2.6.5 Global Transaction ID System Variables

The MySQL Server system variables described in this section are used to monitor and control Global Transaction Identifiers (GTIDs). For additional information, see Section 2.3, “Replication with Global Transaction Identifiers”.

• binlog_gtid_simple_recovery

  Command-Line Format	--binlog-gtid-simple-recovery[={OFF|ON}]
  System Variable	binlog_gtid_simple_recovery
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	ON

  This variable controls how binary log files are iterated during the search for GTIDs when MySQL starts or restarts.

  When binlog_gtid_simple_recovery=TRUE, which is the default in MySQL 8.0, the values of gtid_executed and gtid_purged are computed at startup based on the values of Previous_gtids_log_event in the most recent and oldest binary log files. For a description of the computation, see The gtid_purged System Variable. This setting accesses only two binary log files during server restart. If all binary logs on the server were generated using MySQL 5.7.8 or later, binlog_gtid_simple_recovery=TRUE can always safely be used.

  If any binary logs from MySQL 5.7.7 or older are present on the server (for example, following an upgrade of an older server to MySQL 8.0), with binlog_gtid_simple_recovery=TRUE, gtid_executed and gtid_purged might be initialized incorrectly in the following two situations:

  • The newest binary log was generated by MySQL 5.7.5 or earlier, and gtid_mode was ON for some binary logs but OFF for the newest binary log.

  • A SET @@GLOBAL.gtid_purged statement was issued on MySQL 5.7.7 or earlier, and the binary log that was active at the time of the SET @@GLOBAL.gtid_purged statement has not yet been purged.

  If an incorrect GTID set is computed in either situation, it remains incorrect even if the server is later restarted with binlog_gtid_simple_recovery=FALSE. If either of these situations apply or might apply on the server, set binlog_gtid_simple_recovery=FALSE before starting or restarting the server.

  When binlog_gtid_simple_recovery=FALSE is set, the method of computing gtid_executed and gtid_purged as described in The gtid_purged System Variable is changed to iterate the binary log files as follows:

  • Instead of using the value of Previous_gtids_log_event and GTID log events from the newest binary log file, the computation for gtid_executed iterates from the newest binary log file, and uses the value of Previous_gtids_log_event and any GTID log events from the first binary log file where it finds a Previous_gtids_log_event value. If the server's most recent binary log files do not have GTID log events, for example if gtid_mode=ON was used but the server was later changed to gtid_mode=OFF, this process can take a long time.

  • Instead of using the value of Previous_gtids_log_event from the oldest binary log file, the computation for gtid_purged iterates from the oldest binary log file, and uses the value of Previous_gtids_log_event from the first binary log file where it finds either a nonempty Previous_gtids_log_event value, or at least one GTID log event (indicating that the use of GTIDs starts at that point). If the server's older binary log files do not have GTID log events, for example if gtid_mode=ON was only set recently on the server, this process can take a long time.

• enforce_gtid_consistency

  Command-Line Format	--enforce-gtid-consistency[=value]
  System Variable	enforce_gtid_consistency
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Enumeration
  Default Value	OFF
  Valid Values	OFF ON WARN

  Depending on the value of this variable, the server enforces GTID consistency by allowing execution of only statements that can be safely logged using a GTID. You must set this variable to ON before enabling GTID based replication.

  The values that enforce_gtid_consistency can be configured to are:

  • OFF: all transactions are allowed to violate GTID consistency.

  • ON: no transaction is allowed to violate GTID consistency.

  • WARN: all transactions are allowed to violate GTID consistency, but a warning is generated in this case.

  --enforce-gtid-consistency only takes effect if binary logging takes place for a statement. If binary logging is disabled on the server, or if statements are not written to the binary log because they are removed by a filter, GTID consistency is not checked or enforced for the statements that are not logged.

  Only statements that can be logged using GTID safe statements can be logged when enforce_gtid_consistency is set to ON, so the operations listed here cannot be used with this option:

  • CREATE TEMPORARY TABLE or DROP TEMPORARY TABLE statements inside transactions.

