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Replication enables data from one MySQL database server (called the
master) to be replicated to one or more MySQL database servers
(slaves). Replication is asynchronous - your replication slaves do
not need to be connected permanently to receive updates from the
master, which means that updates can occur over long-distance
connections and even temporary solutions such as a dial-up service.
Depending on the configuration, you can replicate all databases,
selected databases, or even selected tables within a database.
The target uses for replication in MySQL include:
Scale-out solutions - spreading the load among multiple slaves
to improve performance. In this environment, all writes and
updates must take place on the master server. Reads, however,
may take place on one or more slaves. This model can improve the
performance of writes (since the master is dedicated to
updates), while dramatically increasing read speed across an
increasing number of slaves.
Data security - because data is replicated to the slave, and the
slave can pause the replication process, it is possible to run
backup services on the slave without corrupting the
corresponding master data.
Analytics - live data can be created on the master, while the
analysis of the information can take place on the slave without
affecting the performance of the master.
Long-distance data distribution - if a branch office would like
to work with a copy of your main data, you can use replication
to create a local copy of the data for their use without
requiring permanent access to the master.
Replication in MySQL features support for one-way, asynchronous
replication, in which one server acts as the master, while one or
more other servers act as slaves. This is in contrast to the
synchronous replication which is a
characteristic of MySQL Cluster (see
Chapter 17, MySQL Cluster).
There are a number of solutions available for setting up replication
between two servers, but the best method to use depends on the
presence of data and the engine types you are using. For more
information on the available options, see
Section 16.1.1, “How to Set Up Replication”.
Replication is controlled through a number of different options and
variables. These control the core operation of the replication,
timeouts, and the databases and filters that can be applied on
databases and tables. For more information on the available options,
see Section 16.1.2, “Replication and Binary Logging Options and Variables”.
You can use replication to solve a number of different problems,
including problems with performance, supporting the backup of
different databases, and as part of a larger solution to alleviate
system failures. For information on how to address these issues, see
Section 16.2, “Replication Solutions”.
For notes and tips on how different data types and statements are
treated during replication, including details of replication
features, version compatibility, upgrades, and problems and their
resolution, including an FAQ, see
Section 16.3, “Replication Notes and Tips”.
For detailed information on the implementation of replication, how
replication works, the process and contents of the binary log,
background threads and the rules used to decide how statements are
recorded and replication, see
Section 16.4, “Replication Implementation”.
MySQL Enterprise
The MySQL Enterprise Monitor provides numerous advisors that give
immediate feedback about replication-related problems. For more
information, see
http://www.mysql.com/products/enterprise/advisors.html.
16.1. Replication Configuration
Replication between servers in MySQL is based on the binary logging
mechanism. The MySQL instance operating as the master (the source of
the database changes) writes updates and changes as
“events” to the binary log. The information in the
binary log is stored in different logging formats according to the
database changes being recorded. Slaves are configured to read the
binary log from the master and to execute the events in the binary
log on the slave's local database.
The master is “dumb” in this scenario. Once binary
logging has been enabled, all statements are recorded in the binary
log. Each slave receives a copy of the entire contents of the binary
log. It is the responsibility of the slave to decide which
statements in the binary log should be executed; you cannot
configure the master to log only certain events. If you do not
specify otherwise, all events in the master binary log are executed
on the slave. If required, you can configure the slave to process
only events that apply to particular databases or tables.
Each slave keeps a record of the binary log coordinates: The file
name and position within the file that it has read and processed
from the master. This means that multiple slaves can be connected to
the master and executing different parts of the same binary log.
Because the slaves control this process, individual slaves can be
connected and disconnected from the server without affecting the
master's operation. Also, because each slave remembers the position
within the binary log, it is possible for slaves to be disconnected,
reconnect and then “catch up” by continuing from the
recorded position.
Both the master and each slave must be configured with a unique ID
(using the server-id option). In
addition, each slave must be configured with information about the
master host name, log file name, and position within that file.
These details can be controlled from within a MySQL session using
the CHANGE MASTER TO statement on the
slave. The details are stored within the slave's
master.info file.
This section describes the setup and configuration required for a
replication environment, including step-by-step instructions for
creating a new replication environment. The major components of this
section are:
16.1.1. How to Set Up Replication
This section describes how to set up complete replication of a
MySQL server. There are a number of different methods for setting
up replication, and the exact method to use depends on how you are
setting up replication, and whether you already have data within
your master database.
There are some generic tasks that may be required for all
replication setups:
You may want to create a separate user that will be used by
your slaves to authenticate with the master to read the binary
log for replication. The step is optional. See
Section 16.1.1.1, “Creating a User for Replication”.
On the master, you must enable binary logging and configure a
unique server ID. See
Section 16.1.1.2, “Setting the Replication Master Configuration”.
On each slave that you want to connect to the master, you must
configure a unique server ID. See
Section 16.1.1.4, “Setting the Replication Slave Configuration”.
Before starting a data snapshot or the replication process,
you should record the position of the binary log on the
master. You will need this information when configuring the
slave so that the slave knows where within the binary log to
start executing events. See
Section 16.1.1.3, “Obtaining the Replication Master Binary Log Coordinates”.
If you already have data on your master and you want to
synchronize your slave using it, then you will need to create
a data snapshot. You can create a snapshot using
mysqldump (see
Section 16.1.1.5, “Creating a Data Snapshot Using mysqldump”) or by copying
the data files directly (see
Section 16.1.1.6, “Creating a Data Snapshot Using Raw Data Files”).
You will need to configure the slave with settings for
connecting to the master, such as the host name, login
credentials, and binary log file name and position. See
Section 16.1.1.10, “Setting the Master Configuration on the Slave”.
Once you have configured the basic options, you will need to
follow the instructions for your replication setup. A number of
alternatives are provided:
If you are establishing a new MySQL master and one or more
slaves, then you need only set up the configuration, as you
have no data to exchange. For guidance on setting up
replication in this situation, see
Section 16.1.1.7, “Setting Up Replication with New Master and Slaves”.
If you are already running a MySQL server, and therefore
already have data that must be transferred to your slaves
before replication starts, have not previously configured the
binary log and are able to shut down your MySQL server for a
short period during the process, see
Section 16.1.1.8, “Setting Up Replication with Existing Data”.
If you are adding slaves to an existing replication
environment, you can set up the slaves without affecting the
master. See
Section 16.1.1.9, “Introducing Additional Slaves to an Existing Replication Environment”.
If you will be administering MySQL replication servers, we suggest
that you read this entire chapter through and try all statements
mentioned in Section 12.6.1, “SQL Statements for Controlling Master Servers”, and
Section 12.6.2, “SQL Statements for Controlling Slave Servers”. You should also
familiarize yourself with the replication startup options
described in Section 16.1.2, “Replication and Binary Logging Options and Variables”.
Note
Note that certain steps within the setup process require the
SUPER privilege. If you do not
have this privilege then enabling replication may not be
possible.
16.1.1.1. Creating a User for Replication
Each slave must connect to the master using a MySQL user name
and password, so there must be a user account on the master that
the slave can use to connect. Any account can be used for this
operation, providing it has been granted the
REPLICATION SLAVE privilege. You
may wish to create a different account for each slave, or
connect to the master using the same account for each slave.
You do not need to create an account specifically for
replication. However, you should be aware that the user name and
password will be stored in plain text within the
master.info file. Therefore, you may want to
create a separate account that has privileges only for the
replication process, to minimize the possibility of compromise
to other accounts.
To create a new acccount, use CREATE
USER. To grant this account the privileges required
for replication, use the GRANT
statement. If you create a user solely for the purposes of
replication, that user needs only the
REPLICATION SLAVE privilege. For
example, to set up a new user, repl, that can
connect for replication from any host within the
mydomain.com domain, issue these statements
on the master:
mysql> CREATE USER 'repl'@'%.mydomain.com' IDENTIFIED BY 'slavepass';
mysql> GRANT REPLICATION SLAVE ON *.*
-> TO 'repl'@'%.mydomain.com';
See Section 12.5.1, “Account Management Statements”, for more
information on statements for manipulation of user accounts.
16.1.1.2. Setting the Replication Master Configuration
For replication to work, you must enable
binary logging on the master because the binary log is the basie
for sending data changes from the master to its slaves. If
binary logging is not enabled, replication will not be possible.
Each server within a replication group must be configured with a
unique server-id value. The
server ID is used to identify individual servers within the
group, and must be a positive integer between 1 and
(232)–1. How you organize and
select the numbers is entirely up to you.
To configure the binary log and server ID options, you will need
to shut down your MySQL server and edit the configuration of the
my.cnf or my.ini file.
You will need to add the following options to the configuration
file within the [mysqld] section. If these
options already exist, but are commented out, uncomment the
options and alter them according to your needs. For example, to
enable binary logging, using a log file name prefix of
mysql-bin, and a server ID of 1:
[mysqld]
log-bin=mysql-bin
server-id=1
Note
For the greatest possible durability and consistency in a
replication setup using InnoDB with
transactions, you should use
innodb_flush_log_at_trx_commit=1 and
sync_binlog=1 in the master
my.cnf file.
Note
Ensure that the skip-networking
option is not enabled on your replication master. If
networking has been disabled, your slave will not able to
communicate with the master and replication will fail.
16.1.1.3. Obtaining the Replication Master Binary Log Coordinates
To configure replication on the slave you must determine the
master's current coordinates within its binary log. You will
need this information so that when the slave starts the
replication process, it is able to start processing events from
the binary log at the correct point.
If you have existing data on your master that you want to
synchronize on your slaves before starting the replication
process, you must stop processing statements on the master, and
then obtain its current binary log coordinates and dump its
data, before allowing the master to continue executing
statements. If you do not stop the execution of statements, the
data dump and the master status information that you use will
not match and you will end up with inconsistent or corrupted
databases on the slaves.
To obtain the master binary log coordinates, follow these steps:
Start a session on the master by connecting to it with the
command-line client, and flush all tables and block write
statements by executing the
FLUSH TABLES WITH
READ LOCK statement:
mysql> FLUSH TABLES WITH READ LOCK;
For InnoDB tables, note that
FLUSH TABLES WITH
READ LOCK also blocks
COMMIT operations.
Warning
Leave the client from which you issued the
FLUSH
TABLES statement running so that the read lock
remains in effect. If you exit the client, the lock is
released.
In a different session on the master, use the
SHOW MASTER STATUS statement
to determine the current binary log file name and offset:
mysql > SHOW MASTER STATUS;
+---------------+----------+--------------+------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+---------------+----------+--------------+------------------+
| mysql-bin.003 | 73 | test | manual,mysql |
+---------------+----------+--------------+------------------+
The File column shows the name of the log
file and Position shows the offset within
the file. In this example, the binary log file is
mysql-bin.003 and the offset is 73.
Record these values. You need them later when you are
setting up the slave. They represent the replication
coordinates at which the slave should begin processing new
updates from the master.
If the master has been running previously without binary
logging enabled, the log file name and position values
displayed by SHOW MASTER
STATUS or mysqldump
--master-data will be empty. In that case, the
values that you need to use later when specifying the
slave's log file and position are the empty string
('') and 4.
You now have the information you need to enable the slave to
start reading from the binary log in the correct place to start
replication.
If you have existing data that needs be to synchronized with the
slave before you start replication, leave the client running so
that the lock remains in place and then proceed to
Section 16.1.1.5, “Creating a Data Snapshot Using mysqldump”, or
Section 16.1.1.6, “Creating a Data Snapshot Using Raw Data Files”. The idea here is to
prevent any further changes so that the data copied to the
slaves is in synchrony with the master.
If you are setting up a brand new master and slave replication
group, you can exit the first session to release the read lock.
16.1.1.4. Setting the Replication Slave Configuration
The only option you must configure on the slave is to set the
unique server ID. If this option is not already set, or the
current value conflicts with the value that you have chosen for
the master server, then you should shut down your slave server,
and edit the configuration to specify a unique server ID. For
example:
[mysqld]
server-id=2
If you are setting up multiple slaves, each one must have a
unique server-id value that
differs from that of the master and from each of the other
slaves. Think of server-id values
as something similar to IP addresses: These IDs uniquely
identify each server instance in the community of replication
partners.
If you do not specify a server-id
value, it defaults to 0.
Note
If you omit server-id (or set
it explicitly to 0), a master refuses connections from all
slaves, and a slave refuses to connect to a master. Thus,
omitting server-id is good only
for backup with a binary log.
You do not have to enable binary logging on the slave for
replication to be enabled. However, if you enable binary logging
on the slave, you can use the binary log for data backups and
crash recovery on the slave, and also use the slave as part of a
more complex replication topology (for example, where the slave
acts as a master to other slaves).
16.1.1.5. Creating a Data Snapshot Using mysqldump
One way to create a snapshot of the data in an existing master
database is to use the mysqldump tool. Once
the data dump has been completed, you then import this data into
the slave before starting the replication process.
To obtain a snapshot of the data using
mysqldump:
If you have not already locked the tables on the server to
prevent statements that update data from executing:
Start a session on the server by connecting to it with the
command-line client, and flush all tables and block write
statements by executing the
FLUSH TABLES WITH
READ LOCK statement:
mysql> FLUSH TABLES WITH READ LOCK;
Remember to use SHOW MASTER
STATUS and record the binary log details for use
when starting up the slave. The point in time of your
snapshot and the binary log position must match. See
Section 16.1.1.3, “Obtaining the Replication Master Binary Log Coordinates”.
In another session, use mysqldump to
create a dump either of all the databases you want to
replicate, or of selected individual databases. For example:
shell> mysqldump --all-databases --lock-all-tables >dbdump.db
An alternative to using a bare dump, is to use the
--master-data option, which automatically
appends the CHANGE MASTER TO
statement required on the slave to start the replication
process.
shell> mysqldump --all-databases --master-data >dbdump.db
In the client where you acquired the read lock, release the
lock:
mysql> UNLOCK TABLES;
Alternatively, exit the first session to release the read
lock.
When choosing databases to include in the dump, remember that
you will need to filter out databases on each slave that you do
not want to include in the replication process.
You will need either to copy the dump file to the slave, or to
use the file from the master when connecting remotely to the
slave to import the data.
16.1.1.6. Creating a Data Snapshot Using Raw Data Files
If your database is particularly large, copying the raw data
files may be more efficient than using
mysqldump and importing the file on each
slave.
However, using this method with tables in storage engines with
complex caching or logging algorithms may not give you a perfect
“in time” snapshot as cache information and logging
updates may not have been applied, even if you have acquired a
global read lock. How the storage engine responds to this
depends on its crash recovery abilities.
In addition, this method does not work reliably if the master
and slave have different values for
ft_stopword_file,
ft_min_word_len, or
ft_max_word_len and you are
copying tables having full-text indexes.
If you are using InnoDB tables, you can use
the InnoDB Hot Backup tool
to obtain a consistent snapshot. This tool records the log name
and offset corresponding to the snapshot to be later used on the
slave. Hot Backup is a nonfree (commercial)
tool that is not included in the standard MySQL distribution.
See the InnoDB Hot Backup
home page at
http://www.innodb.com/wp/products/hot-backup/ for
detailed information.
Otherwise, you can obtain a reliable binary snapshot of
InnoDB tables only after shutting down the
MySQL Server.
To create a raw data snapshot of MyISAM
tables you can use standard copy tools such as
cp or copy, a remote copy
tool such as scp or rsync,
an archiving tool such as zip or
tar, or a file system snapshot tool such as
dump, providing that your MySQL data files
exist on a single file system. If you are replicating only
certain databases then make sure you copy only those files that
related to those tables. (For InnoDB, all
tables in all databases are stored in the shared tablespace
files, unless you have the
innodb_file_per_table option enabled.)
You may want to specifically exclude the following files from
your archive:
To get the most consistent results with a raw data snapshot you
should shut down the master server during the process, as
follows:
Acquire a read lock and get the master's status. See
Section 16.1.1.3, “Obtaining the Replication Master Binary Log Coordinates”.
In a separate session, shut down the master server:
shell> mysqladmin shutdown
Make a copy of the MySQL data files. The following examples
show common ways to do this. You need to choose only one of
them:
shell> tar cf /tmp/db.tar ./data
shell> zip -r /tmp/db.zip ./data
shell> rsync --recursive ./data /tmp/dbdata
Restart the master server.
If you are not using InnoDB tables, you can
get a snapshot of the system from a master without shutting down
the server as described in the following steps:
Acquire a read lock and get the master's status. See
Section 16.1.1.3, “Obtaining the Replication Master Binary Log Coordinates”.
Make a copy of the MySQL data files. The following examples
show common ways to do this. You need to choose only one of
them:
shell> tar cf /tmp/db.tar ./data
shell> zip -r /tmp/db.zip ./data
shell> rsync --recursive ./data /tmp/dbdata
In the client where you acquired the read lock, release the
lock:
mysql> UNLOCK TABLES;
Once you have created the archive or copy of the database, you
will need to copy the files to each slave before starting the
slave replication process.
16.1.1.7. Setting Up Replication with New Master and Slaves
The easiest and most straightforward method for setting up
replication is to use new master and slave servers.
You can also use this method if you are setting up new servers
but have an existing dump of the databases from a different
server that you want to load into your replication
configuration. By loading the data into a new master, the data
will be automatically replicated to the slaves.
To set up replication between a new master and slave:
Perform the slave setup steps on each slave.
Because there is no data to load or exchange on a new server
configuration you do not need to copy or import any information.
If you are setting up a new replication environment using the
data from a different existing database server, you will now
need to run the dump file generated from that server on the new
master. The database updates will automatically be propagated to
the slaves:
shell> mysql -h master < fulldb.dump
16.1.1.8. Setting Up Replication with Existing Data
When setting up replication with existing data, you will need to
decide how best to get the data from the master to the slave
before starting the replication service.
The basic process for setting up replication with existing data
is as follows:
With the MySQL master running, create a user to be used by
the slave when connecting to the master during replication.
