So we have a bunch of websites for different markets, running wordpress, which we would ideally like hosted in their home market. BUT, we want to be able to fail them over to a different country, should the servers in their country go down. Failover in a MySQL master-slave relationship is always a bit of a pain (as it is with any DB engine) – once you’ve failed, you really can’t ‘go back’ to the original master, until you’ve re-synced it all. Which isn’t overly easy when you only have a 1 hour per 24-hr-period maintenance window, across all the markets your company operates in.
Enter MySQL Multi-Master replication. Make a change on on server? It appears on the other. Make a change on the other server? It appears on the first!
The way this works, is that each MySQL server can be both a Master, AND a Slave. So Take two servers, A and B. Any changes made on A are played via logs to the B server. Similarly, any changes on the B server are pushed to the A server. Well, actually it’s a little more than that, as Server A will send the updates it receives from Server B, on to Server B. Why does it do this?
Well, we might have six Masters! Going Master A -> Master B -> Master C -> Master D -> Master E -> Master F. And Master F is feeding Master A. All a nice big circle. So when you make a change on Master B, it propagates to C, D, E, F, and then to A. AND back to B. But B knows not to replicate its own changes on again, and they stop there.
It’s easiest to set this all up with fresh, clean, servers.
I installed MySQL-server on two clean Debian VM’s, one in Australia, one in Ireland.
Configuration
Server A – /etc/my.cnf
Add the following:
[mysqld]
# ... other configuration, tuning, etc ...
server-id = 10
# Make sure this partition has space to log bin, relay and whatever else!
log-bin = /var/lib/mysql/bin.log
relay-log = /var/lib/mysql/slave-relay.log
relay-log-index = /var/lib/mysql/slave-relay-log.index
# Creating some room between pk ids, we can always manually insert if need be.
auto_increment_increment = 10
auto_increment_offset = 1
# This is the default, but let's be safe and ensure it's on
replicate-same-server-id = FALSE
# Want more slaves in the future with writes going to both masters?
log-slave-updates = TRUE
# If there's a reboot, let's not auto start replication. - we need to make sure of where we are, and start it manually..
skip-slave-start = TRUE
Server B – /etc/my.cnf
[mysqld]
# ... other configuration, tuning, etc ...
server-id = 11
log-bin = /var/lib/mysql/bin.log
relay-log = /var/lib/mysql/slave-relay.log
relay-log-index = /var/lib/mysql/slave-relay-log.index
auto_increment_increment = 10
auto_increment_offset = 2
replicate-same-server-id = FALSE
log-slave-updates = TRUE
skip-slave-start = TRUE
You could add multiple more servers here, just increment the server-id, and the auto_increment_offset.
Starting Replication
To start replication, we first need to create a replication user on both servers, then setup the replication attributes.
First, create the replication user on both servers:
Server A
# mysql -u root -p
mysql> GRANT REPLICATION SLAVE, REPLICATION CLIENT ON *.* TO repluser@'serverb.ip.address' IDENTIFIED BY 'replpassword';
Server B
# mysql -u root -p
mysql> GRANT REPLICATION SLAVE, REPLICATION CLIENT ON *.* TO repluser@'servera.ip.address' IDENTIFIED BY 'replpassword';
Find the master info on Server B:
mysql> show master status;
+------------+----------+--------------+------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+------------+----------+--------------+------------------+
| bin.000001 | 294 | | |
+------------+----------+--------------+------------------+
1 row in set (0.00 sec)
Now we want to start the replication on Server A, using the info from Server B:
mysql>CHANGE MASTER TO
MASTER_HOST='92.1.1.1',
MASTER_USER='repluser',
MASTER_PASSWORD='replpassword',
MASTER_LOG_FILE='bin.000001',
MASTER_LOG_POS=294;
mysql>start slave;
mysql>show slave status\G
You may need to run the show slave status\G a few times, before the slave drops into the standard ‘Waiting for master to send event’ state.
Once this is done, you can then work on repeating this process to start Server B slaving from Server A.
Find the master info on Server A:
mysql> show master status;
+------------+----------+--------------+------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+------------+----------+--------------+------------------+
| bin.000001 | 293 | | |
+------------+----------+--------------+------------------+
1 row in set (0.00 sec)
Now we want to start the replication on Server A, using the info from Server B:
mysql>CHANGE MASTER TO
MASTER_HOST='202.62.1.1',
MASTER_USER='repluser',
MASTER_PASSWORD='replpassword',
MASTER_LOG_FILE='bin.000001',
MASTER_LOG_POS=293;
mysql>start slave;
mysql>show slave status\G
Exporting/Importing the Data
Now you want to create any databases, users, grants, and then import any data you want. Keep an eye on the ‘show slave status\G’ on the server opposite to where you’re doing all this, to make sure it is replicating correctly 🙂
And you’re done!