  • Transactions or statements that update both transactional and nontransactional tables. There is an exception that nontransactional DML is allowed in the same transaction or in the same statement as transactional DML, if all nontransactional tables are temporary.

  • CREATE TABLE ... SELECT statements, prior to MySQL 8.0.21. From MySQL 8.0.21, CREATE TABLE ... SELECT statements are allowed for storage engines that support atomic DDL.

  For more information, see Section 2.3.7, “Restrictions on Replication with GTIDs”.

  Prior to MySQL 5.7 and in early releases in that release series, the boolean enforce_gtid_consistency defaulted to OFF. To maintain compatibility with these earlier releases, the enumeration defaults to OFF, and setting --enforce-gtid-consistency without a value is interpreted as setting the value to ON. The variable also has multiple textual aliases for the values: 0=OFF=FALSE, 1=ON=TRUE,2=WARN. This differs from other enumeration types but maintains compatibility with the boolean type used in previous releases. These changes impact on what is returned by the variable. Using SELECT @@ENFORCE_GTID_CONSISTENCY, SHOW VARIABLES LIKE 'ENFORCE_GTID_CONSISTENCY', and SELECT * FROM INFORMATION_SCHEMA.VARIABLES WHERE 'VARIABLE_NAME' = 'ENFORCE_GTID_CONSISTENCY', all return the textual form, not the numeric form. This is an incompatible change, since @@ENFORCE_GTID_CONSISTENCY returns the numeric form for booleans but returns the textual form for SHOW and the Information Schema.

• gtid_executed

  System Variable	gtid_executed
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	String
  Unit	set of GTIDs

  When used with global scope, this variable contains a representation of the set of all transactions executed on the server and GTIDs that have been set by a SET gtid_purged statement. This is the same as the value of the Executed_Gtid_Set column in the output of SHOW MASTER STATUS and SHOW REPLICA STATUS. The value of this variable is a GTID set, see GTID Sets for more information.

  When the server starts, @@GLOBAL.gtid_executed is initialized. See binlog_gtid_simple_recovery for more information on how binary logs are iterated to populate gtid_executed. GTIDs are then added to the set as transactions are executed, or if any SET gtid_purged statement is executed.

  The set of transactions that can be found in the binary logs at any given time is equal to GTID_SUBTRACT(@@GLOBAL.gtid_executed, @@GLOBAL.gtid_purged); that is, to all transactions in the binary log that have not yet been purged.

  Issuing RESET MASTER causes the global value (but not the session value) of this variable to be reset to an empty string. GTIDs are not otherwise removed from this set other than when the set is cleared due to RESET MASTER.

• gtid_executed_compression_period

  Command-Line Format	--gtid-executed-compression-period=#
  System Variable	gtid_executed_compression_period
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value (≥ 8.0.23)	0
  Default Value (≤ 8.0.22)	1000
  Minimum Value	0
  Maximum Value	4294967295

  Compress the mysql.gtid_executed table each time this many transactions have been processed. When binary logging is enabled on the server, this compression method is not used, and instead the mysql.gtid_executed table is compressed on each binary log rotation. When binary logging is disabled on the server, the compression thread sleeps until the specified number of transactions have been executed, then wakes up to perform compression of the mysql.gtid_executed table. Setting the value of this system variable to 0 means that the thread never wakes up, so this explicit compression method is not used. Instead, compression occurs implicitly as required.

  From MySQL 8.0.17, InnoDB transactions are written to the mysql.gtid_executed table by a separate process to non-InnoDB transactions. If the server has a mix of InnoDB transactions and non-InnoDB transactions, the compression controlled by this system variable interferes with the work of this process and can slow it significantly. For this reason, from that release it is recommended that you set gtid_executed_compression_period to 0.

  From MySQL 8.0.23, InnoDB and non-InnoDB transactions are written to the mysql.gtid_executed table by the same process, and the gtid_executed_compression_period default value is 0.

  See mysql.gtid_executed Table Compression for more information.

