MGR单主之自增序列参数设置

一、官网说明
When Group Replication is started on a server, the value of auto_increment_increment is changed to the value of group_replication_auto_increment_increment, which defaults to 7, and the value of auto_increment_offset is changed to the server ID. The changes are reverted when Group Replication is stopped. These settings avoid the selection of duplicate auto-increment values for writes on group members, which causes rollback of transactions. The default auto increment value of 7 for Group Replication represents a balance between the number of usable values and the permitted maximum size of a replication group (9 members).

The changes are only made and reverted if auto_increment_increment and auto_increment_offset each have their default value of 1. If their values have already been modified from the default, Group Replication does not alter them. From MySQL 8.0, the system variables are also not modified when Group Replication is in single-primary mode, where only one server writes.

敲黑板，划重点，在MySQL8.0的版本中，组复制在单主模式下（只有一个写服务器），系统变量（自增序列参数）是不会自动修改的。这是新版本中增加的一段话，我也是在测试中发现了了这个问题，在官网中得到了证实。知识就是知识或者别人告诉你，或者你自己找答案，找答案的过程是很有意思的。

二、验证过程
1、系统默认参数

2、自增序列表，自增是按照默认参数自增的，并没有修改

三、建议
1、建议大家还是用MGR中的单主模式
2、要对你要修改的每一个参数了如指掌，不然就不要改动。
3、纸上得来终觉浅，多做测试和实验也要多分享。