我们行MGR年底要上线了，每天都要看官方文档学习，做测试，坚持每天写一个小知识点，有想一起学习的么~

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 127.0.0.1mysql: [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 857246Server version: 5.7.26-29-log Percona Server (GPL)Copyright (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> status--------------mysqlVer 14.14 Distrib 5.7.26-29, for Linux (x86_64) usingEditLine wrapperConnection id:857246Current database: Current user: root@127.0.0.1SSL:Not in useCurrent pager:stdoutUsing outfile:''Using delimiter:;Server version: 5.7.26-29-log Percona Server (GPL)Protocol version: 10Connection: 127.0.0.1 via TCP/IPServer characterset:utf8mb4Db characterset:utf8mb4Client characterset:utf8Conn.characterset:utf8TCP port: 3306Uptime: 39 days 8 hours 53 min 39 secThreads: 4Questions: 4989552Slow queries: 731837Opens: 930Flush tables: 1Open tables: 924Queries 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>