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6.3 Replication Implementation Details
======================================
Menu
* master-thread-states Replication Master Thread States
* slave-io-thread-states Replication Slave I/O Thread States
* slave-sql-thread-states Replication Slave SQL Thread States
* slave-logs Replication Relay and Status Files
MySQL replication capabilities are implemented using three threads (one
on the master server and two on the slave). When a `START SLAVE'
statement is issued on a slave server, the slave creates an I/O thread,
which connects to the master and asks it to send the updates recorded
in its binary logs. The master creates a thread to send the binary log
contents to the slave. This thread can be identified as the `Binlog
Dump' thread in the output of `SHOW PROCESSLIST' on the master. The
slave I/O thread reads the updates that the master `Binlog Dump' thread
sends and copies them to local files, known as _relay logs_, in the
slave's data directory. The third thread is the SQL thread, which the
slave creates to read the relay logs and to execute the updates they
contain.
In the preceding description, there are three threads per master/slave
connection. A master that has multiple slaves creates one thread for
each currently-connected slave, and each slave has its own I/O and SQL
threads.
The slave uses two threads so that reading updates from the master and
executing them can be separated into two independent tasks. Thus, the
task of reading statements is not slowed down if statement execution is
slow. For example, if the slave server has not been running for a
while, its I/O thread can quickly fetch all the binary log contents
from the master when the slave starts, even if the SQL thread lags far
behind. If the slave stops before the SQL thread has executed all the
fetched statements, the I/O thread has at least fetched everything so
that a safe copy of the statements is stored locally in the slave's
relay logs, ready for execution the next time that the slave starts.
This enables the master server to purge its binary logs sooner because
it no longer needs to wait for the slave to fetch their contents.
The `SHOW PROCESSLIST' statement provides information that tells you
what is happening on the master and on the slave regarding replication.
The following example illustrates how the three threads show up in the
output from `SHOW PROCESSLIST'.
On the master server, the output from `SHOW PROCESSLIST' looks like
this:
mysql> SHOW PROCESSLIST\G
*************************** 1. row ***************************
Id: 2
User: root
Host: localhost:32931
db: NULL
Command: Binlog Dump
Time: 94
State: Has sent all binlog to slave; waiting for binlog to
be updated
Info: NULL
Here, thread 2 is a `Binlog Dump' replication thread for a connected
slave. The `State' information indicates that all outstanding updates
have been sent to the slave and that the master is waiting for more
updates to occur. If you see no `Binlog Dump' threads on a master
server, this means that replication is not running -- that is, that no
slaves are currently connected.
On the slave server, the output from `SHOW PROCESSLIST' looks like this:
mysql> SHOW PROCESSLIST\G
*************************** 1. row ***************************
Id: 10
User: system user
Host:
db: NULL
Command: Connect
Time: 11
State: Waiting for master to send event
Info: NULL
*************************** 2. row ***************************
Id: 11
User: system user
Host:
db: NULL
Command: Connect
Time: 11
State: Has read all relay log; waiting for the slave I/O
thread to update it
Info: NULL
This information indicates that thread 10 is the I/O thread that is
communicating with the master server, and thread 11 is the SQL thread
that is processing the updates stored in the relay logs. At the time
that the `SHOW PROCESSLIST' was run, both threads were idle, waiting
for further updates.
The value in the `Time' column can show how late the slave is compared
to the master. See replication-faq.
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