MySQL 5.7 MGR单主确定主节点是哪个，我们可以通过成员ID来判断，然后结合read_only参数来确认。
[root@localhost ~]# mysql -uroot -p -P 3306 -h 127.0.0.1
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 857246
Server version: 5.7.26-29-log Percona 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> status
--------------
mysql Ver 14.14 Distrib 5.7.26-29, for Linux (x86_64) using EditLine wrapper
Connection id: 857246
Current database:
Current user: root@127.0.0.1
SSL: Not in use
Current pager: stdout
Using outfile: ''
Using delimiter: ;
Server version: 5.7.26-29-log Percona Server (GPL)
Protocol version: 10
Connection: 127.0.0.1 via TCP/IP
Server characterset: utf8mb4
Db characterset: utf8mb4
Client characterset: utf8
Conn. characterset: utf8
TCP port: 3306
Uptime: 39 days 8 hours 53 min 39 sec
Threads: 4 Questions: 4989552 Slow queries: 731837 Opens: 930 Flush tables: 1 Open tables: 924 Queries per second avg: 1.466
--------------
mysql> SHOW STATUS LIKE 'group_replication_primary_member';
+----------------------------------+--------------------------------------+
| Variable_name | Value |
+----------------------------------+--------------------------------------+
| group_replication_primary_member | 59d7f183-b8f6-11e9-863b-005056875165 |
+----------------------------------+--------------------------------------+
1 row in set (0.00 sec)
mysql> select * from performance_schema.replication_group_members;
+---------------------------+--------------------------------------+-------------+-------------+--------------+
| CHANNEL_NAME | MEMBER_ID | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE |
+---------------------------+--------------------------------------+-------------+-------------+--------------+
| group_replication_applier | 500dd0aa-b8f6-11e9-85ba-005056875a61 | 196.55.7.37 | 3306 | ONLINE |
| group_replication_applier | 59d7f183-b8f6-11e9-863b-005056875165 | 196.55.7.36 | 3306 | ONLINE |
| group_replication_applier | 69e1c26d-b8f6-11e9-91c1-00505687032a | 196.55.7.38 | 3306 | ONLINE |
+---------------------------+--------------------------------------+-------------+-------------+--------------+
3 rows in set (0.00 sec)
mysql> show variables like '%read_only';
+-----------------------+-------+
| Variable_name | Value |
+-----------------------+-------+
| innodb_read_only | OFF |
| read_only | OFF |
| super_read_only | OFF |
| transaction_read_only | OFF |
| tx_read_only | OFF |
+-----------------------+-------+
5 rows in set (0.01 sec)
mysql>