See Section 16.1.1.1, “Creating a User for Replication”.
If you have not already configured the
server-id and enabled binary
logging on the master server, you will need to shut it down
to configure these options. See
Section 16.1.1.2, “Setting the Replication Master Configuration”.
If you have to shut down your master server, this is a good
opportunity to take a snapshot of its databases. You should
obtain the master status (see
Section 16.1.1.3, “Obtaining the Replication Master Binary Log Coordinates”) before
taking down the master, updating the configuration and
taking a snapshot. For information on how to create a
snapshot using raw data files, see
Section 16.1.1.6, “Creating a Data Snapshot Using Raw Data Files”.
If your master server is already correctly configured,
obtain its status (see
Section 16.1.1.3, “Obtaining the Replication Master Binary Log Coordinates”) and then
use mysqldump to take a snapshot (see
Section 16.1.1.5, “Creating a Data Snapshot Using mysqldump”) or take a raw
snapshot of the live server using the guide in
Section 16.1.1.6, “Creating a Data Snapshot Using Raw Data Files”.
Update the configuration of the slave. See
Section 16.1.1.4, “Setting the Replication Slave Configuration”.
The next step depends on how you created the snapshot of
data on the master.
If you used mysqldump:
Start the slave, using the
--skip-slave-start option
so that replication does not start.
Import the dump file:
shell> mysql < fulldb.dump
If you created a snapshot using the raw data files:
Extract the data files into your slave data directory.
For example:
shell> tar xvf dbdump.tar
You may need to set permissions and ownership on the
files so that the slave server can access and modify
them.
Start the slave, using the
--skip-slave-start option
so that replication does not start.
Configure the slave with the replication coordinates from
the master. This tells the slave the binary log file and
position within the file where replication needs to start.
Also, configure the slave with the login credentials and
host name of the master. For more information on the
CHANGE MASTER TO statement
required, see Section 16.1.1.10, “Setting the Master Configuration on the Slave”.
Start the slave threads:
mysql> START SLAVE;
After you have performed this procedure, the slave should
connect to the master and catch up on any updates that have
occurred since the snapshot was taken.
If you have forgotten to set the
server-id option for the master,
slaves cannot connect to it.
If you have forgotten to set the
server-id option for the slave,
you get the following error in the slave's error log:
Warning: You should set server-id to a non-0 value if master_host
is set; we will force server id to 2, but this MySQL server will
not act as a slave.
You also find error messages in the slave's error log if it is
not able to replicate for any other reason.
Once a slave is replicating, you can find in its data directory
one file named master.info and another
named relay-log.info. The slave uses these
two files to keep track of how much of the master's binary log
it has processed. Do not remove or edit
these files unless you know exactly what you are doing and fully
understand the implications. Even in that case, it is preferred
that you use the CHANGE MASTER TO
statement to change replication parameters. The slave will use
the values specified in the statement to update the status files
automatically.
A single snapshot of the master suffices for multiple slaves. To
set up additional slaves, use the same master snapshot and
follow the slave portion of the procedure just described.
16.1.1.9. Introducing Additional Slaves to an Existing Replication Environment
To add another slave to an existing replication configuration,
you can do so without stopping the master. Instead, set up the
new slave by making a copy of an existing slave, except that you
configure the new slave with a different
server-id value.
To duplicate an existing slave:
Shut down the existing slave:
shell> mysqladmin shutdown
Copy the data directory from the existing slave to the new
slave. You can do this by creating an archive using
tar or WinZip, or by
performing a direct copy using a tool such as
cp or rsync. Ensure
that you also copy the log files and relay log files.
Note
A common problem that is encountered when adding new
replication slaves is that the new slave fails with a
series of warning and error messages like these:
071118 16:44:10 [Warning] Neither --relay-log nor --relay-log-index were used; so
replication may break when this MySQL server acts as a slave and has his hostname
changed!! Please use '--relay-log=new_slave_hostname-relay-bin' to avoid this problem.
071118 16:44:10 [ERROR] Failed to open the relay log './old_slave_hostname-relay-bin.003525'
(relay_log_pos 22940879)
071118 16:44:10 [ERROR] Could not find target log during relay log initialization
071118 16:44:10 [ERROR] Failed to initialize the master info structure
This is due to the fact that, if the
--relay-log option is not
specified, the relay log files contain the host name as
part of their file names. (This is also true of the relay
log index file if the
--relay-log-index option is
not used. See Section 16.1.2, “Replication and Binary Logging Options and Variables”, for
more information about these options.)
To avoid this problem, use the same value for
--relay-log on the new
slave that was used on the existing slave. (If this option
was not set explicitly on the existing slave, use
existing_slave_hostname-relay-bin--relay-log-index
option on the new slave to match what was used on the
existing slave. (If this option was not set explicitly on
the existing slave, use
existing_slave_hostname-relay-bin.index
If you have not already done so, issue a
STOP SLAVE on the new
slave.
If you have already started the existing slave
again, issue a STOP
SLAVE on the existing slave as well.
Copy the contents of the existing slave's relay
log index file into the new slave's relay log
index file, making sure to overwrite any content
already in the file.
Proceed with the remaining steps in this section.
Copy the master.info and
relay-log.info files from the existing
slave to the new slave if they were not located in the data
directory. These files hold the current log coordinates for
the master's binary log and the slave's relay log.
Start the existing slave.
On the new slave, edit the configuration and give the new
slave a unique server-id not
used by the master or any of the existing slaves.
Start the new slave. The slave will use the information in
its master.info file to start the
replication process.
16.1.1.10. Setting the Master Configuration on the Slave
To set up the slave to communicate with the master for
replication, you must tell the slave the necessary connection
information. To do this, execute the following statement on the
slave, replacing the option values with the actual values
relevant to your system:
mysql> CHANGE MASTER TO
-> MASTER_HOST='master_host_name',
-> MASTER_USER='replication_user_name',
-> MASTER_PASSWORD='replication_password',
-> MASTER_LOG_FILE='recorded_log_file_name',
-> MASTER_LOG_POS=recorded_log_position;
Note
Replication cannot use Unix socket files. You must be able to
connect to the master MySQL server using TCP/IP.
The CHANGE MASTER TO statement
has other options as well. For example, it is possible to set up
secure replication using SSL. For a full list of options, and
information about the maximum allowable length for the
string-valued options, see Section 12.6.2.1, “CHANGE MASTER TO Syntax”.
16.1.2. Replication and Binary Logging Options and Variables
The next few sections contain information about
mysqld options and server variables that are used
in replication and for controlling the binary log. Options and
variables for use on replication masters and replication slaves are
covered separately, as are options and variables relating to binary
logging. A set of quick-reference tables providing basic information
about these options and variables is also included (in the next
section following this one).
Of particular importance is the
--server-id option.
This option is common to both master and slave replication servers,
and is used in replication to enable master and slave servers to
identify themselves uniquely. For additional information, see
Section 16.1.2.2, “Replication Master Options and Variables”, and
Section 16.1.2.3, “Replication Slave Options and Variables”.
On the master and each slave, you must use the
--server-id option to establish a
unique replication ID in the range from 1 to
232 – 1. “Unique”,
means that each ID must be different from every other ID in use by
any other replication master or slave. Example:
server-id=3.
If you omit --server-id, the default
ID is 0, in which case a master refuses connections from all slaves,
and a slave refuses to connect to a master. For more information,
see Section 16.1.1.4, “Setting the Replication Slave Configuration”.
16.1.2.1. Replication and Binary Logging Option and Variable Reference
The following tables list basic information about the MySQL
command-line options and system variables applicable to
replication and the binary log.
Table 16.1. Replication Option/Variable Summary
Section 16.1.2.2, “Replication Master Options and Variables”, provides more
detailed information about options and variables relating to
replication master servers. For more information about options and
variables relating to replication slaves
Section 16.1.2.3, “Replication Slave Options and Variables”.
Table 16.2. Binary Logging Option/Variable Summary
Section 16.1.2.4, “Binary Log Options and Variables”, provides more
detailed information about options and variables relating to
binary logging. For additional general information about the
binary log, see Section 5.2.3, “The Binary Log”.
For a table showing all command-line options,
system and status variables used with mysqld,
see Section 5.1.1, “Server Option and Variable Reference”.
16.1.2.2. Replication Master Options and Variables
This section describes the server options and system variables
that you can use on replication master servers. You can specify
the options either on the
command line or in an
option file. You can specify
system variable values using
SET.
On the master and each slave, you must use the
server-id option to establish a
unique replication ID. For each server, you should pick a unique
positive integer in the range from 1 to
232 – 1, and each ID must be
different from every other ID in use by any other replication
master or slave. Example: server-id=3.
For options used on the master for controlling binary logging, see
Section 16.1.2.4, “Binary Log Options and Variables”.
auto_increment_increment
auto_increment_increment and
auto_increment_offset are
intended for use with master-to-master replication, and can be
used to control the operation of
AUTO_INCREMENT columns. Both variables have
global and session values, and each can assume an integer
value between 1 and 65,535 inclusive. Setting the value of
either of these two variables to 0 causes its value to be set
to 1 instead. Attempting to set the value of either of these
two variables to an integer greater than 65,535 or less than 0
causes its value to be set to 65,535 instead. Attempting to
set the value of
auto_increment_increment or
auto_increment_offset to a
noninteger value gives rise to an error, and the actual value
of the variable remains unchanged.
These two variables affect AUTO_INCREMENT
column behavior as follows:
auto_increment_increment
controls the interval between successive column values.
For example:
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
+--------------------------+-------+
2 rows in set (0.00 sec)
mysql> CREATE TABLE autoinc1
-> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
Query OK, 0 rows affected (0.04 sec)
mysql> SET @@auto_increment_increment=10;
Query OK, 0 rows affected (0.00 sec)
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 10 |
| auto_increment_offset | 1 |
+--------------------------+-------+
2 rows in set (0.01 sec)
mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
| 1 |
| 11 |
| 21 |
| 31 |
+-----+
4 rows in set (0.00 sec)
auto_increment_offset
determines the starting point for the
AUTO_INCREMENT column value. Consider
the following, assuming that these statements are executed
during the same session as the example given in the
description for
auto_increment_increment:
mysql> SET @@auto_increment_offset=5;
Query OK, 0 rows affected (0.00 sec)
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 10 |
| auto_increment_offset | 5 |
+--------------------------+-------+
2 rows in set (0.00 sec)
mysql> CREATE TABLE autoinc2
-> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
Query OK, 0 rows affected (0.06 sec)
mysql> INSERT INTO autoinc2 VALUES (NULL), (NULL), (NULL), (NULL);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> SELECT col FROM autoinc2;
+-----+
| col |
+-----+
| 5 |
| 15 |
| 25 |
| 35 |
+-----+
4 rows in set (0.02 sec)
If the value of
auto_increment_offset is
greater than that of
auto_increment_increment,
the value of
auto_increment_offset is
ignored.
Should one or both of these variables be changed and then new
rows inserted into a table containing an
AUTO_INCREMENT column, the results may seem
counterintuitive because the series of
AUTO_INCREMENT values is calculated without
regard to any values already present in the column, and the
next value inserted is the least value in the series that is
greater than the maximum existing value in the
AUTO_INCREMENT column. In other words, the
series is calculated like so:
auto_increment_offset + N
? auto_increment_increment
where N is a positive integer value
in the series [1, 2, 3, ...]. For example:
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 10 |
| auto_increment_offset | 5 |
+--------------------------+-------+
2 rows in set (0.00 sec)
mysql> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
| 1 |
| 11 |
| 21 |
| 31 |
+-----+
4 rows in set (0.00 sec)
mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
| 1 |
| 11 |
| 21 |
| 31 |
| 35 |
| 45 |
| 55 |
| 65 |
+-----+
8 rows in set (0.00 sec)
The values shown for
auto_increment_increment and
auto_increment_offset
generate the series 5 + N ?
10, that is, [5, 15, 25, 35, 45, ...]. The greatest value
present in the col column prior to the
INSERT is 31, and the next
available value in the AUTO_INCREMENT
series is 35, so the inserted values for
col begin at that point and the results are
as shown for the SELECT query.
It is not possible to confine the effects of these two
variables to a single table, and thus they do not take the
place of the sequences offered by some other database
management systems; these variables control the behavior of
all AUTO_INCREMENT columns in
all tables on the MySQL server. If the
global value of either variable is set, its effects persist
until the global value is changed or overridden by setting the
session value, or until mysqld is
restarted. If the local value is set, the new value affects
AUTO_INCREMENT columns for all tables into
which new rows are inserted by the current user for the
duration of the session, unless the values are changed during
that session.
The auto_increment_increment
variable was added in MySQL 5.0.2. Its default value is 1. See
Section 16.3.1.1, “Replication and AUTO_INCREMENT”.
auto_increment_increment is
supported for use with NDB tables
beginning with MySQL 5.0.46. Previously, setting it when using
MySQL Cluster tables produced unpredictable results.
auto_increment_offset
This variable was introduced in MySQL 5.0.2. Its default value
is 1. For particulars, see the description for
auto_increment_increment.
auto_increment_offset is
supported for use with NDB tables
beginning with MySQL 5.0.46. Previously, setting it when using
MySQL Cluster tables produced unpredictable results.
16.1.2.3. Replication Slave Options and Variables
This section describes the server options and system variables
that you can use on slave replication servers. You can specify the
options either on the command
line or in an option
file. Many of the options can be reset while the server is
running by using the CHANGE MASTER
TO statement. You can specify system variable values
using SET.
Server ID.
On the master and each slave, you must use the
server-id option to establish a
unique replication ID in the range from 1 to
232 – 1. “Unique”,
means that each ID must be different from every other ID in use
by any other replication master or slave. Example:
server-id=3.
Some slave server replication options are handled in a special
way, in the sense that each is ignored if a
master.info file exists when the slave starts
and contains a value for the option. The following options are
handled this way:
The master.info file format included values
corresponding to the SSL options, and includes as its first line
the number of lines in the file. (See
Section 16.4.2, “Replication Relay and Status Files”.) If you upgrade an older server
(that is, a server running a MySQL version prior to 4.1.1), the
new server upgrades the master.info file to
the new format automatically when it starts. However, if you
downgrade a newer server to a version older than 4.1.1, you should
manually remove the first line before starting the older server
for the first time. Note that, in this case, the downgraded server
can no longer use an SSL connection to communicate with the
master.
If no master.info file exists when the slave
server starts, it uses the values for those options that are
specified in option files or on the command line. This occurs when
you start the server as a replication slave for the very first
time, or when you have run RESET
SLAVE and then have shut down and restarted the slave.
If the master.info file exists when the slave
server starts, the server uses its contents and ignores any
options that correspond to the values listed in the file. Thus, if
you start the slave server with different values of the startup
options that correspond to values in the
master.info file, the different values have
no effect because the server continues to use the
master.info file. To use different values,
the preferred method is to use the CHANGE
MASTER TO statement to reset the values while the slave
is running. Alternatively, you can stop the server, remove the
master.info file, and restart the server with
different option values.
Suppose that you specify this option in your
my.cnf file:
[mysqld]
master-host=some_host
The first time you start the server as a replication slave, it
reads and uses that option from the my.cnf
file. The server then records the value in the
master.info file. The next time you start the
server, it reads the master host value from the
master.info file only and ignores the value
in the option file. If you modify the my.cnf
file to specify a different master host of
some_other_host, the change still has
no effect. You should use CHANGE MASTER
TO instead.
MySQL Enterprise
For expert advice regarding slave startup options, subscribe to
the MySQL Enterprise Monitor. For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
Because the server gives an existing
master.info file precedence over the startup
options just described, you might prefer not to use startup
options for these values at all, and instead specify them by using
the CHANGE MASTER TO statement. See
Section 12.6.2.1, “CHANGE MASTER TO Syntax”.
This example shows a more extensive use of startup options to
configure a slave server:
[mysqld]
server-id=2
master-host=db-master.mycompany.com
master-port=3306
master-user=pertinax
master-password=freitag
master-connect-retry=60
report-host=db-slave.mycompany.com
Startup options for replication slaves.
The following list describes startup options for controlling
replication slave servers. Many of these options can be reset
while the server is running by using the
CHANGE MASTER TO statement.
Others, such as the --replicate-* options, can
be set only when the slave server starts. Replication-related
system variables are discussed later in this section.
--abort-slave-event-count
When this option is set to some positive integer
value other than 0 (the default) it
affects replication behavior as follows: After the slave SQL
thread has started, value log
events are allowed to be executed; after that, the slave SQL
thread does not receive any more events, just as if the
network connection from the master were cut. The slave thread
continues to run, and the output from
SHOW SLAVE STATUS displays
Yes in both the
Slave_IO_Running and the
Slave_SQL_Running columns, but no further
events are read from the relay log.
This option is used internally by the MySQL test suite for
replication testing and debugging. It is not intended for use
in a production setting.
--disconnect-slave-event-count
This option is used internally by the MySQL test suite for
replication testing and debugging.
--log-slave-updates
Normally, a slave does not log to its own binary log any
updates that are received from a master server. This option
tells the slave to log the updates performed by its SQL thread
to its own binary log. For this option to have any effect, the
slave must also be started with the
--log-bin option to enable
binary logging.
--log-slave-updates is used
when you want to chain replication servers. For example, you
might want to set up replication servers using this
arrangement:
A -> B -> C
Here, A serves as the master for the slave
B, and B serves as the
master for the slave C. For this to work,
B must be both a master
and a slave. You must start both
A and B with
--log-bin to enable binary
logging, and B with the
--log-slave-updates option so
that updates received from A are logged by
B to its binary log.
--log-warnings[=level]
This option causes a server to print more messages to the
error log about what it is doing. With respect to replication,
the server generates warnings that it succeeded in
reconnecting after a network/connection failure, and informs
you as to how each slave thread started. This option is
enabled by default; to disable it, use
--skip-log-warnings.
Aborted connections are not logged to the error log unless the
value is greater than 1.
