In this mode 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 ). This happens automatically. The primary is typically the first server to boostrap the group, all other servers that join automatically learn about the primary server and are set to read only.

Figure 21.5 New Primary Election

When in single-primary mode, some of the checks deployed in multi-primary mode are disabled, because the system enforces that only a single writer server is in the group at a time. For example, changes to tables that have cascading foreign keys are allowed, whereas in multi-primary mode they are not. Upon primary member failure, an automatic primary election mechanism chooses the next primary member. The method of choosing the next primary depends on the server's server_uuid variable and the group_replication_member_weight variable.

In the event that the primary member leaves the group, then an election is performed and a new primary is chosen from the remaining servers in the group. This election is performed by looking at the new view, and ordering the potential new primaries based on the value of group_replication_member_weight. The member with the highest value for group_replication_member_weight is elected as the new primary. In the event that multiple servers have the same group_replication_member_weight, the servers are then prioritised based on their server_uuid in lexicographical order and by picking the first one. Once a new primary is elected, it is automatically set to read-write and the other secondaries remain as secondaries, and as such, read-only.

It is a good practice to wait for the new primary to apply its replication related relay-log before re-routing the client applications to it.