2.6.3 Replica Server Options and Variables

This section explains the server options and system variables that apply to replica servers and contains the following:

• Startup Options for Replica Servers

• System Variables Used on Replica Servers

Specify the options either on the command line or in an option file. Many of the options can be set while the server is running by using the CHANGE REPLICATION SOURCE TO statement (from MySQL 8.0.23) or CHANGE MASTER TO statement (before MySQL 8.0.23). Specify system variable values using SET.

Server ID.  On the source and each replica, you must set the server_id system variable to establish a unique replication ID in the range from 1 to 2³² − 1. “Unique” means that each ID must be different from every other ID in use by any other source or replica in the replication topology. Example my.cnf file:

[mysqld]
server-id=3

Startup Options for Replica Servers

This section explains startup options for controlling replica servers. Many of these options can be set while the server is running by using the CHANGE REPLICATION SOURCE TO statement (from MySQL 8.0.23) or CHANGE MASTER TO statement (before MySQL 8.0.23). Others, such as the --replicate-* options, can be set only when the replica server starts. Replication-related system variables are discussed later in this section.

• --master-info-file=file_name

  Command-Line Format	--master-info-file=file_name
  Deprecated	8.0.18
  Type	File name
  Default Value	master.info

  The use of this option is now deprecated. It was used to set the file name for the replica's connection metadata repository if master_info_repository=FILE was set. --master-info-file and the use of the master_info_repository system variable are deprecated because the use of a file for the connection metadata repository has been superseded by crash-safe tables. For information about the connection metadata repository, see Section 5.4.2, “Replication Metadata Repositories”.

• --master-retry-count=count

  Command-Line Format	--master-retry-count=#
  Deprecated	Yes
  Type	Integer
  Default Value	86400
  Minimum Value	0
  Maximum Value (64-bit platforms)	18446744073709551615
  Maximum Value (32-bit platforms)	4294967295

  The number of times that the replica tries to reconnect to the source before giving up. The default value is 86400 times. A value of 0 means “infinite”, and the replica attempts to connect forever. Reconnection attempts are triggered when the replica reaches its connection timeout (specified by the replica_net_timeout or slave_net_timeout system variable) without receiving data or a heartbeat signal from the source. Reconnection is attempted at intervals set by the SOURCE_CONNECT_RETRY | MASTER_CONNECT_RETRY option of the CHANGE REPLICATION SOURCE TO | CHANGE MASTER TO statement (which defaults to every 60 seconds).

  This option is deprecated; expect it to be removed in a future MySQL release. Use the SOURCE_RETRY_COUNT | MASTER_RETRY_COUNT option of the CHANGE REPLICATION SOURCE TO | CHANGE MASTER TO statement instead.

• --max-relay-log-size=size

  Command-Line Format	--max-relay-log-size=#
  System Variable	max_relay_log_size
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	0
  Minimum Value	0
  Maximum Value	1073741824
  Unit	bytes
  Block Size	4096

  The size at which the server rotates relay log files automatically. If this value is nonzero, the relay log is rotated automatically when its size exceeds this value. If this value is zero (the default), the size at which relay log rotation occurs is determined by the value of max_binlog_size. For more information, see Section 5.4.1, “The Relay Log”.

• --relay-log-purge={0|1}

  Command-Line Format	--relay-log-purge[={OFF|ON}]
  System Variable	relay_log_purge
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	ON

  Disable or enable automatic purging of relay logs as soon as they are no longer needed. The default value is 1 (enabled). This is a global variable that can be changed dynamically with SET GLOBAL relay_log_purge = N. Disabling purging of relay logs when enabling the --relay-log-recovery option risks data consistency and is therefore not crash-safe.

• --relay-log-space-limit=size

  Command-Line Format	--relay-log-space-limit=#
  System Variable	relay_log_space_limit
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	0
  Minimum Value	0
  Maximum Value	18446744073709551615
  Unit	bytes

  This option places an upper limit on the total size in bytes of all relay logs on the replica. A value of 0 means “no limit”. This is useful for a replica server host that has limited disk space. When the limit is reached, the I/O (receiver) thread stops reading binary log events from the source 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 (applier) thread needs more events before it can delete relay logs. In that case, the receiver thread exceeds the limit until it becomes possible for the applier 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 receiver thread waits for free space because --relay-log-space-limit is exceeded, but the applier thread has no relay log to purge and is unable to satisfy the receiver thread. This forces the receiver thread to ignore --relay-log-space-limit temporarily.

• --replicate-do-db=db_name

  Command-Line Format	--replicate-do-db=name
  Type	String

  Creates a replication filter using the name of a database. Such filters can also be created using CHANGE REPLICATION FILTER REPLICATE_DO_DB.

  This option supports channel specific replication filters, enabling multi-source replicas to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-do-db:channel_1:db_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 5.5.4, “Replication Channel Based Filters” for more information.

  Note

  Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replica to a source that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

  The precise effect of this replication filter depends on whether statement-based or row-based replication is in use.

  Statement-based replication.  Tell the replication SQL thread 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; however, doing so does not replicate cross-database statements such as UPDATE some_db.some_table SET foo='bar' while a different database (or no database) is selected.

  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 is treated as the name of a single database.

  An example of what does not work as you might expect when using statement-based replication: If the replica is started with --replicate-do-db=sales and you issue the following statements on the source, the UPDATE statement is not replicated:

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

  The main reason for this “check just the default database” behavior is that it is difficult from the statement alone to know whether it should be replicated (for example, if you are using multiple-table DELETE 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.

  Row-based replication.  Tells the replication SQL thread to restrict replication to database db_name. Only tables belonging to db_name are changed; the current database has no effect on this. Suppose that the replica is started with --replicate-do-db=sales and row-based replication is in effect, and then the following statements are run on the source:

  USE prices;
  UPDATE sales.february SET amount=amount+100;

  The february table in the sales database on the replica is changed in accordance with the UPDATE statement; this occurs whether or not the USE statement was issued. However, issuing the following statements on the source has no effect on the replica when using row-based replication and --replicate-do-db=sales:

  USE prices;
  UPDATE prices.march SET amount=amount-25;

  Even if the statement USE prices were changed to USE sales, the UPDATE statement's effects would still not be replicated.

  Another important difference in how --replicate-do-db is handled in statement-based replication as opposed to row-based replication occurs with regard to statements that refer to multiple databases. Suppose that the replica is started with --replicate-do-db=db1, and the following statements are executed on the source:

  USE db1;
  UPDATE db1.table1, db2.table2 SET db1.table1.col1 = 10, db2.table2.col2 = 20;

  If you are using statement-based replication, then both tables are updated on the replica. However, when using row-based replication, only table1 is affected on the replica; since table2 is in a different database, table2 on the replica is not changed by the UPDATE. Now suppose that, instead of the USE db1 statement, a USE db4 statement had been used:

  USE db4;
  UPDATE db1.table1, db2.table2 SET db1.table1.col1 = 10, db2.table2.col2 = 20;

  In this case, the UPDATE statement would have no effect on the replica when using statement-based replication. However, if you are using row-based replication, the UPDATE would change table1 on the replica, but not table2—in other words, only tables in the database named by --replicate-do-db are changed, and the choice of default database has no effect on this behavior.