Note that the effects of this option are not limited to
replication. It produces warnings across a spectrum of server
activities.
--master-connect-retry=seconds
The number of seconds that the slave thread sleeps before
trying to reconnect to the master in case the master goes down
or the connection is lost. The value in the
master.info file takes precedence if it
can be read. If not set, the default is 60. Connection retries
are not invoked until the slave times out reading data from
the master according to the value of
--slave-net-timeout. The number
of reconnection attempts is limited by the
--master-retry-count option.
--master-host=host_name
The host name or IP number of the master replication server.
The value in master.info takes precedence
if it can be read. If no master host is specified, the slave
thread does not start.
--master-info-file=file_name
The name to use for the file in which the slave records
information about the master. The default name is
master.info in the data directory.
--master-password=password
The password of the account that the slave thread uses for
authentication when it connects to the master. The value in
the master.info file takes precedence if
it can be read. If not set, an empty password is assumed.
--master-port=port_number
The TCP/IP port number that the master is listening on. The
value in the master.info file takes
precedence if it can be read. If not set, the compiled-in
setting is assumed (normally 3306).
--master-retry-count=count
The number of times that the slave tries to connect to the
master before giving up. Reconnects are attempted at intervals
set by the CHANGE MASTER TO
statement or
--master-connect-retry option
and reconnects are triggered when data reads by the slave time
out according to the
--slave-net-timeout option. The
default value is 86400.
--master-ssl,
--master-ssl-ca=file_name,
--master-ssl-capath=directory_name,
--master-ssl-cert=file_name,
--master-ssl-cipher=cipher_list,
--master-ssl-key=file_name
These options are used for setting up a secure replication
connection to the master server using SSL. Their meanings are
the same as the corresponding
--ssl,
--ssl-ca,
--ssl-capath,
--ssl-cert,
--ssl-cipher,
--ssl-key options that are
described in Section 5.5.7.3, “SSL Command Options”. The values in the
master.info file take precedence if they
can be read.
--master-user=user_name
The user name of the account that the slave thread uses for
authentication when it connects to the master. This account
must have the REPLICATION SLAVE
privilege. FILE privilege
instead.) The value in the master.info
file takes precedence if it can be read. If the master user
name is not set, the name test is assumed.
--read-only
Cause the slave to allow no updates except from slave threads
or from users having the SUPER
privilege. On a slave server, this can be useful to ensure
that the slave accepts updates only from its master server and
not from clients. This variable does not apply to
TEMPORARY tables.
--relay-log=file_name
The basename for the relay log. The default basename is
host_name-relay-bin
Due to the manner in which MySQL parses server options, if you
specify this option, you must supply a value; the
default basename is used only if the option is not actually
specified. If you use the
--relay-log option without
specifying a value, unexpected behavior is likely to result;
this behavior depends on the other options used, the order in
which they are specified, and whether they are specified on
the command line or in an option file. For more information
about how MySQL handles server options, see
Section 4.2.3, “Specifying Program Options”.
If you specify this option, the value specified is also used
as the basename for the relay log index file. You can override
this behavior by specifying a different relay log index file
basename using the
--relay-log-index option.
You may find the --relay-log
option useful in performing the following tasks:
Creating relay logs whose names are independent of host
names.
If you need to put the relay logs in some area other than
the data directory because your relay logs tend to be very
large and you do not want to decrease
max_relay_log_size.
To increase speed by using load-balancing between disks.
--relay-log-index=file_name
The name to use for the relay log index file. The default name
is
host_name-relay-bin.indexhost_name is the name of the slave
server.
Due to the manner in which MySQL parses server options, if you
specify this option, you must supply a value; the
default basename is used only if the option is not actually
specified. If you use the
--relay-log-index option
without specifying a value, unexpected behavior is likely to
result; this behavior depends on the other options used, the
order in which they are specified, and whether they are
specified on the command line or in an option file. For more
information about how MySQL handles server options, see
Section 4.2.3, “Specifying Program Options”.
If you specify this option, the value specified is also used
as the basename for the relay logs. You can override this
behavior by specifying a different relay log file basename
using the --relay-log option.
--relay-log-info-file=file_name
The name to use for the file in which the slave records
information about the relay logs. The default name is
relay-log.info in the data directory.
--relay-log-purge={0|1}
Disable or enable automatic purging of relay logs as soon as
they are no longer needed. The default value is 1 (enabled).
This is a global variable that can be changed dynamically with
SET GLOBAL relay_log_purge =
N.
--max-relay-log-size=size
The size at which the server rotates relay log files
automatically. For more information, see
Section 16.4.2, “Replication Relay and Status Files”. Default is 1GB.
--relay-log-space-limit=size
This option places an upper limit on the total size in bytes
of all relay logs on the slave. A value of 0 means “no
limit.” This is useful for a slave server host that has
limited disk space. When the limit is reached, the I/O thread
stops reading binary log events from the master server until
the SQL thread has caught up and deleted some unused relay
logs. Note that this limit is not absolute: There are cases
where the SQL thread needs more events before it can delete
relay logs. In that case, the I/O thread exceeds the limit
until it becomes possible for the SQL thread to delete some
relay logs because not doing so would cause a deadlock. You
should not set
--relay-log-space-limit to less
than twice the value of
--max-relay-log-size (or
--max-binlog-size if
--max-relay-log-size is 0). In
that case, there is a chance that the I/O thread waits for
free space because
--relay-log-space-limit is
exceeded, but the SQL thread has no relay log to purge and is
unable to satisfy the I/O thread. This forces the I/O thread
to ignore
--relay-log-space-limit
temporarily.
--replicate-do-db=db_name
Tell the slave to restrict replication to statements where the
default database (that is, the one selected by
USE) is
db_name. To specify more than one
database, use this option multiple times, once for each
database. Note that this does not replicate cross-database
statements such as UPDATE
some_db.some_table SET
foo='bar' while having selected a different database
or no database.
Warning
To specify multiple databases you must
use multiple instances of this option. Because database
names can contain commas, if you supply a comma separated
list then the list will be treated as the name of a single
database.
An example of what does not work as you might expect: If the
slave is started with
--replicate-do-db=sales and you
issue the following statements on the master, the
UPDATE statement is
not replicated:
USE prices;
UPDATE sales.january SET amount=amount+1000;
The main reason for this “check just the default
database” behavior is that it is difficult from the
statement alone to know whether it should be replicated (for
example, if you are using multiple-table
DELETE or multiple-table
UPDATE statements that go
across multiple databases). It is also faster to check only
the default database rather than all databases if there is no
need.
If you need cross-database updates to work, use
--replicate-wild-do-table=db_name.%
instead. See Section 16.4.3, “How Servers Evaluate Replication Filtering Rules”.
Beginning with MySQL 5.0.84, this option has no effect on
BEGIN,
COMMIT, or
ROLLBACK
statements. (Bug#43263)
--replicate-ignore-db=db_name
Tells the slave to not replicate any statement where the
default database (that is, the one selected by
USE) is
db_name. To specify more than one
database to ignore, use this option multiple times, once for
each database. You should not use this option if you are using
cross-database updates and you do not want these updates to be
replicated. See Section 16.4.3, “How Servers Evaluate Replication Filtering Rules”.
An example of what does not work as you might expect: If the
slave is started with
--replicate-ignore-db=sales and
you issue the following statements on the master, the
UPDATE statement
is replicated:
USE prices;
UPDATE sales.january SET amount=amount+1000;
Note
In the preceding example the statement is replicated because
--replicate-ignore-db only
applies to the default database (set through the
USE statement). Because the
sales database was specified explicitly
in the statement, the statement has not been filtered.
If you need cross-database updates to work, use
--replicate-wild-ignore-table=db_name.%
instead. See Section 16.4.3, “How Servers Evaluate Replication Filtering Rules”.
Beginning with MySQL 5.0.84, this option has no effect on
BEGIN,
COMMIT, or
ROLLBACK
statements. (Bug#43263)
--replicate-do-table=db_name.tbl_name
Tells the slave thread to restrict replication to the
specified table. To specify more than one table, use this
option multiple times, once for each table. This works for
both cross-database updates and default database updates, in
contrast to --replicate-do-db.
See Section 16.4.3, “How Servers Evaluate Replication Filtering Rules”.
This option affects only statements that apply to tables. It
does not affect statements that apply only to other database
objects, such as stored routines. To filter statements
operating on stored routines, use one or more of the
--replicate-*-db options.
--replicate-ignore-table=db_name.tbl_name
Tells the slave thread to not replicate any statement that
updates the specified table, even if any other tables might be
updated by the same statement. To specify more than one table
to ignore, use this option multiple times, once for each
table. This works for cross-database updates, in contrast to
--replicate-ignore-db. See
Section 16.4.3, “How Servers Evaluate Replication Filtering Rules”.
This option affects only statements that apply to tables. It
does not affect statements that apply only to other database
objects, such as stored routines. To filter statements
operating on stored routines, use one or more of the
--replicate-*-db options.
--replicate-rewrite-db=from_name->to_name
Tells the slave to translate the default database (that is,
the one selected by USE) to
to_name if it was
from_name on the master. Only
statements involving tables are affected (not statements such
as CREATE DATABASE,
DROP DATABASE, and
ALTER DATABASE), and only if
from_name is the default database
on the master. This does not work for cross-database updates.
To specify multiple rewrites, use this option multiple times.
The server uses the first one with a
from_name value that matches. The
database name translation is done before
the --replicate-* rules are tested.
If you use this option on the command line and the
“>” character is special to
your command interpreter, quote the option value. For example:
shell> mysqld --replicate-rewrite-db="olddb->newdb"
--replicate-same-server-id
To be used on slave servers. Usually you should use the
default setting of 0, to prevent infinite loops caused by
circular replication. If set to 1, the slave does not skip
events having its own server ID. Normally, this is useful only
in rare configurations. Cannot be set to 1 if
--log-slave-updates is used. By
default, the slave I/O thread does not write binary log events
to the relay log if they have the slave's server ID (this
optimization helps save disk usage). If you want to use
--replicate-same-server-id, be
sure to start the slave with this option before you make the
slave read its own events that you want the slave SQL thread
to execute.
--replicate-wild-do-table=db_name.tbl_name
Tells the slave thread to restrict replication to statements
where any of the updated tables match the specified database
and table name patterns. Patterns can contain the
“%” and
“_” wildcard characters, which
have the same meaning as for the
LIKE pattern-matching operator.
To specify more than one table, use this option multiple
times, once for each table. This works for cross-database
updates. See Section 16.4.3, “How Servers Evaluate Replication Filtering Rules”.
This option applies to tables, views, and triggers. It does
not apply to stored functions and procedures. To filter
statements operating on the latter objects, use one or more of
the --replicate-*-db options.
Example:
--replicate-wild-do-table=foo%.bar%
replicates only updates that use a table where the database
name starts with foo and the table name
starts with bar.
If the table name pattern is %, it matches
any table name and the option also applies to database-level
statements (CREATE DATABASE,
DROP DATABASE, and
ALTER DATABASE). For example,
if you use
--replicate-wild-do-table=foo%.%,
database-level statements are replicated if the database name
matches the pattern foo%.
To include literal wildcard characters in the database or
table name patterns, escape them with a backslash. For
example, to replicate all tables of a database that is named
my_own%db, but not replicate tables from
the my1ownAABCdb database, you should
escape the “_” and
“%” characters like this:
--replicate-wild-do-table=my\_own\%db.
If you use the option on the command line, you might need to
double the backslashes or quote the option value, depending on
your command interpreter. For example, with the
bash shell, you would need to type
--replicate-wild-do-table=my\\_own\\%db.
--replicate-wild-ignore-table=db_name.tbl_name
Tells the slave thread not to replicate a statement where any
table matches the given wildcard pattern. To specify more than
one table to ignore, use this option multiple times, once for
each table. This works for cross-database updates. See
Section 16.4.3, “How Servers Evaluate Replication Filtering Rules”.
Example:
--replicate-wild-ignore-table=foo%.bar%
does not replicate updates that use a table where the database
name starts with foo and the table name
starts with bar.
For information about how matching works, see the description
of the
--replicate-wild-do-table
option. The rules for including literal wildcard characters in
the option value are the same as for
--replicate-wild-ignore-table
as well.
--report-host=host_name
The host name or IP number of the slave to be reported to the
master during slave registration. This value appears in the
output of SHOW SLAVE HOSTS on
the master server. Leave the value unset if you do not want
the slave to register itself with the master. Note that it is
not sufficient for the master to simply read the IP number of
the slave from the TCP/IP socket after the slave connects. Due
to NAT and other routing issues, that IP may not be valid for
connecting to the slave from the master or other hosts.
--report-password=password
The account password of the slave to be reported to the master
during slave registration. This value appears in the output of
SHOW SLAVE HOSTS on the master
server if the
--show-slave-auth-info option
is given.
--report-port=slave_port_num
The TCP/IP port number for connecting to the slave, to be
reported to the master during slave registration. Set this
only if the slave is listening on a nondefault port or if you
have a special tunnel from the master or other clients to the
slave. If you are not sure, do not use this option.
--report-user=user_name
The account user name of the slave to be reported to the
master during slave registration. This value appears in the
output of SHOW SLAVE HOSTS on
the master server if the
--show-slave-auth-info option
is given.
--show-slave-auth-info
Display slave user names and passwords in the output of
SHOW SLAVE HOSTS on the master
server for slaves started with the
--report-user and
--report-password options.
--skip-slave-start
Tells the slave server not to start the slave threads when the
server starts. To start the threads later, use a
START SLAVE statement.
--slave_compressed_protocol={0|1}
If this option is set to 1, use compression for the
slave/master protocol if both the slave and the master support
it. The default is 0 (no compression).
--slave-load-tmpdir=file_name
The name of the directory where the slave creates temporary
files. This option is by default equal to the value of the
tmpdir system variable. When the slave SQL
thread replicates a
LOAD DATA
INFILE statement, it extracts the file to be loaded
from the relay log into temporary files, and then loads these
into the table. If the file loaded on the master is huge, the
temporary files on the slave are huge, too. Therefore, it
might be advisable to use this option to tell the slave to put
temporary files in a directory located in some file system
that has a lot of available space. In that case, the relay
logs are huge as well, so you might also want to use the
--relay-log option to place the
relay logs in that file system.
The directory specified by this option should be located in a
disk-based file system (not a memory-based file system)
because the temporary files used to replicate
LOAD DATA
INFILE must survive machine restarts. The directory
also should not be one that is cleared by the operating system
during the system startup process.
--slave-net-timeout=seconds
The number of seconds to wait for more data from the master
before the slave considers the connection broken, aborts the
read, and tries to reconnect. The first retry occurs
immediately after the timeout. The interval between retries is
controlled by the CHANGE MASTER
TO statement or
--master-connect-retry option
and the number of reconnection attempts is limited by the
--master-retry-count option.
The default is 3600 seconds (one hour).
--slave-skip-errors=[err_code1,err_code2,...|all]
Normally, replication stops when an error occurs on the slave.
This gives you the opportunity to resolve the inconsistency in
the data manually. This option tells the slave SQL thread to
continue replication when a statement returns any of the
errors listed in the option value.
Do not use this option unless you fully understand why you are
getting errors. If there are no bugs in your replication setup
and client programs, and no bugs in MySQL itself, an error
that stops replication should never occur. Indiscriminate use
of this option results in slaves becoming hopelessly out of
synchrony with the master, with you having no idea why this
has occurred.
For error codes, you should use the numbers provided by the
error message in your slave error log and in the output of
SHOW SLAVE STATUS.
Appendix B, Errors, Error Codes, and Common Problems, lists server error codes.
You can also (but should not) use the very nonrecommended
value of all to cause the slave to ignore
all error messages and keeps going regardless of what happens.
Needless to say, if you use all, there are
no guarantees regarding the integrity of your data. Please do
not complain (or file bug reports) in this case if the slave's
data is not anywhere close to what it is on the master.
You have been warned.
Examples:
--slave-skip-errors=1062,1053
--slave-skip-errors=all
System variables used on replication slaves.
The following list describes system variables for controlling
replication slave servers. They can be set at server startup and
some of them can be changed at runtime using
SET.
Server options used with replication slaves are listed earlier
in this section.
init_slave
This variable is similar to
init_connect, but is a string
to be executed by a slave server each time the SQL thread
starts. The format of the string is the same as for the
init_connect variable.
rpl_recovery_rank
This variable is unused.
slave_compressed_protocol
Whether to use compression of the slave/master protocol if
both the slave and the master support it.
slave_load_tmpdir
The name of the directory where the slave creates temporary
files for replicating
LOAD DATA
INFILE statements.
slave_net_timeout
The number of seconds to wait for more data from a
master/slave connection before aborting the read. This timeout
applies only to TCP/IP connections, not to connections made
via Unix socket files, named pipes, or shared memory.
slave_skip_errors
Normally, replication stops when an error occurs on the slave.
This gives you the opportunity to resolve the inconsistency in
the data manually. This variable tells the slave SQL thread to
continue replication when a statement returns any of the
errors listed in the variable value.
slave_transaction_retries
If a replication slave SQL thread fails to execute a
transaction because of an InnoDB
deadlock or because the transaction's execution time
exceeded InnoDB's
innodb_lock_wait_timeout or
NDBCLUSTER's
TransactionDeadlockDetectionTimeout or
TransactionInactiveTimeout, it
automatically retries
slave_transaction_retries
times before stopping with an error. Prior to MySQL 5.0.3, the
default is 0, and you must explicitly set the value greater
than 0 to enable the “retry” behavior. In MySQL
5.0.3 or newer, the default is 10.
sql_slave_skip_counter
The number of events from the master that a slave server
should skip.
Important
If skipping the number of events specified by setting this
variable would cause the slave to begin in the middle of an
event group, the slave continues to skip until it finds the
beginning of the next event group and begins from that
point. For more information, see
Section 12.6.2.6, “SET GLOBAL sql_slave_skip_counter Syntax”.
