Configuring MariaDB Replication between Two MariaDB Galera Clusters

MariaDB replication can be used to replication between two MariaDB Galera Clusters. This article will discuss how to do that.

Configuring the Clusters

Before we set up replication, we need to ensure that the clusters are configured properly. This involves the following steps:

Configuring Wsrep GTID Mode

If you want to use GTID replication, then you also need to configure some things to enable wsrep GTID mode. For example:

  • wsrep_gtid_domain_id needs to be set to the same value on all nodes in a given cluster, so that each cluster node uses the same domain when assigning GTIDs for Galera Cluster's write sets. Each cluster should have this set to a different value, so that each cluster uses different domains when assigning GTIDs for their write sets.
  • log_bin needs to be set to the same path on all nodes in the cluster. See MDEV-9856 about that.

And as an extra safety measure:

  • gtid_domain_id should be set to a different value on all nodes in a given cluster, and each of these values should be different than the configured wsrep_gtid_domain_id value. This is to prevent a node from using the same domain used for Galera Cluster's write sets when assigning GTIDs for non-Galera transactions, such as DDL executed with wsrep_sst_method=RSU set or DML executed with wsrep_on=OFF set.

Setting up Replication

Our process to set up replication is going to be similar to the process described at Setting up a Replication Slave with Mariabackup, but it will be modified a bit to work in this context.

Start the First Cluster

The very first step is to start the nodes in the first cluster. The first node will have to be bootstrapped. The other nodes can be started normally.

Once the nodes are started, you need to pick a specific node that will act as the replication master for the second cluster.

Backup the Database on the First Cluster's Master Node and Prepare It

The first step is to simply take and prepare a fresh full backup of the node that you have chosen to be the replication master. For example:

$ mariabackup --backup \
   --target-dir=/var/mariadb/backup/ \
   --user=mariabackup --password=mypassword

And then you would prepare the backup as you normally would. For example:

$ mariabackup --prepare \
   --target-dir=/var/mariadb/backup/ 

Copy the Backup to the Second Cluster's Slave

Once the backup is done and prepared, you can copy it to the node in the second cluster that will be acting as slave. For example:

$ rsync -avrP /var/mariadb/backup c2dbserver:/var/mariadb/backup

Restore the Backup on the Second Cluster's Slave

At this point, you can restore the backup to the datadir, as you normally would. For example:

$ mariabackup --copy-back \
   --target-dir=/var/mariadb/backup/

And adjusting file permissions, if necessary:

$ chown -R mysql:mysql /var/lib/mysql/

Bootstrap the Second Cluster's Slave

Now that the backup has been restored to the second cluster's slave, you can start the server by bootstrapping the node.

Create a Replication User on the First Cluster's Master

Before the second cluster's slave can begin replicating from the first cluster's master, you need to create a user account on the master that the slave can use to connect, and you need to grant the user account the REPLICATION SLAVE privilege. For example:

CREATE USER 'repl'@'c2dbserver1' IDENTIFIED BY 'password';
GRANT REPLICATION SLAVE ON *.*  TO 'repl'@'c2dbserver1';

Start Replication on the Second Cluster's Slave

At this point, you need to get the replication coordinates of the master from the original backup.

The coordinates will be in the xtrabackup_binlog_info file.

Mariabackup dumps replication coordinates in two forms: GTID strings and binary log file and position coordinates, like the ones you would normally see from SHOW MASTER STATUS output. In this case, it is probably better to use the GTID coordinates.

For example:

mariadb-bin.000096 568 0-1-2

Regardless of the coordinates you use, you will have to set up the master connection using CHANGE MASTER TO and then start the replication threads with START SLAVE.

GTIDs

If you want to use GTIDs, then you will have to first set gtid_slave_pos to the GTID coordinates that we pulled from the xtrabackup_binlog_info file, and we would set MASTER_USE_GTID=slave_pos in the CHANGE MASTER TO command. For example:

SET GLOBAL gtid_slave_pos = "0-1-2";
CHANGE MASTER TO 
   MASTER_HOST="c1dbserver1", 
   MASTER_PORT=3310, 
   MASTER_USER="repl",  
   MASTER_PASSWORD="password", 
   MASTER_USE_GTID=slave_pos;
START SLAVE;

File and Position

If you want to use the binary log file and position coordinates, then you would set MASTER_LOG_FILE and MASTER_LOG_POS in the CHANGE MASTER TO command to the file and position coordinates that we pulled the xtrabackup_binlog_info file. For example:

CHANGE MASTER TO 
   MASTER_HOST="c1dbserver1", 
   MASTER_PORT=3310, 
   MASTER_USER="repl",  
   MASTER_PASSWORD="password", 
   MASTER_LOG_FILE='mariadb-bin.000096',
   MASTER_LOG_POS=568,
START SLAVE;

Check the Status of the Second Cluster's Slave

You should be done setting up the slave now, so you should check its status with SHOW SLAVE STATUS. For example:

SHOW SLAVE STATUS\G

Start the Second Cluster

If the slave is replicating normally, then the next step would be to start the MariaDB Server process on the other nodes in the second cluster.

Now that the second cluster is up, ensure that it does not start accepting writes yet if you want to set up circular replication between the two clusters.

Setting up Circular Replication

You can also set up circular replication between the two clusters, which means that the second cluster replicates from the first cluster, and the first cluster also replicates from the second cluster.

Create a Replication User on the Second Cluster's Master

Before circular replication can begin, you also need to create a user account on the second cluster's master that the first cluster's slave can use to connect, and you need to grant the user account the REPLICATION SLAVE privilege. For example:

CREATE USER 'repl'@'c1dbserver1' IDENTIFIED BY 'password';
GRANT REPLICATION SLAVE ON *.*  TO 'repl'@'c1dbserver1';

Start Circular Replication on the First Cluster

How this is done would depend on whether you want to use the GTID coordinates or the binary log file and position coordinates.

Regardless, you need to ensure that the second cluster is not accepting any writes other than those that it replicates from the first cluster at this stage.

GTIDs

To get the GTID coordinates on the second cluster, you can check gtid_current_pos by executing:

SHOW GLOBAL VARIABLES LIKE 'gtid_current_pos';

Then on the first cluster, you can set up replication by setting gtid_slave_pos to the GTID that was returned and then executing CHANGE MASTER TO:

SET GLOBAL gtid_slave_pos = "0-1-2";
CHANGE MASTER TO 
   MASTER_HOST="c2dbserver1", 
   MASTER_PORT=3310, 
   MASTER_USER="repl",  
   MASTER_PASSWORD="password", 
   MASTER_USE_GTID=slave_pos;
START SLAVE;

File and Position

To get the binary log file and position coordinates on the second cluster, you can execute SHOW MASTER STATUS:

SHOW MASTER STATUS

Then on the first cluster, you would set master_log_file and master_log_pos in the CHANGE MASTER TO command. For example:

CHANGE MASTER TO 
   MASTER_HOST="c2dbserver1", 
   MASTER_PORT=3310, 
   MASTER_USER="repl",  
   MASTER_PASSWORD="password", 
   MASTER_LOG_FILE='mariadb-bin.000096',
   MASTER_LOG_POS=568;
START SLAVE;

Check the Status of the Circular Replication

You should be done setting up the circular replication on the node in the first cluster now, so you should check its status with SHOW SLAVE STATUS. For example:

SHOW SLAVE STATUS\G

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