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: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 2087
Server version: 8.0.17 MySQL Community Server - GPL
Copyright (c) 2009-2019 Percona LLC and/or its affiliates
Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
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.