16.1.2.4. Binary Log Options and Variables
You can use the mysqld options and system
variables that are described in this section to affect the
operation of the binary log as well as to control which statements
are written to the binary log. For additional information about
the binary log, see Section 5.2.3, “The Binary Log”. For additional
information about using MySQL server options and system variables,
see Section 5.1.2, “Server Command Options”, and
Section 5.1.3, “Server System Variables”.
Startup options used with binary logging.
The following list describes startup options for enabling and
configuring the binary log. System variables used with binary
logging are discussed later in this section.
--log-bin[=base_name]
Enable binary logging. The server logs all statements that
change data to the binary log, which is used for backup and
replication. See Section 5.2.3, “The Binary Log”.
The option value, if given, is the basename for the log
sequence. The server creates binary log files in sequence by
adding a numeric suffix to the basename. It is recommended
that you specify a basename (see
Additional Known Issues, for the reason).
Otherwise, MySQL uses
host_name-bin
--log-bin-index[=file_name]
The index file for binary log file names. See
Section 5.2.3, “The Binary Log”. If you omit the file name, and
if you did not specify one with
--log-bin, MySQL uses
host_name-bin.index
--log-bin-trust-function-creators[={0|1}]
This option sets the corresponding
log_bin_trust_function_creators
system variable. If no argument is given, the option sets the
variable to 1.
log_bin_trust_function_creators
affects how MySQL enforces restrictions on stored function and
trigger creation. See
Section 18.5, “Binary Logging of Stored Programs”.
This option was added in MySQL 5.0.16.
--log-bin-trust-routine-creators[={0|1}]
This is the old name for
--log-bin-trust-function-creators.
Before MySQL 5.0.16, it also applies to stored procedures, not
just stored functions and sets the
log_bin_trust_routine_creators system
variable. As of 5.0.16, this option is deprecated. It is
recognized for backward compatibility but its use results in a
warning.
This option was added in MySQL 5.0.6.
Statement selection options.
The options in the following list affect which statements are
written to the binary log, and thus sent by a replication master
server to its slaves. There are also options for slave servers
that control which statements received from the master should be
executed or ignored. For details, see
Section 16.1.2.3, “Replication Slave Options and Variables”.
--binlog-do-db=db_name
This option affects binary logging in a manner similar to the
way that --replicate-do-db
affects replication.
Tell the server to restrict binary logging to updates for
which the default database is
db_name (that is, the database
selected by USE). All other
databases that are not explicitly mentioned are ignored. If
you use this option, you should ensure that you do updates
only in the default database.
There is an exception to this for CREATE
DATABASE, ALTER
DATABASE, and DROP
DATABASE statements. The server uses the database
named in the statement (not the default database) to decide
whether it should log the statement.
An example of what does not work as you might expect: If the
server is started with binlog-do-db=sales,
and you run USE prices; UPDATE sales.january SET
amount=amount+1000;, this statement is
not written into the binary log.
Important
To log multiple databases, use this option multiple times,
specifying the option once for each database to be logged.
Because database names can contain commas, the list will be
treated as the name of a single database if you supply a
comma-separated list.
--binlog-ignore-db=db_name
This option affects binary logging in a manner similar to the
way that --replicate-ignore-db
affects replication.
Tell the server to suppress binary logging of updates for
which the default database is
db_name (that is, the database
selected by USE). If you use
this option, you should ensure that you do updates only in the
default database.
As with the --binlog-do-db
option, there is an exception for the
CREATE DATABASE,
ALTER DATABASE, and
DROP DATABASE statements. The
server uses the database named in the statement (not the
default database) to decide whether it should log the
statement.
An example of what does not work as you might expect: If the
server is started with
binlog-ignore-db=sales, and you run
USE prices; UPDATE sales.january SET amount = amount
+ 1000;, this statement is
written into the binary log.
Important
To ignore multiple databases, use this option multiple
times, specifying the option once for each database to be
ignored. Because database names can contain commas, the list
will be treated as the name of a single database if you
supply a comma-separated list.
Testing and debugging options.
The following binary log options are used in replication testing
and debugging. They are not intended for use in normal
operations.
System variables used with the binary log.
The following list describes system variables for controlling
binary logging. They can be set at server startup and some of
them can be changed at runtime using
SET.
Server options used to control binary logging are listed earlier
in this section.
log_slave_updates
Whether updates received by a slave server from a master
server should be logged to the slave's own binary log. Binary
logging must be enabled on the slave for this variable to have
any effect. See Section 16.1.2.3, “Replication Slave Options and Variables”.
max_binlog_cache_size
If a multiple-statement transaction requires more than this
many bytes of memory, the server generates a
Multi-statement transaction required more than
'max_binlog_cache_size' bytes of storage error.
The minimum value is 4096; the maximum and default values are
4GB on 32-bit platforms and 16 PB (petabytes) on 64-bit
platforms.
max_binlog_size
If a write to the binary log causes the current log file size
to exceed the value of this variable, the server rotates the
binary logs (closes the current file and opens the next one).
The minimum value is 4096 bytes. The maximum and default value
is 1GB.
A transaction is written in one chunk to the binary log, so it
is never split between several binary logs. Therefore, if you
have big transactions, you might see binary log files larger
than max_binlog_size.
If max_relay_log_size is 0,
the value of max_binlog_size
applies to relay logs as well.
sync_binlog
If the value of this variable is greater than 0, the MySQL
server synchronizes its binary log to disk (using
fdatasync()) after every
sync_binlog writes to the
binary log. There is one write to the binary log per statement
if autocommit is enabled, and one write per transaction
otherwise. The default value of
sync_binlog is 0, which does
no synchronizing to disk. A value of 1 is the safest choice
because in the event of a crash you lose at most one statement
or transaction from the binary log. However, it is also the
slowest choice (unless the disk has a battery-backed cache,
which makes synchronization very fast).
If the value of sync_binlog
is 0 (the default), no extra flushing is done. The server
relies on the operating system to flush the file contents
occasionally as for any other file.
16.1.3. Common Replication Administration Tasks
Once replication has been started it should execute without
requiring much regular administration. Depending on your
replication environment, you will want to check the replication
status of each slave periodically, daily, or even more frequently.
MySQL Enterprise
For regular reports regarding the status of your slaves,
subscribe to the MySQL Enterprise Monitor. For more information,
see http://www.mysql.com/products/enterprise/advisors.html.
16.1.3.1. Checking Replication Status
The most common task when managing a replication process is to
ensure that replication is taking place and that there have been
no errors between the slave and the master. The primary
statement for this is SHOW SLAVE
STATUS, which you must execute on each slave:
mysql> SHOW SLAVE STATUS\G
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: master1
Master_User: root
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: mysql-bin.000004
Read_Master_Log_Pos: 931
Relay_Log_File: slave1-relay-bin.000056
Relay_Log_Pos: 950
Relay_Master_Log_File: mysql-bin.000004
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Replicate_Do_DB:
Replicate_Ignore_DB:
Replicate_Do_Table:
Replicate_Ignore_Table:
Replicate_Wild_Do_Table:
Replicate_Wild_Ignore_Table:
Last_Errno: 0
Last_Error:
Skip_Counter: 0
Exec_Master_Log_Pos: 931
Relay_Log_Space: 1365
Until_Condition: None
Until_Log_File:
Until_Log_Pos: 0
Master_SSL_Allowed: No
Master_SSL_CA_File:
Master_SSL_CA_Path:
Master_SSL_Cert:
Master_SSL_Cipher:
Master_SSL_Key:
Seconds_Behind_Master: 0
The key fields from the status report to examine are:
Slave_IO_State — The current status
of the slave. See Section 7.5.6.6, “Replication Slave I/O Thread States”,
and Section 7.5.6.7, “Replication Slave SQL Thread States”, for more
information.
Slave_IO_Running — Whether the I/O
thread for reading the master's binary log is running.
Normally, you want this to be Yes unless
you have not yet started replication or have explicitly
stopped it with STOP SLAVE.
Slave_SQL_Running — Whether the SQL
thread for executing events in the relay log is running. As
with the I/O thread, this should normally be
Yes.
Last_Error — The last error
registered when processing the relay log. Ideally this
should be blank, indicating no error.
Seconds_Behind_Master — The number
of seconds that the slave SQL thread is behind processing
the master binary log. A high number (or an increasing one)
can indicate that the slave is unable to handle events from
the master in a timely fashion.
A value of 0 for Seconds_Behind_Master
can usually be interpreted as meaning that the slave has
caught up with the master, but there are some cases where
this is not strictly true. For example, this can occur if
the network connection between master and slave is broken
but the slave I/O thread has not yet noticed this —
that is, slave_net_timeout
has not yet elapsed.
It is also possible that transient values for
Seconds_Behind_Master may not reflect the
situation accurately. When the slave SQL thread has caught
up on I/O, Seconds_Behind_Master displays
0; but when the slave I/O thread is still queuing up a new
event, Seconds_Behind_Master may show a
large value until the SQL thread finishes executing the new
event. This is especially likely when the events have old
timestamps; in such cases, if you execute
SHOW SLAVE STATUS several
times in a relatively short period, you may see this value
change back and forth repeatedly between 0 and a relatively
large value.
Several pairs of fields provide information about the progress
of the slave in reading events from the master binary log and
processing them in the relay log:
(Master_Log_file,
Read_Master_Log_Pos) — Coordinates
in the master binary log indicating how far the slave I/O
thread has read events from that log.
(Relay_Master_Log_File,
Exec_Master_Log_Pos) — Coordinates
in the master binary log indicating how far the slave SQL
thread has executed events received from that log.
(Relay_Log_File,
Relay_Log_Pos) — Coordinates in the
slave relay log indicating how far the slave SQL thread has
executed the relay log. These correspond to the preceding
coordinates, but are expressed in slave relay log
coordinates rather than master binary log coordinates.
On the master, you can check the status of connected slaves
using SHOW PROCESSLIST to examine
the list of running processes. Slave connections have
Binlog Dump in the Command
field:
mysql> SHOW PROCESSLIST \G;
*************************** 4. row ***************************
Id: 10
User: root
Host: slave1:58371
db: NULL
Command: Binlog Dump
Time: 777
State: Has sent all binlog to slave; waiting for binlog to be updated
Info: NULL
Because it is the slave that drives the replication process,
very little information is available in this report.
For slaves that were started with the
--report-host 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 option, the
connecting port, and master ID:
mysql> SHOW SLAVE HOSTS;
+-----------+--------+------+-------------------+-----------+
| Server_id | Host | Port | Rpl_recovery_rank | Master_id |
+-----------+--------+------+-------------------+-----------+
| 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 either the I/O or SQL threads
individually by specifying the thread type:
mysql> STOP SLAVE IO_THREAD;
mysql> STOP 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 allows the events in the relay log
to be executed up until the point where the relay log has ceased
to receive new events. This can be useful when you want to pause
execution to allow the slave to catch up with events from the
master, when you want to perform administration on the slave but
also ensure you have the latest updates to a specific point.
This method can also be used to pause execution on the slave
while you conduct administration on the master while ensuring
that there is not a massive backlog of events to be executed
when replication is started again.
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;
16.2. Replication Solutions
Replication can be used in many different environments for a range
of purposes. This section provides general notes and advice on using
replication for specific solution types.
For information on using replication in a backup environment,
including notes on the setup, backup procedure, and files to back
up, see Section 16.2.1, “Using Replication for Backups”.
For advice and tips on using different storage engines on the master
and slaves, see Section 16.2.2, “Using Replication with Different Master and Slave Storage Engines”.
Using replication as a scale-out solution requires some changes in
the logic and operation of applications that use the solution. See
Section 16.2.3, “Using Replication for Scale-Out”.
For performance or data distribution reasons, you may want to
replicate different databases to different replication slaves. See
Section 16.2.4, “Replicating Different Databases to Different Slaves”
As the number of replication slaves increases, the load on the
master can increase and lead to reduced performance (because of the
need to replicate the binary log to each slave). For tips on
improving your replication performance, including using a single
secondary server as an replication master, see
Section 16.2.5, “Improving Replication Performance”.
For guidance on switching masters, or converting slaves into masters
as part of an emergency failover solution, see
Section 16.2.6, “Switching Masters During Failover”.
To secure your replication communication, you can use SSL to encrypt
the communication channel. For step-by-step instructions, see
Section 16.2.7, “Setting Up Replication Using SSL”.
16.2.1. Using Replication for Backups
To use replication as a backup solution, replicate data from the
master to a slave, and then back up the data slave. The slave can
be paused and shut down without affecting the running operation of
the master, so you can produce an effective snapshot of
“live” data that would otherwise require the master
to be shut down.
How you back up a database depends on its size and whether you are
backing up only the data, or the data and the replication slave
state so that you can rebuild the slave in the event of failure.
There are therefore two choices:
16.2.1.1. Backing Up a Slave Using mysqldump
Using mysqldump to create a copy of a
database enables you to capture all of the data in the database
in a format that allows the information to be imported into
another instance of MySQL Server (see
Section 4.5.4, “mysqldump — A Database Backup Program”). Because the format of the
information is SQL statements, the file can easily be
distributed and applied to running servers in the event that you
need access to the data in an emergency. However, if the size of
your data set is very large, mysqldump may be
impractical.
When using mysqldump, you should stop
replication on the slave before starting the dump process to
ensure that the dump contains a consistent set of data:
Stop the slave from processing requests. You can stop
replication completely on the slave using
mysqladmin:
shell> mysqladmin stop-slave
Alternatively, you can stop only the slave SQL thread to
pause event execution:
shell> mysql -e 'STOP SLAVE SQL_THREAD;'
This allows the slave to continue to receive data change
events from the master's binary log and store them in the
relay logs via the I/O thread, but prevents the slave from
executing these events and changing its data. Within busy
replication environments, allowing the I/O thread to run
during backup may speed up the catch-up process when you
restart the slave SQL thread.
Run mysqldump to dump your databases. You
may either dump all databases or select databases to be
dumped. For example, to dump all databases:
shell> mysqldump --all-databases > fulldb.dump
Once the dump has completed, start slave operations again:
shell> mysqladmin start-slave
In the preceding example, you may want to add login credentials
(user name, password) to the commands, and bundle the process up
into a script that you can run automatically each day.
If you use this approach, make sure you monitor the slave
replication process to ensure that the time taken to run the
backup does not affect the slave's ability to keep up with
events from the master. See
Section 16.1.3.1, “Checking Replication Status”. If the
slave is unable to keep up, you may want to add another slave
and distribute the backup process. For an example of how to
configure this scenario, see
Section 16.2.4, “Replicating Different Databases to Different Slaves”.
16.2.1.2. Backing Up Raw Data from a Slave
To guarantee the integrity of the files that are copied, backing
up the raw data files on your MySQL replication slave should
take place while your slave server is shut down. If the MySQL
server is still running, background tasks may still be updating
the database files, particularly those involving storage engines
with background processes such as InnoDB.
With InnoDB, these problems should be
resolved during crash recovery, but since the slave server can
be shut down during the backup process without affecting the
execution of the master it makes sense to take advantage of this
capability.
To shut down the server and back up the files:
Shut down the slave MySQL server:
shell> mysqladmin shutdown
Copy the data files. You can use any suitable copying or
archive utility, including cp,
tar or WinZip. For
example, assuming that the data directory is located under
the current directory, you can archive the entire directory
as follows:
shell> tar cf /tmp/dbbackup.tar ./data
Start the MySQL server again. Under Unix:
shell> mysqld_safe &
Under Windows:
C:\> "C:\Program Files\MySQL\MySQL Server 5.0\bin\mysqld"
Normally you should back up the entire data directory for the
slave MySQL server. If you want to be able to restore the data
and operate as a slave (for example, in the event of failure of
the slave), then in addition to the slave's data, you should
also back up the slave status files,
master.info and
relay-log.info, along with the relay log
files. These files are needed to resume replication after you
restore the slave's data.
If you lose the relay logs but still have the
relay-log.info file, you can check it to
determine how far the SQL thread has executed in the master
binary logs. Then you can use CHANGE MASTER
TO with the MASTER_LOG_FILE and
MASTER_LOG_POS options to tell the slave to
re-read the binary logs from that point. This requires that the
binary logs still exist on the master server.
If your slave is replicating
LOAD DATA
INFILE statements, you should also back up any
SQL_LOAD-* files that exist in the
directory that the slave uses for this purpose. The slave needs
these files to resume replication of any interrupted
LOAD DATA
INFILE operations. The location of this directory is
the value of the
--slave-load-tmpdir option. If
the server was not started with that option, the directory
location is the value of the tmpdir system
variable.
16.2.2. Using Replication with Different Master and Slave Storage Engines
It does not matter for the replication process whether the source
table on the master and the replicated table on the slave use
different engine types. In fact, the system variables
storage_engine and
table_type are not replicated.
This provides a number of benefits in the replication process in
that you can take advantage of different engine types for
different replication scenarios. For example, in a typical
scale-out scenario (see
Section 16.2.3, “Using Replication for Scale-Out”), you want to use
InnoDB tables on the master to take advantage
of the transactional functionality, but use
MyISAM on the slaves where transaction support
is not required because the data is only read. When using
replication in a data-logging environment you may want to use the
Archive storage engine on the slave.
Configuring different engines on the master and slave depends on
how you set up the initial replication process:
If you used mysqldump to create the
database snapshot on your master, you could edit the dump file
text to change the engine type used on each table.
Another alternative for mysqldump is to
disable engine types that you do not want to use on the slave
before using the dump to build the data on the slave. For
example, you can add the
--skip-innodb
option on your slave to disable the InnoDB
engine. If a specific engine does not exist for a table to be
created, MySQL will use the default engine type, usually
MyISAM. (This requires that the
NO_ENGINE_SUBSTITUTION SQL
mode is not enabled.) If you want to disable additional
engines in this way, you may want to consider building a
special binary to be used on the slave that only supports the
engines you want.
If you are using raw data files (a binary backup) to set up
the slave, you will be unable to change the initial table
format. Instead, use ALTER
TABLE to change the table types after the slave has
been started.
