Here’s a very quick example for how to setup Multi-Master replication with MariaDB.  It’s light on detail here to focus only on the multi-master aspects of the setup. Have a good read of the documentation before attempting this. This example also uses GTIDs so you’ll need some understanding of these as well.

The example here is pretty simple. There are two master servers. We will replicate two servers to a single slave. From  master1 we will replicate all databases to the slave. From  master2 we will replicate a single database to the same slave as  master1.  I have assumed you’re working on the slave itself as I reference it by 127.0.0.1.

First dump and load the databases we want…

# all databases from one server
mysqldump -h master1 -P3306 --master-data=2 --routines --single-transaction --all-databases -u dba -pXXXXXXXX > all_databases.sql
cat all_databases.sql | mysql -h 127.0.0.1 -P3306

# Single database from another master
mysqldump -h master2 -P3306 --master-data=2 --routines --single-transaction --databases mydb -u dba -pXXXXXXXX > single_database.sql
cat single_database.sql | mysql -h 127.0.0.1 -P3306

Extract the GTID co-ordinates…

head -n 30 all_databases.sql | grep -i gtid
head -n 30 single_database.sql | grep -i gtid

You’ll see something like this displayed…

-- GTID to start replication from
-- SET GLOBAL gtid_slave_pos='XXXX-XXXX-XXXX';

-- GTID to start replication from
-- SET GLOBAL gtid_slave_pos='XXXX-XXXX-XXXX';

Extract the GTID positions and combine them in a statement like below to execute on the slave…

SET GLOBAL gtid_slave_pos='XXXX-XXXX-XXXX,XXXX-XXXX-XXXX';

Configure the slave to point at both masters…

CHANGE MASTER 'mirror_master1' TO
			MASTER_HOST='master1',
			MASTER_PORT=3306,
			MASTER_USER='replication',
			MASTER_PASSWORD='XXXXXXXX',
			MASTER_USE_GTID=slave_pos;

CHANGE MASTER 'single_database' TO
			MASTER_HOST='master2',
			MASTER_PORT=3307,
			MASTER_USER='replication',
			MASTER_PASSWORD='XXXXXXXX',
			MASTER_USE_GTID=slave_pos;

Run the following statement so the slave will only process binlog entries for the appropriate database… (you’d also want this in your cnf file)

SET GLOBAL single_database.replicate_do_db=mydb;

Now you can start the slave threads…

START ALL SLAVES;

Double check the status to make sure it has worked…

SHOW SLAVE STATUS \G