Multi-Primary and Single-Primary Modes The following examples are typical use cases for Group Replication. 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. From MySQL 8.0.13, 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 8.0, a proxy, a connector, or the application itself. In single-primary mode ( group_replication_single_primary_mode=ON ) the group has a single primary server that is set to read-write mode. All the other members in the group are set to read-only mode (with  super_read_only=ON ). The primary is typically the first server to bootstrap the group. All other servers that join the group learn about the primary server and are automatically set to read-only mode. In single-primary mode, Group Replication enforces that only a single server writes to the group, so compared to multi-primary mode, consistency checking can be less strict and DDL statements do not need to be handled with any extra care. The option group_replication_enforce_update_everywhere_checks  enables or disables strict consistency checks for a group. When deploying in single-primary mode, or changing the group to single-primary mode, this system variable must be set to  OFF . The member that is designated as the primary server can change in the following ways: 3693 18.1.3.1 Single-Primary Mode 18.1.3 Multi-Primary and Single-Primary Modes • Autonomic Systems  - Additionally, you can deploy MySQL Group Replication purely for the automation that is built into the replication protocol (described already in this and previous chapters). • Alternative to asynchronous Source-Replica replication  - In certain situations, using a single source server makes it a single point of contention. Writing to an entire group may prove more scalable under certain circumstances. • Highly Available Shards  - Sharding is a popular approach to achieve write scale-out. Use MySQL Group Replication to implement highly available shards, where each shard maps to a replication group. • Elastic Replication  - Environments that require a very fluid replication infrastructure, where the number of servers has to grow or shrink dynamically and with as few side-effects as possible. For instance, database services for the cloud.