  If you need cross-database updates to work, use --replicate-wild-do-table=db_name.% instead. See Section 5.5, “How Servers Evaluate Replication Filtering Rules”.

  Note

  This option affects replication in the same manner that --binlog-do-db affects binary logging, and the effects of the replication format on how --replicate-do-db affects replication behavior are the same as those of the logging format on the behavior of --binlog-do-db.

  This option has no effect on BEGIN, COMMIT, or ROLLBACK statements.

• --replicate-ignore-db=db_name

  Command-Line Format	--replicate-ignore-db=name
  Type	String

  Creates a replication filter using the name of a database. Such filters can also be created using CHANGE REPLICATION FILTER REPLICATE_IGNORE_DB.

  This option supports channel specific replication filters, enabling multi-source replicas to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-ignore-db:channel_1:db_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 5.5.4, “Replication Channel Based Filters” for more information.

  Note

  Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replica to a source that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

  To specify more than one database to ignore, use this option multiple times, once for each database. Because database names can contain commas, if you supply a comma-separated list, it is treated as the name of a single database.

  As with --replicate-do-db, the precise effect of this filtering depends on whether statement-based or row-based replication is in use, and are described in the next several paragraphs.

  Statement-based replication.  Tells the replication SQL thread not to replicate any statement where the default database (that is, the one selected by USE) is db_name.

  Row-based replication.  Tells the replication SQL thread not to update any tables in the database db_name. The default database has no effect.

  When using statement-based replication, the following example does not work as you might expect. Suppose that the replica is started with --replicate-ignore-db=sales and you issue the following statements on the source:

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

  The UPDATE statement is replicated in such a case because --replicate-ignore-db applies only to the default database (determined by the USE statement). Because the sales database was specified explicitly in the statement, the statement has not been filtered. However, when using row-based replication, the UPDATE statement's effects are not propagated to the replica, and the replica's copy of the sales.january table is unchanged; in this instance, --replicate-ignore-db=sales causes all changes made to tables in the source's copy of the sales database to be ignored by the replica.

  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 5.5, “How Servers Evaluate Replication Filtering Rules”.

  If you need cross-database updates to work, use --replicate-wild-ignore-table=db_name.% instead. See Section 5.5, “How Servers Evaluate Replication Filtering Rules”.

  Note

  This option affects replication in the same manner that --binlog-ignore-db affects binary logging, and the effects of the replication format on how --replicate-ignore-db affects replication behavior are the same as those of the logging format on the behavior of --binlog-ignore-db.

  This option has no effect on BEGIN, COMMIT, or ROLLBACK statements.

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

  Command-Line Format	--replicate-do-table=name
  Type	String

  Creates a replication filter by telling the replication SQL thread to restrict replication to a given table. To specify more than one table, use this option multiple times, once for each table. This works for both cross-database updates and default database updates, in contrast to --replicate-do-db. See Section 5.5, “How Servers Evaluate Replication Filtering Rules”. You can also create such a filter by issuing a CHANGE REPLICATION FILTER REPLICATE_DO_TABLE statement.

  This option supports channel specific replication filters, enabling multi-source replicas to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-do-table:channel_1:db_name.tbl_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 5.5.4, “Replication Channel Based Filters” for more information.

  Note

  Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replica to a source that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

  This option affects only statements that apply to tables. It does not affect statements that apply only to other database objects, such as stored routines. To filter statements operating on stored routines, use one or more of the --replicate-*-db options.

• --replicate-ignore-table=db_name.tbl_name

  Command-Line Format	--replicate-ignore-table=name
  Type	String

  Creates a replication filter by telling the replication SQL thread not to 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 5.5, “How Servers Evaluate Replication Filtering Rules”. You can also create such a filter by issuing a CHANGE REPLICATION FILTER REPLICATE_IGNORE_TABLE statement.

  This option supports channel specific replication filters, enabling multi-source replicas to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-ignore-table:channel_1:db_name.tbl_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 5.5.4, “Replication Channel Based Filters” for more information.

  Note

  Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replica to a source that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

  This option affects only statements that apply to tables. It does not affect statements that apply only to other database objects, such as stored routines. To filter statements operating on stored routines, use one or more of the --replicate-*-db options.

• --replicate-rewrite-db=from_name->to_name

  Command-Line Format	--replicate-rewrite-db=old_name->new_name
  Type	String

  Tells the replica to create a replication filter that translates the specified database to to_name if it was from_name on the source. Only statements involving tables are affected, not statements such as CREATE DATABASE, DROP DATABASE, and ALTER DATABASE.

  To specify multiple rewrites, use this option multiple times. The server uses the first one with a from_name value that matches. The database name translation is done before the --replicate-* rules are tested. You can also create such a filter by issuing a CHANGE REPLICATION FILTER REPLICATE_REWRITE_DB statement.

  If you use the --replicate-rewrite-db option on the command line and the > character is special to your command interpreter, quote the option value. For example:

  $> mysqld --replicate-rewrite-db="olddb->newdb"

  The effect of the --replicate-rewrite-db option differs depending on whether statement-based or row-based binary logging format is used for the query. With statement-based format, DML statements are translated based on the current database, as specified by the USE statement. With row-based format, DML statements are translated based on the database where the modified table exists. DDL statements are always filtered based on the current database, as specified by the USE statement, regardless of the binary logging format.

  To ensure that rewriting produces the expected results, particularly in combination with other replication filtering options, follow these recommendations when you use the --replicate-rewrite-db option:

  • Create the from_name and to_name databases manually on the source and the replica with different names.

  • If you use statement-based or mixed binary logging format, do not use cross-database queries, and do not specify database names in queries. For both DDL and DML statements, rely on the USE statement to specify the current database, and use only the table name in queries.

  • If you use row-based binary logging format exclusively, for DDL statements, rely on the USE statement to specify the current database, and use only the table name in queries. For DML statements, you can use a fully qualified table name (db.table) if you want.

  If these recommendations are followed, it is safe to use the --replicate-rewrite-db option in combination with table-level replication filtering options such as --replicate-do-table.

  This option supports channel specific replication filters, enabling multi-source replicas to use specific filters for different sources. Specify the channel name followed by a colon, followed by the filter specification. The first colon is interpreted as a separator, and any subsequent colons are interpreted as literal colons. For example, to configure a channel specific replication filter on a channel named channel_1, use:

  $> mysqld --replicate-rewrite-db=channel_1:db_name1->db_name2

  If you use a colon but do not specify a channel name, the option configures the replication filter for the default replication channel. See Section 5.5.4, “Replication Channel Based Filters” for more information.

  Note

  Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replica to a source that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

• --replicate-same-server-id

  Command-Line Format	--replicate-same-server-id[={OFF|ON}]
  Type	Boolean
  Default Value	OFF

  This option is for use on replicas. The default is 0 (FALSE). With this option set to 1 (TRUE), the replica does not skip events that have its own server ID. This setting is normally useful only in rare configurations.

