怎么防止超卖

1. mysql的排它锁（配合失误）

 UPDATE seller_product_goods SET product_stock = product_stock - #{number} WHERE id = #{id} AND product_stock > (#{number} - 1)

2. 版本号

  select version from goods WHERE id= 1001
  update goods set num = num - 1, version = version + 1 WHERE id= 1001 AND num > 0 AND version = @version(上面查到的version);

3. redis的预减库存

  每一个用户线程进来，key值就减1，等减到0的时候，全部拒绝剩下的请求