介绍一下分库分表

// 计算分片索引
public ShardingResult sharding(Long userId, int dbCount, int tableCount) {
    // 先分库
    long dbIndex = userId % dbCount;
    // 再分表
    long tableIndex = (userId / dbCount) % tableCount;
    
    return ShardingResult.builder()
            .dbName("order_db_" + dbIndex)
            .tableName("order_" + tableIndex)
            .build();
}

public class SnowflakeIdGenerator {
    // 起始时间戳 (2020-01-01)
    private static final long START_TIMESTAMP = 1577808000000L;
    
    // 各部分位数
    private static final long WORKER_ID_BITS = 5L;
    private static final long DATA_CENTER_ID_BITS = 5L;
    private static final long SEQUENCE_BITS = 12L;
    
    // 最大值
    private static final long MAX_WORKER_ID = ~(-1L << WORKER_ID_BITS);
    private static final long MAX_DATA_CENTER_ID = ~(-1L << DATA_CENTER_ID_BITS);
    
    // 偏移量
    private static final long WORKER_ID_SHIFT = SEQUENCE_BITS;
    private static final long DATA_CENTER_ID_SHIFT = SEQUENCE_BITS + WORKER_ID_BITS;
    private static final long TIMESTAMP_SHIFT = SEQUENCE_BITS + WORKER_ID_BITS + DATA_CENTER_ID_BITS;
    
    private final long workerId;
    private final long dataCenterId;
    private long sequence = 0L;
    private long lastTimestamp = -1L;

    public SnowflakeIdGenerator(long workerId, long dataCenterId) {
        if (workerId > MAX_WORKER_ID || workerId < 0) {
            throw new IllegalArgumentException("Worker ID超出范围");
        }
        if (dataCenterId > MAX_DATA_CENTER_ID || dataCenterId < 0) {
            throw new IllegalArgumentException("DataCenter ID超出范围");
        }
        this.workerId = workerId;
        this.dataCenterId = dataCenterId;
    }

    public synchronized long nextId() {
        long currentTimestamp = System.currentTimeMillis();
        
        // 技术亮点：时钟回拨处理
        if (currentTimestamp < lastTimestamp) {
            long offset = lastTimestamp - currentTimestamp;
            // 回拨小于5ms，等待
            if (offset <= 5) {
                try {
                    wait(offset << 1);
                    currentTimestamp = System.currentTimeMillis();
                    if (currentTimestamp < lastTimestamp) {
                        throw new RuntimeException("时钟回拨严重，无法生成ID");
                    }
                } catch (InterruptedException e) {
                    throw new RuntimeException("线程中断", e);
                }
            } else {
                throw new RuntimeException("时钟回拨超过5ms，拒绝生成ID");
            }
        }
        
        // 同一毫秒内，序列号自增
        if (currentTimestamp == lastTimestamp) {
            sequence = (sequence + 1) & ~(-1L << SEQUENCE_BITS);
            // 序列号溢出，等待下一毫秒
            if (sequence == 0) {
                currentTimestamp = waitNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }
        
        lastTimestamp = currentTimestamp;
        
        // 拼接ID
        return ((currentTimestamp - START_TIMESTAMP) << TIMESTAMP_SHIFT)
                | (dataCenterId << DATA_CENTER_ID_SHIFT)
                | (workerId << WORKER_ID_SHIFT)
                | sequence;
    }

    private long waitNextMillis(long lastTimestamp) {
        long timestamp = System.currentTimeMillis();
        while (timestamp <= lastTimestamp) {
            timestamp = System.currentTimeMillis();
        }
        return timestamp;
    }
}

@Service
public class DataMigrationService {
    // 迁移状态枚举
    public enum MigrationStatus {
        INIT,       // 初始状态
        DOUBLE_WRITE, // 双写阶段
        DATA_CHECK,   // 数据校验阶段
        SWITCH_READ,  // 切换读流量
        FINISH        // 迁移完成
    }

    @Autowired
    private OldOrderMapper oldOrderMapper;
    @Autowired
    private NewOrderMapper newOrderMapper;
    @Autowired
    private RedisTemplate<String, String> redisTemplate;

    // 技术亮点：根据状态自动路由写入
    public void createOrder(Order order) {
        MigrationStatus status = getMigrationStatus();
        
        // 先写旧库
        oldOrderMapper.insert(order);
        
        // 双写阶段：再写新库
        if (status == MigrationStatus.DOUBLE_WRITE || 
            status == MigrationStatus.DATA_CHECK ||
            status == MigrationStatus.SWITCH_READ) {
            try {
                newOrderMapper.insert(order);
            } catch (Exception e) {
                // 记录失败日志，后续补偿
                log.error("双写新库失败，orderId:{}", order.getId(), e);
                compensateWrite(order);
            }
        }
    }

    // 技术亮点：根据状态自动路由查询
    public Order getOrderById(Long orderId) {
        MigrationStatus status = getMigrationStatus();
        
        if (status == MigrationStatus.SWITCH_READ || status == MigrationStatus.FINISH) {
            return newOrderMapper.selectById(orderId);
        } else {
            return oldOrderMapper.selectById(orderId);
        }
    }

    // 数据校验：对比新旧库数据一致性
    public boolean checkDataConsistency(Long startId, Long endId) {
        List<Order> oldOrders = oldOrderMapper.selectByIdRange(startId, endId);
        List<Order> newOrders = newOrderMapper.selectByIdRange(startId, endId);
        