  When binary logging is enabled on a replica, the combination of the --replicate-same-server-id and --log-slave-updates options on the replica can cause infinite loops in replication if the server is part of a circular replication topology. (In MySQL 8.0, binary logging is enabled by default, and replica update logging is the default when binary logging is enabled.) However, the use of global transaction identifiers (GTIDs) prevents this situation by skipping the execution of transactions that have already been applied. If gtid_mode=ON is set on the replica, you can start the server with this combination of options, but you cannot change to any other GTID mode while the server is running. If any other GTID mode is set, the server does not start with this combination of options.

  By default, the replication I/O (receiver) thread does not write binary log events to the relay log if they have the replica's server ID (this optimization helps save disk usage). If you want to use --replicate-same-server-id, be sure to start the replica with this option before you make the replica read its own events that you want the replication SQL (applier) thread to execute.

• --replicate-wild-do-table=db_name.tbl_name

  Command-Line Format	--replicate-wild-do-table=name
  Type	String

  Creates a replication filter by telling the replication SQL (applier) 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 5.5, “How Servers Evaluate Replication Filtering Rules”. You can also create such a filter by issuing a CHANGE REPLICATION FILTER REPLICATE_WILD_DO_TABLE statement.

  This option supports channel specific replication filters, enabling multi-source replicas to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-wild-do-table:channel_1:db_name.tbl_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 5.5.4, “Replication Channel Based Filters” for more information.

  Important

  Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replica to a source that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

  The replication filter specified by the --replicate-wild-do-table option applies to tables, views, and triggers. It does not apply to stored procedures and functions, or events. To filter statements operating on the latter objects, use one or more of the --replicate-*-db options.

  As an 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%.

  Important

  Table-level replication filters are only applied to tables that are explicitly mentioned and operated on in the query. They do not apply to tables that are implicitly updated by the query. For example, a GRANT statement, which updates the mysql.user system table but does not mention that table, is not affected by a filter that specifies mysql.% as the wildcard pattern.

  To include literal wildcard characters in the database or table name patterns, escape them with a backslash. For example, to replicate all tables of a database that is named my_own%db, but not replicate tables from the my1ownAABCdb database, you should escape the _ and % characters like this: --replicate-wild-do-table=my\_own\%db. If you use the option on the command line, you might need to double the backslashes or quote the option value, depending on your command interpreter. For example, with the bash shell, you would need to type --replicate-wild-do-table=my\\_own\\%db.

• --replicate-wild-ignore-table=db_name.tbl_name

  Command-Line Format	--replicate-wild-ignore-table=name
  Type	String

  Creates a replication filter which keeps the replication SQL thread from replicating a statement in which 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 5.5, “How Servers Evaluate Replication Filtering Rules”. You can also create such a filter by issuing a CHANGE REPLICATION FILTER REPLICATE_WILD_IGNORE_TABLE statement.

  This option supports channel specific replication filters, enabling multi-source replicas to use specific filters for different sources. To configure a channel specific replication filter on a channel named channel_1 use --replicate-wild-ignore:channel_1:db_name.tbl_name. In this case, the first colon is interpreted as a separator and subsequent colons are literal colons. See Section 5.5.4, “Replication Channel Based Filters” for more information.

  Important

  Global replication filters cannot be used on a MySQL server instance that is configured for Group Replication, because filtering transactions on some servers would make the group unable to reach agreement on a consistent state. Channel specific replication filters can be used on replication channels that are not directly involved with Group Replication, such as where a group member also acts as a replica to a source that is outside the group. They cannot be used on the group_replication_applier or group_replication_recovery channels.

  As an 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.

  Important

  Table-level replication filters are only applied to tables that are explicitly mentioned and operated on in the query. They do not apply to tables that are implicitly updated by the query. For example, a GRANT statement, which updates the mysql.user system table but does not mention that table, is not affected by a filter that specifies mysql.% as the wildcard pattern.

  If you need to filter out GRANT statements or other administrative statements, a possible workaround is to use the --replicate-ignore-db filter. This filter operates on the default database that is currently in effect, as determined by the USE statement. You can therefore create a filter to ignore statements for a database that is not replicated, then issue the USE statement to switch the default database to that one immediately before issuing any administrative statements that you want to ignore. In the administrative statement, name the actual database where the statement is applied.

  For example, if --replicate-ignore-db=nonreplicated is configured on the replica server, the following sequence of statements causes the GRANT statement to be ignored, because the default database nonreplicated is in effect:

  USE nonreplicated;
  GRANT SELECT, INSERT ON replicated.t1 TO 'someuser'@'somehost';

• --skip-replica-start

  Command-Line Format	--skip-replica-start[={OFF|ON}]
  Introduced	8.0.26
  System Variable	skip_replica_start
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  From MySQL 8.0.26, use --skip-replica-start in place of --skip-slave-start, which is deprecated from that release. In releases before MySQL 8.0.26, use --skip-slave-start.

  --skip-replica-start tells the replica server not to start the replication I/O (receiver) and SQL (applier) threads when the server starts. To start the threads later, use a START REPLICA statement.

  You can use the skip_replica_start system variable in place of the command line option to allow access to this feature using MySQL Server’s privilege structure, so that database administrators do not need any privileged access to the operating system.

• --skip-slave-start

  Command-Line Format	--skip-slave-start[={OFF|ON}]
  Deprecated	8.0.26
  System Variable	skip_slave_start
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  From MySQL 8.0.26, --skip-slave-start is deprecated and the alias --skip-replica-start should be used instead. In releases before MySQL 8.0.26, use --skip-slave-start.

  Tells the replica server not to start the replication I/O (receiver) and SQL (applier) threads when the server starts. To start the threads later, use a START REPLICA statement.

  From MySQL 8.0.24, you can use the skip_slave_start system variable in place of the command line option to allow access to this feature using MySQL Server’s privilege structure, so that database administrators do not need any privileged access to the operating system.

• --slave-skip-errors=[err_code1,err_code2,...|all|ddl_exist_errors]

  Command-Line Format	--slave-skip-errors=name
  Deprecated	8.0.26
  System Variable	slave_skip_errors
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	String
  Default Value	OFF
  Valid Values	OFF [list of error codes] all ddl_exist_errors

  Normally, replication stops when an error occurs on the replica, which gives you the opportunity to resolve the inconsistency in the data manually. This option causes the replication 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 replicas becoming hopelessly out of synchrony with the source, with you having no idea why this has occurred.

  For error codes, you should use the numbers provided by the error message in your replica's error log and in the output of SHOW REPLICA STATUS. Error Messages and Common Problems, lists server error codes.

  The shorthand value ddl_exist_errors is equivalent to the error code list 1007,1008,1050,1051,1054,1060,1061,1068,1094,1146.

  You can also (but should not) use the very nonrecommended value of all to cause the replica 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 replica's data is not anywhere close to what it is on the source. You have been warned.

