MySQL 5.7 Release Notes
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 the
--log-bin option, and if not using
GTIDs then they should also be run without enabling the
log_slave_updates system
variable. 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 16.4, “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 16.3.4, “Using Replication for Scale-Out”.
Each MySQL Slave (Slave 1, Slave
2, and Slave 3) is a slave running
with --log-bin and without enabling
the log_slave_updates system
variable. Because updates received by a slave from the master are
not logged in the binary log unless
log_slave_updates is enabled, 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 without
log_slave_updates enabled 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, 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 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 16.5, “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.