就是毫秒级时间41位+机器ID 10位+毫秒内序列12位。

核心代码为其IdWorker这个类实现，其原理结构如下，我分别用一个0表示一位，用—分割开部分的作用：

0---0000000000 0000000000 0000000000 0000000000 0 --- 00000 ---00000 ---000000000000

在上面的字符串中，第一位为未使用（实际上也可作为long的符号位），接下来的41位为毫秒级时间，然后5位datacenter标识位，5位机器ID（并不算标识符，实际是为线程标识），然后12位该毫秒内的当前毫秒内的计数，加起来刚好64位，为一个Long型。

这样的好处是，整体上按照时间自增排序，并且整个分布式系统内不会产生ID碰撞（由datacenter和机器ID作区分），并且效率较高，经测试，snowflake每秒能够产生26万ID左右，完全满足需要。

且看其核心代码：

/** Copyright 2010-2012 Twitter, Inc.*/
package com.twitter.service.snowflake

import com.twitter.ostrich.stats.Stats
import com.twitter.service.snowflake.gen._
import java.util.Random
import com.twitter.logging.Logger

/**
 * An object that generates IDs.
 * This is broken into a separate class in case
 * we ever want to support multiple worker threads
 * per process
 */
class IdWorker(val workerId: Long, val datacenterId: Long, private val reporter: Reporter, var sequence: Long = 0L)
extends Snowflake.Iface {
  private[this] def genCounter(agent: String) = {
    Stats.incr("ids_generated")
    Stats.incr("ids_generated_%s".format(agent))
  }
  private[this] val exceptionCounter = Stats.getCounter("exceptions")
  private[this] val log = Logger.get
  private[this] val rand = new Random

  val twepoch = 1288834974657L

 //机器标识位数

  private[this] val workerIdBits = 5L

//数据中心标识位数
  private[this] val datacenterIdBits = 5L

//机器ID最大值
  private[this] val maxWorkerId = -1L ^ (-1L << workerIdBits)

//数据中心ID最大值
  private[this] val maxDatacenterId = -1L ^ (-1L << datacenterIdBits)

//毫秒内自增位
  private[this] val sequenceBits = 12L

//机器ID偏左移12位

  private[this] val workerIdShift = sequenceBits

//数据中心ID左移17位
  private[this] val datacenterIdShift = sequenceBits + workerIdBits

//时间毫秒左移22位
  private[this] val timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits
  private[this] val sequenceMask = -1L ^ (-1L << sequenceBits)

  private[this] var lastTimestamp = -1L

  // sanity check for workerId
  if (workerId > maxWorkerId || workerId < 0) {
    exceptionCounter.incr(1)
    throw new IllegalArgumentException("worker Id can't be greater than %d or less than 0".format(maxWorkerId))
  }

  if (datacenterId > maxDatacenterId || datacenterId < 0) {
    exceptionCounter.incr(1)
    throw new IllegalArgumentException("datacenter Id can't be greater than %d or less than 0".format(maxDatacenterId))
  }

  log.info("worker starting. timestamp left shift %d, datacenter id bits %d, worker id bits %d, sequence bits %d, workerid %d",
    timestampLeftShift, datacenterIdBits, workerIdBits, sequenceBits, workerId)

  def get_id(useragent: String): Long = {
    if (!validUseragent(useragent)) {
      exceptionCounter.incr(1)
      throw new InvalidUserAgentError
    }

    val id = nextId()
    genCounter(useragent)

    reporter.report(new AuditLogEntry(id, useragent, rand.nextLong))
    id
  }

  def get_worker_id(): Long = workerId
  def get_datacenter_id(): Long = datacenterId
  def get_timestamp() = System.currentTimeMillis

  protected[snowflake] def nextId(): Long = synchronized {
    var timestamp = timeGen()

 //时间错误

    if (timestamp < lastTimestamp) {
      exceptionCounter.incr(1)
      log.error("clock is moving backwards.  Rejecting requests until %d.", lastTimestamp);
      throw new InvalidSystemClock("Clock moved backwards.  Refusing to generate id for %d milliseconds".format(
        lastTimestamp - timestamp))
    }

    if (lastTimestamp == timestamp) {
//当前毫秒内，则+1
      sequence = (sequence + 1) & sequenceMask
      if (sequence == 0) {
//当前毫秒内计数满了，则等待下一秒
        timestamp = tilNextMillis(lastTimestamp)
      }
    } else {
      sequence = 0
    }

    lastTimestamp = timestamp
//ID偏移组合生成最终的ID，并返回ID   

((timestamp - twepoch) << timestampLeftShift) |
      (datacenterId << datacenterIdShift) |
      (workerId << workerIdShift) |
      sequence
  }

//等待下一个毫秒的到来 

protected def tilNextMillis(lastTimestamp: Long): Long = {
    var timestamp = timeGen()
    while (timestamp <= lastTimestamp) {
      timestamp = timeGen()
    }
    timestamp
  }

  protected def timeGen(): Long = System.currentTimeMillis()

  val AgentParser = """([a-zA-Z][a-zA-Z\-0-9]*)""".r

  def validUseragent(useragent: String): Boolean = useragent match {
    case AgentParser(_) => true
    case _ => false
  }
}

