leaf SnowflakeIDGenImpl 源碼解析

leaf SnowflakeIDGenImpl

1. leaf SnowflakeIDGenImpl 對應的是Leaf-snowflake的實現 ，源碼位置
   https://github.com/Meituan-Dianping/Leaf/blob/0dc819c9fd8505bf0ec8e23b40ab3465c451c28b/leaf-core/src/main/java/com/sankuai/inf/leaf/snowflake/SnowflakeIDGenImpl.java
2. Leaf-snowflake設計詳情可以看美團同學的文章
   https://tech.meituan.com/2017/04/21/mt-leaf.html

snowflake 優點

• 全局唯一性
• 有序性
• 高效性

snowflake 缺點

• 時鐘回撥
• 業務團隊不好設置workerid

美團leaf 針對於兩個snowflake 缺點做了優化
從SnowflakeIDGenImpl的get方法 可以看出 id生成邏輯比較簡單
1 . 判斷是否發生時鐘回撥 ，如果發生 ，如果offset 超過5ms ，返回new Result(-3, Status.EXCEPTION); ，如果 5ms 之內 則 wait 左移一位時間。重新判斷是否發生時鐘回撥 ，是 ，拋出異常 。否 進入正常邏輯 。

2. sequence 是全局變量的，記錄上次序列id，以及本次序列id ，他的生成邏輯很簡單，具體看下面的源碼 ， 他和key查詢參數毫無關聯 ， 可以對方法級別synchronize 做優化 ，但是snowflake協議 需要加上 key 對應的業務id 。可以作爲優化的點，提升併發度 ，但是會破壞協議 ，新的業務可以用，不兼容原本snowflake
3. workerID 是從Zookeeper拿取workerID，每個leaf instance 會拿到自己的workerid ， 而且動態擴展，無需開發人員配置的 。並且會在本機文件系統上緩存一個workerID文件 。
4. id長度最少23bit =
   long id = ((timestamp - twepoch) << timestampLeftShift) | (workerId << workerIdShift) | sequence;

package com.sankuai.inf.leaf.snowflake;

import com.google.common.base.Preconditions;
import com.sankuai.inf.leaf.IDGen;
import com.sankuai.inf.leaf.common.Result;
import com.sankuai.inf.leaf.common.Status;
import com.sankuai.inf.leaf.common.Utils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.Random;

public class SnowflakeIDGenImpl implements IDGen {

    @Override
    public boolean init() {
        return true;
    }

    private static final Logger LOGGER = LoggerFactory.getLogger(SnowflakeIDGenImpl.class);

    private final long twepoch;
    private final long workerIdBits = 10L;
    private final long maxWorkerId = ~(-1L << workerIdBits);//最大能夠分配的workerid =1023
    private final long sequenceBits = 12L;
    private final long workerIdShift = sequenceBits;
    private final long timestampLeftShift = sequenceBits + workerIdBits;
    private final long sequenceMask = ~(-1L << sequenceBits);
    private long workerId;
    private long sequence = 0L;
    private long lastTimestamp = -1L;
    private static final Random RANDOM = new Random();

    public SnowflakeIDGenImpl(String zkAddress, int port) {
        //Thu Nov 04 2010 09:42:54 GMT+0800 (中國標準時間) 
        this(zkAddress, port, 1288834974657L);
    }

    /**
     * @param zkAddress zk地址
     * @param port      snowflake監聽端口
     * @param twepoch   起始的時間戳
     */
    public SnowflakeIDGenImpl(String zkAddress, int port, long twepoch) {
        this.twepoch = twepoch;
        Preconditions.checkArgument(timeGen() > twepoch, "Snowflake not support twepoch gt currentTime");
        final String ip = Utils.getIp();
        SnowflakeZookeeperHolder holder = new SnowflakeZookeeperHolder(ip, String.valueOf(port), zkAddress);
        LOGGER.info("twepoch:{} ,ip:{} ,zkAddress:{} port:{}", twepoch, ip, zkAddress, port);
        boolean initFlag = holder.init();
        if (initFlag) {
            workerId = holder.getWorkerID();
            LOGGER.info("START SUCCESS USE ZK WORKERID-{}", workerId);
        } else {
            Preconditions.checkArgument(initFlag, "Snowflake Id Gen is not init ok");
        }
        Preconditions.checkArgument(workerId >= 0 && workerId <= maxWorkerId, "workerID must gte 0 and lte 1023");
    }

    @Override
    public synchronized Result get(String key) {
        long timestamp = timeGen();
        if (timestamp < lastTimestamp) {
            long offset = lastTimestamp - timestamp;
            if (offset <= 5) {
                try {
                    wait(offset << 1);
                    timestamp = timeGen();
                    if (timestamp < lastTimestamp) {
                        return new Result(-1, Status.EXCEPTION);
                    }
                } catch (InterruptedException e) {
                    LOGGER.error("wait interrupted");
                    return new Result(-2, Status.EXCEPTION);
                }
            } else {
                return new Result(-3, Status.EXCEPTION);
            }
        }
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                //seq 爲0的時候表示是下一毫秒時間開始對seq做隨機
                sequence = RANDOM.nextInt(100);
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            //如果是新的ms開始
            sequence = RANDOM.nextInt(100);
        }
        lastTimestamp = timestamp;
        long id = ((timestamp - twepoch) << timestampLeftShift) | (workerId << workerIdShift) | sequence;
        return new Result(id, Status.SUCCESS);

    }

    protected long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    protected long timeGen() {
        return System.currentTimeMillis();
    }

    public long getWorkerId() {
        return workerId;
    }

}