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17.1.6.2 Replication Master Options and Variables

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This section describes the server options and system variables that you can use on replication master servers. You can specify the options either on the command line or in an option file. You can specify system variable values using SET.

On the master and each slave, you must use the server-id option to establish a unique replication ID. For each server, you should pick a unique positive integer in the range from 1 to 232 − 1, and each ID must be different from every other ID in use by any other replication master or slave. Example: server-id=3.

For options used on the master for controlling binary logging, see Section 17.1.6.4, “Binary Logging Options and Variables”.

Startup Options for Replication Masters

The following list describes startup options for controlling replication master servers. Replication-related system variables are discussed later in this section.

 

System Variables Used on Replication Masters

The following system variables are used to control replication masters:

  • auto_increment_increment

    Property Value
    System Variable auto_increment_increment
    Scope Global, Session
    Dynamic Yes
    SET_VAR Hint Applies Yes
    Type Integer
    Default Value 1
    Minimum Value 1
    Maximum Value 65535

    auto_increment_increment and auto_increment_offset are intended for use with master-to-master replication, and can be used to control the operation of AUTO_INCREMENT columns. Both variables have global and session values, and each can assume an integer value between 1 and 65,535 inclusive. Setting the value of either of these two variables to 0 causes its value to be set to 1 instead. Attempting to set the value of either of these two variables to an integer greater than 65,535 or less than 0 causes its value to be set to 65,535 instead. Attempting to set the value of auto_increment_increment or auto_increment_offset to a noninteger value produces an error, and the actual value of the variable remains unchanged.

    As of MySQL 8.0.14, setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    Note

    auto_increment_increment is also supported for use with NDB tables.

    These two variables affect AUTO_INCREMENT column behavior as follows:

    • auto_increment_increment controls the interval between successive column values. For example:

      1. mysql> SHOW VARIABLES LIKE 'auto_inc%';
      2. +--------------------------+-------+
      3. | Variable_name            | Value |
      4. +--------------------------+-------+
      5. | auto_increment_increment | 1     |
      6. | auto_increment_offset    | 1     |
      7. +--------------------------+-------+
      8. 2 rows in set (0.00 sec)
      9.  
      10. mysql> CREATE TABLE autoinc1
      11.     -> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
      12.   Query OK, 0 rows affected (0.04 sec)
      13.  
      14. mysql> SET @@auto_increment_increment=10;
      15. Query OK, 0 rows affected (0.00 sec)
      16.  
      17. mysql> SHOW VARIABLES LIKE 'auto_inc%';
      18. +--------------------------+-------+
      19. | Variable_name            | Value |
      20. +--------------------------+-------+
      21. | auto_increment_increment | 10    |
      22. | auto_increment_offset    | 1     |
      23. +--------------------------+-------+
      24. 2 rows in set (0.01 sec)
      25.  
      26. mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
      27. Query OK, 4 rows affected (0.00 sec)
      28. Records: 4  Duplicates: 0  Warnings: 0
      29.  
      30. mysql> SELECT col FROM autoinc1;
      31. +-----+
      32. | col |
      33. +-----+
      34. |   1 |
      35. |  11 |
      36. |  21 |
      37. |  31 |
      38. +-----+
      39. 4 rows in set (0.00 sec)

    • auto_increment_offset determines the starting point for the AUTO_INCREMENT column value. Consider the following, assuming that these statements are executed during the same session as the example given in the description for auto_increment_increment:

      1. mysql> SET @@auto_increment_offset=5;
      2. Query OK, 0 rows affected (0.00 sec)
      3.  
      4. mysql> SHOW VARIABLES LIKE 'auto_inc%';
      5. +--------------------------+-------+
      6. | Variable_name            | Value |
      7. +--------------------------+-------+
      8. | auto_increment_increment | 10    |
      9. | auto_increment_offset    | 5     |
      10. +--------------------------+-------+
      11. 2 rows in set (0.00 sec)
      12.  
      13. mysql> CREATE TABLE autoinc2
      14.     -> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
      15. Query OK, 0 rows affected (0.06 sec)
      16.  
      17. mysql> INSERT INTO autoinc2 VALUES (NULL), (NULL), (NULL), (NULL);
      18. Query OK, 4 rows affected (0.00 sec)
      19. Records: 4  Duplicates: 0  Warnings: 0
      20.  
      21. mysql> SELECT col FROM autoinc2;
      22. +-----+
      23. | col |
      24. +-----+
      25. |   5 |
      26. |  15 |
      27. |  25 |
      28. |  35 |
      29. +-----+
      30. 4 rows in set (0.02 sec)

      When the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored.