For new master/slave replication setups where there are
currently no tables on the master, avoid specifying the engine
type when creating new tables.
If you are already running a replication solution and want to
convert your existing tables to another engine type, follow these
steps:
Stop the slave from running replication updates:
mysql> STOP SLAVE;
This will enable you to change engine types without
interruptions.
Execute an ALTER TABLE ...
ENGINE='engine_type' for
each table to be changed.
Start the slave replication process again:
mysql> START SLAVE;
Although the storage_engine and
table_type variables are not
replicated, be aware that CREATE
TABLE and ALTER TABLE
statements that include the engine specification will be correctly
replicated to the slave. For example, if you have a CSV table and
you execute:
mysql> ALTER TABLE csvtable Engine='MyISAM';
The above statement will be replicated to the slave and the engine
type on the slave will be converted to MyISAM,
even if you have previously changed the table type on the slave to
an engine other than CSV. If you want to retain engine differences
on the master and slave, you should be careful to use the
storage_engine variable on the
master when creating a new table. For example, instead of:
mysql> CREATE TABLE tablea (columna int) Engine=MyISAM;
Use this format:
mysql> SET storage_engine=MyISAM;
mysql> CREATE TABLE tablea (columna int);
When replicated, the
storage_engine variable will be
ignored, and the CREATE TABLE
statement will execute on the slave using the slave's default
engine.
16.2.3. Using Replication for Scale-Out
You can use replication as a scale-out solution; that is, where
you want to split up the load of database queries across multiple
database servers, within some reasonable limitations.
Because replication works from the distribution of one master to
one or more slaves, using replication for scale-out works best in
an environment where you have a high number of reads and low
number of writes/updates. Most Web sites fit into this category,
where users are browsing the Web site, reading articles, posts, or
viewing products. Updates only occur during session management, or
when making a purchase or adding a comment/message to a forum.
Replication in this situation enables you to distribute the reads
over the replication slaves, while still allowing your web servers
to communicate with the replication master when a write is
required. You can see a sample replication layout for this
scenario in Figure 16.1, “Using Replication to Improve Performance During Scale-Out”.
If the part of your code that is responsible for database access
has been properly abstracted/modularized, converting it to run
with a replicated setup should be very smooth and easy. Change the
implementation of your database access to send all writes to the
master, and to send reads to either the master or a slave. If your
code does not have this level of abstraction, setting up a
replicated system gives you the opportunity and motivation to
clean it up. Start by creating a wrapper library or module that
implements the following functions:
safe_writer_connect()
safe_reader_connect()
safe_reader_statement()
safe_writer_statement()
safe_ in each function name means that the
function takes care of handling all error conditions. You can use
different names for the functions. The important thing is to have
a unified interface for connecting for reads, connecting for
writes, doing a read, and doing a write.
Then convert your client code to use the wrapper library. This may
be a painful and scary process at first, but it pays off in the
long run. All applications that use the approach just described
are able to take advantage of a master/slave configuration, even
one involving multiple slaves. The code is much easier to
maintain, and adding troubleshooting options is trivial. You need
modify only one or two functions; for example, to log how long
each statement took, or which statement among those issued gave
you an error.
If you have written a lot of code, you may want to automate the
conversion task by using the replace utility
that comes with standard MySQL distributions, or write your own
conversion script. Ideally, your code uses consistent programming
style conventions. If not, then you are probably better off
rewriting it anyway, or at least going through and manually
regularizing it to use a consistent style.
16.2.4. Replicating Different Databases to Different Slaves
There may be situations where you have a single master and want to
replicate different databases to different slaves. For example,
you may want to distribute different sales data to different
departments to help spread the load during data analysis. A sample
of this layout is shown in
Figure 16.2, “Using Replication to Replicate Databases to Separate Replication Slaves”.
You can achieve this separation by configuring the master and
slaves as normal, and then limiting the binary log statements that
each slave processes by using the
--replicate-wild-do-table
configuration option on each slave.
Important
You should not use
--replicate-do-db for this
purpose, since its affects vary according to the database that
is currently selected.
For example, to support the separation as shown in
Figure 16.2, “Using Replication to Replicate Databases to Separate Replication Slaves”, you should
configure each replication slave as follows, before executing
START SLAVE:
Replication slave 1 should use
--replicate-wild-do-table=databaseA.%.
Replication slave 2 should use
--replicate-wild-do-table=databaseB.%.
Replication slave 3 should use
--replicate-wild-do-table=databaseC.%.
Each slave in this configuration receives the entire binary log
from the master, but executes only those events from the binary
log that apply to the databases and tables included by the
--replicate-wild-do-table option in
effect on that slave.
If you have data that must be synchronized to the slaves before
replication starts, you have a number of choices:
Synchronize all the data to each slave, and delete the
databases, tables, or both that you do not want to keep.
Use mysqldump to create a separate dump
file for each database and load the appropriate dump file on
each slave.
Use a raw data file dump and include only the specific files
and databases that you need for each slave.
16.2.5. Improving Replication Performance
As the number of slaves connecting to a master increases, the
load, although minimal, also increases, as each slave uses a
client connection to the master. Also, as each slave must receive
a full copy of the master binary log, the network load on the
master may also increase and create a bottleneck.
If you are using a large number of slaves connected to one master,
and that master is also busy processing requests (for example, as
part of a scale-out solution), then you may want to improve the
performance of the replication process.
One way to improve the performance of the replication process is
to create a deeper replication structure that enables the master
to replicate to only one slave, and for the remaining slaves to
connect to this primary slave for their individual replication
requirements. A sample of this structure is shown in
Figure 16.3, “Using an Additional Replication Host to Improve Performance”.
For this to work, you must configure the MySQL instances as
follows:
Master 1 is the primary master where all changes and updates
are written to the database. Binary logging should be enabled
on this machine.
Master 2 is the slave to the Master 1 that provides the
replication functionality to the remainder of the slaves in
the replication structure. Master 2 is the only machine
allowed to connect to Master 1. Master 2 also has binary
logging enabled, and the
--log-slave-updates option so
that replication instructions from Master 1 are also written
to Master 2's binary log so that they can then be replicated
to the true slaves.
Slave 1, Slave 2, and Slave 3 act as slaves to Master 2, and
replicate the information from Master 2, which actually
consists of the upgrades logged on Master 1.
The above solution reduces the client load and the network
interface load on the primary master, which should improve the
overall performance of the primary master when used as a direct
database solution.
If your slaves are having trouble keeping up with the replication
process on the master, there are a number of options available:
If possible, put the relay logs and the data files on
different physical drives. To do this, use the
--relay-log option to specify
the location of the relay log.
If the slaves are significantly slower than the master, you
may want to divide up the responsibility for replicating
different databases to different slaves. See
Section 16.2.4, “Replicating Different Databases to Different Slaves”.
If your master makes use of transactions and you are not
concerned about transaction support on your slaves, use
MyISAM or another nontransactional engine
on the slaves. See
Section 16.2.2, “Using Replication with Different Master and Slave Storage Engines”.
If your slaves are not acting as masters, and you have a
potential solution in place to ensure that you can bring up a
master in the event of failure, then you can switch off
--log-slave-updates. This
prevents “dumb” slaves from also logging events
they have executed into their own binary log.
16.2.6. 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 executing
events from the specified log and position 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, 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 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 16.2.3, “Using Replication for Scale-Out”.
Each MySQL Slave (Slave 1, Slave
2, and Slave 3) are slaves 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
Slave 1's binary log or binary log position to
read from: We know it is the first binary log 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 their
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 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.
16.2.7. Setting Up Replication Using SSL
To use SSL for encrypting the transfer of the binary log required
during replication, both the master and the slave must support SSL
network connections. If either host does not support SSL
connections (because it has not been compiled or configured for
SSL), replication through an SSL connection is not possible.
Setting up replication using an SSL connection is similar to
setting up a server and client using SSL. You must obtain (or
create) a suitable security certificate that you can use on the
master, and a similar certificate (from the same certificate
authority) on each slave.
For more information on setting up a server and client for SSL
connectivity, see Section 5.5.7.2, “Using SSL Connections”.
To enable SSL on the master you must create or obtain suitable
certificates, and then add the following configuration options to
the master's configuration within the [mysqld]
section of the master's my.cnf file:
[mysqld]
ssl-ca=cacert.pem
ssl-cert=server-cert.pem
ssl-key=server-key.pem
The options are as follows:
ssl-ca identifies the Certificate Authority
(CA) certificate.
ssl-cert identifies the server public key.
This can be sent to the client and authenticated against the
CA certificate that it has.
ssl-key identifies the server private key.
On the slave, you have two options available for setting the SSL
information. You can either add the slave certificates to the
[client] section of the slave's
my.cnf file, or you can explicitly specify
the SSL information using the CHANGE MASTER
TO statement:
To addthe slave certificates using an option file, add the
following lines to the [client] section of
the slave's my.cnf file:
[client]
ssl-ca=cacert.pem
ssl-cert=client-cert.pem
ssl-key=client-key.pem
Restart the slave server, using the
--skip-slave-start to prevent
the slave from connecting to the master. Use
CHANGE MASTER TO to specify the
master configuration, using the MASTER_SSL
option to enable SSL connectivity:
mysql> CHANGE MASTER TO
-> MASTER_HOST='master_hostname',
-> MASTER_USER='replicate',
-> MASTER_PASSWORD='password',
-> MASTER_SSL=1;
To specify the SSL certificate options using the
CHANGE MASTER TO statement,
append the SSL options:
mysql> CHANGE MASTER TO
-> MASTER_HOST='master_hostname',
-> MASTER_USER='replicate',
-> MASTER_PASSWORD='password',
-> MASTER_SSL=1,
-> MASTER_SSL_CA = 'ca_file_name',
-> MASTER_SSL_CAPATH = 'ca_directory_name',
-> MASTER_SSL_CERT = 'cert_file_name',
-> MASTER_SSL_KEY = 'key_file_name';
After the master information has been updated, start the slave
replication process:
mysql> START SLAVE;
You can use the SHOW SLAVE STATUS
statement to confirm that the SSL connection was established
successfully.
For more information on the CHANGE MASTER
TO statement, see Section 12.6.2.1, “CHANGE MASTER TO Syntax”.
If you want to enforce the use of SSL connections during
replication, then create a user with the
REPLICATION SLAVE privilege and use
the REQUIRE SSL option for that user. For
example:
mysql> CREATE USER 'repl'@'%.mydomain.com' IDENTIFIED BY 'slavepass';
mysql> GRANT REPLICATION SLAVE ON *.*
-> TO 'repl'@'%.mydomain.com' REQUIRE SSL;
If the account already exists, you can add REQUIRE
SSL to it with this statement:
mysql> GRANT USAGE ON *.*
-> TO 'repl'@'%.mydomain.com' REQUIRE SSL;
16.3. Replication Notes and Tips16.3.1. Replication Features and Issues
The following sections provide information about what is supported
and what is not in MySQL replication, and about specific issues
and situations that may occur when replicating certain statements.
Statement-based replication depends on compatibility at the SQL
level between the master and slave. In others, successful SBR
requires that any SQL features used be supported by both the
master and the slave servers. For example, if you use a feature on
the master server that is available only in MySQL 5.0
(or later), you cannot replicate to a slave that uses MySQL
4.1 (or earlier).
Such incompatibilities also can occur within a release series when
using pre-production releases of MySQL. For example, the
SLEEP() function is available
beginning with MySQL 5.0.12. If you use this function on the
master, you cannot replicate to a slave that uses MySQL 5.0.11 or
earlier.
For this reason, use Generally Available (GA) releases of MySQL
for statement-based replication in a production setting, since we
do not introduce new SQL statements or change their behavior
within a given release series once that series reaches GA release
status.
If you are planning to use replication between MySQL
5.0 and a previous MySQL release series, it is also a
good idea to consult the edition of the MySQL Reference
Manual corresponding to the earlier release series for
information regarding the replication characteristics of that
series.
For additional information specific to replication and
InnoDB, see
Section 13.2.4.5, “InnoDB and MySQL Replication”.
16.3.1.1. Replication and AUTO_INCREMENT
Replication of AUTO_INCREMENT,
LAST_INSERT_ID(), and
TIMESTAMP values is done
correctly, subject to the following exceptions.
INSERT DELAYED ...
VALUES(LAST_INSERT_ID()) inserts a different value
on the master and the slave. (Bug#20819) This is fixed in
MySQL 5.1 when using row-based or mixed-format
binary logging. For more information, see
Replication Formats.
Before MySQL 5.0.26, a stored procedure that uses
LAST_INSERT_ID() does not
replicate properly.
When a statement uses a stored function that inserts into an
AUTO_INCREMENT column, the generated
AUTO_INCREMENT value is not written into
the binary log, so a different value can in some cases be
inserted on the slave. This is also true of a trigger that
causes an INSERT into an
AUTO_INCREMENT column.
An insert into an AUTO_INCREMENT column
caused by a stored routine or trigger running on a master
that uses MySQL 5.0.60 or earlier does not replicate
correctly to a slave running MySQL 5.1.12 through 5.1.23
(inclusive). (Bug#33029)
Adding an AUTO_INCREMENT column to a
table with ALTER TABLE might
not produce the same ordering of the rows on the slave and
the master. This occurs because the order in which the rows
are numbered depends on the specific storage engine used for
the table and the order in which the rows were inserted. If
it is important to have the same order on the master and
slave, the rows must be ordered before assigning an
AUTO_INCREMENT number. Assuming that you
want to add an AUTO_INCREMENT column to a
table t1 that has columns
col1 and col2, the
following statements produce a new table
t2 identical to t1 but
with an AUTO_INCREMENT column:
CREATE TABLE t2 LIKE t1;
ALTER TABLE t2 ADD id INT AUTO_INCREMENT PRIMARY KEY;
INSERT INTO t2 SELECT * FROM t1 ORDER BY col1, col2;
Important
To guarantee the same ordering on both master and slave,
the ORDER BY clause must name
all columns of t1.
The instructions just given are subject to the limitations
of CREATE TABLE ... LIKE: Foreign key
definitions are ignored, as are the DATA
DIRECTORY and INDEX DIRECTORY
table options. If a table definition includes any of those
characteristics, create t2 using a
CREATE TABLE statement that
is identical to the one used to create
t1, but with the addition of the
AUTO_INCREMENT column.
Regardless of the method used to create and populate the
copy having the AUTO_INCREMENT column,
the final step is to drop the original table and then rename
the copy:
DROP t1;
ALTER TABLE t2 RENAME t1;
See also Section B.5.7.1, “Problems with ALTER TABLE”.
16.3.1.2. Replication and Character Sets
The following applies to replication between MySQL servers that
use different character sets:
If the master uses MySQL 4.1, you must
always use the same
global character set and collation on
the master and the slave, regardless of the slave MySQL
version. (These are controlled by the
--character-set-server and
--collation-server options.)
Otherwise, you may get duplicate-key errors on the slave,
because a key that is unique in the master character set
might not be unique in the slave character set. Note that
this is not a cause for concern when master and slave are
both MySQL 5.0 or later.
If the master is older than MySQL 4.1.3, the character set
of any client should never be made different from its global
value because this character set change is not known to the
slave. In other words, clients should not use SET
NAMES, SET CHARACTER SET, and
so forth. If both the master and the slave are 4.1.3 or
newer, clients can freely set session values for character
set variables because these settings are written to the
binary log and so are known to the slave. That is, clients
can use SET NAMES or SET
CHARACTER SET or can set variables such as
collation_client or
collation_server. However,
clients are prevented from changing the
global value of these variables; as
stated previously, the master and slave must always have
identical global character set values.
If you have databases on the master with character sets that
differ from the global
character_set_server value,
you should design your CREATE
TABLE statements so that tables in those databases
do not implicitly rely on the database default character
set. A good workaround is to state the table character set
and collation explicitly in CREATE
TABLE statements.
16.3.1.3. Replication of CREATE
TABLE ... SELECT Statements
This section discusses the rules that are applied when a
CREATE TABLE ...
SELECT statement is replicated.
Statement succeeds.
A successful
CREATE TABLE ...
SELECT is itself replicated.
Statement fails.
A failed CREATE
TABLE ... SELECT replicates as follows:
Statement does not use IF NOT EXISTS.
The statement has no effect. However, the implicit
commit caused by the statement is logged. This is true
regardless of the storage engine used and the reason
for which the statement failed.
Statement uses IF NOT EXISTS.
The CREATE TABLE IF NOT EXISTS ...
SELECT is logged with an error.
16.3.1.4. Replication and DIRECTORY Table Options
If a DATA DIRECTORY or INDEX
DIRECTORY table option is used in a
CREATE TABLE statement on the
master server, the table option is also used on the slave. This
can cause problems if no corresponding directory exists in the
slave host file system or if it exists but is not accessible to
the slave server. This can be overridden by using the
NO_DIR_IN_CREATE server SQL
mode on the slave, which causes the slave to ignore the
DATA DIRECTORY and INDEX
DIRECTORY table options when replicating
CREATE TABLE statements. The
result is that MyISAM data and index files
are created in the table's database directory.
For more information, see Section 5.1.7, “Server SQL Modes”.
16.3.1.5. Replication of DROP ... IF EXISTS Statements
The DROP DATABASE
IF EXISTS,
DROP TABLE IF
EXISTS, and
DROP VIEW IF
EXISTS statements are always replicated, even if the
database, table, or view to be dropped does not exist on the
master. This is to ensure that the object to be dropped no
longer exists on either the master or the slave, once the slave
has caught up with the master.
Beginning with MySQL 5.0.82, DROP ... IF
EXISTS statements for stored programs (stored
functions and procedures, triggers, and events) are also
replicated, even if the stored program to be dropped does not
exist on the master. (Bug#13684)
16.3.1.6. Replication with Floating-Point Values
With statement-based replication, values are converted from
decimal to binary. Because conversions between decimal and
binary representations of them may be approximate, comparisons
involving floating-point values are inexact. This is true for
operations that use floating-point values explicitly, or that
use values that are converted to floating-point implicitly.
