MySQL 5.7 Release Notes
The first step is to deploy at least three instances of MySQL Server, this procedure demonstrates using multiple hosts for the instances, named s1, s2, and s3. It is assumed that MySQL Server is installed on each host (see Chapter 2, Installing and Upgrading MySQL). The Group Replication plugin is provided with MySQL Server 5.7.17 and later; no additional software is required, although the plugin must be installed in the running MySQL server. See Section 17.2.1.1, “Deploying Instances for Group Replication”; for additional information, see Section 5.5, “MySQL Server Plugins”.
In this example, three instances are used for the group, which is the minimum number of instances to create a group. Adding more instances increases the fault tolerance of the group. For example if the group consists of three members, in event of failure of one instance the group can continue. But in the event of another failure the group can no longer continue processing write transactions. By adding more instances, the number of servers which can fail while the group continues to process transactions also increases. The maximum number of instances which can be used in a group is nine. For more information see Section 17.1.3.2, “Failure Detection”.