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。

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 2087Server version: 8.0.17 MySQL Community Server - GPLCopyright (c) 2009-2019 Percona LLC and/or its affiliatesCopyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.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_memberShows 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.