  This option does not work in the same way when replicating between NDB Clusters, due to the internal NDB mechanism for checking epoch sequence numbers; normally, as soon as NDB detects an epoch number that is missing or otherwise out of sequence, it immediately stops the replica applier thread. Beginning with NDB 8.0.28, you can override this behavior by also specifying --ndb-applier-allow-skip-epoch together with --slave-skip-errors; doing so causes NDB to ignore skipped epoch transactions.

  Examples:

  --slave-skip-errors=1062,1053
  --slave-skip-errors=all
  --slave-skip-errors=ddl_exist_errors

• --slave-sql-verify-checksum={0|1}

  Command-Line Format	--slave-sql-verify-checksum[={OFF|ON}]
  Type	Boolean
  Default Value	ON

  When this option is enabled, the replica examines checksums read from the relay log. In the event of a mismatch, the replica stops with an error.

The following options are used internally by the MySQL test suite for replication testing and debugging. They are not intended for use in a production setting.

• --abort-slave-event-count

  Command-Line Format	--abort-slave-event-count=#
  Deprecated	8.0.29
  Type	Integer
  Default Value	0
  Minimum Value	0

  When this option is set to some positive integer value other than 0 (the default) it affects replication behavior as follows: After the replication SQL thread has started, value log events are permitted to be executed; after that, the replication SQL thread does not receive any more events, just as if the network connection from the source were cut. The replication SQL thread continues to run, and the output from SHOW REPLICA STATUS displays Yes in both the Replica_IO_Running and the Replica_SQL_Running columns, but no further events are read from the relay log.

  This option is used internally by the MySQL test suite for replication testing and debugging. It is not intended for use in a production setting. Beginning with MySQL 8.0.29, it is deprecated, and subject to removal in a future version of MySQL.

• --disconnect-slave-event-count

  Command-Line Format	--disconnect-slave-event-count=#
  Deprecated	8.0.29
  Type	Integer
  Default Value	0

  This option is used internally by the MySQL test suite for replication testing and debugging. It is not intended for use in a production setting. Beginning with MySQL 8.0.29, it is deprecated, and subject to removal in a future version of MySQL.

System Variables Used on Replica Servers

The following list describes system variables for controlling replica servers. They can be set at server startup and some of them can be changed at runtime using SET. Server options used with replicas are listed earlier in this section.

• init_replica

  Command-Line Format	--init-replica=name
  Introduced	8.0.26
  System Variable	init_replica
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	String

  From MySQL 8.0.26, use init_replica in place of init_slave, which is deprecated from that release. In releases before MySQL 8.0.26, use init_slave.

  init_replica is similar to init_connect, but is a string to be executed by a replica server each time the replication SQL thread starts. The format of the string is the same as for the init_connect variable. The setting of this variable takes effect for subsequent START REPLICA statements.

  Note

  The replication SQL thread sends an acknowledgment to the client before it executes init_replica. Therefore, it is not guaranteed that init_replica has been executed when START REPLICA returns. See START REPLICA Statement for more information.

• init_slave

  Command-Line Format	--init-slave=name
  Deprecated	8.0.26
  System Variable	init_slave
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	String

  From MySQL 8.0.26, init_slave is deprecated and the alias init_replica should be used instead. In releases before MySQL 8.0.26, use init_slave.

  init_slave is similar to init_connect, but is a string to be executed by a replica server each time the replication SQL thread starts. The format of the string is the same as for the init_connect variable. The setting of this variable takes effect for subsequent START REPLICA statements.

  Note

  The replication SQL thread sends an acknowledgment to the client before it executes init_slave. Therefore, it is not guaranteed that init_slave has been executed when START REPLICA returns. See START REPLICA Statement for more information.

• log_slow_replica_statements

  Command-Line Format	--log-slow-replica-statements[={OFF|ON}]
  Introduced	8.0.26
  System Variable	log_slow_replica_statements
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  From MySQL 8.0.26, use log_slow_replica_statements in place of log_slow_slave_statements, which is deprecated from that release. In releases before MySQL 8.0.26, use log_slow_slave_statements.

  When the slow query log is enabled, log_slow_replica_statements enables logging for queries that have taken more than long_query_time seconds to execute on the replica. Note that if row-based replication is in use (binlog_format=ROW), log_slow_replica_statements has no effect. Queries are only added to the replica's slow query log when they are logged in statement format in the binary log, that is, when binlog_format=STATEMENT is set, or when binlog_format=MIXED is set and the statement is logged in statement format. Slow queries that are logged in row format when binlog_format=MIXED is set, or that are logged when binlog_format=ROW is set, are not added to the replica's slow query log, even if log_slow_replica_statements is enabled.

  Setting log_slow_replica_statements has no immediate effect. The state of the variable applies on all subsequent START REPLICA statements. Also note that the global setting for long_query_time applies for the lifetime of the SQL thread. If you change that setting, you must stop and restart the replication SQL thread to implement the change there (for example, by issuing STOP REPLICA and START REPLICA statements with the SQL_THREAD option).

• log_slow_slave_statements

  Command-Line Format	--log-slow-slave-statements[={OFF|ON}]
  Deprecated	8.0.26
  System Variable	log_slow_slave_statements
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  From MySQL 8.0.26, log_slow_slave_statements is deprecated and the alias log_slow_replica_statements should be used instead. In releases before MySQL 8.0.26, use log_slow_slave_statements.

  When the slow query log is enabled, log_slow_slave_statements enables logging for queries that have taken more than long_query_time seconds to execute on the replica. Note that if row-based replication is in use (binlog_format=ROW), log_slow_slave_statements has no effect. Queries are only added to the replica's slow query log when they are logged in statement format in the binary log, that is, when binlog_format=STATEMENT is set, or when binlog_format=MIXED is set and the statement is logged in statement format. Slow queries that are logged in row format when binlog_format=MIXED is set, or that are logged when binlog_format=ROW is set, are not added to the replica's slow query log, even if log_slow_slave_statements is enabled.

  Setting log_slow_slave_statements has no immediate effect. The state of the variable applies on all subsequent START REPLICA statements. Also note that the global setting for long_query_time applies for the lifetime of the SQL thread. If you change that setting, you must stop and restart the replication SQL thread to implement the change there (for example, by issuing STOP REPLICA and START REPLICA statements with the SQL_THREAD option).

• master_info_repository

  Command-Line Format	--master-info-repository={FILE|TABLE}
  Deprecated	8.0.23
  System Variable	master_info_repository
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	String
  Default Value	TABLE
  Valid Values	FILE TABLE

  The use of this system variable is now deprecated. The setting TABLE is the default, and is required when multiple replication channels are configured. The alternative setting FILE was previously deprecated.

  With the default setting, the replica records metadata about the source, consisting of status and connection information, to an InnoDB table in the mysql system database named mysql.slave_master_info. For more information on the connection metadata repository, see Section 5.4, “Relay Log and Replication Metadata Repositories”.

  The FILE setting wrote the replica's connection metadata repository to a file, which was named master.info by default. The name could be changed using the --master-info-file option.