Comparisons of floating-point values might yield different
results on master and slave servers due to differences in
computer architecture, the compiler used to build MySQL, and so
forth. See Section 11.2.2, “Type Conversion in Expression Evaluation”, and
Section B.5.5.8, “Problems with Floating-Point Comparisons”.
MySQL Enterprise
For expert advice regarding replication, subscribe to the
MySQL Enterprise Monitor. For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
16.3.1.7. Replication and FLUSH
Some forms of the FLUSH statement
are not logged because they could cause problems if replicated
to a slave: FLUSH
LOGS, FLUSH
MASTER, FLUSH
SLAVE, and
FLUSH TABLES WITH READ
LOCK. For a syntax example, see
Section 12.5.6.2, “FLUSH Syntax”. The
FLUSH TABLES,
ANALYZE TABLE,
OPTIMIZE TABLE, and
REPAIR TABLE statements are
written to the binary log and thus replicated to slaves. This is
not normally a problem because these statements do not modify
table data.
However, this behavior can cause difficulties under certain
circumstances. If you replicate the privilege tables in the
mysql database and update those tables
directly without using GRANT, you
must issue a FLUSH
PRIVILEGES on the slaves to put the new privileges
into effect. In addition, if you use
FLUSH TABLES
when renaming a MyISAM table that is part of
a MERGE table, you must issue
FLUSH TABLES
manually on the slaves. These statements are written to the
binary log unless you specify
NO_WRITE_TO_BINLOG or its alias
LOCAL.
16.3.1.8. Replication and System Functions
Certain functions do not replicate well under some conditions:
The USER(),
CURRENT_USER(),
UUID(),
VERSION(), and
LOAD_FILE() functions are
replicated without change and thus do not work reliably on
the slave.
For NOW(), the binary log
includes the timestamp. This means that the value
as returned by the call to this function on the
master is replicated to the slave. This can lead
to a possibly unexpected result when replicating between
MySQL servers in different time zones. Suppose that the
master is located in New York, the slave is located in
Stockholm, and both servers are using local time. Suppose
further that, on the master, you create a table
mytable, perform an
INSERT statement on this
table, and then select from the table, as shown here:
mysql> CREATE TABLE mytable (mycol TEXT);
Query OK, 0 rows affected (0.06 sec)
mysql> INSERT INTO mytable VALUES ( NOW() );
Query OK, 1 row affected (0.00 sec)
mysql> SELECT * FROM mytable;
+---------------------+
| mycol |
+---------------------+
| 2009-09-01 12:00:00 |
+---------------------+
1 row in set (0.00 sec)
Local time in Stockholm is 6 hours later than in New York;
so, if you issue SELECT NOW() on the
slave at that exact same instant, the value
2009-09-01 18:00:00 is returned. For this
reason, if you select from the slave's copy of
mytable after the
CREATE TABLE and
INSERT statements just shown
have been replicated, you might expect
mycol to contain the value
2009-09-01 18:00:00. However, this is not
the case; when you select from the slave's copy of
mytable, you obtain exactly the same
result as on the master:
mysql> SELECT * FROM mytable;
+---------------------+
| mycol |
+---------------------+
| 2009-09-01 12:00:00 |
+---------------------+
1 row in set (0.00 sec)
As of MySQL 5.0.13, the
SYSDATE() function is no
longer equivalent to NOW().
Implications are that
SYSDATE() is not
replication-safe because it is not affected by SET
TIMESTAMP statements in the binary log and is
nondeterministic. To avoid this, you can start the server
with the --sysdate-is-now
option to cause SYSDATE() to
be an alias for NOW().
See also Section 16.3.1.23, “Replication and Time Zones”.
The GET_LOCK(),
RELEASE_LOCK(),
IS_FREE_LOCK(), and
IS_USED_LOCK() functions that
handle user-level locks are replicated without the slave
knowing the concurrency context on master. Therefore, these
functions should not be used to insert into a master's table
because the content on the slave would differ. For example,
do not issue a statement such as INSERT INTO
mytable VALUES(GET_LOCK(...)).
As a workaround for the preceding limitations, you can use the
strategy of saving the problematic function result in a user
variable and referring to the variable in a later statement. For
example, the following single-row
INSERT is problematic due to the
reference to the UUID() function:
INSERT INTO t VALUES(UUID());
To work around the problem, do this instead:
SET @my_uuid = UUID();
INSERT INTO t VALUES(@my_uuid);
That sequence of statements replicates because the value of
@my_uuid is stored in the binary log as a
user-variable event prior to the
INSERT statement and is available
for use in the INSERT.
The same idea applies to multiple-row inserts, but is more
cumbersome to use. For a two-row insert, you can do this:
SET @my_uuid1 = UUID(); @my_uuid2 = UUID();
INSERT INTO t VALUES(@my_uuid1),(@my_uuid2);
However, if the number of rows is large or unknown, the
workaround is difficult or impracticable. For example, you
cannot convert the following statement to one in which a given
individual user variable is associated with each row:
INSERT INTO t2 SELECT UUID(), * FROM t1;
Non-delayed INSERT statements
that refer to RAND() or
user-defined variables replicate correctly. However, changing
the statements to use INSERT
DELAYED can result in different results on master and
slave.
Within a stored function, RAND()
replicates correctly as long as it is invoked only once during
the execution of the function. (You can consider the function
execution timestamp and random number seed as implicit inputs
that are identical on the master and slave.)
The FOUND_ROWS() and
ROW_COUNT() functions are also
not replicated reliably. A workaround is to store the result of
the function call in a user variable, and then use that in the
INSERT statement. For example, if
you wish to store the result in a table named
mytable, you might normally do so like this:
SELECT SQL_CALC_FOUND_ROWS FROM mytable LIMIT 1;
INSERT INTO mytable VALUES( FOUND_ROWS() );
However, if you are replicating mytable, you
should use SELECT INTO, and then store the
variable in the table, like this:
SELECT SQL_CALC_FOUND_ROWS INTO @found_rows FROM mytable LIMIT 1;
INSERT INTO mytable VALUES(@found_rows);
In this way, the user variable is replicated as part of the
context, and applied on the slave correctly.
16.3.1.9. Replication and LIMIT
Replication of LIMIT clauses in
DELETE,
UPDATE, and
INSERT ...
SELECT statements is not guaranteed, since the order
of the rows affected is not defined. Such statements can be
replicated correctly only if they also contain an ORDER
BY clause.
16.3.1.10. Replication and LOAD Operations
Using LOAD TABLE FROM MASTER where the master
is running MySQL 4.1 and the slave is running MySQL 5.0 may
corrupt the table data, and is not supported. (Bug#16261)
The LOAD DATA
INFILE statement's CONCURRENT
option is not replicated; that is, LOAD DATA CONCURRENT
INFILE is replicated as
LOAD DATA
INFILE, and LOAD DATA CONCURRENT LOCAL
INFILE is replicated as
LOAD DATA LOCAL
INFILE. (Bug#34628)
The following applies only if either the master or the
slave is running MySQL version 5.0.3 or older: If on
the master a LOAD
DATA INFILE is interrupted (integrity constraint
violation, killed connection, and so on), the slave skips the
LOAD DATA
INFILE entirely. This means that if this command
permanently inserted or updated table records before being
interrupted, these modifications are not replicated to the
slave.
16.3.1.11. Replication and the Slow Query Log
Replication slaves do not write replicated queries to the slow
query log, even if the same queries were written to the slow
query log on the master.
This is a known issue which we intend to fix in a future version
of MySQL. (Bug#23300)
16.3.1.12. Replication During a Master Crash
A crash on the master side can result in the master's binary log
having a final position less than the most recent position read
by the slave, due to the master's binary log file not being
flushed. This can cause the slave not to be able to replicate
when the master comes back up. Setting
sync_binlog=1 in the master
my.cnf file helps to minimize this problem
because it causes the master to flush its binary log more
frequently.
16.3.1.13. Replication During a Master Shutdown
It is safe to shut down a master server and restart it later.
When a slave loses its connection to the master, the slave tries
to reconnect immediately and retries periodically if that fails.
The default is to retry every 60 seconds. This may be changed
with the CHANGE MASTER TO
statement or
--master-connect-retry option. A
slave also is able to deal with network connectivity outages.
However, the slave notices the network outage only after
receiving no data from the master for
slave_net_timeout seconds. If
your outages are short, you may want to decrease
slave_net_timeout. See
Section 5.1.3, “Server System Variables”.
16.3.1.14. Replication with MEMORY Tables
When a server shuts down and restarts, its
MEMORY (HEAP) tables
become empty. To replication this effect to slaves, the first
time that the master uses a given MEMORY
table after startup, it logs an event that notifies the slaves
that the table needs to be emptied by writing a
DELETE statement for that table
to the binary log. See Section 13.4, “The MEMORY (HEAP) Storage Engine”,
for more information about MEMORY tables.
16.3.1.15. Replication of the System mysql Database
User privileges are replicated only if the
mysql database is replicated. That is, the
GRANT,
REVOKE, SET
PASSWORD, CREATE USER,
and DROP USER statements take
effect on the slave only if the replication setup includes the
mysql database.
16.3.1.16. Replication and the Query Optimizer
It is possible for the data on the master and slave to become
different if a statement is designed in such a way that the data
modification is nondeterministic; that is, left up the query
optimizer. (This is in general not a good practice, even outside
of replication.) Examples of nondeterministic statements include
DELETE or
UPDATE statements that use
LIMIT with no ORDER BY
clause; see Section 16.3.1.9, “Replication and LIMIT”, for a
detailed discussion of these.
Also see Additional Known Issues.
16.3.1.17. Replication and Reserved Words
You can encounter problems when you attempt to replicate from an
older master to a newer slave and you make use of identifiers on
the master that are reserved words in the newer MySQL version
running on the slave. An example of this is using a table column
named current_user on a 4.0 master that is
replicating to a 4.1 or higher slave because
CURRENT_USER is a reserved word beginning in
MySQL 4.1. Replication can fail in such cases with Error 1064
You have an error in your SQL syntax...,
even if a database or table named using the reserved
word or a table having a column named using the reserved word is
excluded from replication. This is due to the fact
that each SQL event must be parsed by the slave prior to
execution, so that the slave knows which database object or
objects would be affected; only after the event is parsed can
the slave apply any filtering rules defined by
--replicate-do-db,
--replicate-do-table,
--replicate-ignore-db, and
--replicate-ignore-table.
To work around the problem of database, table, or column names
on the master which would be regarded as reserved words by the
slave, do one of the following:
Use one or more ALTER TABLE
statements on the master to change the names of any database
objects where these names would be considered reserved words
on the slave, and change any SQL statements that use the old
names to use the new names instead.
In any SQL statements using these database object names,
quote the names using backtick characters
(`).
For listings of reserved words by MySQL version, see
Reserved
Words, in the MySQL Server Version
Reference.
16.3.1.18. Slave Errors during Replication
If a statement produces the same error (identical error code) on
both the master and the slave, the error is logged, but
replication continues.
If a statement produces different errors on the master and the
slave, the slave SQL thread terminates, and the slave writes a
message to its error log. This includes the case that a
statement produces an error on the master or the slave, but not
both. To address the issue, connect to the slave manually and
determine the cause of the problem. SHOW
SLAVE STATUS is useful for this. Then fix the problem
and run START SLAVE. For example,
you might need to create a nonexistent table before you can
start the slave again.
Note
If you are replicating between tables that use different
storage engines, you should keep in mind that the same
statement might produce a different error when run against one
version of the table, but not the other, or might cause an
error for one version of the table, but not the other. For
example, since MyISAM ignores foreign key
constraints, an INSERT or
UPDATE statement accessing an
InnoDB table on the master might cause a
foreign key violation but the same statement performed on a
MyISAM version of the same table on the
slave would produce no such error, causing replication to
stop.
16.3.1.19. Replication during a Slave Shutdown
Shutting down the slave (cleanly) is also safe because it keeps
track of where it left off. Unclean shutdowns might produce
problems, especially if the disk cache was not flushed to disk
before the system went down. Your system fault tolerance is
greatly increased if you have a good uninterruptible power
supply. Unclean shutdowns of the master may cause
inconsistencies between the content of tables and the binary log
on the master; this can be avoided by using
InnoDB tables and the
--innodb_safe_binlog option on
the master. See Section 5.2.3, “The Binary Log”.
Note
--innodb_safe_binlog is
unneeded as of MySQL 5.0.3, having been made obsolete by the
introduction of XA transaction support.
16.3.1.20. Replication and Server SQL Mode
Using different server SQL mode settings on the master and the
slave may cause the same INSERT
statements to be handled differently on the master and the
slave, leading the master and slave to diverge. For best
results, you should always use the same server SQL mode on the
master and on the slave.
For more information, see Section 5.1.7, “Server SQL Modes”.
16.3.1.21. Replication and Temporary TablesSafe shutdown of slaves when using temporary tables.
Temporary tables are replicated except in the case where you
stop the slave server (not just the slave threads) and you
have replicated temporary tables that are used in updates that
have not yet been executed on the slave. If you stop the slave
server, the temporary tables needed by those updates are no
longer available when the slave is restarted. To avoid this
problem, do not shut down the slave while it has temporary
tables open. Instead, use the following procedure:
Issue a STOP SLAVE SQL_THREAD statement.
Use SHOW STATUS to check the
value of the
Slave_open_temp_tables
variable.
If the value is not 0, restart the slave SQL thread with
START SLAVE SQL_THREAD and repeat the
procedure later.
When the value is 0, issue a mysqladmin
shutdown command to stop the slave.
Temporary tables and replication options.
By default, all temporary tables are replicated; this happens
whether or not there are any matching
--replicate-do-db,
--replicate-do-table, or
--replicate-wild-do-table
options in effect. However, the
--replicate-ignore-table and
--replicate-wild-ignore-table
options are honored for temporary tables.
A recommended practice when using replication is to designate a
prefix for exclusive use in naming temporary tables that you do
not want replicated, then employ a matching
--replicate-wild-ignore-table
option. For example, you might give all such tables names
beginning with norep (such as
norepmytable,
norepyourtable, and so on), then use
--replicate-wild-ignore-table=norep%
to prevent the replication of these tables.
16.3.1.22. Replication Retries and Timeouts
In MySQL 5.0 (starting from 5.0.3), there is a
global system variable
slave_transaction_retries: If
the slave SQL thread fails to execute a transaction because of
an InnoDB deadlock or because it exceeded the
InnoDB
innodb_lock_wait_timeout or the
NDBCLUSTER
TransactionDeadlockDetectionTimeout or
TransactionInactiveTimeout value, the slave
automatically retries the transaction
slave_transaction_retries times
before stopping with an error. The default value is 10. Starting
from MySQL 5.0.4, the total retry count can be seen in the
output of SHOW STATUS; see
Section 5.1.6, “Server Status Variables”.
16.3.1.23. Replication and Time Zones
The same system time zone should be set for both master and
slave. Otherwise statements depending on the local time on the
master are not replicated properly, such as statements that use
the NOW() or
FROM_UNIXTIME() functions. You
can set the time zone in which MySQL server runs by using the
--timezone=timezone_name
option of the mysqld_safe script or by
setting the TZ environment variable. See also
Section 16.3.1.8, “Replication and System Functions”.
If the master is MySQL 4.1 or earlier, then both master and
slave should also have the same default connection time zone
setting; that is, the
--default-time-zone parameter
should have the same value for both master and slave.
CONVERT_TZ(...,...,@@session.time_zone)
is properly replicated only if both master and slave are running
MySQL 5.0.4 or newer.
16.3.1.24. Replication and Transactions
It is possible to replicate transactional tables on the master
using nontransactional tables on the slave. For example, you can
replicate an InnoDB master table as a
MyISAM slave table. However, if you do this,
there are problems if the slave is stopped in the middle of a
BEGIN/COMMIT
block because the slave restarts at the beginning of the
BEGIN block.
In situations where transactions mix updates to transactional
and nontransactional tables, the order of statements in the
binary log is correct, and all needed statements are written to
the binary log even in case of a
ROLLBACK.
However, when a second connection updates the nontransactional
table before the first connection's transaction is complete,
statements can be logged out of order because the second
connection's update is written immediately after it is
performed, regardless of the state of the transaction being
performed by the first connection.
Due to the nontransactional nature of MyISAM
tables, it is possible to have a statement that only partially
updates a table and returns an error code. This can happen, for
example, on a multiple-row insert that has one row violating a
key constraint, or if a long update statement is killed after
updating some of the rows. If that happens on the master, the
slave thread exits and waits for the database administrator to
decide what to do about it unless the error code is legitimate
and execution of the statement results in the same error code on
the slave. If this error code validation behavior is not
desirable, some or all errors can be masked out (ignored) with
the --slave-skip-errors option.
If you update transactional tables from nontransactional tables
inside a
BEGIN/COMMIT
sequence, updates to the binary log may be out of synchrony with
table states if the nontransactional table is updated before the
transaction commits. This occurs because the transaction is
written to the binary log only when it is committed.
In situations where transactions mix updates to transactional
and nontransactional tables, the order of statements in the
binary log is correct, and all needed statements are written to
the binary log even in case of a
ROLLBACK.
However, when a second connection updates the nontransactional
table before the first connection's transaction is complete,
statements can be logged out of order because the second
connection's update is written immediately after it is
performed, regardless of the state of the transaction being
performed by the first connection.
Caution
You should avoid transactions that update both transactional
and nontransactional tables in a replication environment.
When the storage engine type of the slave is nontransactional,
transactions on the master that mix updates of transactional and
nontransactional tables should be avoided because they can cause
inconsistency of the data between the master's transactional
table and the slave's nontransactional table. That is, such
transactions can lead to master storage engine-specific behavior
with the possible effect of replication going out of synchrony.
