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。(转换成字符串后长度最多19)
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。经测试snowflake每秒能够产生26万个ID
C#
public class IdWorker
{
//机器ID
private static long workerId;
private static long twepoch = 687888001020L; //唯一时间，这是一个避免重复的随机量，自行设定不要大于当前时间戳
private static long sequence = 0L;
private static int workerIdBits = 4; //机器码字节数。4个字节用来保存机器码(定义为Long类型会出现，最大偏移64位，所以左移64位没有意义)
public static long maxWorkerId = -1L ^ -1L << workerIdBits; //最大机器ID
private static int sequenceBits = 10; //计数器字节数，10个字节用来保存计数码
private static int workerIdShift = sequenceBits; //机器码数据左移位数，就是后面计数器占用的位数
private static int timestampLeftShift = sequenceBits + workerIdBits; //时间戳左移动位数就是机器码和计数器总字节数
public static long sequenceMask = -1L ^ -1L << sequenceBits; //一微秒内可以产生计数，如果达到该值则等到下一微妙在进行生成
private long lastTimestamp = -1L;
/// <summary>
/// 机器码
/// </summary>
/// <param name="workerId"></param>
public IdWorker(long workerId)
{
if (workerId > maxWorkerId || workerId < 0)
throw new Exception(string.Format("worker Id can't be greater than {0} or less than 0 ", workerId));
IdWorker.workerId = workerId;
}
public long nextId()
{
lock (this)
{
long timestamp = timeGen();
if (this.lastTimestamp == timestamp)
{ //同一微妙中生成ID
IdWorker.sequence = (IdWorker.sequence + 1) & IdWorker.sequenceMask; //用&运算计算该微秒内产生的计数是否已经到达上限
if (IdWorker.sequence == 0)
{
//一微妙内产生的ID计数已达上限，等待下一微妙
timestamp = tillNextMillis(this.lastTimestamp);
}
}
else
{ //不同微秒生成ID
IdWorker.sequence = 0; //计数清0
}
if (timestamp < lastTimestamp)
{ //如果当前时间戳比上一次生成ID时时间戳还小，抛出异常，因为不能保证现在生成的ID之前没有生成过
throw new Exception(string.Format("Clock moved backwards. Refusing to generate id for {0} milliseconds",
this.lastTimestamp - timestamp));
}
this.lastTimestamp = timestamp; //把当前时间戳保存为最后生成ID的时间戳
long nextId = (timestamp - twepoch << timestampLeftShift) | IdWorker.workerId << IdWorker.workerIdShift | IdWorker.sequence;
return nextId;
}
}
/// <summary>
/// 获取下一微秒时间戳
/// </summary>
/// <param name="lastTimestamp"></param>
/// <returns></returns>
private long tillNextMillis(long lastTimestamp)
{
long timestamp = timeGen();
while (timestamp <= lastTimestamp)
{
timestamp = timeGen();
}
return timestamp;
}
/// <summary>
/// 生成当前时间戳
/// </summary>
/// <returns></returns>
private long timeGen()
{
return (long)(DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc)).TotalMilliseconds;
}
}C#调用
IdWorker idworker = new IdWorker(1);
for (int i = 0; i < 1000; i++)
{
Response.Write(idworker.nextId() + "<br/>");
}java
public class IdWorker {
private final long workerId;
private final static long twepoch = 1288834974657L;
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();
}
//调用
public static void main(String[] args){
IdWorker worker2 = new IdWorker(2);
System.out.println(worker2.nextId());
}
}