MGR变量group_replication_primary

组复制的状态变量只有一个，目前在8.0.17这个版本上还是存在的，未来会被废弃掉，我们在单主的模式下，可以用来查看哪个节点是读写节点。

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mysql> show status like 'group_replication_primary_member';
+----------------------------------+--------------------------------------+
| Variable_name                    | Value                                |
+----------------------------------+--------------------------------------+
| group_replication_primary_member | 1fb894d8-e752-11e9-a0a1-00505687032a |
+----------------------------------+--------------------------------------+
1 row in set (0.00 sec)

group_replication_primary_member

Shows the primary member's UUID when the group is operating in single-primary mode. If the group is operating in multi-primary mode, shows an empty string.

The group_replication_primary_member status variable has been deprecated and is scheduled to be removed in a future version.