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MySQL在8.0.13后支持在线切换single primary和multi primary两种模式，本文章讲解 MGR（组复制）的模式切换。
使用 group_replication_switch_to_single_primary_mode() UDF通过发出以下命令，将以多主要模式运行的组更改为单主要模式：
SELECT group_replication_switch_to_single_primary_mode();
mysql> SELECT group_replication_switch_to_single_primary_mode();+---------------------------------------------------+| group_replication_switch_to_single_primary_mode() |+---------------------------------------------------+| Mode switched to single-primary successfully. |+---------------------------------------------------+1 row in set (0.01 sec)mysql> SELECT * FROM performance_schema.replication_group_members;+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+| CHANNEL_NAME | MEMBER_ID | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE | MEMBER_ROLE | MEMBER_VERSION |+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+| group_replication_applier | 47dfdfb5-32b5-11ea-a738-02000aba3d28 | 10.186.61.40 | 3306 | ONLINE | PRIMARY | 8.0.15 || group_replication_applier | 897b6353-32b9-11ea-a7e9-02000aba3d2e | 10.186.61.46 | 3306 | ONLINE | SECONDARY | 8.0.15 || group_replication_applier | a1796612-32b6-11ea-a32b-02000aba3d29 | 10.186.61.41 | 3306 | ONLINE | SECONDARY | 8.0.15 |+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+
在操作运行期间，您可以通过发出以下命令来检查其进度：
SELECT event_name, work_completed, work_estimated FROM performance_schema.events_stages_current WHERE event_name LIKE "%stage/group_rpl%";+----------------------------------------------------------------------------+----------------+----------------+| event_name | work_completed | work_estimated |+----------------------------------------------------------------------------+----------------+----------------+| stage/group_rpl/Primary Switch: waiting for pending transactions to finish | 4 | 20 |+----------------------------------------------------------------------------+----------------+----------------+
使用 group_replication_switch_to_multi_primary_mode() UDF，通过发出以下命令，将以单主要模式运行的组更改为多主要模式：
SELECT group_replication_switch_to_multi_primary_mode();
mysql> SELECT group_replication_switch_to_multi_primary_mode();+--------------------------------------------------+| group_replication_switch_to_multi_primary_mode() |+--------------------------------------------------+| Mode switched to multi-primary successfully. |+--------------------------------------------------+1 row in set (0.01 sec)mysql> SELECT * FROM performance_schema.replication_group_members;+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+| CHANNEL_NAME | MEMBER_ID | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE | MEMBER_ROLE | MEMBER_VERSION |+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+| group_replication_applier | 47dfdfb5-32b5-11ea-a738-02000aba3d28 | 10.186.61.40 | 3306 | ONLINE | PRIMARY | 8.0.15 || group_replication_applier | 897b6353-32b9-11ea-a7e9-02000aba3d2e | 10.186.61.46 | 3306 | ONLINE | PRIMARY | 8.0.15 || group_replication_applier | a1796612-32b6-11ea-a32b-02000aba3d29 | 10.186.61.41 | 3306 | ONLINE | PRIMARY | 8.0.15 |+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+经过一些协调的组操作以确保数据的安全性和一致性之后，属于该组的所有成员都将成为主成员。
当您将以单主要模式运行的组更改为以多主要模式运行时，如果运行的MySQL服务器版本高于最低版本的成员，则运行MySQL 8.0.17或更高版本的成员将自动置于只读模式。在组中。运行MySQL 8.0.16或更低版本的成员不执行此检查，并且始终处于读写模式。
在操作运行期间，您可以通过发出以下命令来检查其进度：
SELECT event_name, work_completed, work_estimated FROM performance_schema.events_stages_current WHERE event_name LIKE "%stage/group_rpl%";+----------------------------------------------------------------------+----------------+----------------+| event_name | work_completed | work_estimated |+----------------------------------------------------------------------+----------------+----------------+| stage/group_rpl/Multi-primary Switch: applying buffered transactions | 0 | 1 |+----------------------------------------------------------------------+----------------+----------------+
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