Twitter 全局序列ID生成算法

 

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。

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。

public class IdSeqGenerator {
    private final static long twepoch            = 1463108596098L;             //日期起始点

    private final long        workerId;
    private long              sequence           = 0L;
    private long              lastTimestamp      = -1L;

    private final static long workerIdBits       = 10L;                        //机器ID占用10bits
    private final static long sequenceBits       = 12L;                        //序列占用12bits

    public final static long  maxWorkerId        = -1L ^ -1L << workerIdBits;  //机器ID 最大值

    private final static long timestampLeftShift = sequenceBits + workerIdBits;//时间偏移位
    private final static long workerIdShift      = sequenceBits;               //机器ID偏移位

    public final static long  sequenceMask       = -1L ^ -1L << sequenceBits;  //序列掩码

    public IdSeqGenerator(final long workerId) {
        super();
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
        }
        LogKit.log().info(String.format("worker Id : greater than %d or less than 0", maxWorkerId));
        this.workerId = workerId;
    }
    //生成ID
    public synchronized long nextId() {
        long timestamp = this.timeGen();
        //如果是同一时间生成的，则自增
        if (this.lastTimestamp == timestamp) {
            this.sequence = (this.sequence + 1) & sequenceMask;
            if (this.sequence == 0) {
                //生成下一个毫秒级的序列
                timestamp = this.tilNextMillis(this.lastTimestamp);
            }
        } else {
            //如果发现是下一个时间单位，则自增序列回0，重新自增 
            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 << workerIdShift) | (this.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) {
        Map<Long, Long> store = new ConcurrentHashMap<>();

        final int MAX_POOL_SIZE = 4;
        ExecutorService service = Executors.newFixedThreadPool(MAX_POOL_SIZE);
        for (int i = 0; i < MAX_POOL_SIZE; i++) {
            final int x = i;
            service.execute(() -> {
                IdSeqGenerator seqKit = new IdSeqGenerator(x);
                for (int j = 0; j < 100; j++) {
                    long id = seqKit.nextId();
                    if (store.containsKey(id)) {
                        System.out.println("dublicate:" + id);
                    } else {
                        store.put(id, id);
                        System.out.println(id);
                    }
                }

                System.out.println("thread" + x + " end.");
            });
        }

        service.shutdown();
    }
}