17.3.6. Switching Masters During Failover

There is currently no official solution for providing failover between master and slaves in the event of a failure. With the currently available features, you would have to set up a master and a slave (or several slaves), and to write a script that monitors the master to check whether it is up. Then instruct your applications and the slaves to change master in case of failure.

Remember that you can tell a slave to change its master at any time, using the CHANGE MASTER TO statement. The slave will not check whether the databases on the master are compatible with the slave, it will just start reading and executing events from the specified binary log coordinates on the new master. 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 database structure or integrity providing you are careful.

Run your slaves with the --log-bin option and without --log-slave-updates. In this way, the slave is ready to become a master as soon as you issue STOP SLAVE; RESET MASTER, and CHANGE MASTER TO statement on the other slaves. For example, assume that you have the structure shown in Figure 17.4, “Redundancy Using Replication, Initial Structure”.

Figure 17.4. Redundancy Using Replication, Initial Structure

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 17.3.3, “Using Replication for Scale-Out”.

Each MySQL Slave (Slave 1, Slave 2, and Slave 3) is a slave running with --log-bin and without --log-slave-updates. Because updates received by a slave from the master are not logged in the binary log unless --log-slave-updates 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 will log updates to its binary log. Slave 2 and Slave 3 should then replicate from Slave 1.

The reason for running the slave without --log-slave-updates is to prevent slaves from receiving updates twice in case you cause one of the slaves to become the new master. Suppose that Slave 1 has --log-slave-updates enabled. Then it will write updates that it receives from Master to its own binary log. 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). In CHANGE MASTER TO, there is no need to specify the name of the Slave 1 binary log file or log position to read from: We know it is the first binary log file and position 4, which are the defaults for CHANGE MASTER TO. Finally, use START SLAVE on Slave 2 and Slave 3.

Once the new replication is in place, you will then need to instruct each Web Client to direct its statements to Slave 1. From that point on, all updates 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 17.5, “Redundancy Using Replication, After Master Failure”.

Figure 17.5. Redundancy Using Replication, After Master Failure

Redundancy using replication, after master
          failure

When Master is up again, you must issue on it the same CHANGE MASTER TO as that issued on Slave 2 and Slave 3, so that Master becomes a slave of S1 and picks up each Web Client writes that it missed while it was down.

To make Master a master again (for example, because it is the most powerful machine), 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. Otherwise, they may pick up old Web Client writes from before the point at which Master became unavailable.

Note that there is no synchronization between the different slaves to a master. Some slaves might be ahead of others. This means that the concept outlined in the previous example might not work. In practice, however, the relay logs of different slaves will most likely not be far behind the master, so it would work, anyway (but there is no guarantee).

A good way to keep your applications informed as to the location of the master is by having a dynamic DNS entry for the master. With bind you can use nsupdate to dynamically update your DNS.

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