Chapter 6. Replication
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## Chapter 6. Replication

This chapter describes the various replication features provided by MySQL. It introduces replication concepts, shows how to set up replication servers, and serves as a reference to the available replication options. It also provides a list of frequently asked questions (with answers), and troubleshooting advice for solving replication problems.

MySQL Enterprise The MySQL Network Monitoring and Advisory Service provides numerous advisors that provide immediate feedback about replication-related problems. For more information see, http://www.mysql.com/products/enterprise/advisors.html.

For a description of the syntax of replication-related SQL statements, see Section 13.6, “Replication Statements”.

## 6.1. Introduction to Replication

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 15, MySQL Cluster).

In single-master replication, the master server writes updates to its binary log files and maintains an index of those files to keep track of log rotation. The binary log files serve as a record of updates to be sent to any slave servers. When a slave connects to its master, it informs the master of the position up to which the slave read the logs at its last successful update. The slave receives any updates that have taken place since that time, and then blocks and waits for the master to notify it of new updates.

A slave server can itself serve as a master if you want to set up chained replication servers.

Multiple-master replication is possible, but raises issues not present in single-master replication. See Section 6.13, “Auto-Increment in Multiple-Master Replication”.

When you are using replication, all updates to the tables that are replicated should be performed on the master server. Otherwise, you must always be careful to avoid conflicts between updates that users make to tables on the master and updates that they make to tables on the slave.

Replication offers benefits for robustness, speed, and system administration:

- Robustness is increased with a master/slave setup. In the event of problems with the master, you can switch to the slave as a backup.

- Better response time for clients can be achieved by splitting the load for processing client queries between the master and slave servers. `SELECT` queries may be sent to the slave to reduce the query processing load of the master. Statements that modify data should still be sent to the master so that the master and slave do not get out of synchrony. This load-balancing strategy is effective if non-updating queries dominate, but that is the normal case.

- Another benefit of using replication is that you can perform database backups using a slave server without disturbing the master. The master continues to process updates while the backup is being made. See Section 5.10.1, “Database Backups”.

## 6.2. Replication Implementation Overview

MySQL replication is based on the master server keeping track of all changes to your databases (updates, deletes, and so on) in its binary logs. Therefore, to use replication, you must enable binary logging on the master server. See Section 5.12.3, “The Binary Log”.

Each slave server receives from the master the saved updates that the master has recorded in its binary log, so that the slave can execute the same updates on its copy of the data.

It is extremely important to realize that the binary log is simply a record starting from the fixed point in time at which you enable binary logging. Any slaves that you set up need copies of the databases on your master as they existed at the moment you enabled binary logging on the master. If you start your slaves with databases that are not in the same state as those on the master when the binary log was started, your slaves are quite likely to fail.

After the slave has been set up with a copy of the master's data, it connects to the master and waits for updates to process. If the master fails, or the slave loses connectivity with your master, the slave keeps trying to connect periodically until it is able to resume listening for updates. The `--master-connect-retry` option controls the retry interval. The default is 60 seconds.

Each slave keeps track of where it left off when it last read from its master server. The master has no knowledge of how many slaves it has or which ones are up to date at any given time.

## 6.3. Replication Implementation Details

MySQL replication capabilities are implemented using three threads (one on the master server and two on the slave). When a `START SLAVE` statement is issued on a slave server, the slave creates an I/O thread, which connects to the master and asks it to send the updates recorded in its binary logs. The master creates a thread to send the binary log contents to the slave. This thread can be identified as the `Binlog Dump` thread in the output of `SHOW PROCESSLIST` on the master. The slave I/O thread reads the updates that the master `Binlog Dump` thread sends and copies them to local files, known as relay logs, in the slave's data directory. The third thread is the SQL thread, which the slave creates to read the relay logs and to execute the updates they contain.

In the preceding description, there are three threads per master/slave connection. A master that has multiple slaves creates one thread for each currently-connected slave, and each slave has its own I/O and SQL threads.

The slave uses two threads so that reading updates from the master and executing them can be separated into two independent tasks. Thus, the task of reading statements is not slowed down if statement execution is slow. For example, if the slave server has not been running for a while, its I/O thread can quickly fetch all the binary log contents from the master when the slave starts, even if the SQL thread lags far behind. If the slave stops before the SQL thread has executed all the fetched statements, the I/O thread has at least fetched everything so that a safe copy of the statements is stored locally in the slave's relay logs, ready for execution the next time that the slave starts. This enables the master server to purge its binary logs sooner because it no longer needs to wait for the slave to fetch their contents.