MySQL does not issue a warning about this currently, so extra
care should be taken when replicating transactional tables from
the master to nontransactional tables on the slaves.
16.3.1.25. Replication and Triggers
Known issue: In MySQL 5.0.17,
the syntax for CREATE TRIGGER
changed to include a DEFINER clause for
specifying which access privileges to check at trigger
invocation time. (See Section 12.1.11, “CREATE TRIGGER Syntax”, for more
information.) However, if you attempt to replicate from a master
server older than MySQL 5.0.17 to a slave running MySQL 5.0.17
through 5.0.19, replication of CREATE
TRIGGER statements fails on the slave with a
Definer not fully qualified error. A
workaround is to create triggers on the master using a
version-specific comment embedded in each
CREATE TRIGGER statement:
CREATE /*!50017 DEFINER = 'root'@'localhost' */ TRIGGER ... ;
CREATE TRIGGER statements written
this way will replicate to newer slaves, which pick up the
DEFINER clause from the comment and execute
successfully.
This slave problem is fixed as of MySQL 5.0.20.
16.3.1.26. Replication and User Privileges
User privileges are replicated only if the
mysql database is replicated. That is, the
GRANT,
REVOKE, SET
PASSWORD, CREATE USER,
and DROP USER statements take
effect on the slave only if the replication setup includes the
mysql database.
If you're replicating all databases, but don't want statements
that affect user privileges to be replicated, set up the slave
to not replicate the mysql database, using
the
--replicate-wild-ignore-table=mysql.%
option. The slave recognizes that issuing privilege-related SQL
statements have no effect, and thus it does not execute those
statements.
16.3.1.27. Replication and Variables
The foreign_key_checks,
unique_checks, and
sql_auto_is_null variables are
all replicated.
sql_mode is also replicated
except for the
NO_DIR_IN_CREATE mode.
However, when mysqlbinlog parses a
SET @@sql_mode =
mode statement, the full
mode value, including
NO_DIR_IN_CREATE, is passed to
the receiving server.
The storage_engine system
variable is not replicated, which is a good thing for
replication between different storage engines.
Starting from MySQL 5.0.3 (master and slave), replication works
even if the master and slave have different global character set
variables. Starting from MySQL 5.0.4 (master and slave),
replication works even if the master and slave have different
global time zone variables.
Session variables are not replicated properly when used in
statements that update tables. For example, SET
max_join_size=1000 followed by INSERT INTO
mytable VALUES(@@max_join_size) will not insert the
same data on the master and the slave. This does not apply to
the common sequence of SET time_zone=...
followed by INSERT INTO mytable
VALUES(CONVERT_TZ(...,...,@@time_zone)), which
replicates correctly as of MySQL 5.0.4.
Update statements that refer to user-defined variables (that is,
variables of the form
@var_name) are
replicated correctly in MySQL 5.0. However, this is
not true for versions prior to 4.1. Note that user variable
names are case insensitive starting in MySQL 5.0. You should
take this into account when setting up replication between MySQL
5.0 and older versions.
In MySQL 5.0.46 and later, the following session variables are
written to the binary log and honored by the replication slave
when parsing the binary log, regardless of the logging format:
Important
Even though session variables relating to character sets and
collations are written to the binary log, replication between
different character sets is not supported.
16.3.1.28. Replication and Views
Views are always replicated to slaves. Views are filtered by
their own name, not by the tables they refer to. This means that
a view can be replicated to the slave even if the view contains
a table that would normally be filtered out by
replication-ignore-table rules. Care should
therefore be taken to ensure that views do not replicate table
data that would normally be filtered for security reasons.
16.3.2. Replication Compatibility Between MySQL Versions
MySQL supports replication from one major version to the next
higher major version. For example, you can replicate from a master
running MySQL 4.1 to a slave running MySQL 5.0, from a master
running MySQL 5.0 to a slave running MySQL 5.1, and so on.
In some cases, it is also possible to replicate between a master
and a slave that is more than one major version newer than the
master. However, there are known issues with trying to replicate
from a master running MySQL 4.1 or earlier to a slave running
MySQL 5.1 or later. To work around such problems, you can insert a
MySQL server running an intermediate version between the two; for
example, rather than replicating directly from a MySQL 4.1 master
to a MySQL 5.1 slave, it is possible to replicate from a MySQL 4.1
server to a MySQL 5.0 server, and then from the MySQL 5.0 server
to a MySQL 5.1 server.
Important
It is strongly recommended to use the most recent release
available within a given MySQL major version because replication
(and other) capabilities are continually being improved. It is
also recommended to upgrade masters and slaves that use early
releases of a major version of MySQL to GA (production) releases
when the latter become available for that major version.
Replication from newer masters to older slaves may be possible,
but is generally not supported. This is due to a number of
factors:
Binary log format changes.
The binary log format can change between major releases.
While we attempt to maintain backward-compatiblity, this is
not always possible. Major changes were made in MySQL 5.0.3
(for improvements to handling of character sets and
LOAD DATA
INFILE) and 5.0.4 (for improvements to handling of
time zones). Because of these changes, replication from a
MySQL 5.0.3 or later master to a MySQL 5.0.2 or earlier
slave is not supported. This also means that replication
from a MySQL 5.0.3 (or later) master to any MySQL 4.1 (or
earlier) slave is generally not supported.
This also has significant implications for upgrading
replication servers; see
Section 16.3.3, “Upgrading a Replication Setup”, for more information.
Use of row-based replication.
Row-based replication was implemented in MySQL 5.1.5, so you
cannot replicate using row-based replication from any MySQL
5.0 or later master to a slave older than MySQL
5.1.5.
Note
Row-based replication is not available in MySQL
5.0. For more information about row-based
replication in MySQL 5.1, see
Replication Formats.
SQL incompatiblities.
You cannot replicate from a newer master to an older slave
using statement-based replication if the statements to be
replicated use SQL features available on the master but not
on the slave.
For more information on potential replication issues, see
Section 16.3.1, “Replication Features and Issues”.
16.3.3. Upgrading a Replication Setup
When you upgrade servers that participate in a replication setup,
the procedure for upgrading depends on the current server versions
and the version to which you are upgrading.
This section applies to upgrading replication from older versions
of MySQL to MySQL 5.0. A 4.0 server should be 4.0.3
or newer.
When you upgrade a master to 5.0 from an earlier
MySQL release series, you should first ensure that all the slaves
of this master are using the same 5.0.x release. If
this is not the case, you should first upgrade the slaves. To
upgrade each slave, shut it down, upgrade it to the appropriate
5.0.x version, restart it, and restart replication.
The 5.0 slave is able to read the old relay logs
written prior to the upgrade and to execute the statements they
contain. Relay logs created by the slave after the upgrade are in
5.0 format.
After the slaves have been upgraded, shut down the master, upgrade
it to the same 5.0.x release as the slaves, and
restart it. The 5.0 master is able to read the old
binary logs written prior to the upgrade and to send them to the
5.0 slaves. The slaves recognize the old format and
handle it properly. Binary logs created by the master subsequent
to the upgrade are in 5.0 format. These too are
recognized by the 5.0 slaves.
In other words, when upgrading to MySQL 5.0, the
slaves must be MySQL 5.0 before you can upgrade the
master to 5.0. Note that downgrading from
5.0 to older versions does not work so simply: You
must ensure that any 5.0 binary log or relay log has
been fully processed, so that you can remove it before proceeding
with the downgrade.
Some upgrades may require that you drop and re-create database
objects when you move from one MySQL series to the next. For
example, collation changes might require that table indexes be
rebuilt. Such operations, if necessary, will be detailed at
Section 2.18.1.2, “Upgrading from MySQL 4.1 to 5.0”. It is safest to
perform these operations separately on the slaves and the master,
and to disable replication of these operations from the master to
the slave. To achieve this, use the following procedure:
Stop all the slaves and upgrade them. Restart them with the
--skip-slave-start option so
that they do not connect to the master. Perform any table
repair or rebuilding operations needed to re-create database
objects, such as use of REPAIR TABLE or
ALTER TABLE, or dumping and reloading
tables or triggers.
Disable the binary log on the master. To do this without
restarting the master, execute a SET sql_log_bin =
0 statement. Alternatively, stop the master and
restart it without the
--log-bin option. If you
restart the master, you might also want to disallow client
connections. For example, if all clients connect via TCP/IP,
use the --skip-networking
option when you restart the master.
With the binary log disabled, perform any table repair or
rebuilding operations needed to re-create database objects.
The binary log must be disabled during this step to prevent
these operations from being logged and sent to the slaves
later.
Re-enable the binary log on the master. If you set
sql_log_bin to 0 earlier,
execute a SET sql_log_bin = 1 statement. If
you restarted the master to disable the binary log, restart it
with --log-bin, and without
--skip-networking so that
clients and slaves can connect.
Restart the slaves, this time without the
--skip-slave-start option.
Questions 16.3.4.1:
How do I configure a slave if the master is running and I do
not want to stop it?
16.3.4.2:
Does the slave need to be connected to the master all the
time?
16.3.4.3:
How do I know how late a slave is compared to the master? In
other words, how do I know the date of the last statement
replicated by the slave?
16.3.4.4:
How do I force the master to block updates until the slave
catches up?
16.3.4.5:
What issues should I be aware of when setting up two-way
replication?
16.3.4.6:
How can I use replication to improve performance of my
system?
16.3.4.7:
What should I do to prepare client code in my own
applications to use performance-enhancing replication?
16.3.4.8:
When and how much can MySQL replication improve the
performance of my system?
16.3.4.9:
How do I prevent GRANT and REVOKE statements from
replicating to slave machines?
16.3.4.10:
Does replication work on mixed operating systems (for
example, the master runs on Linux while slaves run on Mac OS
X and Windows)?
16.3.4.11:
Does replication work on mixed hardware architectures (for
example, the master runs on a 64-bit machine while slaves
run on 32-bit machines)?
Questions and Answers 16.3.4.1:
How do I configure a slave if the master is running and I do
not want to stop it?
There are several possibilities. If you have taken a
snapshot backup of the master at some point and recorded the
binary log file name and offset (from the output of
SHOW MASTER STATUS)
corresponding to the snapshot, use the following procedure:
Make sure that the slave is assigned a unique server ID.
Execute the following statement on the slave, filling in
appropriate values for each option:
mysql> CHANGE MASTER TO
-> MASTER_HOST='master_host_name',
-> MASTER_USER='master_user_name',
-> MASTER_PASSWORD='master_pass',
-> MASTER_LOG_FILE='recorded_log_file_name',
-> MASTER_LOG_POS=recorded_log_position;
Execute START SLAVE on
the slave.
If you do not have a backup of the master server, here is a
quick procedure for creating one. All steps should be
performed on the master host.
Issue this statement to acquire a global read lock:
mysql> FLUSH TABLES WITH READ LOCK;
With the lock still in place, execute this command (or a
variation of it):
shell> tar zcf /tmp/backup.tar.gz /var/lib/mysql
Issue this statement and record the output, which you
will need later:
mysql> SHOW MASTER STATUS;
Release the lock:
mysql> UNLOCK TABLES;
An alternative to using the preceding procedure to make a
binary copy is to make an SQL dump of the master. To do
this, you can use mysqldump --master-data
on your master and later load the SQL dump into your slave.
However, this is slower than making a binary copy.
Regardless of which of the two methods you use, afterward
follow the instructions for the case when you have a
snapshot and have recorded the log file name and offset. You
can use the same snapshot to set up several slaves. Once you
have the snapshot of the master, you can wait to set up a
slave as long as the binary logs of the master are left
intact. The two practical limitations on the length of time
you can wait are the amount of disk space available to
retain binary logs on the master and the length of time it
takes the slave to catch up.
16.3.4.2:
Does the slave need to be connected to the master all the
time?
No, it does not. The slave can go down or stay disconnected
for hours or even days, and then reconnect and catch up on
updates. For example, you can set up a master/slave
relationship over a dial-up link where the link is up only
sporadically and for short periods of time. The implication
of this is that, at any given time, the slave is not
guaranteed to be in synchrony with the master unless you
take some special measures.
16.3.4.3:
How do I know how late a slave is compared to the master? In
other words, how do I know the date of the last statement
replicated by the slave?
You can read the Seconds_Behind_Master
column in SHOW SLAVE STATUS.
See Section 16.4.1, “Replication Implementation Details”.
When the slave SQL thread executes an event read from the
master, it modifies its own time to the event timestamp.
(This is why TIMESTAMP is
well replicated.) In the Time column in
the output of SHOW
PROCESSLIST, the number of seconds displayed for
the slave SQL thread is the number of seconds between the
timestamp of the last replicated event and the real time of
the slave machine. You can use this to determine the date of
the last replicated event. Note that if your slave has been
disconnected from the master for one hour, and then
reconnects, you may immediately see Time
values like 3600 for the slave SQL thread in
SHOW PROCESSLIST. This is
because the slave is executing statements that are one hour
old.
16.3.4.4:
How do I force the master to block updates until the slave
catches up?
Use the following procedure:
On the master, execute these statements:
mysql> FLUSH TABLES WITH READ LOCK;
mysql> SHOW MASTER STATUS;
Record the replication coordinates (the log file name
and offset) from the output of the
SHOW statement.
On the slave, issue the following statement, where the
arguments to the
MASTER_POS_WAIT()
function are the replication coordinate values obtained
in the previous step:
mysql> SELECT MASTER_POS_WAIT('log_name', log_offset);
The SELECT statement
blocks until the slave reaches the specified log file
and offset. At that point, the slave is in synchrony
with the master and the statement returns.
On the master, issue the following statement to allow
the master to begin processing updates again:
mysql> UNLOCK TABLES;
16.3.4.5:
What issues should I be aware of when setting up two-way
replication?
MySQL replication currently does not support any locking
protocol between master and slave to guarantee the atomicity
of a distributed (cross-server) update. In other words, it
is possible for client A to make an update to co-master 1,
and in the meantime, before it propagates to co-master 2,
client B could make an update to co-master 2 that makes the
update of client A work differently than it did on co-master
1. Thus, when the update of client A makes it to co-master
2, it produces tables that are different from what you have
on co-master 1, even after all the updates from co-master 2
have also propagated. This means that you should not chain
two servers together in a two-way replication relationship
unless you are sure that your updates can safely happen in
any order, or unless you take care of mis-ordered updates
somehow in the client code.
You should also realize that two-way replication actually
does not improve performance very much (if at all) as far as
updates are concerned. Each server must do the same number
of updates, just as you would have a single server do. The
only difference is that there is a little less lock
contention because the updates originating on another server
are serialized in one slave thread. Even this benefit might
be offset by network delays.
16.3.4.6:
How can I use replication to improve performance of my
system?
You should set up one server as the master and direct all
writes to it. Then configure as many slaves as you have the
budget and rackspace for, and distribute the reads among the
master and the slaves. You can also start the slaves with
the
--skip-innodb,
--skip-bdb,
--low-priority-updates, and
--delay-key-write=ALL options
to get speed improvements on the slave end. In this case,
the slave uses nontransactional MyISAM
tables instead of InnoDB and
BDB tables to get more speed by
eliminating transactional overhead.
16.3.4.7:
What should I do to prepare client code in my own
applications to use performance-enhancing replication?
If the part of your code that is responsible for database
access has been properly abstracted/modularized, converting
it to run with a replicated setup should be very smooth and
easy. Change the implementation of your database access to
send all writes to the master, and to send reads to either
the master or a slave. If your code does not have this level
of abstraction, setting up a replicated system gives you the
opportunity and motivation to it clean up. Start by creating
a wrapper library or module that implements the following
functions:
safe_writer_connect()
safe_reader_connect()
safe_reader_statement()
safe_writer_statement()
safe_ in each function name means that
the function takes care of handling all error conditions.
You can use different names for the functions. The important
thing is to have a unified interface for connecting for
reads, connecting for writes, doing a read, and doing a
write.
Then convert your client code to use the wrapper library.
This may be a painful and scary process at first, but it
pays off in the long run. All applications that use the
approach just described are able to take advantage of a
master/slave configuration, even one involving multiple
slaves. The code is much easier to maintain, and adding
troubleshooting options is trivial. You need modify only one
or two functions; for example, to log how long each
statement took, or which statement among those issued gave
you an error.
If you have written a lot of code, you may want to automate
the conversion task by using the replace
utility that comes with standard MySQL distributions, or
write your own conversion script. Ideally, your code uses
consistent programming style conventions. If not, then you
are probably better off rewriting it anyway, or at least
going through and manually regularizing it to use a
consistent style.
16.3.4.8:
When and how much can MySQL replication improve the
performance of my system?
MySQL replication is most beneficial for a system that
processes frequent reads and infrequent writes. In theory,
by using a single-master/multiple-slave setup, you can scale
the system by adding more slaves until you either run out of
network bandwidth, or your update load grows to the point
that the master cannot handle it.
To determine how many slaves you can use before the added
benefits begin to level out, and how much you can improve
performance of your site, you need to know your query
patterns, and to determine empirically by benchmarking the
relationship between the throughput for reads (reads per
second, or reads) and for writes
(writes) on a typical master and a
typical slave. The example here shows a rather simplified
calculation of what you can get with replication for a
hypothetical system.
Let's say that system load consists of 10% writes and 90%
reads, and we have determined by benchmarking that
reads is 1200 – 2 ?
writes. In other words, the system can do
1,200 reads per second with no writes, the average write is
twice as slow as the average read, and the relationship is
linear. Let us suppose that the master and each slave have
the same capacity, and that we have one master and
N slaves. Then we have for each
server (master or slave):
reads = 1200 – 2 ? writes
reads = 9 ? writes /
(N + 1) (reads are
split, but writes go to all servers)
9 ? writes / (N +
1) + 2 ? writes = 1200
writes = 1200 / (2 +
9/(N+1))
The last equation indicates the maximum number of writes for
N slaves, given a maximum
possible read rate of 1,200 per minute and a ratio of nine
reads per write.