• max_relay_log_size

  Command-Line Format	--max-relay-log-size=#
  System Variable	max_relay_log_size
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	0
  Minimum Value	0
  Maximum Value	1073741824
  Unit	bytes
  Block Size	4096

  If a write by a replica to its relay log causes the current log file size to exceed the value of this variable, the replica rotates the relay logs (closes the current file and opens the next one). If max_relay_log_size is 0, the server uses max_binlog_size for both the binary log and the relay log. If max_relay_log_size is greater than 0, it constrains the size of the relay log, which enables you to have different sizes for the two logs. You must set max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. See Section 5.3, “Replication Threads”.

• relay_log

  Command-Line Format	--relay-log=file_name
  System Variable	relay_log
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	File name

  The base name for relay log files. For the default replication channel, the default base name for relay logs is host_name-relay-bin. For non-default replication channels, the default base name for relay logs is host_name-relay-bin-channel, where channel is the name of the replication channel recorded in this relay log.

  The server writes the file in the data directory unless the base name is given with a leading absolute path name to specify a different directory. The server creates relay log files in sequence by adding a numeric suffix to the base name.

  The relay log and relay log index on a replication server cannot be given the same names as the binary log and binary log index, whose names are specified by the --log-bin and --log-bin-index options. The server issues an error message and does not start if the binary log and relay log file base names would be the same.

  Due to the manner in which MySQL parses server options, if you specify this variable at server startup, you must supply a value; the default base name is used only if the option is not actually specified. If you specify the relay_log system variable at server startup without specifying a value, unexpected behavior is likely to result; this behavior depends on the other options used, the order in which they are specified, and whether they are specified on the command line or in an option file. For more information about how MySQL handles server options, see Specifying Program Options.

  If you specify this variable, the value specified is also used as the base name for the relay log index file. You can override this behavior by specifying a different relay log index file base name using the relay_log_index system variable.

  When the server reads an entry from the index file, it checks whether the entry contains a relative path. If it does, the relative part of the path is replaced with the absolute path set using the relay_log system variable. An absolute path remains unchanged; in such a case, the index must be edited manually to enable the new path or paths to be used.

  You may find the relay_log system variable useful in performing the following tasks:

  • Creating relay logs whose names are independent of host names.

  • If you need to put the relay logs in some area other than the data directory because your relay logs tend to be very large and you do not want to decrease max_relay_log_size.

  • To increase speed by using load-balancing between disks.

  You can obtain the relay log file name (and path) from the relay_log_basename system variable.

• relay_log_basename

  System Variable	relay_log_basename
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	File name
  Default Value	datadir + '/' + hostname + '-relay-bin'

  Holds the base name and complete path to the relay log file. The maximum variable length is 256. This variable is set by the server and is read only.

• relay_log_index

  Command-Line Format	--relay-log-index=file_name
  System Variable	relay_log_index
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	File name
  Default Value	*host_name*-relay-bin.index

  The name for the relay log index file. The maximum variable length is 256. If you do not specify this variable, but the relay_log system variable is specified, its value is used as the default base name for the relay log index file. If relay_log is also not specified, then for the default replication channel, the default name is host_name-relay-bin.index, using the name of the host machine. For non-default replication channels, the default name is host_name-relay-bin-channel.index, where channel is the name of the replication channel recorded in this relay log index.

  The default location for relay log files is the data directory, or any other location that was specified using the relay_log system variable. You can use the relay_log_index system variable to specify an alternative location, by adding a leading absolute path name to the base name to specify a different directory.

  The relay log and relay log index on a replication server cannot be given the same names as the binary log and binary log index, whose names are specified by the --log-bin and --log-bin-index options. The server issues an error message and does not start if the binary log and relay log file base names would be the same.

  Due to the manner in which MySQL parses server options, if you specify this variable at server startup, you must supply a value; the default base name is used only if the option is not actually specified. If you specify the relay_log_index system variable at server startup without specifying a value, unexpected behavior is likely to result; this behavior depends on the other options used, the order in which they are specified, and whether they are specified on the command line or in an option file. For more information about how MySQL handles server options, see Specifying Program Options.

• relay_log_info_file

  Command-Line Format	--relay-log-info-file=file_name
  Deprecated	8.0.18
  System Variable	relay_log_info_file
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	File name
  Default Value	relay-log.info

  The use of this system variable is now deprecated. It was used to set the file name for the replica's applier metadata repository if relay_log_info_repository=FILE was set. relay_log_info_file and the use of the relay_log_info_repository system variable are deprecated because the use of a file for the applier metadata repository has been superseded by crash-safe tables. For information about the applier metadata repository, see Section 5.4.2, “Replication Metadata Repositories”.

• relay_log_info_repository

  Command-Line Format	--relay-log-info-repository=value
  Deprecated	8.0.23
  System Variable	relay_log_info_repository
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	String
  Default Value	TABLE
  Valid Values	FILE TABLE

  The use of this system variable is now deprecated. The setting TABLE is the default, and is required when multiple replication channels are configured. The TABLE setting for the replica's applier metadata repository is also required to make replication resilient to unexpected halts. See Section 3.2, “Handling an Unexpected Halt of a Replica” for more information. The alternative setting FILE was previously deprecated.

  With the default setting, the replica stores its applier metadata repository as an InnoDB table in the mysql system database named mysql.slave_relay_log_info. For more information on the applier metadata repository, see Section 5.4, “Relay Log and Replication Metadata Repositories”.

  The FILE setting wrote the replica's applier metadata repository to a file, which was named relay-log.info by default. The name could be changed using the relay_log_info_file system variable.

• relay_log_purge

  Command-Line Format	--relay-log-purge[={OFF|ON}]
  System Variable	relay_log_purge
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	ON

  Disables or enables automatic purging of relay log files as soon as they are not needed any more. The default value is 1 (ON).

• relay_log_recovery

  Command-Line Format	--relay-log-recovery[={OFF|ON}]
  System Variable	relay_log_recovery
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  If enabled, this variable enables automatic relay log recovery immediately following server startup. The recovery process creates a new relay log file, initializes the SQL (applier) thread position to this new relay log, and initializes the I/O (receiver) thread to the applier thread position. Reading of the relay log from the source then continues. If SOURCE_AUTO_POSITION=1 was set for the replication channel using the CHANGE REPLICATION SOURCE TO option, the source position used to start replication might be the one received in the connection and not the ones assigned in this process.

  This global variable is read-only at runtime. Its value can be set with the --relay-log-recovery option at replica server startup, which should be used following an unexpected halt of a replica to ensure that no possibly corrupted relay logs are processed, and must be used in order to guarantee a crash-safe replica. The default value is 0 (disabled). For information on the combination of settings on a replica that is most resilient to unexpected halts, see Section 3.2, “Handling an Unexpected Halt of a Replica”.