• gtid_mode

  Command-Line Format	--gtid-mode=MODE
  System Variable	gtid_mode
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Enumeration
  Default Value	OFF
  Valid Values	OFF OFF_PERMISSIVE ON_PERMISSIVE ON

  Controls whether GTID based logging is enabled and what type of transactions the logs can contain. You must have privileges sufficient to set global system variables. See System Variable Privileges. enforce_gtid_consistency must be set to ON before you can set gtid_mode=ON. Before modifying this variable, see Section 2.4, “Changing GTID Mode on Online Servers”.

  Logged transactions can be either anonymous or use GTIDs. Anonymous transactions rely on binary log file and position to identify specific transactions. GTID transactions have a unique identifier that is used to refer to transactions. The different modes are:

  • OFF: Both new and replicated transactions must be anonymous.

  • OFF_PERMISSIVE: New transactions are anonymous. Replicated transactions can be either anonymous or GTID transactions.

  • ON_PERMISSIVE: New transactions are GTID transactions. Replicated transactions can be either anonymous or GTID transactions.

  • ON: Both new and replicated transactions must be GTID transactions.

  Changes from one value to another can only be one step at a time. For example, if gtid_mode is currently set to OFF_PERMISSIVE, it is possible to change to OFF or ON_PERMISSIVE but not to ON.

  The values of gtid_purged and gtid_executed are persistent regardless of the value of gtid_mode. Therefore even after changing the value of gtid_mode, these variables contain the correct values.

• gtid_next

  System Variable	gtid_next
  Scope	Session
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Enumeration
  Default Value	AUTOMATIC
  Valid Values	AUTOMATIC ANONYMOUS <UUID>:<NUMBER>

  This variable is used to specify whether and how the next GTID is obtained.

  Setting the session value of this system variable is a restricted operation. The session user must have either the REPLICATION_APPLIER privilege (see Replication Privilege Checks), or privileges sufficient to set restricted session variables (see System Variable Privileges).

  gtid_next can take any of the following values:

  • AUTOMATIC: Use the next automatically-generated global transaction ID.

  • ANONYMOUS: Transactions do not have global identifiers, and are identified by file and position only.

  • A global transaction ID in UUID:NUMBER format.

  Exactly which of the above options are valid depends on the setting of gtid_mode, see Section 2.4.1, “Replication Mode Concepts” for more information. Setting this variable has no effect if gtid_mode is OFF.

  After this variable has been set to UUID:NUMBER, and a transaction has been committed or rolled back, an explicit SET GTID_NEXT statement must again be issued before any other statement.

  DROP TABLE or DROP TEMPORARY TABLE fails with an explicit error when used on a combination of nontemporary tables with temporary tables, or of temporary tables using transactional storage engines with temporary tables using nontransactional storage engines.

• gtid_owned

  System Variable	gtid_owned
  Scope	Global, Session
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	String
  Unit	set of GTIDs

  This read-only variable is primarily for internal use. Its contents depend on its scope.

  • When used with global scope, gtid_owned holds a list of all the GTIDs that are currently in use on the server, with the IDs of the threads that own them. This variable is mainly useful for a multi-threaded replica to check whether a transaction is already being applied on another thread. An applier thread takes ownership of a transaction's GTID all the time it is processing the transaction, so @@global.gtid_owned shows the GTID and owner for the duration of processing. When a transaction has been committed (or rolled back), the applier thread releases ownership of the GTID.

  • When used with session scope, gtid_owned holds a single GTID that is currently in use by and owned by this session. This variable is mainly useful for testing and debugging the use of GTIDs when the client has explicitly assigned a GTID for the transaction by setting gtid_next. In this case, @@session.gtid_owned displays the GTID all the time the client is processing the transaction, until the transaction has been committed (or rolled back). When the client has finished processing the transaction, the variable is cleared. If gtid_next=AUTOMATIC is used for the session, gtid_owned is populated only briefly during the execution of the commit statement for the transaction, so it cannot be observed from the session concerned, although it is listed if @@global.gtid_owned is read at the right point. If you have a requirement to track the GTIDs that are handled by a client in a session, you can enable the session state tracker controlled by the session_track_gtids system variable.

