Twitter的分布式自增ID算法snowflake (Java版)

来源：互联网发布：淘宝商城入驻费用编辑：程序博客网时间：2024/04/30 03:08

概述

分布式系统中，有一些需要使用全局唯一ID的场景，这种时候为了防止ID冲突可以使用36位的UUID，但是UUID有一些缺点，首先他相对比较长，另外UUID一般是无序的。

有些时候我们希望能使用一种简单一些的ID，并且希望ID能够按照时间有序生成。

而twitter的snowflake解决了这种需求，最初Twitter把存储系统从MySQL迁移到Cassandra，因为Cassandra没有顺序ID生成机制，所以开发了这样一套全局唯一ID生成服务。

结构

snowflake的结构如下(每部分用-分开):

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

第一位为未使用，接下来的41位为毫秒级时间(41位的长度可以使用69年)，然后是5位datacenterId和5位workerId(10位的长度最多支持部署1024个节点），最后12位是毫秒内的计数（12位的计数顺序号支持每个节点每毫秒产生4096个ID序号）

一共加起来刚好64位，为一个Long型。(转换成字符串长度为18)

snowflake生成的ID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workerId作区分），并且效率较高。据说：snowflake每秒能够产生26万个ID。

源码

(JAVA版本的源码)

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/** Snowflake */
public class IdWorker {
 
  private final long twepoch = 1288834974657L;
  private final long workerIdBits = 5L;
  private final long datacenterIdBits = 5L;
  private final long maxWorkerId = -1L ^ (-1L << workerIdBits);
  private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
  private final long sequenceBits = 12L;
  private final long workerIdShift = sequenceBits;
  private final long datacenterIdShift = sequenceBits + workerIdBits;
  private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
  private final long sequenceMask = -1L ^ (-1L << sequenceBits);
 
  private long workerId;
  private long datacenterId;
  private long sequence = 0L;
  private long lastTimestamp = -1L;
 
  public IdWorker(long workerId, long datacenterId) {
    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));
    }
    this.workerId = workerId;
    this.datacenterId = datacenterId;
  }
 
  public synchronized long nextId() {
    long timestamp = timeGen();
    if (timestamp < 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 = 0L;
    }
 
    lastTimestamp = timestamp;
 
    return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift) | (workerId << workerIdShift) | sequence;
  }
 
  protected long tilNextMillis(long lastTimestamp) {
    long timestamp = timeGen();
    while (timestamp <= lastTimestamp) {
      timestamp = timeGen();
    }
    return timestamp;
  }
 
  protected long timeGen() {
    return System.currentTimeMillis();
  }
 
  public static void main(String[] args) {
    IdWorker idWorker = new IdWorker(0, 0);
    for (int i = 0; i < 1000; i++) {
      long id = idWorker.nextId();
      System.out.println(id);
    }
  }
}

(JAVA版本2的源码)

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/** Snowflake */
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());

}

}

参考

https://github.com/twitter/snowflake

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