MySQL 9.0 Release Notes
Group Replication operates either in single-primary mode or in
multi-primary mode. The group's mode is a group-wide configuration
setting, specified by the
group_replication_single_primary_mode
system variable, which must be the same on all members.
ON means single-primary mode, which is the
default mode, and OFF means multi-primary mode.
It is not possible to have members of the group deployed in
different modes, for example one member configured in
multi-primary mode while another member is in single-primary mode.
You cannot change the value of
group_replication_single_primary_mode
manually while Group Replication is running. You can use the
group_replication_switch_to_single_primary_mode()
and
group_replication_switch_to_multi_primary_mode()
functions to move a group from one mode to another while Group
Replication is still running. These functions manage the process
of changing the group's mode and ensure the safety and consistency
of your data. In earlier releases, to change the group's mode you
must stop Group Replication and change the value of
group_replication_single_primary_mode
on all members. Then carry out a full reboot of the group (a
bootstrap by a server with
group_replication_bootstrap_group=ON)
to implement the change to the new operating configuration. You do
not need to restart the servers.
Regardless of the deployed mode, Group Replication does not handle client-side failover. That must be handled by a middleware framework such as MySQL Router 9.0, a proxy, a connector, or the application itself.