• gtid_purged

  System Variable	gtid_purged
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	String
  Unit	set of GTIDs

  The global value of the gtid_purged system variable (@@GLOBAL.gtid_purged) is a GTID set consisting of the GTIDs of all the transactions that have been committed on the server, but do not exist in any binary log file on the server. gtid_purged is a subset of gtid_executed. The following categories of GTIDs are in gtid_purged:

  • GTIDs of replicated transactions that were committed with binary logging disabled on the replica.

  • GTIDs of transactions that were written to a binary log file that has now been purged.

  • GTIDs that were added explicitly to the set by the statement SET @@GLOBAL.gtid_purged.

  When the server starts, the global value of gtid_purged is initialized to a set of GTIDs. For information on how this GTID set is computed, see The gtid_purged System Variable. If binary logs from MySQL 5.7.7 or older are present on the server, you might need to set binlog_gtid_simple_recovery=FALSE in the server's configuration file to produce the correct computation. See the description for binlog_gtid_simple_recovery for details of the situations in which this setting is needed.

  Issuing RESET MASTER causes the value of gtid_purged to be reset to an empty string.

  You can set the value of gtid_purged in order to record on the server that the transactions in a certain GTID set have been applied, although they do not exist in any binary log on the server. An example use case for this action is when you are restoring a backup of one or more databases on a server, but you do not have the relevant binary logs containing the transactions on the server.

  Important

  GTIDs are only available on a server instance up to the number of non-negative values for a signed 64-bit integer (2 to the power of 63, minus 1). If you set the value of gtid_purged to a number that approaches this limit, subsequent commits can cause the server to run out of GTIDs and take the action specified by binlog_error_action. From MySQL 8.0.23, a warning message is issued when the server instance is approaching the limit.

  From MySQL 8.0, there are two ways to set the value of gtid_purged. You can either replace the value of gtid_purged with your specified GTID set, or you can append your specified GTID set to the GTID set that is already held by gtid_purged. If the server has no existing GTIDs, for example an empty server that you are provisioning with a backup of an existing database, both methods have the same result. If you are restoring a backup that overlaps the transactions that are already on the server, for example replacing a corrupted table with a partial dump from the source made using mysqldump (which includes the GTIDs of all the transactions on the server, even though the dump is partial), use the first method of replacing the value of gtid_purged. If you are restoring a backup that is disjoint from the transactions that are already on the server, for example provisioning a multi-source replica using dumps from two different servers, use the second method of adding to the value of gtid_purged.

  • To replace the value of gtid_purged with your specified GTID set, use the following statement:

    SET @@GLOBAL.gtid_purged = 'gtid_set'

    gtid_set must be a superset of the current value of gtid_purged, and must not intersect with gtid_subtract(gtid_executed,gtid_purged). In other words, the new GTID set must include any GTIDs that were already in gtid_purged, and must not include any GTIDs in gtid_executed that have not yet been purged. gtid_set also cannot include any GTIDs that are in @@global.gtid_owned, that is, the GTIDs for transactions that are currently being processed on the server.

    The result is that the global value of gtid_purged is set equal to gtid_set, and the value of gtid_executed becomes the union of gtid_set and the previous value of gtid_executed.

  • To append your specified GTID set to gtid_purged, use the following statement with a plus sign (+) before the GTID set:

    SET @@GLOBAL.gtid_purged = '+gtid_set'

    gtid_set must not intersect with the current value of gtid_executed. In other words, the new GTID set must not include any GTIDs in gtid_executed, including transactions that are already also in gtid_purged. gtid_set also cannot include any GTIDs that are in @@global.gtid_owned, that is, the GTIDs for transactions that are currently being processed on the server.

    The result is that gtid_set is added to both gtid_executed and gtid_purged.

Note

If any binary logs from MySQL 5.7.7 or older are present on the server (for example, following an upgrade of an older server to MySQL 8.0), after issuing a SET @@GLOBAL.gtid_purged statement, you might need to set binlog_gtid_simple_recovery=FALSE in the server's configuration file before restarting the server, otherwise gtid_purged can be computed incorrectly. See the description for binlog_gtid_simple_recovery for details of the situations in which this setting is needed.