我掌握的新兴技术：分布式 ID 生成系统 Leaf 的设计思路，源码解读

private void updateCacheFromDb() {
        logger.info("update cache from db");
        StopWatch sw = new Slf4JStopWatch();
        try {
            // 执行 SELECT biz_tag FROM leaf_alloc 语句，获取所有的 业务字段。
            List<String> dbTags = dao.getAllTags();
            if (dbTags == null || dbTags.isEmpty()) {
                return;
            }
            // 缓存中的 biz_tag
            List<String> cacheTags = new ArrayList<String>(cache.keySet());
            // 要插入的 db 中的 biz_tag
            Set<String> insertTagsSet = new HashSet<>(dbTags);
            // 要移除的缓存中的 biz_tag 
            Set<String> removeTagsSet = new HashSet<>(cacheTags);

            // 缓存中有的话，不用再插入，从 insertTagsSet 中移除
            for (int i = 0; i < cacheTags.size(); i++) {
                String tmp = cacheTags.get(i);
                if (insertTagsSet.contains(tmp)) {
                    insertTagsSet.remove(tmp);
                }
            }
            
            // 为新增的 biz_tag 创建缓存 SegmentBuffer
            for (String tag : insertTagsSet) {
                SegmentBuffer buffer = new SegmentBuffer();
                buffer.setKey(tag);
                Segment segment = buffer.getCurrent();
                segment.setValue(new AtomicLong(0));
                segment.setMax(0);
                segment.setStep(0);
                cache.put(tag, buffer);
                logger.info("Add tag {} from db to IdCache, SegmentBuffer {}", tag, buffer);
            }

            
            // db中存在的，从要移除的 removeTagsSet 移除。
            for (int i = 0; i < dbTags.size(); i++) {
                String tmp = dbTags.get(i);
                if (removeTagsSet.contains(tmp)) {
                    removeTagsSet.remove(tmp);
                }
            }
            
            // 从 cache 中移除不存在的 bit_tag。
            for (String tag : removeTagsSet) {
                cache.remove(tag);
                logger.info("Remove tag {} from IdCache", tag);
            }
        } catch (Exception e) {
            logger.warn("update cache from db exception", e);
        } finally {
            sw.stop("updateCacheFromDb");
        }
    }

Add tag leaf-segment-test from db to IdCache, SegmentBuffer SegmentBuffer{key='leaf-segment-test', segments=[Segment(value:0,max:0,step:0), Segment(value:0,max:0,step:0)], currentPos=0, nextReady=false, initOk=false, threadRunning=false, step=0, minStep=0, updateTimestamp=0}

@Override
    public synchronized Result get(String key) {
        long timestamp = timeGen();
        //  发生了回拨，此刻时间小于上次发号时间
        if (timestamp < lastTimestamp) {
            long offset = lastTimestamp - timestamp;
            if (offset <= 5) {
                try {
                    //时间偏差大小小于5ms，则等待两倍时间
                    wait(offset << 1);
                    timestamp = timeGen();
                    //还是小于，抛异常并上报
                    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) {
            // sequenceMask = ~(-1L << 12 ) = 4095 二进制即 12 个1
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                //seq 为0的时候表示是下一毫秒时间开始对seq做随机
                sequence = RANDOM.nextInt(100);
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            //如果是新的ms开始
            sequence = RANDOM.nextInt(100);
        }
        lastTimestamp = timestamp;
        // timestampLeftShift = 22， workerIdShift = 12 
        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();
    }