The `SHOW PROCESSLIST` statement provides information that tells you what is happening on the master and on the slave regarding replication. The following example illustrates how the three threads show up in the output from `SHOW PROCESSLIST`.

On the master server, the output from `SHOW PROCESSLIST` looks like this:

    mysql> SHOW PROCESSLIST\G
    *************************** 1. row ***************************
         Id: 2
       User: root
       Host: localhost:32931
         db: NULL
    Command: Binlog Dump
       Time: 94
      State: Has sent all binlog to slave; waiting for binlog to
             be updated
       Info: NULL

Here, thread 2 is a `Binlog Dump` replication thread for a connected slave. The `State` information indicates that all outstanding updates have been sent to the slave and that the master is waiting for more updates to occur. If you see no `Binlog Dump` threads on a master server, this means that replication is not running — that is, that no slaves are currently connected.

On the slave server, the output from `SHOW PROCESSLIST` looks like this:

    mysql> SHOW PROCESSLIST\G
    *************************** 1. row ***************************
         Id: 10
       User: system user
       Host:
         db: NULL
    Command: Connect
       Time: 11
      State: Waiting for master to send event
       Info: NULL
    *************************** 2. row ***************************
         Id: 11
       User: system user
       Host:
         db: NULL
    Command: Connect
       Time: 11
      State: Has read all relay log; waiting for the slave I/O
             thread to update it
       Info: NULL

This information indicates that thread 10 is the I/O thread that is communicating with the master server, and thread 11 is the SQL thread that is processing the updates stored in the relay logs. At the time that the `SHOW PROCESSLIST` was run, both threads were idle, waiting for further updates.

The value in the `Time` column can show how late the slave is compared to the master. See Section 6.10, “Replication FAQ”.

### 6.3.1. Replication Master Thread States

The following list shows the most common states you may see in the `State` column for the master's `Binlog Dump` thread. 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.

- `Sending binlog event to slave`

  Binary logs consist of events, where an event is usually an update plus some other information. The thread has read an event from the binary log and is now sending it to the slave.

- `Finished reading one binlog; switching to next binlog`

  The thread has finished reading a binary log file and is opening the next one to send to the slave.

- `Has sent all binlog to slave; waiting for binlog to be updated`

  The thread has read all outstanding updates from the binary logs and sent them to the slave. The thread is now idle, waiting for new events to appear in the binary log resulting from new updates occurring on the master.

- `Waiting to finalize termination`

  A very brief state that occurs as the thread is stopping.

### 6.3.2. Replication Slave I/O Thread States

The following list shows the most common states you see in the `State` column for a slave server I/O thread. This state also appears in the `Slave_IO_State` column displayed by `SHOW SLAVE STATUS`, so you can get a good view of what is happening by using that statement.

- `Connecting to master`

  The thread is attempting to connect to the master.

- `Checking master version`

  A state that occurs very briefly, after the connection to the master is established.

- `Registering slave on master`

  A state that occurs very briefly after the connection to the master is established.

- `Requesting binlog dump`

  A state that occurs very briefly, after the connection to the master is established. The thread sends to the master a request for the contents of its binary logs, starting from the requested binary log filename and position.

- `Waiting to reconnect after a failed binlog dump request`

  If the binary log dump request failed (due to disconnection), the thread goes into this state while it sleeps, then tries to reconnect periodically. The interval between retries can be specified using the `--master-connect-retry` option.

- `Reconnecting after a failed binlog dump request`

  The thread is trying to reconnect to the master.

- `Waiting for master to send event`

  The thread has connected to the master and is waiting for binary log events to arrive. This can last for a long time if the master is idle. If the wait lasts for `slave_net_timeout` seconds, a timeout occurs. At that point, the thread considers the connection to be broken and makes an attempt to reconnect.

- `Queueing master event to the relay log`

  The thread has read an event and is copying it to the relay log so that the SQL thread can process it.

- `Waiting to reconnect after a failed master event read`

  An error occurred while reading (due to disconnection). The thread is sleeping for `master-connect-retry` seconds before attempting to reconnect.

- `Reconnecting after a failed master event read`

  The thread is trying to reconnect to the master. When connection is established again, the state becomes `Waiting for master to send event`.

- `Waiting for the slave SQL thread to free enough relay log space`

  You are using a non-zero `relay_log_space_limit` value, and the relay logs have grown large enough that their combined size exceeds this value. The I/O thread is waiting until the SQL thread frees enough space by processing relay log contents so that it can delete some relay log files.

