3.8 Switching Masters During Failover

You can tell a slave to change to a new master using the CHANGE MASTER TO statement. The slave does not check whether the databases on the master are compatible with those on the slave; it simply begins reading and executing events from the specified coordinates in the new master's binary log. In a failover situation, all the servers in the group are typically executing the same events from the same binary log file, so changing the source of the events should not affect the structure or integrity of the database, provided that you exercise care in making the change.

Slaves should be run with binary logging enabled (the --log-bin option), which is the default. If you are not using GTIDs for replication, then the slaves should also be run with --log-slave-updates=OFF (logging slave updates is the default). In this way, the slave is ready to become a master without restarting the slave mysqld. Assume that you have the structure shown in Figure 3.4, “Redundancy Using Replication, Initial Structure”.

Figure 3.4 Redundancy Using Replication, Initial Structure

In this diagram, the MySQL Master holds the master database, the MySQL Slave hosts are replication slaves, and the Web Client machines are issuing database reads and writes. Web clients that issue only reads (and would normally be connected to the slaves) are not shown, as they do not need to switch to a new server in the event of failure. For a more detailed example of a read/write scale-out replication structure, see Section 3.5, “Using Replication for Scale-Out”.

Each MySQL Slave (Slave 1, Slave 2, and Slave 3) is a slave running with binary logging enabled, and with --log-slave-updates=OFF. Because updates received by a slave from the master are not logged in the binary log when --log-slave-updates=OFF is specified, the binary log on each slave is empty initially. If for some reason MySQL Master becomes unavailable, you can pick one of the slaves to become the new master. For example, if you pick Slave 1, all Web Clients should be redirected to Slave 1, which writes the updates to its binary log. Slave 2 and Slave 3 should then replicate from Slave 1.

The reason for running the slave with --log-slave-updates=OFF is to prevent slaves from receiving updates twice in case you cause one of the slaves to become the new master. If Slave 1 has --log-slave-updates enabled, which is the default, it writes any updates that it receives from Master in its own binary log. This means that, when Slave 2 changes from Master to Slave 1 as its master, it may receive updates from Slave 1 that it has already received from Master.

Make sure that all slaves have processed any statements in their relay log. On each slave, issue STOP SLAVE IO_THREAD, then check the output of SHOW PROCESSLIST until you see Has read all relay log. When this is true for all slaves, they can be reconfigured to the new setup. On the slave Slave 1 being promoted to become the master, issue STOP SLAVE and RESET MASTER.

On the other slaves Slave 2 and Slave 3, use STOP SLAVE and CHANGE MASTER TO MASTER_HOST='Slave1' (where 'Slave1' represents the real host name of Slave 1). To use CHANGE MASTER TO, add all information about how to connect to Slave 1 from Slave 2 or Slave 3 (user, password, port). When issuing the CHANGE MASTER TO statement in this, there is no need to specify the name of the Slave 1 binary log file or log position to read from, since the first binary log file and position 4, are the defaults. Finally, execute START SLAVE on Slave 2 and Slave 3.

Once the new replication setup is in place, you need to tell each Web Client to direct its statements to Slave 1. From that point on, all update statements sent by Web Client to Slave 1 are written to the binary log of Slave 1, which then contains every update statement sent to Slave 1 since Master died.

The resulting server structure is shown in Figure 3.5, “Redundancy Using Replication, After Master Failure”.

Figure 3.5 Redundancy Using Replication, After Master Failure

When Master becomes available again, you should make it a slave of Slave 1. To do this, issue on Master the same CHANGE MASTER TO statement as that issued on Slave 2 and Slave 3 previously. Master then becomes a slave of S1ave 1 and picks up the Web Client writes that it missed while it was offline.

To make Master a master again, use the preceding procedure as if Slave 1 was unavailable and Master was to be the new master. During this procedure, do not forget to run RESET MASTER on Master before making Slave 1, Slave 2, and Slave 3 slaves of Master. If you fail to do this, the slaves may pick up stale writes from the Web Client applications dating from before the point at which Master became unavailable.

You should be aware that there is no synchronization between slaves, even when they share the same master, and thus some slaves might be considerably ahead of others. This means that in some cases the procedure outlined in the previous example might not work as expected. In practice, however, relay logs on all slaves should be relatively close together.

One way to keep applications informed about the location of the master is to have a dynamic DNS entry for the master. With bind you can use nsupdate to update the DNS dynamically.