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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 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 between group members. 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 server:

mysql-5.7/bin/mysqld --defaults-file=data/s1/s1.cnf

Create a MySQL user with the REPLICATION-SLAVE privilege. This process should not be captured in the binary log to avoid the changes being propagated to other server instances. 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. Connect to server s1 and issue the following statements:

mysql> SET SQL_LOG_BIN=0;
mysql> CREATE USER rpl_user@'%' IDENTIFIED BY 'password';
mysql> GRANT REPLICATION SLAVE ON *.* TO rpl_user@'%';
mysql> SET SQL_LOG_BIN=1;

Once the user has been configured as above, 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. If these credentials are not set correctly as shown, the server cannot run the recovery process and gain synchrony with the other group members, and hence ultimately cannot join the group. Similarly, if the member 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.

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