Use the group_replication_switch_to_single_primary_mode() function to change a group running in multi-primary mode to single-primary mode by issuing:

SELECT group_replication_switch_to_single_primary_mode()

When you change to single-primary mode, strict consistency checks are also disabled on all group members, as required in single-primary mode (group_replication_enforce_update_everywhere_checks=OFF).

If no string is passed in, the election of the new primary in the resulting single-primary group follows the election policies described in Section 20.1.3.1, “Single-Primary Mode”. To override the election process and configure a specific member of the multi-primary group as the new primary in the process, get the server_uuid of the member and pass it to group_replication_switch_to_single_primary_mode(). For example, issue:

SELECT group_replication_switch_to_single_primary_mode(member_uuid);

Use the group_replication_switch_to_multi_primary_mode() function to change a group running in single-primary mode to multi-primary mode by issuing:

SELECT group_replication_switch_to_multi_primary_mode()

After some coordinated group operations to ensure the safety and consistency of your data, all members which belong to the group become primaries.

When you change a group that was running in single-primary mode to run in multi-primary mode, members running MySQL 8.0.17 or higher are automatically placed in read-only mode if they are running a higher MySQL server version than the lowest version present in the group. Members running MySQL 8.0.16 or lower do not carry out this check, and are always placed in read-write mode.

While the action runs, you can check its progress by issuing:

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 |
+----------------------------------------------------------------------+----------------+----------------+