This analysis yields the following conclusions:
If N = 0 (which means we have
no replication), our system can handle about 1200/11 =
109 writes per second.
If N = 1, we get up to 184
writes per second.
If N = 8, we get up to 400
writes per second.
If N = 17, we get up to 480
writes per second.
Eventually, as N approaches
infinity (and our budget negative infinity), we can get
very close to 600 writes per second, increasing system
throughput about 5.5 times. However, with only eight
servers, we increase it nearly four times.
Note that these computations assume infinite network
bandwidth and neglect several other factors that could be
significant on your system. In many cases, you may not be
able to perform a computation similar to the one just shown
that accurately predicts what will happen on your system if
you add N replication slaves.
However, answering the following questions should help you
decide whether and by how much replication will improve the
performance of your system:
What is the read/write ratio on your system?
How much more write load can one server handle if you
reduce the reads?
For how many slaves do you have bandwidth available on
your network?
16.3.4.9:
How do I prevent GRANT and REVOKE statements from
replicating to slave machines?
Start the server with the
--replicate-wild-ignore-table=mysql.%
option.
16.3.4.10:
Does replication work on mixed operating systems (for
example, the master runs on Linux while slaves run on Mac OS
X and Windows)?
Yes.
16.3.4.11:
Does replication work on mixed hardware architectures (for
example, the master runs on a 64-bit machine while slaves
run on 32-bit machines)?
Yes.
16.3.5. Troubleshooting Replication
If you have followed the instructions, and your replication setup
is not working, the first thing to do is check the error
log for messages. Many users have lost time by not
doing this soon enough after encountering problems.
If you cannot tell from the error log what the problem was, try
the following techniques:
Verify that the master has binary logging enabled by issuing a
SHOW MASTER STATUS statement.
If logging is enabled, Position is nonzero.
If binary logging is not enabled, verify that you are running
the master with the --log-bin
and --server-id options.
Verify that the slave is running. Use
SHOW SLAVE STATUS to check
whether the Slave_IO_Running and
Slave_SQL_Running values are both
Yes. If not, verify the options that were
used when starting the slave server. For example,
--skip-slave-start prevents the
slave threads from starting until you issue a
START SLAVE statement.
If the slave is running, check whether it established a
connection to the master. Use SHOW
PROCESSLIST, find the I/O and SQL threads and check
their State column to see what they
display. See
Section 16.4.1, “Replication Implementation Details”. If the
I/O thread state says Connecting to master,
verify the privileges for the replication user on the master,
the master host name, your DNS setup, whether the master is
actually running, and whether it is reachable from the slave.
If the slave was running previously but has stopped, the
reason usually is that some statement that succeeded on the
master failed on the slave. This should never happen if you
have taken a proper snapshot of the master, and never modified
the data on the slave outside of the slave thread. If the
slave stops unexpectedly, it is a bug or you have encountered
one of the known replication limitations described in
Section 16.3.1, “Replication Features and Issues”. If it is a bug, see
Section 16.3.6, “How to Report Replication Bugs or Problems”, for instructions on how to
report it.
MySQL Enterprise
For immediate notification whenever a slave stops, subscribe
to the MySQL Enterprise Monitor. For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
If a statement that succeeded on the master refuses to run on
the slave, try the following procedure if it is not feasible
to do a full database resynchronization by deleting the
slave's databases and copying a new snapshot from the master:
Determine whether the affected table on the slave is
different from the master table. Try to understand how
this happened. Then make the slave's table identical to
the master's and run START
SLAVE.
If the preceding step does not work or does not apply, try
to understand whether it would be safe to make the update
manually (if needed) and then ignore the next statement
from the master.
If you decide that you can skip the next statement from
the master, issue the following statements:
mysql> SET GLOBAL sql_slave_skip_counter = N;
mysql> START SLAVE;
The value of N should be 1 if
the next statement from the master does not use
AUTO_INCREMENT or
LAST_INSERT_ID().
Otherwise, the value should be 2. The reason for using a
value of 2 for statements that use
AUTO_INCREMENT or
LAST_INSERT_ID() is that
they take two events in the binary log of the master.
See also
Section 12.6.2.6, “SET GLOBAL sql_slave_skip_counter Syntax”.
If you are sure that the slave started out perfectly
synchronized with the master, and that no one has updated
the tables involved outside of the slave thread, then
presumably the discrepancy is the result of a bug. If you
are running the most recent version of MySQL, please
report the problem. If you are running an older version,
try upgrading to the latest production release to
determine whether the problem persists.
16.3.6. How to Report Replication Bugs or Problems
When you have determined that there is no user error involved, and
replication still either does not work at all or is unstable, it
is time to send us a bug report. We need to obtain as much
information as possible from you to be able to track down the bug.
Please spend some time and effort in preparing a good bug report.
If you have a repeatable test case that demonstrates the bug,
please enter it into our bugs database using the instructions
given in Section 1.7, “How to Report Bugs or Problems”. If you have a
“phantom” problem (one that you cannot duplicate at
will), use the following procedure:
Verify that no user error is involved. For example, if you
update the slave outside of the slave thread, the data goes
out of synchrony, and you can have unique key violations on
updates. In this case, the slave thread stops and waits for
you to clean up the tables manually to bring them into
synchrony. This is not a replication problem. It is
a problem of outside interference causing replication to
fail.
Run the slave with the
--log-slave-updates and
--log-bin options. These
options cause the slave to log the updates that it receives
from the master into its own binary logs.
Save all evidence before resetting the replication state. If
we have no information or only sketchy information, it becomes
difficult or impossible for us to track down the problem. The
evidence you should collect is:
All binary logs from the master
All binary logs from the slave
The output of SHOW MASTER
STATUS from the master at the time you
discovered the problem
The output of SHOW SLAVE
STATUS from the slave at the time you discovered
the problem
Error logs from the master and the slave
Use mysqlbinlog to examine the binary logs.
The following should be helpful to find the problem statement.
log_pos and
log_file are the
Master_Log_File and
Read_Master_Log_Pos values from
SHOW SLAVE STATUS.
shell> mysqlbinlog --start-position=log_pos log_file | head
After you have collected the evidence for the problem, try to
isolate it as a separate test case first. Then enter the problem
with as much information as possible into our bugs database using
the instructions at Section 1.7, “How to Report Bugs or Problems”.
16.4. Replication Implementation
The basic mechanics of replication is based on the master server
keeping track of all changes to your databases (updates, deletes,
and so on) in its binary logs. The binary log serves as a written
record of each to the database from the moment the database was
started. The binary log contains records of all the statements which
edit or modify either the database structure or the data that the
structure contains. Typically SELECT
statements are not recorded, as they do not modify the database data
or structure.
Each slave that connects to the master receives a copy of the binary
log, and executes the events within the binary log. This has the
effect of repeating the original statements and changes just as they
were made on the master. Tables are created or their structure
modified, and data is inserted, deleted and updated according to the
statements that were originally executed on the master.
Because each slave is independent, the replaying of the statements
in the masters binary log can occur on each slave that is connected
to the master. In addition, because each slave only receives a copy
of the binary log by requesting it from the master (it pulls the
data from the master, rather than the master pushing the data to the
slave), the slave is able to read and update the copy of the
database at its own pace and rate and can start and stop the
replication process at will without affecting the master or the
slaves ability to update to the latest database status.
For more information on the specifics of the replication
implementation, see
Section 16.4.1, “Replication Implementation Details”.
Slaves and masters report their status in respect of the replication
process regularly so that you can monitor the situation. For
information on slave states, see
Section 7.5.6.6, “Replication Slave I/O Thread States”, and
Section 7.5.6.7, “Replication Slave SQL Thread States”. For master states, see
Section 7.5.6.5, “Replication Master Thread States”.
The master binary log is written to a local relay log on the slave
before it is processed. The slave also records information about the
current position with the master's binary log and the local relayed
log. See Section 16.4.2, “Replication Relay and Status Files”.
Databases and tables are updated on the slave according to a set of
rules that are applied according to the various configuration
options and variables that control statement evaluation. For details
on how these rules are applied, see
Section 16.4.3, “How Servers Evaluate Replication Filtering Rules”.
16.4.1. Replication Implementation Details
MySQL replication capabilities are implemented using three threads
(one on the master server and two on the slave):
Slave I/O thread.
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 slave I/O thread reads the updates that the master'
Binlog Dump thread sends (see next item)
and copies them to local files — known as
relay logs ? in the slave's
data directory.
The state of this thread is shown as
Slave_IO_running in the output of
SHOW SLAVE STATUS or as
Slave_running in the output
of SHOW STATUS.
Binlog dump thread.
The master creates a thread to send the binary log contents
to the slave. This thread can be identified in the output of
SHOW PROCESSLIST on the
master as the Binlog Dump thread.
The binlog dump thread acquires a lock on the master's
binary log for reading each event that is to be sent to the
slave. As soon as the event has been read, the lock is
released, even before the event is sent to the slave.
Slave SQL thread.
The slave creates this thread to read the relay logs that
were written by the slave I/O thread. The slave
SQL thread is also used to execute the updates
contained in the relay logs.
MySQL Enterprise
For constant monitoring of the status of slaves, subscribe to
the MySQL Enterprise Monitor. For more information, see
http://www.mysql.com/products/enterprise/advisors.html.
In the preceding description, there are three threads per
master/slave connection. A master that has multiple slaves creates
one binlog dump 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. See
Section 7.5.6, “Examining Thread Information”, for descriptions of all
replicated-related states.
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
Section 16.3.4, “Replication FAQ”. The amount of time that the
slave lags behind the master that is required before the master
determines that the slave is no longer connected — as with
any other client connection — is dependent on the values of
net_write_timeout and
net_retry_count; for more information about
these, see Section 5.1.3, “Server System Variables”.
16.4.2. Replication Relay and Status Files
By default, relay log file names have the form
host_name-relay-bin.nnnnnnhost_name is the name of the
slave server host and nnnnnn is a
sequence number. Successive relay log files are created using
successive sequence numbers, beginning with
000001. The slave uses an index file to track
the relay log files currently in use. The default relay log index
file name is
host_name-relay-bin.index
The default file names for relay logs and relay log index files
can be overridden with, respectively, the
--relay-log and
--relay-log-index server options
(see Section 16.1.2, “Replication and Binary Logging Options and Variables”). For this reason,
changing a replication slave's host name can cause
replication to fail with the errors Failed to open the
relay log and Could not find target log
during relay log initialization. This is a known issue
which we intend to fix in a future MySQL release (see Bug#2122).
If you anticipate that a slave's host name may change in the
future (for example, if networking is set up on the slave such
that its host name can be modified via DHCP), then you can use
these options to prevent this problem from occurring. However, if
you encounter this issue, one way to work around it is to stop the
slave server, prepend the contents of the old relay log index file
to the new one, then restart the slave. On a Unix system, this can
be done as shown here, where
new_host_name is the new host name and
old_host_name is the old one:
shell> cat new_host_name-relay-bin.index >> old_host_name-relay-bin.index
shell> mv old_host_name-relay-bin.index new_host_name-relay-bin.index
Relay logs have the same format as binary logs and can be read
using mysqlbinlog. The SQL thread automatically
deletes each relay log file as soon as it has executed all events
in the file and no longer needs it. There is no explicit mechanism
for deleting relay logs because the SQL thread takes care of doing
so. However, FLUSH
LOGS rotates relay logs, which influences when the SQL
thread deletes them.
A slave server creates a new relay log file under the following
conditions:
Each time the I/O thread starts.
When the logs are flushed; for example, with
FLUSH LOGS or
mysqladmin flush-logs.
When the size of the current relay log file becomes too large.
The meaning of “too large” is determined as
follows:
A slave replication server creates two additional small files in
the data directory. These status files are
named master.info and
relay-log.info by default. Their names can be
changed by using the
--master-info-file and
--relay-log-info-file options. See
Section 16.1.2, “Replication and Binary Logging Options and Variables”.
The two status files contain information like that shown in the
output of the SHOW SLAVE STATUS
statement, which is discussed in
Section 12.6.2, “SQL Statements for Controlling Slave Servers”. Because the status files
are stored on disk, they survive a slave server's shutdown. The
next time the slave starts up, it reads the two files to determine
how far it has proceeded in reading binary logs from the master
and in processing its own relay logs.
The I/O thread updates the master.info file.
The following table shows the correspondence between the lines in
the file and the columns displayed by SHOW
SLAVE STATUS.
The SQL thread updates the relay-log.info
file. The following table shows the correspondence between the
lines in the file and the columns displayed by
SHOW SLAVE STATUS.
The contents of the relay-log.info file and
the states shown by the SHOW SLAVE STATES
command may not match if the relay-log.info
file has not been flushed to disk. Ideally, you should only view
relay-log.info on a slave that is offline
(that is, mysqld is not running). For a running
system, SHOW SLAVE STATUS should be
used.
When you back up the slave's data, you should back up these two
status files as well, along with the relay log files. They are
needed to resume replication after you restore the slave's data.
If you lose the relay logs but still have the
relay-log.info file, you can check it to
determine how far the SQL thread has executed in the master binary
logs. Then you can use CHANGE MASTER
TO with the MASTER_LOG_FILE and
MASTER_LOG_POS options to tell the slave to
re-read the binary logs from that point. Of course, this requires
that the binary logs still exist on the master server.
If your slave is subject to replicating
LOAD DATA
INFILE statements, you should also back up any
SQL_LOAD-* files that exist in the directory
that the slave uses for this purpose. The slave needs these files
to resume replication of any interrupted
LOAD DATA
INFILE operations. The directory location is specified
using the --slave-load-tmpdir
option. If this option is not specified, the directory location is
the value of the tmpdir system variable.
16.4.3. How Servers Evaluate Replication Filtering Rules
If a master server does not write a statement to its binary log,
the statement is not replicated. If the server does log the
statement, the statement is sent to all slaves and each slave
determines whether to execute it or ignore it.
On the master you can control which databases write events to the
binary log using the --binlog-do-db
and --binlog-ignore-db options to
control binary logging. For a description of the rules that
servers use in evaluating these options, see
Section 16.4.3.1, “Evaluation of Database-Level Replication and Binary Logging Options”. You should not use
these options to control the databases and tables that are
replicated, instead, use filtering on the slave to control the
events that are executed on the slave.
On the slave side, decisions about whether to execute or ignore
statements received from the master are made according to the
--replicate-* options that the slave was started
with. (See Section 16.1.2, “Replication and Binary Logging Options and Variables”.) The slave
evaluates these options using the following procedure, which first
checks the database-level options and then the table-level
options.
In the simplest case, when there are no
--replicate-* options, the procedure yields the
result that the slave executes all statements that it receives
from the master. Otherwise, the result depends on the particular
options given. In general, to make it easier to determine what
effect an option set will have, it is recommended that you avoid
mixing “do” and “ignore” options, or
wildcard and nonwildcard options.
Database-level options
(--replicate-do-db,
--replicate-ignore-db) are checked
first; see Section 16.4.3.1, “Evaluation of Database-Level Replication and Binary Logging Options”, for a
description of this process. If no matching database-level options
are found, option checking proceeds to any table-level options
that may be in use, as discussed in
Section 16.4.3.2, “Evaluation of Table-Level Replication Options”.
16.4.3.1. Evaluation of Database-Level Replication and Binary Logging Options
When evaluating replication options, the slave begins by
checking to see whether there are any
--replicate-do-db or
--replicate-ignore-db options
that apply. When using
--binlog-do-db or
--binlog-ignore-db, the process
is similar, but the options are checked on the master.
Checking of the database-level options proceeds as shown in the
following diagram.
The steps involved are listed here:
Are there any
--replicate-do-db options?
Are there any
--replicate-ignore-db
options?
Proceed to checking the table-level replication options, if
there are any. For a description of how these options are
checked, see
Section 16.4.3.2, “Evaluation of Table-Level Replication Options”.
Important
A statement that is not yet disallowed at this stage is
not yet actually executed. The statement is not executed
until all table-level options (if any) have also been
checked, and the outcome of that process permits execution
of the statement.
For binary logging, the steps involved are listed here:
Important
An exception is made in the rules just given for the
CREATE DATABASE,
ALTER DATABASE, and
DROP DATABASE statements. In
those cases, the database being created, altered, or
dropped replaces the default database when
determining whether to log or to ignore updates.
--binlog-do-db can sometimes mean
“ignore other databases”. For example, a server
running with only
--binlog-do-db=sales does not
write to the binary log statements for which the default
database differs from sales.
16.4.3.2. Evaluation of Table-Level Replication Options
The slave checks for and evaluates table options only if no
matching database options were found (see
Section 16.4.3.1, “Evaluation of Database-Level Replication and Binary Logging Options”).
First, as a preliminary condition, the slave checks whether the
statement occurs within a stored function, in which case the
slave executes the statement and exits.
Having reached this point, if there are no table options, the
slave simply executes all statements. If there are any
--replicate-do-table or
--replicate-wild-do-table
options, the statement must match one these if it is to be
executed; otherwise, it is ignored. If there are any
--replicate-ignore-table or
--replicate-wild-ignore-table
options, all statements are executed except those that match any
of these options. This process is illustrated in the following
diagram:
The following steps describe this evaluation in more detail:
16.4.3.3. Replication Rule Application
This section provides additional explanation and examples of
usage for different combinations of replication filtering
options.
Some typical combinations of replication filter rule types are
given in the following table:
A more complex example follows.
Suppose that we have two tables mytbl1 in
database db1 and mytbl2 in
database db2 on the master, and the slave is
running with the following options (and no other replication
filtering options):
replicate-ignore-db = db1
replicate-do-table = db2.tbl2
Now we execute the following statements on the master:
USE db1;
INSERT INTO db2.tbl2 VALUES (1);
The outcome may not match initial expectations, because the
USE statement causes db1
to be the default database. Thus the
--replicate-ignore-db option
matches, which causes the INSERT
statement to be ignored. Because there was a match with a
database-level option, the table options are not checked;
processing immediately moves to the next statement executed on
the master.
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