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MySQL 5.7 Reference Manual
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Excerpts from this Manual User Credentials

Group Replication uses the asynchronous replication protocol to achieve Section 17.9.5, “Distributed Recovery”, synchronizing group members before joining them to the group. The distributed recovery process relies on a replication channel named group_replication_recovery which is used to transfer transactions from donor members to members that join the group. Therefore you need to set up a replication user with the correct permissions so that Group Replication can establish direct member-to-member recovery replication channels.

Start the MySQL server instance and then connect a client to it. Create a MySQL user with the REPLICATION SLAVE privilege. This process can be captured in the binary log and then you can rely on distributed recovery to replicate the statements used to create the user. Alternatively, you can disable binary logging using SET SQL_LOG_BIN=0; and then create the user manually on each member, for example if you want to avoid the changes being propagated to other server instances. If you do decide to disable binary logging, ensure you renable it once you have configured the user.

In the following example the user rpl_user with the password password is shown. When configuring your servers use a suitable user name and password.

mysql> CREATE USER rpl_user@'%' IDENTIFIED BY 'password';
mysql> GRANT REPLICATION SLAVE ON *.* TO rpl_user@'%';

If binary logging was disabled, enable it again once the user has been created using SET SQL_LOG_BIN=1;.

Once the user has been configured, use the CHANGE MASTER TO statement to configure the server to use the given credentials for the group_replication_recovery replication channel the next time it needs to recover its state from another member. Issue the following, replacing rpl_user and password with the values used when creating the user.

mysql> CHANGE MASTER TO MASTER_USER='rpl_user', MASTER_PASSWORD='password' \\
		      FOR CHANNEL 'group_replication_recovery';

Distributed recovery is the first step taken by a server that joins the group and does not have the same set of transactions as the group members. If these credentials are not set correctly for the group_replication_recovery replication channel and the rpl_user as shown, the server cannot connect to the donor members and run the distributed recovery process to gain synchrony with the other group members, and hence ultimately cannot join the group. See Section 17.9.5, “Distributed Recovery”.

Similarly, if the server cannot correctly identify the other members via the server's hostname the recovery process can fail. It is recommended that operating systems running MySQL have a properly configured unique hostname, either using DNS or local settings. This hostname can be verified in the Member_host column of the performance_schema.replication_group_members table. If multiple group members externalize a default hostname set by the operating system, there is a chance of the member not resolving to the correct member address and not being able to join the group. In such a situation use report_host to configure a unique hostname to be externalized by each of the servers.