- `Waiting for slave mutex on exit`

  A state that occurs briefly as the thread is stopping.

### 6.3.3. Replication Slave SQL Thread States

The following list shows the most common states you may see in the `State` column for a slave server SQL thread:

- `Reading event from the relay log`

  The thread has read an event from the relay log so that the event can be processed.

- `Has read all relay log; waiting for the slave I/O thread to update it`

  The thread has processed all events in the relay log files, and is now waiting for the I/O thread to write new events to the relay log.

- `Waiting for slave mutex on exit`

  A very brief state that occurs as the thread is stopping.

The `State` column for the I/O thread may also show the text of a statement. This indicates that the thread has read an event from the relay log, extracted the statement from it, and is executing it.

### 6.3.4. Replication Relay and Status Files

By default, relay logs filenames have the form ``host_name`-relay-bin.`nnnnnn``, where `host_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 filename is ``host_name`-relay-bin.index`. By default, the slave server creates relay log files in its data directory. The default filenames can be overridden with the `--relay-log` and `--relay-log-index` server options. See Section 6.8, “Replication Startup Options”.

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:

  - If the value of `max_relay_log_size` is greater than 0, that is the maximum relay log file size.

  - If the value of `max_relay_log_size` is 0, `max_binlog_size` determines the maximum relay log file size.

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 6.8, “Replication Startup Options”.

The two status files contain information like that shown in the output of the `SHOW SLAVE STATUS` statement, which is discussed in Section 13.6.2, “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`.

Line | Description
1 | Number of lines in the file
2 | `Master_Log_File`
3 | `Read_Master_Log_Pos`
4 | `Master_Host`
5 | `Master_User`
6 | Password (not shown by `SHOW SLAVE STATUS`)
7 | `Master_Port`
8 | `Connect_Retry`
9 | `Master_SSL_Allowed`
10 | `Master_SSL_CA_File`
11 | `Master_SSL_CA_Path`
12 | `Master_SSL_Cert`
13 | `Master_SSL_Cipher`
14 | `Master_SSL_Key`

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`.

Line | Description
1 | `Relay_Log_File`
2 | `Relay_Log_Pos`
3 | `Relay_Master_Log_File`
4 | `Exec_Master_Log_Pos`

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 (i.e. `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.

## 6.4. 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 6.10, “Replication FAQ”.

If you want to administer a MySQL replication setup, we suggest that you read this entire chapter through and try all statements mentioned in Section 13.6.1, “SQL Statements for Controlling Master Servers”, and Section 13.6.2, “SQL Statements for Controlling Slave Servers”. You should also familiarize yourself with the replication startup options described in Section 6.8, “Replication Startup Options”.

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

1. Make sure that the versions of MySQL installed on the master and slave are compatible according to the table shown in Section 6.5, “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.

2. 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.

   MySQL Enterprise Subscribers to the MySQL Network Monitoring and Advisory Service are quickly notified if there is a replication master and no account with the `REPLICATION SLAVE` privilege. For more information see, http://www.mysql.com/products/enterprise/advisors.html.

   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';

   For additional information about setting up user accounts and privileges, see Section 5.9, “MySQL User Account Management”.

3. 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 insuring 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, 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 re-enable 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.

4. Make sure that the `[mysqld]` section of the `my.cnf` file on the master host includes a `log-bin` 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.)

5. 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.

6. 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.

7. 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.

8. 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

9. 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 6.8, “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.

## 6.5. Replication Compatibility Between MySQL Versions

The binary log format as implemented in MySQL 5.0 is considerably different from that used in previous versions. 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).

We recommend using the most recent MySQL version available because replication capabilities are continually being improved. We also recommend using the same version for both the master and the slave. We recommend upgrading masters and slaves running alpha or beta versions to new (production) versions. Replication from a 5.0.3 master to a 5.0.2 slave will fail; from a 5.0.4 master to a 5.0.3 slave will also fail. In general, slaves running MySQL 5.0.x may be used with older masters (even those running MySQL 3.23, 4.0, or 4.1), but not the reverse. For more information on potential issues, see Section 6.7, “Replication Features and Known Problems”.

Note: You cannot replicate from a master that uses a newer binary log format to a slave that uses an older format (for example, from MySQL 5.0 to MySQL 4.1.) This has significant implications for upgrading replication servers, as described in Section 6.6, “Upgrading a Replication Setup”.