    If either of these variables is changed, and then new rows inserted into a table containing an AUTO_INCREMENT column, the results may seem counterintuitive because the series of AUTO_INCREMENT values is calculated without regard to any values already present in the column, and the next value inserted is the least value in the series that is greater than the maximum existing value in the AUTO_INCREMENT column. The series is calculated like this:

    auto_increment_offset + N × auto_increment_increment

    where N is a positive integer value in the series [1, 2, 3, ...]. For example:

    1. mysql> SHOW VARIABLES LIKE 'auto_inc%';
    2. +--------------------------+-------+
    3. | Variable_name            | Value |
    4. +--------------------------+-------+
    5. | auto_increment_increment | 10    |
    6. | auto_increment_offset    | 5     |
    7. +--------------------------+-------+
    8. 2 rows in set (0.00 sec)
    9.  
    10. mysql> SELECT col FROM autoinc1;
    11. +-----+
    12. | col |
    13. +-----+
    14. |   1 |
    15. |  11 |
    16. |  21 |
    17. |  31 |
    18. +-----+
    19. 4 rows in set (0.00 sec)
    20.  
    21. mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
    22. Query OK, 4 rows affected (0.00 sec)
    23. Records: 4  Duplicates: 0  Warnings: 0
    24.  
    25. mysql> SELECT col FROM autoinc1;
    26. +-----+
    27. | col |
    28. +-----+
    29. |   1 |
    30. |  11 |
    31. |  21 |
    32. |  31 |
    33. |  35 |
    34. |  45 |
    35. |  55 |
    36. |  65 |
    37. +-----+
    38. 8 rows in set (0.00 sec)

    The values shown for auto_increment_increment and auto_increment_offset generate the series 5 + N × 10, that is, [5, 15, 25, 35, 45, ...]. The highest value present in the col column prior to the INSERT is 31, and the next available value in the AUTO_INCREMENT series is 35, so the inserted values for col begin at that point and the results are as shown for the SELECT query.

    It is not possible to restrict the effects of these two variables to a single table; these variables control the behavior of all AUTO_INCREMENT columns in all tables on the MySQL server. If the global value of either variable is set, its effects persist until the global value is changed or overridden by setting the session value, or until mysqld is restarted. If the local value is set, the new value affects AUTO_INCREMENT columns for all tables into which new rows are inserted by the current user for the duration of the session, unless the values are changed during that session.

    The default value of auto_increment_increment is 1. See Section 17.4.1.1, “Replication and AUTO_INCREMENT”.

  • auto_increment_offset

    Property Value
    System Variable auto_increment_offset
    Scope Global, Session
    Dynamic Yes
    SET_VAR Hint Applies Yes
    Type Integer
    Default Value 1
    Minimum Value 1
    Maximum Value 65535

    This variable has a default value of 1. For more information, see the description for auto_increment_increment.

    As of MySQL 8.0.14, setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”.

    Note

    auto_increment_offset is also supported for use with NDB tables.

  • immediate_server_version

    Property Value
    Introduced 8.0.14
    System Variable immediate_server_version
    Scope Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer

    For internal use by replication. This session system variable holds the MySQL Server release number of the server that is the immediate master in a replication topology (for example, 80014 for a MySQL 8.0.14 server instance). If this immediate server is at a release that does not support the session system variable, the value of the variable is set to 0 (UNKNOWN_SERVER_VERSION).

    The value of the variable is replicated from a master to a slave. With this information the slave can correctly process data originating from a master at an older release, by recognizing where syntax changes or semantic changes have occurred between the releases involved and handling these appropriately. The information can also be used in a Group Replication environment where one or more members of the replication group is at a newer release than the others. The value of the variable can be viewed in the binary log for each transaction (as part of the Gtid_log_event, or Anonymous_gtid_log_event if GTIDs are not in use on the server), and could be helpful in debugging cross-version replication issues.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”. However, note that the variable is not intended for users to set; it is set automatically by the replication infrastructure.

  • original_server_version

    Property Value
    Introduced 8.0.14
    System Variable original_server_version
    Scope Session
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer

    For internal use by replication. This session system variable holds the MySQL Server release number of the server where a transaction was originally committed (for example, 80014 for a MySQL 8.0.14 server instance). If this original server is at a release that does not support the session system variable, the value of the variable is set to 0 (UNKNOWN_SERVER_VERSION). Note that when a release number is set by the original server, the value of the variable is reset to 0 if the immediate server or any other intervening server in the replication topology does not support the session system variable, and so does not replicate its value.

    The value of the variable is set and used in the same ways as for the immediate_server_version system variable. If the value of the variable is the same as that for the immediate_server_version system variable, only the latter is recorded in the binary log, with an indicator that the original server version is the same.

    In a Group Replication environment, view change log events, which are special transactions queued by each group member when a new member joins the group, are tagged with the server version of the group member queuing the transaction. This ensures that the server version of the original donor is known to the joining member. Because the view change log events queued for a particular view change have the same GTID on all members, for this case only, instances of the same GTID might have a different original server version.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.9.1, “System Variable Privileges”. However, note that the variable is not intended for users to set; it is set automatically by the replication infrastructure.

