Section 5.5. How to Set Up Replication

5.5. How to Set Up Replication

This section briefly describes how to set up complete replication of a MySQL server. It assumes that you want to replicate all databases on the master and have not previously configured replication. You must shut down your master server briefly to complete the steps outlined here.

This procedure is written in terms of setting up a single slave, but you can repeat it to set up multiple slaves.

Although this method is the most straightforward way to set up a slave, it is not the only one. For example, if you have a snapshot of the master's data, and the master already has its server ID set and binary logging enabled, you can set up a slave without shutting down the master or even blocking updates to it. For more details, please see Section 5.11, "Replication FAQ."

If you want to administer a MySQL replication setup, we suggest that you read this entire chapter through and try all replication-related SQL statements mentioned in the "MySQL Language Reference." You should also familiarize yourself with the replication startup options described in Section 5.9, "Replication Startup Options."

Note: This procedure and some of the replication SQL statements shown in later sections require the SUPER privilege.

Make sure that the versions of MySQL installed on the master and slave are compatible according to the table shown in Section 5.6, "Replication Compatibility Between MySQL Versions." Ideally, you should use the most recent version of MySQL on both master and slave.

If you encounter a problem, please do not report it as a bug until you have verified that the problem is present in the latest MySQL release.

Set up an account on the master server that the slave server can use to connect. This account must be given the REPLICATION SLAVE privilege. If the account is used only for replication (which is recommended), you don't need to grant any additional privileges.

Suppose that your domain is mydomain.com and that you want to create an account with a username of repl such that slave servers can use the account to access the master server from any host in your domain using a password of slavepass. To create the account, use this GRANT statement:

mysql> GRANT REPLICATION SLAVE ON *.*
        -> TO '  repl  @' %.mydomain.com    IDENTIFIED BY '  slavepass  ;

If you plan to use the LOAD TABLE FROM MASTER or LOAD DATA FROM MASTER statements from the slave host, you must grant this account additional privileges:

Grant the account the SUPER and RELOAD global privileges.
Grant the SELECT privilege for all tables that you want to load. Any master tables from which the account cannot SELECT will be ignored by LOAD DATA FROM MASTER.

For additional information about setting up user accounts and privileges, see Section 4.9, "MySQL User Account Management."

Flush all the tables and block write statements by executing a 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. When you have acquired a global read lock, you can start a filesystem snapshot of your InnoDB tables. Internally (inside the InnoDB storage engine) the snapshot won't be consistent (because the InnoDB caches are not flushed), but this is not a cause for concern, because InnoDB resolves this at startup and delivers a consistent result. This means that InnoDB can perform crash recovery when started on this snapshot, without corruption. However, there is no way to stop the MySQL server while ensuring a consistent snapshot of your InnoDB tables.

Leave running the client from which you issue the FLUSH TABLES statement so that the read lock remains in effect. (If you exit the client, the lock is released.) Then take a snapshot of the data on your master server.

The easiest way to create a snapshot is to use an archiving program to make a binary backup of the databases in your master's data directory. For example, use tar on Unix, or PowerArchiver, WinRAR, WinZip, or any similar software on Windows. To use tar to create an archive that includes all databases, change location into the master server's data directory, and then execute this command:

shell> tar -cvf /tmp/mysql-snapshot.tar .

If you want the archive to include only a database called this_db, use this command instead:

shell> tar -cvf /tmp/mysql-snapshot.tar ./this_db

Then copy the archive file to the /tmp directory on the slave server host. On that machine, change location into the slave's data directory, and unpack the archive file using this command:

shell> tar -xvf /tmp/mysql-snapshot.tar

You may not want to replicate the mysql database if the slave server has a different set of user accounts from those that exist on the master. In this case, you should exclude it from the archive. You also need not include any log files in the archive, or the master.info or relay-log.info files.

While the read lock placed by FLUSH TABLES WITH READ LOCK is in effect, read the value of the current binary log name and offset on the master:

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 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 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.

After you have taken the snapshot and recorded the log name and offset, you can reenable write activity on the master:

mysql> UNLOCK TABLES;

If you are using InnoDB tables, ideally you should use the InnoDB Hot Backup tool, which takes a consistent snapshot without acquiring any locks on the master server, and records the log name and offset corresponding to the snapshot to be later used on the slave. Hot Backup is an additional non-free (commercial) tool that is not included in the standard MySQL distribution. See the InnoDB Hot Backup home page at http://www.innodb.com/manual.php for detailed information.

Without the Hot Backup tool, the quickest way to take a binary snapshot of InnoDB tables is to shut down the master server and copy the InnoDB data files, log files, and table format files (.frm files). To record the current log file name and offset, you should issue the following statements before you shut down the server:

mysql> FLUSH TABLES WITH READ LOCK;
mysql> SHOW MASTER STATUS;

Then record the log name and the offset from the output of SHOW MASTER STATUS as was shown earlier. After recording the log name and the offset, shut down the server without unlocking the tables to make sure that the server goes down with the snapshot corresponding to the current log file and offset:

shell> mysqladmin -u root shutdown

An alternative that works for both MyISAM and InnoDB tables is to take an SQL dump of the master instead of a binary copy as described in the preceding discussion. For this, you can use mysqldump --master-data on your master and later load the SQL dump file into your slave. However, this is slower than doing a binary copy.

Make sure that the [mysqld] section of the my.cnf file on the master host includes a logbin option. The section should also have a server-id=master_id option, where master_id must be a positive integer value from 1 to 2³² 1. For example:

[mysqld]
log-bin=mysql-bin
server-id=1

If those options are not present, add them and restart the server. The server cannot act as a replication master unless binary logging is enabled.

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, sync_binlog=1, and, before MySQL 5.0.3, innodb_safe_binlog, in the master my.cnf file. (innodb_safe_binlog is not needed from 5.0.3 on.)

Stop the server that is to be used as a slave and add the following lines to its my.cnf file:

[mysqld]
server-id=slave_id

The slave_id value, like the master_id value, must be a positive integer value from 1 to 2³² 1. In addition, it is necessary that the ID of the slave be different from the ID of the master. 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 is set to 1 if you have not defined master-host; otherwise it is set to 2. Note that in the case of server-id omission, 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.

If you made a binary backup of the master server's data, copy it to the slave server's data directory before starting the slave. Make sure that the privileges on the files and directories are correct. The system account that you use to run the slave server must be able to read and write the files, just as on the master.

If you made a backup using mysqldump, start the slave first. The dump file is loaded in a later step.

Start the slave server. If it has been replicating previously, start the slave server with the --skip-slave-start option so that it doesn't immediately try to connect to its master. You also may want to start the slave server with the --log-warnings option to get more messages in the error log about problems (for example, network or connection problems). The option is enabled by default, but aborted connections are not logged to the error log unless the option value is greater than 1.

If you made a backup of the master server's data using mysqldump, load the dump file into the slave server:

shell> mysql -u root -p < dump_file.sql

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;

The following table shows the maximum allowable length for the string-valued options:

MASTER_HOST
60
MASTER_USER
16
MASTER_PASSWORD
32
MASTER_LOG_FILE
255

10.

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.

Note: The content of master.info overrides some of the server options specified on the command line or in my.cnf. See Section 5.9, "Replication Startup Options," for more details.

Once you have a snapshot of the master, you can use it to set up other slaves by following the slave portion of the procedure just described. You do not need to take another snapshot of the master; you can use the same one for each slave.