The preceding information pertains to replication compatibility at the protocol level. However, there can be other constraints, such as SQL-level compatibility issues. For example, a 5.0 master cannot replicate to a 4.1 slave if the replicated statements use SQL features available in 5.0 but not in 4.1. These and other issues are discussed in Section 6.7, “Replication Features and Known Problems”.

## 6.6. 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.

### 6.6.1. Upgrading Replication to 5.0

This section applies to upgrading replication from MySQL 3.23, 4.0, or 4.1 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 following the upgrade are in 5.0 format. These too are recognized by the 5.0 slaves.

In other words, there are no measures to take when upgrading to MySQL 5.0, except that 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 logs or relay logs have been fully processed, so that you can remove them before proceeding with the downgrade.

## 6.7. Replication Features and Known Problems

In general, replication compatibility at the SQL level requires that any features used be supported by both the master and the slave servers. If you use a feature on a master server that is available only as of a given version of MySQL, you cannot replicate to a slave that is older than that version. Such incompatibilities are likely to occur between series, so that, for example, you cannot replicate from MySQL 5.0 to 4.1. However, these incompatibilities also can occur for within-series replication. For example, the `SLEEP()` function is available in MySQL 5.0.12 and up. If you use this function on the master server, you cannot replicate to a slave server that is older than MySQL 5.0.12.

If you are planning to use replication between 5.0 and a previous version of MySQL you should consult the edition of the MySQL Reference Manual corresponding to the earlier release series for information regarding the replication characteristics of that series.

The following list provides details about what is supported and what is not. Additional `InnoDB`-specific information about replication is given in Section 14.2.6.5, “`InnoDB` and MySQL Replication”.

Replication issues with regard to stored routines and triggers is described in Section 17.4, “Binary Logging of Stored Routines 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 18.1, “`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.

- 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.

  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.

  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 the table `t1`, 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;

  This assumes that the table `t1` has columns `col1` and `col2`.

  Important: To guarantee the same ordering on both master and slave, all columns of `t1` must be referenced in the `ORDER BY` clause.

  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.7.1, “Problems with `ALTER TABLE`”.

- Certain functions do not replicate well under some conditions:

  - The `USER()`, `CURRENT_USER()`, `UUID()`, and `LOAD_FILE()` functions are replicated without change and thus do not work reliably on the slave.

  - 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 non-deterministic. To avoid this, you can start the server with the `--sysdate-is-now` option to cause `SYSDATE()` to be an alias for `NOW()`.

  - 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;

- 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 will recognize that issuing privilege-related SQL statements won't have an effect, and thus not execute those statements.

- The `FOREIGN_KEY_CHECKS`, `SQL_MODE`, `UNIQUE_CHECKS`, and `SQL_AUTO_IS_NULL` variables are all replicated in MySQL 5.0. The `storage_engine` system variable (also known as `table_type`) is not yet 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.

- The following applies to replication between MySQL servers that use different character sets:

  1. 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 MySQL version running on the slave. (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.

  2. 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.

  3. 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 (see Bug#2326). A good workaround is to state the character set and collation explicitly in `CREATE TABLE` statements.

- If the master uses MySQL 4.1, the same system time zone should be set for both master and slave. Otherwise some statements will not be 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. 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. Note that this is not necessary when the master is MySQL 5.0 or later.

- `CONVERT_TZ(...,...,@@global.time_zone)` is not properly replicated. `CONVERT_TZ(...,...,@@session.time_zone)` is properly replicated only if the master and slave are from MySQL 5.0.4 or newer.

- 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.

- It is possible to replicate transactional tables on the master using non-transactional 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.

- 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.

- Slaves can connect to masters using SSL.

- 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.

- In MySQL 5.0 (starting from 5.0.3), there is a global system variable `slave_transaction_retries`: If the replication 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 transaction automatically retries `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 5.2.5, “Status Variables”.

- 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 filesystem or if it exists but is not accessible to the slave server. MySQL supports an `sql_mode` option called `NO_DIR_IN_CREATE`. If the slave server is run with this SQL mode enabled, it ignores 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.

- 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 non-deterministic; that is, left to the will of the query optimizer. (This is in general not a good practice, even outside of replication.) For a detailed explanation of this issue, see Section B.8.1, “Open Issues in MySQL”.

- 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 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.

- 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 13.5.5.2, “`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 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`.

- When a server shuts down and restarts, its `MEMORY` (`HEAP`) tables become empty. The master replicates this effect to slaves as follows: The first time that the master uses each `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 14.4, “The `MEMORY` (`HEAP`) Storage Engine”, for more information about `MEMORY` tables.