  For a multithreaded replica (where replica_parallel_workers or slave_parallel_workers is greater than 0), setting --relay-log-recovery at startup automatically handles any inconsistencies and gaps in the sequence of transactions that have been executed from the relay log. These gaps can occur when file position based replication is in use. (For more details, see Section 4.1.34, “Replication and Transaction Inconsistencies”.) The relay log recovery process deals with gaps using the same method as the START REPLICA UNTIL SQL_AFTER_MTS_GAPS statement would. When the replica reaches a consistent gap-free state, the relay log recovery process goes on to fetch further transactions from the source beginning at the SQL (applier) thread position. When GTID-based replication is in use, from MySQL 8.0.18 a multithreaded replica checks first whether MASTER_AUTO_POSITION is set to ON, and if it is, omits the step of calculating the transactions that should be skipped or not skipped, so that the old relay logs are not required for the recovery process.

  Note

  This variable does not affect the following Group Replication channels:

  • group_replication_applier

  • group_replication_recovery

  Any other channels running on a group are affected, such as a channel which is replicating from an outside source or another group.

• relay_log_space_limit

  Command-Line Format	--relay-log-space-limit=#
  System Variable	relay_log_space_limit
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	0
  Minimum Value	0
  Maximum Value	18446744073709551615
  Unit	bytes

  The maximum amount of space to use for all relay logs.

• replica_checkpoint_group

  Command-Line Format	--replica-checkpoint-group=#
  Introduced	8.0.26
  System Variable	replica_checkpoint_group
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	512
  Minimum Value	32
  Maximum Value	524280
  Block Size	8

  From MySQL 8.0.26, use replica_checkpoint_group in place of slave_checkpoint_group, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_checkpoint_group.

  replica_checkpoint_group sets the maximum number of transactions that can be processed by a multithreaded replica before a checkpoint operation is called to update its status as shown by SHOW REPLICA STATUS. Setting this variable has no effect on replicas for which multithreading is not enabled. Setting this variable has no immediate effect. The state of the variable applies to all subsequent START REPLICA statements.

  Previously, multithreaded replicas were not supported by NDB Cluster, which silently ignored the setting for this variable. This restriction was lifted in MySQL 8.0.33.

  This variable works in combination with the replica_checkpoint_period system variable in such a way that, when either limit is exceeded, the checkpoint is executed and the counters tracking both the number of transactions and the time elapsed since the last checkpoint are reset.

  The minimum allowed value for this variable is 32, unless the server was built using -DWITH_DEBUG, in which case the minimum value is 1. The effective value is always a multiple of 8; you can set it to a value that is not such a multiple, but the server rounds it down to the next lower multiple of 8 before storing the value. (Exception: No such rounding is performed by the debug server.) Regardless of how the server was built, the default value is 512, and the maximum allowed value is 524280.

• replica_checkpoint_period

  Command-Line Format	--replica-checkpoint-period=#
  Introduced	8.0.26
  System Variable	replica_checkpoint_period
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	300
  Minimum Value	1
  Maximum Value	4294967295
  Unit	milliseconds

  In MySQL 8.0.26 and later, use replica_checkpoint_period in place of slave_checkpoint_period, which is deprecated from that release; prior to MySQL 8.0.26, use slave_checkpoint_period.

  replica_checkpoint_period sets the maximum time (in milliseconds) that is allowed to pass before a checkpoint operation is called to update the status of a multithreaded replica as shown by SHOW REPLICA STATUS. Setting this variable has no effect on replicas for which multithreading is not enabled. Setting this variable takes effect for all replication channels immediately, including running channels.

  Previously, multithreaded replicas were not supported by NDB Cluster, which silently ignored the setting for this variable. This restriction was lifted in MySQL 8.0.33.

  This variable works in combination with the replica_checkpoint_group system variable in such a way that, when either limit is exceeded, the checkpoint is executed and the counters tracking both the number of transactions and the time elapsed since the last checkpoint are reset.

  The minimum allowed value for this variable is 1, unless the server was built using -DWITH_DEBUG, in which case the minimum value is 0. Regardless of how the server was built, the default value is 300 milliseconds, and the maximum possible value is 4294967295 milliseconds (approximately 49.7 days).

• replica_compressed_protocol

  Command-Line Format	--replica-compressed-protocol[={OFF|ON}]
  Introduced	8.0.26
  System Variable	replica_compressed_protocol
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  From MySQL 8.0.26, use replica_compressed_protocol in place of slave_compressed_protocol, which is deprecated. In releases before MySQL 8.0.26, use slave_compressed_protocol.

  replica_compressed_protocol specifies whether to use compression of the source/replica connection protocol if both source and replica support it. If this variable is disabled (the default), connections are uncompressed. Changes to this variable take effect on subsequent connection attempts; this includes after issuing a START REPLICA statement, as well as reconnections made by a running replication I/O (receiver) thread.

  Binary log transaction compression (available as of MySQL 8.0.20), which is activated by the binlog_transaction_compression system variable, can also be used to save bandwidth. If you use binary log transaction compression in combination with protocol compression, protocol compression has less opportunity to act on the data, but can still compress headers and those events and transaction payloads that are uncompressed. For more information on binary log transaction compression, see Binary Log Transaction Compression.

  If replica_compressed_protocol is enabled, it takes precedence over any SOURCE_COMPRESSION_ALGORITHMS option specified for the CHANGE REPLICATION SOURCE TO statement. In this case, connections to the source use zlib compression if both the source and replica support that algorithm. If replica_compressed_protocol is disabled, the value of SOURCE_COMPRESSION_ALGORITHMS applies. For more information, see Connection Compression Control.

• replica_exec_mode

  Command-Line Format	--replica-exec-mode=mode
  Introduced	8.0.26
  System Variable	replica_exec_mode
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Enumeration
  Default Value	IDEMPOTENT (NDB) STRICT (Other)
  Valid Values	STRICT IDEMPOTENT

  From MySQL 8.0.26, use replica_exec_mode in place of slave_exec_mode, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_exec_mode.

  replica_exec_mode controls how a replication thread resolves conflicts and errors during replication. IDEMPOTENT mode causes suppression of duplicate-key and no-key-found errors; STRICT means no such suppression takes place.

  IDEMPOTENT mode is intended for use in multi-source replication, circular replication, and some other special replication scenarios for NDB Cluster Replication. (See NDB Cluster Replication: Bidirectional and Circular Replication, and NDB Cluster Replication Conflict Resolution, for more information.) NDB Cluster ignores any value explicitly set for replica_exec_mode, and always treats it as IDEMPOTENT.

  In MySQL Server 8.0, STRICT mode is the default value.

  Setting this variable takes immediate effect for all replication channels, including running channels.

  For storage engines other than NDB, IDEMPOTENT mode should be used only when you are absolutely sure that duplicate-key errors and key-not-found errors can safely be ignored. It is meant to be used in fail-over scenarios for NDB Cluster where multi-source replication or circular replication is employed, and is not recommended for use in other cases.

• replica_load_tmpdir

  Command-Line Format	--replica-load-tmpdir=dir_name
  Introduced	8.0.26
  System Variable	replica_load_tmpdir
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Directory name
  Default Value	Value of --tmpdir

  From MySQL 8.0.26, use replica_load_tmpdir in place of slave_load_tmpdir, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_load_tmpdir.

  replica_load_tmpdir specifies the name of the directory where the replica creates temporary files. Setting this variable takes effect for all replication channels immediately, including running channels. The variable value is by default equal to the value of the tmpdir system variable, or the default that applies when that system variable is not specified.