  • rpl_semi_sync_master_enabled

    Property Value
    System Variable rpl_semi_sync_master_enabled
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value OFF

    Controls whether semisynchronous replication is enabled on the master. To enable or disable the plugin, set this variable to ON or OFF (or 1 or 0), respectively. The default is OFF.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_timeout

    Property Value
    System Variable rpl_semi_sync_master_timeout
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 10000

    A value in milliseconds that controls how long the master waits on a commit for acknowledgment from a slave before timing out and reverting to asynchronous replication. The default value is 10000 (10 seconds).

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_trace_level

    Property Value
    System Variable rpl_semi_sync_master_trace_level
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 32

    The semisynchronous replication debug trace level on the master. Four levels are defined:

    • 1 = general level (for example, time function failures)

    • 16 = detail level (more verbose information)

    • 32 = net wait level (more information about network waits)

    • 64 = function level (information about function entry and exit)

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_wait_for_slave_count

    Property Value
    System Variable rpl_semi_sync_master_wait_for_slave_count
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Integer
    Default Value 1
    Minimum Value 1
    Maximum Value 65535

    The number of slave acknowledgments the master must receive per transaction before proceeding. By default rpl_semi_sync_master_wait_for_slave_count is 1, meaning that semisynchronous replication proceeds after receiving a single slave acknowledgment. Performance is best for small values of this variable.

    For example, if rpl_semi_sync_master_wait_for_slave_count is 2, then 2 slaves must acknowledge receipt of the transaction before the timeout period configured by rpl_semi_sync_master_timeout for semisynchronous replication to proceed. If less slaves acknowledge receipt of the transaction during the timeout period, the master reverts to normal replication.

    Note

    This behavior also depends on rpl_semi_sync_master_wait_no_slave

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_wait_no_slave

    Property Value
    System Variable rpl_semi_sync_master_wait_no_slave
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Boolean
    Default Value ON

    Controls whether the master waits for the timeout period configured by rpl_semi_sync_master_timeout to expire, even if the slave count drops to less than the number of slaves configured by rpl_semi_sync_master_wait_for_slave_count during the timeout period.

    When the value of rpl_semi_sync_master_wait_no_slave is ON (the default), it is permissible for the slave count to drop to less than rpl_semi_sync_master_wait_for_slave_count during the timeout period. As long as enough slaves acknowledge the transaction before the timeout period expires, semisynchronous replication continues.

    When the value of rpl_semi_sync_master_wait_no_slave is OFF, if the slave count drops to less than the number configured in rpl_semi_sync_master_wait_for_slave_count at any time during the timeout period configured by rpl_semi_sync_master_timeout, the master reverts to normal replication.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

  • rpl_semi_sync_master_wait_point

    Property Value
    System Variable rpl_semi_sync_master_wait_point
    Scope Global
    Dynamic Yes
    SET_VAR Hint Applies No
    Type Enumeration
    Default Value AFTER_SYNC
    Valid Values

    AFTER_SYNC

    AFTER_COMMIT

    This variable controls the point at which a semisynchronous replication master waits for slave acknowledgment of transaction receipt before returning a status to the client that committed the transaction. These values are permitted:

    • AFTER_SYNC (the default): The master writes each transaction to its binary log and the slave, and syncs the binary log to disk. The master waits for slave acknowledgment of transaction receipt after the sync. Upon receiving acknowledgment, the master commits the transaction to the storage engine and returns a result to the client, which then can proceed.

    • AFTER_COMMIT: The master writes each transaction to its binary log and the slave, syncs the binary log, and commits the transaction to the storage engine. The master waits for slave acknowledgment of transaction receipt after the commit. Upon receiving acknowledgment, the master returns a result to the client, which then can proceed.

    The replication characteristics of these settings differ as follows:

    • With AFTER_SYNC, all clients see the committed transaction at the same time: After it has been acknowledged by the slave and committed to the storage engine on the master. Thus, all clients see the same data on the master.

      In the event of master failure, all transactions committed on the master have been replicated to the slave (saved to its relay log). A crash of the master and failover to the slave is lossless because the slave is up to date. Note, however, that the master cannot be restarted in this scenario and must be discarded, because its binary log might contain uncommitted transactions that would cause a conflict with the slave when externalized after binary log recovery.

    • With AFTER_COMMIT, the client issuing the transaction gets a return status only after the server commits to the storage engine and receives slave acknowledgment. After the commit and before slave acknowledgment, other clients can see the committed transaction before the committing client.

      If something goes wrong such that the slave does not process the transaction, then in the event of a master crash and failover to the slave, it is possible that such clients will see a loss of data relative to what they saw on the master.

    This variable is available only if the master-side semisynchronous replication plugin is installed.

    With the addition of rpl_semi_sync_master_wait_point in MySQL 5.7, a version compatibility constraint was created because it increments the semisynchronous interface version: Servers for MySQL 5.7 and higher do not work with semisynchronous replication plugins from older versions, nor do servers from older versions work with semisynchronous replication plugins for MySQL 5.7 and higher.


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