- Temporary tables are replicated except in the case where you shut down 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 shut down 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:

  1. Issue a `STOP SLAVE` statement.

  2. Use `SHOW STATUS` to check the value of the `Slave_open_temp_tables` variable.

  3. If the value is 0, issue a mysqladmin shutdown command to stop the slave.

  4. If the value is not 0, restart the slave threads with `START SLAVE`.

  5. Repeat the procedure later until the `Slave_open_temp_tables` variable is 0 and you can stop the slave.

- The syntax for multiple-table `DELETE` statements that use table aliases changed between MySQL 4.0 and 4.1. In MySQL 4.0, you should use the true table name to refer to any table from which rows should be deleted:

      DELETE test FROM test AS t1, test2 WHERE ...

  In MySQL 4.1, you must use the alias:

      DELETE t1 FROM test AS t1, test2 WHERE ...

  If you use such `DELETE` statements, the change in syntax means that a 4.0 master cannot replicate to 4.1 (or higher) slaves.

- It is safe to connect servers in a circular master/slave relationship if you use the `--log-slave-updates` option. That means that you can create a setup such as this:

      A -> B -> C -> A

  However, many statements do not work correctly in this kind of setup unless your client code is written to take care of the potential problems that can occur from updates that occur in different sequence on different servers.

  Server IDs are encoded in binary log events, so server A knows when an event that it reads was originally created by itself and does not execute the event (unless server A was started with the `--replicate-same-server-id` option, which is meaningful only in rare cases). Thus, there are no infinite loops. This type of circular setup works only if you perform no conflicting updates between the tables. In other words, if you insert data in both A and C, you should never insert a row in A that may have a key that conflicts with a row inserted in C. You should also not update the same rows on two servers if the order in which the updates are applied is significant.

- If a statement on a slave produces an error, the slave SQL thread terminates, and the slave writes a message to its error log. You should then connect to the slave manually and determine the cause of the problem. (`SHOW SLAVE STATUS` is useful for this.) Then fix the problem (for example, you might need to create a non-existent table) and run `START SLAVE`.

- 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 `--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 5.2.3, “System Variables”.

- 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 in master; this can be avoided by using `InnoDB` tables and the `--innodb-safe-binlog` option on the master. See Section 5.12.3, “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.

- 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.

- Due to the non-transactional 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 non-transactional tables inside a `BEGIN`/`COMMIT` sequence, updates to the binary log may be out of synchrony with table states if the non-transactional 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 non-transactional 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 non-transactional 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.

- Floating-point values are approximate, so comparisons involving them are inexact. This is true for operations that use floating-point values explicitly, or 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 12.1.2, “Type Conversion in Expression Evaluation”, and Section B.5.8, “Problems with Floating-Point Comparisons”.

## 6.8. Replication Startup Options

This section describes the options that you can use on slave replication servers. You can specify these options either on the command line or in an option file.

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. Example: `server-id=3`

Options that you can use on the master server for controlling binary logging are described in Section 5.12.3, “The Binary Log”.

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:

- `--master-host`

- `--master-user`

- `--master-password`

- `--master-port`

- `--master-connect-retry`

- `--master-ssl`

- `--master-ssl-ca`

- `--master-ssl-capath`

- `--master-ssl-cert`

- `--master-ssl-cipher`

- `--master-ssl-key`

The `master.info` file format in MySQL 5.0 includes values corresponding to the SSL options. In addition, the file format includes as its first line the number of lines in the file. (See Section 6.3.4, “Replication Relay and Status Files”.) If you upgrade an older server (before MySQL 4.1.1) to a newer version, the new server upgrades the `master.info` file to the new format automatically when it starts. However, if you downgrade a newer server to an older version, you should remove the first line manually before starting the older server for the first time.

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, you must either restart after removing the `master.info` file or (preferably) use the `CHANGE MASTER TO` statement to reset the values while the slave is running.

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.

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 13.6.2.1, “`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

The following list describes startup options for controlling replication. 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.

- `--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.

- `--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 hostname 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 `--master-connect-retry` 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 5.9.7.3, “SSL Command Options”. The values in the `master.info` file take precedence if they can be read.

- `--master-user=`user_name``

  The username of the account that the slave thread uses for authentication when it connects to the master. This account must have the `REPLICATION SLAVE` privilege. The value in the `master.info` file takes precedence if it can be read. If the master username is not set, the name `test` is assumed.

- `--max-relay-log-size=`size``

  The size at which the server rotates relay log files automatically. For more information, see Section 6.3.4, “Replication Relay and Status Files”. The default size is 1GB.