  When the replication SQL thread replicates a LOAD DATA 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 source is huge, the temporary files on the replica are huge, too. Therefore, it might be advisable to use this option to tell the replica to put temporary files in a directory located in some file system that has a lot of available space. In that case, the relay logs are huge as well, so you might also want to set the relay_log system variable to place the relay logs in that file system.

  The directory specified by this option should be located in a disk-based file system (not a memory-based file system) so that the temporary files used to replicate LOAD DATA statements can survive machine restarts. The directory also should not be one that is cleared by the operating system during the system startup process. However, replication can now continue after a restart if the temporary files have been removed.

• replica_max_allowed_packet

  Command-Line Format	--replica-max-allowed-packet=#
  Introduced	8.0.26
  System Variable	replica_max_allowed_packet
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	1073741824
  Minimum Value	1024
  Maximum Value	1073741824
  Unit	bytes
  Block Size	1024

  From MySQL 8.0.26, use replica_max_allowed_packet in place of slave_max_allowed_packet, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_max_allowed_packet.

  replica_max_allowed_packet sets the maximum packet size in bytes that the replication SQL (applier)and I/O (receiver) threads can handle. Setting this variable takes effect for all replication channels immediately, including running channels. It is possible for a source to write binary log events longer than its max_allowed_packet setting once the event header is added. The setting for replica_max_allowed_packet must be larger than the max_allowed_packet setting on the source, so that large updates using row-based replication do not cause replication to fail.

  This global variable always has a value that is a positive integer multiple of 1024; if you set it to some value that is not, the value is rounded down to the next highest multiple of 1024 for it is stored or used; setting replica_max_allowed_packet to 0 causes 1024 to be used. (A truncation warning is issued in all such cases.) The default and maximum value is 1073741824 (1 GB); the minimum is 1024.

• replica_net_timeout

  Command-Line Format	--replica-net-timeout=#
  Introduced	8.0.26
  System Variable	replica_net_timeout
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	60
  Minimum Value	1
  Maximum Value	31536000
  Unit	seconds

  From MySQL 8.0.26, use replica_net_timeout in place of slave_net_timeout, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_net_timeout.

  replica_net_timeout specifies the number of seconds to wait for more data or a heartbeat signal from the source before the replica considers the connection broken, aborts the read, and tries to reconnect. Setting this variable has no immediate effect. The state of the variable applies on all subsequent START REPLICA commands.

  The default value is 60 seconds (one minute). The first retry occurs immediately after the timeout. The interval between retries is controlled by the SOURCE_CONNECT_RETRY option for the CHANGE REPLICATION SOURCE TO statement, and the number of reconnection attempts is limited by the SOURCE_RETRY_COUNT option.

  The heartbeat interval, which stops the connection timeout occurring in the absence of data if the connection is still good, is controlled by the SOURCE_HEARTBEAT_PERIOD option for the CHANGE REPLICATION SOURCE TO statement. The heartbeat interval defaults to half the value of replica_net_timeout, and it is recorded in the replica's connection metadata repository and shown in the replication_connection_configuration Performance Schema table. Note that a change to the value or default setting of replica_net_timeout does not automatically change the heartbeat interval, whether that has been set explicitly or is using a previously calculated default. If the connection timeout is changed, you must also issue CHANGE REPLICATION SOURCE TO to adjust the heartbeat interval to an appropriate value so that it occurs before the connection timeout.

• replica_parallel_type

  Command-Line Format	--replica-parallel-type=value
  Introduced	8.0.26
  Deprecated	8.0.29
  System Variable	replica_parallel_type
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Enumeration
  Default Value (≥ 8.0.27)	LOGICAL_CLOCK
  Default Value (8.0.26)	DATABASE
  Valid Values	DATABASE LOGICAL_CLOCK

  From MySQL 8.0.26, use replica_parallel_type in place of slave_parallel_type, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_parallel_type.

  For multithreaded replicas (replicas on which replica_parallel_workers or slave_parallel_workers is set to a value greater than 0), replica_parallel_type specifies the policy used to decide which transactions are allowed to execute in parallel on the replica. The variable has no effect on replicas for which multithreading is not enabled. The possible values are:

  • LOGICAL_CLOCK: Transactions are applied in parallel on the replica, based on timestamps which the replication source writes to the binary log. Dependencies between transactions are tracked based on their timestamps to provide additional parallelization where possible.

  • DATABASE: Transactions that update different databases are applied in parallel. This value is only appropriate if data is partitioned into multiple databases which are being updated independently and concurrently on the source. There must be no cross-database constraints, as such constraints may be violated on the replica.

  When replica_preserve_commit_order or slave_preserve_commit_order is enabled, you must use LOGICAL_CLOCK. Before MySQL 8.0.27, DATABASE is the default. From MySQL 8.0.27, multithreading is enabled by default for replica servers (replica_parallel_workers=4 by default), and LOGICAL_CLOCK is the default. (In MySQL 8.0.27 and later, replica_preserve_commit_order is also enabled by default.)

  When the replication topology uses multiple levels of replicas, LOGICAL_CLOCK may achieve less parallelization for each level the replica is away from the source. To compensate for this effect, you should set binlog_transaction_dependency_tracking to WRITESET or WRITESET_SESSION on the source as well as on every intermediate replica to specify that write sets are used instead of timestamps for parallelization where possible.

  When binary log transaction compression is enabled using the binlog_transaction_compression system variable, if replica_parallel_type is set to DATABASE, all the databases affected by the transaction are mapped before the transaction is scheduled. The use of binary log transaction compression with the DATABASE policy can reduce parallelism compared to uncompressed transactions, which are mapped and scheduled for each event.

  replica_parallel_type is deprecated beginning with MySQL 8.0.29, as is support for parallelization of transactions using database partitioning. Expect support for these to be removed in a future release, and for LOGICAL_CLOCK to be used exclusively thereafter.

• replica_parallel_workers

  Command-Line Format	--replica-parallel-workers=#
  Introduced	8.0.26
  System Variable	replica_parallel_workers
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value (≥ 8.0.27)	4
  Default Value (8.0.26)	0
  Minimum Value	0
  Maximum Value	1024

  Beginning with MySQL 8.0.26, slave_parallel_workers is deprecated, and you should use replica_parallel_workers instead. (Prior to MySQL 8.0.26, you must use slave_parallel_workers to set the number of applier threads.)

  replica_parallel_workers enables multithreading on the replica and sets the number of applier threads for executing replication transactions in parallel. When the value is greater than or equal to 1, the replica uses the specified number of worker threads to execute transactions, plus a coordinator thread that reads transactions from the relay log and schedules them to workers. When the value is 0, there is only one thread that reads and applies transactions sequentially. If you are using multiple replication channels, the value of this variable applies to the threads used by each channel.

  Prior to MySQL 8.0.27, the default value of this system variable is 0, so replicas use a single worker thread by default. Beginning with MySQL 8.0.27, the default value is 4, which means that replicas are multithreaded by default.

