秒杀系统超卖记录

Service流程：
1、检查商品余量大于零
2、检查是否存在重复下单；
3、执行减库存。

加了事务不加锁：

    @Transactional
    public Excution executeSeckill(long seckillId, String md5, long userPhone) throws SeckillClosedException, SeckillRepeatException, SeckillException {
        //...
        Seckill seckill = seckillDao.queryById(seckillId);
        if (seckill != null && seckill.getNumber() > 0) {
            int count = seckilledDao.insertSeckilled(seckillId, userPhone);
            if (count > 0) {
                seckillDao.reduceNumber(seckillId, nowTime);
                Seckilled seckilled = seckilledDao.querySeckilled(seckillId, userPhone);
                return new Excution(seckillId, SeckillStatEnum.SUCCESS, seckilled);
            } else {
                throw new SeckillRepeatException("Seckill Repeat");
            }
        } else {
            throw new SeckillClosedException("Seckill Closed");
        }

        //...
    }

超卖

原因：

事务外加锁（结合spring AOP）

主要逻辑部分

    @Override
    @ServiceLock
    @Transactional
    public Excution executeSeckillLock(long seckillId, String md5, long userPhone) throws SeckillClosedException, SeckillRepeatException, SeckillException
    {
        try {
            //...
            Seckill seckill = seckillDao.queryById(seckillId);
            if (seckill != null)System.out.println(seckill.getNumber());
            if (seckill != null && seckill.getNumber() > 0) {

                int count = seckilledDao.insertSeckilled(seckillId, userPhone);
                if (count > 0) {
                    seckillDao.reduceNumber(seckillId, nowTime);
                    Seckilled seckilled = seckilledDao.querySeckilled(seckillId, userPhone);
                    return new Excution(seckillId, SeckillStatEnum.SUCCESS, seckilled);
                } else {
                    throw new SeckillRepeatException("Seckill Repeat");
                }
            } else {
                throw new SeckillClosedException("Seckill Closed");
            }
        }
        catch(Exception e)
        {
            throw new SeckillException();
        }
    }

利用切面加锁

@Component
@Scope
@Aspect
@Order
public class ServiceLockAspect {
    private static Lock lock = new ReentrantLock(true);
    @Pointcut("@annotation(com.***.seckill.aop.annotation.ServiceLock)")
    public void pointCut(){}
    @Around("pointCut()")
    public Object addLock(ProceedingJoinPoint joinPoint)
    {
        lock.lock();
        Object obj = null;
        try {
            obj = joinPoint.proceed();
        }
        catch(Throwable e){
            throw new SeckillException("inner error");
        }
        finally {
            lock.unlock();
        }
        return obj;
    }
}

锁注解

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface ServiceLock {}

不会产生超卖：

事务内加锁

    @Override
    @Transactional
    public Excution executeSeckillLock(long seckillId, String md5, long userPhone) throws SeckillClosedException, SeckillRepeatException, SeckillException
    {
        try {
            lock.lock();
            //...
            Seckill seckill = seckillDao.queryById(seckillId);
            if (seckill != null)System.out.println(seckill.getNumber());
            if (seckill != null && seckill.getNumber() > 0) {

                int count = seckilledDao.insertSeckilled(seckillId, userPhone);
                if (count > 0) {
                    seckillDao.reduceNumber(seckillId, nowTime);
                    Seckilled seckilled = seckilledDao.querySeckilled(seckillId, userPhone);
                    return new Excution(seckillId, SeckillStatEnum.SUCCESS, seckilled);
                } else {
                    throw new SeckillRepeatException("Seckill Repeat");
                }
            } else {
                throw new SeckillClosedException("Seckill Closed");
            }
        }
        catch(Exception e)
        {
            throw new SeckillException();
        }
        finally{
            lock.unlock();
        }
    }

超卖
原因：

另外一些做法是在DAO层改进

1. 判断余量和减库存合并

   UPDATE seckill  
   SET number=number-1 
   WHERE seckill_id=#{seckillId} AND number>0

2. 将判断库存的快照读改成当前读

   SELECT number 
   FROM seckill 
   WHERE seckill_id=#{seckillId} 
   FOR UPDATE

使用队列将发送请求和执行减库存的操作异步化

1. 前端发送请求依次进入阻塞队列，后端将请求入队后就返回,由于只进行请求入队操作，所以速度很快;
2. 服务开启的同时开一个后台消费线程用于消费队列里的请求（减库存），由于消费操作是单线程，不用加锁，不会有超卖问题。