- `--read-only`

  When this option is given, the server allows no updates except from users that have the `SUPER` privilege or (on a slave server) from updates performed by slave threads. On a slave server, this can be useful to ensure that the slave accepts updates only from its master server and not from clients. As of MySQL 5.0.16, this option does not apply to `TEMPORARY` tables.

- `--relay-log=`file_name``

  The name for the relay log. The default name is ``host_name`-relay-bin.`nnnnnn``, where `host_name` is the name of the slave server host and `nnnnnn` indicates that relay logs are created in numbered sequence. You can specify the option to create hostname-independent relay log names, or if your relay logs tend to be big (and you don't want to decrease `max_relay_log_size`) and you need to put them in some area different from the data directory, or if you want to increase speed by balancing load 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.index` in the data directory, where `host_name` is the name of the slave server.

- `--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 not needed any more. The default value is 1 (enabled). This is a global variable that can be changed dynamically with `SET GLOBAL relay_log_purge = `N``.

- `--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 temporarily ignore `--relay-log-space-limit`.

- `--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 “just check 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` statements or multiple-table `UPDATE` statements that act 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 6.9, “How Servers Evaluate Replication Rules”.

- `--replicate-do-table=`db_name.tbl_name``

  Tell 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 cross-database updates, in contrast to `--replicate-do-db`. See Section 6.9, “How Servers Evaluate Replication Rules”.

- `--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 6.9, “How Servers Evaluate Replication 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 not replicated:

      USE prices;
      UPDATE sales.january SET amount=amount+1000;

  If you need cross-database updates to work, use `--replicate-wild-ignore-table=`db_name`.%` instead. See Section 6.9, “How Servers Evaluate Replication Rules”.

- `--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 6.9, “How Servers Evaluate Replication Rules”.

- `--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. 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. Note that by default the slave I/O thread does not even write binary log events to the relay log if they have the slave's server id (this optimization helps save disk usage). So 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 6.9, “How Servers Evaluate Replication Rules”.

  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're using 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 6.9, “How Servers Evaluate Replication 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=`slave_name``

  The hostname 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-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 non-default 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.

- `--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 filesystem 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 filesystem.

  The directory specified by this option should be located in a disk-based filesystem (not a memory-based filesystem) 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 `--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 C, Error Codes and Messages, lists server error codes.

  You can also (but should not) use the very non-recommended 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

## 6.9. How Servers Evaluate Replication 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 side, decisions about which statements to log are based on the `--binlog-do-db` and `--binlog-ignore-db` options that control binary logging. For a description of the rules that servers use in evaluating these options, see Section 5.12.3, “The Binary Log”.

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 6.8, “Replication Startup Options”.) 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 non-wildcard options.

Stage 1. Check the database options.

At this stage, the slave checks whether there are any `--replicate-do-db` or `--replicate-ignore-db` options that specify database-specific conditions:

- No: Permit the statement and proceed to the table-checking stage.

- Yes: Test the options using the same rules as for the `--binlog-do-db` and `--binlog-ignore-db` options to determine whether to permit or ignore the statement. What is the result of the test?

  - Permit: Do not execute the statement immediately. Defer the decision and proceed to the table-checking stage.

  - Ignore: Ignore the statement and exit.

This stage can permit a statement for further option-checking, or cause it to be ignored. However, statements that are permitted at this stage are not actually executed yet. Instead, they pass to the following stage that checks the table options.

Stage 2. Check the table options.

First, as a preliminary condition, the slave checks whether the statement occurs within a stored function or (prior to MySQL 5.0.12) a stored procedure. If so, execute the statement and exit. (Stored procedures are exempt from this test as of MySQL 5.0.12 because procedure logging occurs at the level of statements that are executed within the routine rather than at the `CALL` level.)

Next, the slave checks for table options and evaluates them. If the server reaches this point, it executes all statements if there are no table options. If there are “do” table options, the statement must match one of them if it is to be executed; otherwise, it is ignored. If there are any “ignore” options, all statements are executed except those that match any `ignore` option. The following steps describe how this evaluation occurs in more detail.

1. Are there any `--replicate-*-table` options?

   - No: There are no table restrictions, so all statements match. Execute the statement and exit.

   - Yes: There are table restrictions. Evaluate the tables to be updated against them. There might be multiple tables to update, so loop through the following steps for each table looking for a matching option (first the non-wild options, and then the wild options). Only tables that are to be updated are compared to the options. For example, if the statement is `INSERT INTO sales SELECT * FROM prices`, only `sales` is compared to the options). If several tables are to be updated (multiple-table statement), the first table that matches “do” or “ignore” wins. That is, the server checks the first table against the options. If no decision could be made, it checks the second table against the options, and so on.