        // 技术亮点：用MD5对比数据内容，避免逐字段比较
        Map<Long, String> oldOrderMap = oldOrders.stream()
                .collect(Collectors.toMap(Order::getId, this::calculateOrderMd5));
        Map<Long, String> newOrderMap = newOrders.stream()
                .collect(Collectors.toMap(Order::getId, this::calculateOrderMd5));
        
        return oldOrderMap.equals(newOrderMap);
    }

    private String calculateOrderMd5(Order order) {
        // 将订单所有字段拼接后计算MD5
        String content = JSON.toJSONString(order);
        return DigestUtils.md5Hex(content);
    }

    private MigrationStatus getMigrationStatus() {
        String status = redisTemplate.opsForValue().get("migration:status");
        return status == null ? MigrationStatus.INIT : MigrationStatus.valueOf(status);
    }
}

@Service
public class OrderService {
    @Autowired
    private OrderMapper orderMapper;

    // 技术亮点：游标分页，性能稳定，不受数据量影响
    public PageResult<Order> queryOrdersByCursor(Long lastId, int pageSize, String status) {
        // 查询条件：id > lastId（游标）
        List<Order> orders = orderMapper.selectByCursor(lastId, pageSize, status);
        
        // 计算是否有下一页
        boolean hasNext = orders.size() == pageSize;
        // 获取下一页的游标
        Long nextCursor = hasNext ? orders.get(orders.size() - 1).getId() : null;
        
        return PageResult.<Order>builder()
                .data(orders)
                .hasNext(hasNext)
                .nextCursor(nextCursor)
                .build();
    }
}

// 对应的SQL
@Select("SELECT * FROM order_${tableIndex} " +
        "WHERE id > #{lastId} AND status = #{status} " +
        "ORDER BY id ASC LIMIT #{pageSize}")
List<Order> selectByCursor(@Param("lastId") Long lastId, 
                          @Param("pageSize") int pageSize, 
                          @Param("status") String status);

public class SnowflakeIdWorker {
    // 起始时间戳 (2020-01-01)
    private final long twepoch = 1577808000000L;
    // 机器ID所占位数
    private final long workerIdBits = 5L;
    private final long datacenterIdBits = 5L;
    // 支持的最大机器ID (31)
    private final long maxWorkerId = ~(-1L << workerIdBits);
    private final long maxDatacenterId = ~(-1L << datacenterIdBits);
    // 序列在id中占的位数
    private final long sequenceBits = 12L;
    // 机器ID左移12位
    private final long workerIdShift = sequenceBits;
    // 数据中心ID左移17位
    private final long datacenterIdShift = sequenceBits + workerIdBits;
    // 时间戳左移22位
    private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
    // 序列掩码 (4095)
    private final long sequenceMask = ~(-1L << sequenceBits);

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

    public SnowflakeIdWorker(long workerId, long datacenterId) {
        if (workerId > maxWorkerId || workerId < 0) throw new IllegalArgumentException(...);
        if (datacenterId > maxDatacenterId || datacenterId < 0) throw new IllegalArgumentException(...);
        this.workerId = workerId;
        this.datacenterId = datacenterId;
    }

    public synchronized long nextId() {
        long timestamp = timeGen();
        // 🔥 时钟回拨处理：如果当前时间小于上次生成ID的时间戳，说明时钟回拨了
        if (timestamp < lastTimestamp) {
            long offset = lastTimestamp - timestamp;
            if (offset <= 5) {
                // 回拨5ms以内，等待两倍时间
                try { Thread.sleep(offset << 1); } catch (InterruptedException e) {}
                timestamp = timeGen();
                if (timestamp < lastTimestamp) {
                    throw new RuntimeException("Clock moved backwards, refusing to generate id");
                }
            } else {
                throw new RuntimeException("Clock moved backwards too much");
            }
        }
        // 同一毫秒内，序列递增
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                // 当前毫秒序列已满，等待下一毫秒
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }
        lastTimestamp = timestamp;
        return ((timestamp - twepoch) << timestampLeftShift)
                | (datacenterId << datacenterIdShift)
                | (workerId << workerIdShift)
                | sequence;
    }

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

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

@Aspect
@Component
public class ShardingKeyCheckAspect {
    @Around("@annotation(com.example.annotation.ForceShardingKey)")
    public Object checkShardingKey(ProceedingJoinPoint joinPoint) throws Throwable {
        MethodSignature signature = (MethodSignature) joinPoint.getSignature();
        // 获取参数中的分片键
        Object[] args = joinPoint.getArgs();
        Long shardingKey = null;
        for (Object arg : args) {
            if (arg instanceof ShardingKeyHolder) {
                shardingKey = ((ShardingKeyHolder) arg).getShardingKey();
            }
        }
        // 如果没带分片键，直接抛异常，避免全库扫描
        if (shardingKey == null) {
            throw new BizException("查询必须携带分片键");
        }
        // 使用 HintManager 强制路由到指定分片
        try (HintManager hintManager = HintManager.getInstance()) {
            hintManager.addDatabaseShardingValue("order", shardingKey);
            hintManager.addTableShardingValue("order", shardingKey);
            return joinPoint.proceed();
        }
    }
}