一、问题表现

半同步复制的Mysql集群，从库设置了super_read_only，但突然莫名其妙多出一个事务，如下图；

从库突然出现了自己的事务、或者比主库多出事务。

导致的问题：

1、这个事务不能闪回，这个从库无法作为容灾备用节点，需要人工介入处理；

2、由于主从表数据不一致，会复制异常，如1062、1032的报错；

二、问题原因

实例启动后第一次使用Memory引擎表，Server层会清空该表并写入binlog。

like this:

官网解释如下：

When a replication source server shuts down and restarts, its MEMORY tables become empty. To replicate this effect to replicas, the first time that the source uses a given MEMORY table after startup, it logs an event that notifies replicas that the table must be emptied by writing a DELETE or (from MySQL 8.0.22) TRUNCATE TABLE statement for that table to the binary log. This generated event is identifiable by a comment in the binary log, and if GTIDs are in use on the server, it has a GTID assigned. The statement is always logged in statement format, even if the binary logging format is set to ROW, and it is written even if read_only or super_read_only mode is set on the server.

三、可行的解决办法

对于半同步复制、异步复制的Mysql高可用集群，业务上应当禁用memory引擎表，或通过参数disabled_storage_engines进行限制。