2. Are there any `--replicate-do-table` options?

   - No: Proceed to the next step.

   - Yes: Does the table match any of them?

     - No: Proceed to the next step.

     - Yes: Execute the statement and exit.

3. Are there any `--replicate-ignore-table` options?

   - No: Proceed to the next step.

   - Yes: Does the table match any of them?

     - No: Proceed to the next step.

     - Yes: Ignore the statement and exit.

4. Are there any `--replicate-wild-do-table` options?

   - No: Proceed to the next step.

   - Yes: Does the table match any of them?

     - No: Proceed to the next step.

     - Yes: Execute the statement and exit.

5. Are there any `--replicate-wild-ignore-table` options?

   - No: Proceed to the next step.

   - Yes: Does the table match any of them?

     - No: Proceed to the next step.

     - Yes: Ignore the statement and exit.

6. No `--replicate-*-table` option was matched. Is there another table to test against these options?

   - No: We have now tested all tables to be updated and could not match any option. Are there `--replicate-do-table` or `--replicate-wild-do-table` options?

     - No: There were no “do” table options, so no explicit “do” match is required. Execute the statement and exit.

     - Yes: There were “do” table options, so the statement is executed only with an explicit match to one of them. Ignore the statement and exit.

   - Yes: Loop.

Examples:

- No `--replicate-*` options at all

  The slave executes all statements that it receives from the master.

- `--replicate-*-db` options, but no table options

  The slave permits or ignores statements using the database options. Then it executes all statements permitted by those options because there are no table restrictions.

- `--replicate-*-table` options, but no database options

  All statements are permitted at the database-checking stage because there are no database conditions. The slave executes or ignores statements based on the table options.

- A mix of database and table options

  The slave permits or ignores statements using the database options. Then it evaluates all statements permitted by those options according to the table options. In some cases, this process can yield what might seem a counterintuitive result. Consider the following set of options:

      [mysqld]
      replicate-do-db    = db1
      replicate-do-table = db2.mytbl2

  Suppose that `db1` is the default database and the slave receives this statement:

      INSERT INTO mytbl1 VALUES(1,2,3);

  The database is `db1`, which matches the `--replicate-do-db` option at the database-checking stage. The algorithm then proceeds to the table-checking stage. If there were no table options, the statement would be executed. However, because the options include a “do” table option, the statement must match if it is to be executed. The statement does not match, so it is ignored. (The same would happen for any table in `db1`.)

## 6.10. Replication FAQ

Q: How do I configure a slave if the master is running and I do not want to stop it?

A: There are several possibilities. If you have taken a snapshot backup of the master at some point and recorded the binary log filename and offset (from the output of `SHOW MASTER STATUS`) corresponding to the snapshot, use the following procedure:

1. Make sure that the slave is assigned a unique server ID.

2. 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;

3. 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.

1. Issue this statement to acquire a global read lock:

       mysql> FLUSH TABLES WITH READ LOCK;

2. With the lock still in place, execute this command (or a variation of it):

       shell> tar zcf /tmp/backup.tar.gz /var/lib/mysql

3. Issue this statement and record the output, which you will need later:

       mysql> SHOW MASTER STATUS;

4. 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 filename 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.

Q: Does the slave need to be connected to the master all the time?

A: 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.

Q: 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?

A: You can read the `Seconds_Behind_Master` column in `SHOW SLAVE STATUS`. See Section 6.3, “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.

Q: How do I force the master to block updates until the slave catches up?

A: Use the following procedure:

1. On the master, execute these statements:

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

   Record the replication coordinates (the log filename and offset) from the output of the `SHOW` statement.

2. 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.

3. On the master, issue the following statement to allow the master to begin processing updates again:

       mysql> UNLOCK TABLES;

Q: What issues should I be aware of when setting up two-way replication?

A: 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.

Q: How can I use replication to improve performance of my system?

A: 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 non-transactional `MyISAM` tables instead of `InnoDB` and `BDB` tables to get more speed by eliminating transactional overhead.

Q: What should I do to prepare client code in my own applications to use performance-enhancing replication?

A: 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.

Q: When and how much can MySQL replication improve the performance of my system?

A: 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?

Q: How can I use replication to provide redundancy or high availability?

A: 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. Some suggestions:

- To tell a slave to change its master, use the `CHANGE MASTER TO` statement.

- 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.

