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Replication Implementation

1475

For slaves that were started with the

--report-host

[1460]

option and are connected to the master,

the

SHOW SLAVE HOSTS

statement on the master shows basic information about the slaves. The

output includes the ID of the slave server, the value of the

--report-host

[1460]

option, the

connecting port, and master ID:

mysql>

SHOW SLAVE HOSTS;

+-----------+--------+------+-------------------+-----------+

+-----------+--------+------+-------------------+-----------+

| 10 | slave1 | 3306 | 0 | 1 |

+-----------+--------+------+-------------------+-----------+

1 row in set (0.00 sec)

16.1.3.2. Pausing Replication on the Slave

You can stop and start the replication of statements on the slave using the

STOP SLAVE

and

START

SLAVE

statements.

To stop processing of the binary log from the master, use

STOP SLAVE

mysql>

STOP SLAVE;

When replication is stopped, the slave I/O thread stops reading events from the master binary log and
writing them to the relay log, and the SQL thread stops reading events from the relay log and executing
them. You can pause the I/O or SQL thread individually by specifying the thread type:

mysql>

STOP SLAVE IO_THREAD;

mysql>

STOP SLAVE SQL_THREAD;

To start execution again, use the

START SLAVE

statement:

mysql>

START SLAVE;

To start a particular thread, specify the thread type:

mysql>

START SLAVE IO_THREAD;

mysql>

START SLAVE SQL_THREAD;

For a slave that performs updates only by processing events from the master, stopping only the SQL
thread can be useful if you want to perform a backup or other task. The I/O thread will continue to read
events from the master but they are not executed. This makes it easier for the slave to catch up when
you restart the SQL thread.

Stopping only the I/O thread enables the events in the relay log to be executed by the SQL thread up
to the point where the relay log ends. This can be useful when you want to pause execution to catch
up with events already received from the master, when you want to perform administration on the slave
but also ensure that it has processed all updates to a specific point. This method can also be used
to pause event receipt on the slave while you conduct administration on the master. Stopping the I/O
thread but permitting the SQL thread to run helps ensure that there is not a massive backlog of events
to be executed when replication is started again.

16.2. Replication Implementation

Replication is based on the master server keeping track of all changes to its databases (updates,
deletes, and so on) in its binary log. The binary log serves as a written record of all events that modify
database structure or content (data) from the moment the server was started. Typically,

SELECT

statements are not recorded because they modify neither database structure nor content.

Each slave that connects to the master requests a copy of the binary log. That is, it pulls the data from
the master, rather than the master pushing the data to the slave. The slave also executes the events
from the binary log that it receives. This has the effect of repeating the original changes just as they

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