分布式Id算法-雪花算法实现
分布式Id算法-雪花算法Java实现
参考：
https://gitbook.cn/gitchat/activity/5f43c3e4f9fe2a086081397e
一般情况下需要在应用中配置机器id和数据中心id，由应用程序读取；
或者各个机器启动应用时，在启动参数中传入机器id和数据中心id；
也有使用redis(分布式锁)获取机器id和datacenterid的实践；
本文提供一种简化的基于内存的生成 机器ID(workId)和数据中心ID(datacenterID)的方法。
public class IdWorker {
//下面两个每个5位，加起来就是10位的工作机器id
private long workerId; //机器id
private long datacenterId; //数据中心id
//12位的序列号
private long sequence;
//初始时间戳(取一个离当前时间最近的,从此时间戳开始够用69年)
private long twepoch = 1597766399000L;
//长度为5位
private long workerIdBits = 5L;
private long datacenterIdBits = 5L;
//最大值
private long maxWorkerId = -1L ^ (-1L << workerIdBits);
private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
//序列号id长度
private long sequenceBits = 12L;
//序列号最大值
private long sequenceMask = -1L ^ (-1L << sequenceBits);
//工作id需要左移的位数，12位
private long workerIdShift = sequenceBits;
//数据id需要左移位数 12+5=17位
private long datacenterIdShift = sequenceBits + workerIdBits;
//时间戳需要左移位数 12+5+5=22位
private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
//上次时间戳，初始值为负数
private long lastTimestamp = -1L;
public IdWorker() {
this.datacenterId = getDatacenterId(maxDatacenterId);
this.workerId = getMaxWorkerId(datacenterId, maxWorkerId);
this.sequence = 0L;
System.out.printf("worker starting. timestamp left shift %d, datacenter id bits %d, worker id bits %d, sequence bits %d, datacenterid %d, workerid %d\r\n",
timestampLeftShift, datacenterIdBits, workerIdBits, sequenceBits, datacenterId, workerId);
}
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\r\n", maxWorkerId));
}
if (datacenterId > maxDatacenterId || datacenterId < 0) {
throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0\r\n", maxDatacenterId));
}
System.out.printf("worker starting. timestamp left shift %d, datacenter id bits %d, worker id bits %d, sequence bits %d, datacenterid %d, workerid %d\r\n",
timestampLeftShift, datacenterIdBits, workerIdBits, sequenceBits, datacenterId, workerId);
this.workerId = workerId;
this.datacenterId = datacenterId;
this.sequence = sequence;
}
public long getWorkerId() {
return workerId;
}
public long getDatacenterId() {
return datacenterId;
}
public long getTimestamp() {
return System.currentTimeMillis();
}
//下一个ID生成算法
public synchronized long nextId() {
long timestamp = timeGen();
//获取当前时间戳如果小于上次时间戳，则表示时间戳获取出现异常
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));
}
//获取当前时间戳如果等于上次时间戳（同一毫秒内），则在序列号加一；否则序列号赋值为0，从0开始。
if (lastTimestamp == timestamp) {
sequence = (sequence + 1) & sequenceMask;
if (sequence == 0) {
timestamp = tillNextMillis(lastTimestamp);
}
} else {
sequence = 0;
}
//将上次时间戳值刷新
lastTimestamp = timestamp;
/**
* 返回结果：
* (timestamp - twepoch) << timestampLeftShift) 表示将时间戳减去初始时间戳，再左移相应位数
* (datacenterId << datacenterIdShift) 表示将数据id左移相应位数
* (workerId << workerIdShift) 表示将工作id左移相应位数
* | 是按位或运算符，例如：x | y，只有当x，y都为0的时候结果才为0，其它情况结果都为1。
* 因为个部分只有相应位上的值有意义，其它位上都是0，所以将各部分的值进行 | 运算就能得到最终拼接好的id
*/
return ((timestamp - twepoch) << timestampLeftShift) |
(datacenterId << datacenterIdShift) |
(workerId << workerIdShift) |
sequence;
}
//获取时间戳，并与上次时间戳比较
private long tillNextMillis(long lastTimestamp) {
long timestamp = timeGen();
while (timestamp <= lastTimestamp) {
timestamp = timeGen();
}
return timestamp;
}
//获取系统时间戳
private long timeGen() {
return System.currentTimeMillis();
}
protected static long getMaxWorkerId(long datacenterId, long maxWorkerId) {
StringBuffer mpid = new StringBuffer();
mpid.append(datacenterId);
String name = ManagementFactory.getRuntimeMXBean().getName();
if (!name.isEmpty()) {
/*
* GET jvmPid
*/
mpid.append(name.split("@")[0]);
}
/*
* MAC + PID 的 hashcode 获取16个低位
*/
long workId = (mpid.toString().hashCode() & 0xffff) % (maxWorkerId + 1);
System.out.println("WorkerId:" + workId);
return workId;
}
protected static long getDatacenterId(long maxDatacenterId) {
long datacenterId = 0L;
try {
InetAddress ip = InetAddress.getLocalHost();
NetworkInterface network = NetworkInterface.getByInetAddress(ip);
if (network == null) {
datacenterId = 1L;
} else {
byte[] mac = network.getHardwareAddress();
datacenterId = ((0x000000FF & (long) mac[mac.length - 1])
| (0x0000FF00 & (((long) mac[mac.length - 2]) << 8))) >> 6;
datacenterId = datacenterId % (maxDatacenterId + 1);
}
} catch (Exception e) {
System.out.println(" getDatacenterId: " + e.getMessage());
}
System.out.println("DatacenterId:" + datacenterId);
return datacenterId;
}
//---------------测试---------------
public static void main(String[] args) {
//开100个线程
for (int i = 0; i < 100; i++) {
IdWorker worker = new IdWorker();
new Thread(() -> {
for (int j = 0; j < 10; j++) {
System.out.println(Thread.currentThread().getName() + ":" + worker.nextId());
}
}).start();
}
}
}
如何解决时钟回拨问题?
- 在运行中的时钟回拨
每个worker中记录上一次生成ID用的时间戳lastTimestamp，每次生成成功后更新该值
生成新guid时，只有当前时间戳大于lastTimestamp时方法才能继续
可以采取一些补偿措施，比如当前时间戳小于lastTimestamp时，若偏差在5ms以内，则等待2倍的时间差后开始生成
若两者偏差大于5ms，则立即抛出异常，避免阻塞 - 服务重启时的时钟回拨
通过异步定时的方式，将worker中lastTimestamp更新到redis中
在类加载时，校验本地时间与redis中记录的worker时间戳，只有本地时间大于redis时间，才能继续加载
否则服务启动失败，需要等待一会再重启
参考文献：
https://www.jianshu.com/p/2a27fbd9e71a
https://blog.csdn.net/wmq880204/article/details/107025761
https://msd.misuland.com/pd/4146263777181958818 Redis 选择datacenterId和workerId