- 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 following setup:

             WC
              \
               v
       WC----> M
             / | \
            /  |  \
           v   v   v
          S1   S2  S3

  In this diagram, `M` means the master, `S` the slaves, `WC` the clients issuing database writes and reads; clients that issue only database reads are not represented, because they need not switch. `S1`, `S2`, and `S3` 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 `M` becomes unavailable, you can pick one of the slaves to become the new master. For example, if you pick `S1`, all `WC` should be redirected to `S1`, which will log updates to its binary log. `S2` and `S3` should then replicate from `S1`.

  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 `S1` has `--log-slave-updates` enabled. Then it will write updates that it receives from `M` to its own binary log. When `S2` changes from `M` to `S1` as its master, it may receive updates from `S1` that it has already received from `M`

  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 `S1` being promoted to become the master, issue `STOP SLAVE` and `RESET MASTER`.

  On the other slaves `S2` and `S3`, use `STOP SLAVE` and `CHANGE MASTER TO MASTER_HOST='S1'` (where `'S1'` represents the real hostname of `S1`). To `CHANGE MASTER`, add all information about how to connect to `S1` from `S2` or `S3` (`user`, `password`, `port`). In `CHANGE MASTER`, there is no need to specify the name of `S1`'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`. Finally, use `START SLAVE` on `S2` and `S3`.

  Then instruct all `WC` to direct their statements to `S1`. From that point on, all updates statements sent by `WC` to `S1` are written to the binary log of `S1`, which then contains every update statement sent to `S1` since `M` died.

  The result is this configuration:

             WC
            /
            |
       WC   |  M(unavailable)
        \   |
         \  |
          v v
           S1<--S2  S3
            ^       |
            +-------+

  When `M` is up again, you must issue on it the same `CHANGE MASTER` as that issued on `S2` and `S3`, so that `M` becomes a slave of `S1` and picks up all the `WC` writes that it missed while it was down. To make `M` a master again (because it is the most powerful machine, for example), use the preceding procedure as if `S1` was unavailable and `M` was to be the new master. During this procedure, do not forget to run `RESET MASTER` on `M` before making `S1`, `S2`, and `S3` slaves of `M`. Otherwise, they may pick up old `WC` writes from before the point at which `M` 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).

Q: How do I prevent GRANT and REVOKE statements from replicating to slave machines?

A: Start the server with the `--replicate-wild-ignore-table=mysql.%` option.

Q: Does replication work on mixed operating systems (for example, the master runs on Linux while slaves run on Mac OS X and Windows)?

A: Yes.

Q: Does replication work on mixed hardware architectures (for example, the master runs on a 64-bit machine while slaves run on 32-bit machines)?

A: Yes.

## 6.11. 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 non-zero. 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 6.3, “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 hostname, 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 6.7, “Replication Features and Known Problems”. If it is a bug, see Section 6.12, “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 Network Monitoring and Advisory Service. 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:

  1. 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`.

  2. 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.

  3. 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.

  4. 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.

## 6.12. 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 1.8, “How to Report Bugs or Problems”. If you have a “phantom” problem (one that you cannot duplicate at will), use the following procedure:

1. 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.

2. 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.

3. 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

4. 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 -j 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 1.8, “How to Report Bugs or Problems”.

## 6.13. Auto-Increment in Multiple-Master Replication

When multiple servers are configured as replication masters, special steps must be taken to prevent key collisions when using `AUTO_INCREMENT` columns, otherwise multiple masters may attempt to use the same `AUTO_INCREMENT` value when inserting rows.

The `auto_increment_increment` and `auto_increment_offset` system variables help to accommodate multiple-master replication with `AUTO_INCREMENT` columns. Each of these variables has a default and minimum value of 1, and a maximum value of 65,535. They were introduced in MySQL 5.0.2.

These two variables affect `AUTO_INCREMENT` column behavior as follows:

- `auto_increment_increment` controls the increment between successive `AUTO_INCREMENT` values.

- `auto_increment_offset` determines the starting point for `AUTO_INCREMENT` column values.

By choosing non-conflicting values for these variables on different masters, servers in a multiple-master configuration will not use conflicting `AUTO_INCREMENT` values when inserting new rows into the same table. To set up `N` master servers, set the variables like this:

- Set `auto_increment_increment` to `N` on each master.

- Set each of the `N` masters to have a different `auto_increment_offset`, using the values 1, 2, …, `N`.

For additional information about `auto_increment_increment` and `auto_increment_offset`, see Section 5.2.3, “System Variables”.

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