上述为twitter的实现，下面且看一个Java实现，貌似为淘宝的朋友写的。

public class IdWorker {
 private final long workerId;
 private final static long twepoch = 1361753741828L;
 private long sequence = 0L;
 private final static long workerIdBits = 4L;
 public final static long maxWorkerId = -1L ^ -1L << workerIdBits;
 private final static long sequenceBits = 10L;
 private final static long workerIdShift = sequenceBits;
 private final static long timestampLeftShift = sequenceBits + workerIdBits;
 public final static long sequenceMask = -1L ^ -1L << sequenceBits;
 private long lastTimestamp = -1L;
 public IdWorker(final long workerId) {
  super();
  if (workerId > this.maxWorkerId || workerId < 0) {
   throw new IllegalArgumentException(String.format(
     "worker Id can't be greater than %d or less than 0",
     this.maxWorkerId));
  }
  this.workerId = workerId;
 }
 public synchronized long nextId() {
  long timestamp = this.timeGen();
  if (this.lastTimestamp == timestamp) {
   this.sequence = (this.sequence + 1) & this.sequenceMask;
   if (this.sequence == 0) {
    System.out.println("###########" + sequenceMask);
    timestamp = this.tilNextMillis(this.lastTimestamp);
   }
  } else {
   this.sequence = 0;
  }
  if (timestamp < this.lastTimestamp) {
   try {
    throw new Exception(
      String.format(
        "Clock moved backwards.  Refusing to generate id for %d milliseconds",
        this.lastTimestamp - timestamp));
   } catch (Exception e) {
    e.printStackTrace();
   }
  }
  this.lastTimestamp = timestamp;
  long nextId = ((timestamp - twepoch << timestampLeftShift))
    | (this.workerId << this.workerIdShift) | (this.sequence);
//  System.out.println("timestamp:" + timestamp + ",timestampLeftShift:"
//    + timestampLeftShift + ",nextId:" + nextId + ",workerId:"
//    + workerId + ",sequence:" + sequence);
  return nextId;
 }
 private long tilNextMillis(final long lastTimestamp) {
  long timestamp = this.timeGen();
  while (timestamp <= lastTimestamp) {
   timestamp = this.timeGen();
  }
  return timestamp;
 }
 private long timeGen() {
  return System.currentTimeMillis();
 }
 
 
 public static void main(String[] args){
  IdWorker worker2 = new IdWorker(2);
  System.out.println(worker2.nextId());
  
 }
}

再来看一个php的实现

<?php
class Idwork
{
const debug = 1;
static $workerId;
static $twepoch = 1361775855078;
static $sequence = 0;
const workerIdBits = 4;
static $maxWorkerId = 15;
const sequenceBits = 10;
static $workerIdShift = 10;
static $timestampLeftShift = 14;
static $sequenceMask = 1023;
private  static $lastTimestamp = -1;
function __construct($workId){
if($workId > self::$maxWorkerId || $workId< 0 )
{
throw new Exception("worker Id can't be greater than 15 or less than 0");
}
self::$workerId=$workId;
echo 'logdebug->__construct()->self::$workerId:'.self::$workerId;
echo '</br>';
}
function timeGen(){
//获得当前时间戳
$time = explode(' ', microtime());
$time2= substr($time[0], 2, 3);
$timestramp = $time[1].$time2;
echo 'logdebug->timeGen()->$timestramp:'.$time[1].$time2;
echo '</br>';
return  $time[1].$time2;
}
function  tilNextMillis($lastTimestamp) {
$timestamp = $this->timeGen();
while ($timestamp <= $lastTimestamp) {
$timestamp = $this->timeGen();
}
echo 'logdebug->tilNextMillis()->$timestamp:'.$timestamp;
echo '</br>';
return $timestamp;
}
function  nextId()
{
$timestamp=$this->timeGen();
echo 'logdebug->nextId()->self::$lastTimestamp1:'.self::$lastTimestamp;
echo '</br>';
if(self::$lastTimestamp == $timestamp) {
self::$sequence = (self::$sequence + 1) & self::$sequenceMask;
if (self::$sequence == 0) {
    echo "###########".self::$sequenceMask;
    $timestamp = $this->tilNextMillis(self::$lastTimestamp);
    echo 'logdebug->nextId()->self::$lastTimestamp2:'.self::$lastTimestamp;
    echo '</br>';
  }
} else {
self::$sequence  = 0;
    echo 'logdebug->nextId()->self::$sequence:'.self::$sequence;
    echo '</br>';
}
if ($timestamp < self::$lastTimestamp) {
   throw new Excwption("Clock moved backwards.  Refusing to generate id for ".(self::$lastTimestamp-$timestamp)." milliseconds");
   }
self::$lastTimestamp  = $timestamp;
echo 'logdebug->nextId()->self::$lastTimestamp3:'.self::$lastTimestamp;
echo '</br>';
echo 'logdebug->nextId()->(($timestamp - self::$twepoch << self::$timestampLeftShift )):'.((sprintf('%.0f', $timestamp) - sprintf('%.0f', self::$twepoch) ));
echo '</br>';
$nextId = ((sprintf('%.0f', $timestamp) - sprintf('%.0f', self::$twepoch)  )) | ( self::$workerId << self::$workerIdShift ) | self::$sequence;
echo 'timestamp:'.$timestamp.'-----';
echo 'twepoch:'.sprintf('%.0f', self::$twepoch).'-----';
echo 'timestampLeftShift ='.self::$timestampLeftShift.'-----';
echo 'nextId:'.$nextId.'----';
echo 'workId:'.self::$workerId.'-----';
echo 'workerIdShift:'.self::$workerIdShift.'-----';
return $nextId;
}
}
$Idwork = new Idwork(1);
$a= $Idwork->nextId();
$Idwork = new Idwork(2);
$a= $Idwork->nextId();
?>