背景

线上使用polarDB，基于mysql(5.7)，架构为springboot+mybatisplus+durid连接池，部分业务场景涉及大表更新和查询操作，在查询慢sql且超过一定时间时就会报出"Communications link failure"异常，主要体现在界面查询或定时任务处理大批量数据是执行数据库的更新、查询异常。

现象

不区分业务高峰期，只要慢sql超过一定时间（本例为10秒）无论如何更新设置连接池参数和polarDB实例参数配置，异常依旧存在。

异常如下：

### Cause: com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure

The last packet successfully received from the server was 10,011 milliseconds ago. The last packet sent successfully to the server was 10,011 milliseconds ago.
; Communications link failure

The last packet successfully received from the server was 10,011 milliseconds ago. The last packet sent successfully to the server was 10,011 milliseconds ago.; nested exception is com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure

The last packet successfully received