  As of MySQL 8.0.30, setting this variable to 0 is deprecated, raises a warning, and is subject to removal in a future MySQL release. For a single worker, set replica_parallel_workers to 1 instead.

  When replica_preserve_commit_order (or slave_preserve_commit_order) is set to ON (the default in MySQL 8.0.27 and later), transactions on a replica are externalized on the replica in the same order as they appear in the replica's relay log. The way in which transactions are distributed among applier threads is determined by replica_parallel_type (MySQL 8.0.26 and later) or slave_parallel_type (prior to MySQL 8.0.26). Starting with MySQL 8.0.27, these system variables also have appropriate defaults for multithreading.

  To disable parallel execution, set replica_parallel_workers to 1, in which case the replica uses one coordinator thread which reads transactions, and one worker thread which applies them, which means that transactions are applied sequentially. When replica_parallel_workers is equal to 1, the replica_parallel_type (slave_parallel_type) and replica_preserve_commit_order (slave_preserve_commit_order) system variables have no effect and are ignored. If replica_parallel_workers is equal to 0 while the CHANGE REPLICATION SOURCE TO option GTID_ONLY is enabled, the replica has one coordinator thread and one worker thread, exactly as if replica_parallel_workers had been set to 1. (GTID_ONLY is available in MySQL 8.0.27 and later.) With one parallel worker, the replica_preserve_commit_order (slave_preserve_commit_order) system variable also has no effect.

  Setting replica_parallel_workers has no immediate effect but rather applies to all subsequent START REPLICA statements.

  Multithreaded replicas are supported by NDB Cluster beginning with NDB 8.0.33. (Previously, NDB silently ignored any setting for replica_parallel_workers.) See NDB Cluster Replication Using the Multithreaded Applier, for more information.

  Increasing the number of workers improves the potential for parallelism. Typically, this improves performance up to a certain point, beyond which increasing the number of workers reduces performance due to concurrency effects such as lock contention. The ideal number depends on both hardware and workload; it can be difficult to predict and typically has to be found by testing. Tables without primary keys, which always harm performance, may have even greater negative performance impact on replicas having replica_parallel_workers > 1; so make sure that all tables have primary keys before enabling this option.

• replica_pending_jobs_size_max

  Command-Line Format	--replica-pending-jobs-size-max=#
  Introduced	8.0.26
  System Variable	replica_pending_jobs_size_max
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	128M
  Minimum Value	1024
  Maximum Value	16EiB
  Unit	bytes
  Block Size	1024

  From MySQL 8.0.26, use replica_pending_jobs_size_max in place of slave_pending_jobs_size_max, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_pending_jobs_size_max.

  For multithreaded replicas, this variable sets the maximum amount of memory (in bytes) available to applier queues holding events not yet applied. Setting this variable has no effect on replicas for which multithreading is not enabled. Setting this variable has no immediate effect. The state of the variable applies on all subsequent START REPLICA statements.

  The minimum possible value for this variable is 1024 bytes; the default is 128MB. The maximum possible value is 18446744073709551615 (16 exbibytes). Values that are not exact multiples of 1024 bytes are rounded down to the next lower multiple of 1024 bytes prior to being stored.

  The value of this variable is a soft limit and can be set to match the normal workload. If an unusually large event exceeds this size, the transaction is held until all the worker threads have empty queues, and then processed. All subsequent transactions are held until the large transaction has been completed.

• replica_preserve_commit_order

  Command-Line Format	--replica-preserve-commit-order[={OFF|ON}]
  Introduced	8.0.26
  System Variable	replica_preserve_commit_order
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value (≥ 8.0.27)	ON
  Default Value (8.0.26)	OFF

  From MySQL 8.0.26, use replica_preserve_commit_order in place of slave_preserve_commit_order, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_preserve_commit_order.

  For multithreaded replicas (replicas on which replica_parallel_workers is set to a value greater than 0), setting replica_preserve_commit_order=ON ensures that transactions are executed and committed on the replica in the same order as they appear in the replica's relay log. This prevents gaps in the sequence of transactions that have been executed from the replica's relay log, and preserves the same transaction history on the replica as on the source (with the limitations listed below). This variable has no effect on replicas for which multithreading is not enabled.

  Before MySQL 8.0.27, the default for this system variable is OFF, meaning that transactions may be committed out of order. From MySQL 8.0.27, multithreading is enabled by default for replica servers (replica_parallel_workers=4 by default), so replica_preserve_commit_order=ON is the default, and the setting replica_parallel_type=LOGICAL_CLOCK is also the default. Also from MySQL 8.0.27, the setting for replica_preserve_commit_order is ignored if replica_parallel_workers is set to 1, because in that situation the order of transactions is preserved anyway.

  Binary logging and replica update logging are not required on the replica to set replica_preserve_commit_order=ON, and can be disabled if wanted. Setting replica_preserve_commit_order=ON requires that replica_parallel_type is set to LOGICAL_CLOCK, which is not the default setting before MySQL 8.0.27. Before changing the value of replica_preserve_commit_order and replica_parallel_type, the replication SQL thread (for all replication channels if you are using multiple replication channels) must be stopped.

  When replica_preserve_commit_order=OFF is set, the transactions that a multithreaded replica applies in parallel may commit out of order. Therefore, checking for the most recently executed transaction does not guarantee that all previous transactions from the source have been executed on the replica. There is a chance of gaps in the sequence of transactions that have been executed from the replica's relay log. This has implications for logging and recovery when using a multithreaded replica. See Section 4.1.34, “Replication and Transaction Inconsistencies” for more information.

  When replica_preserve_commit_order=ON is set, the executing worker thread waits until all previous transactions are committed before committing. While a given thread is waiting for other worker threads to commit their transactions, it reports its status as Waiting for preceding transaction to commit. With this mode, a multithreaded replica never enters a state that the source was not in. This supports the use of replication for read scale-out. See Section 3.5, “Using Replication for Scale-Out”.

  Note

  • replica_preserve_commit_order=ON does not prevent source binary log position lag, where Exec_master_log_pos is behind the position up to which transactions have been executed. See Section 4.1.34, “Replication and Transaction Inconsistencies”.

  • replica_preserve_commit_order=ON does not preserve the commit order and transaction history if the replica uses filters on its binary log, such as --binlog-do-db.

  • replica_preserve_commit_order=ON does not preserve the order of non-transactional DML updates. These might commit before transactions that precede them in the relay log, which might result in gaps in the sequence of transactions that have been executed from the replica's relay log.

  • A limitation to preserving the commit order on the replica can occur if statement-based replication is in use, and both transactional and non-transactional storage engines participate in a non-XA transaction that is rolled back on the source. Normally, non-XA transactions that are rolled back on the source are not replicated to the replica, but in this particular situation, the transaction might be replicated to the replica. If this does happen, a multithreaded replica without binary logging does not handle the transaction rollback, so the commit order on the replica diverges from the relay log order of the transactions in that case.

• replica_sql_verify_checksum

  Command-Line Format	--replica-sql-verify-checksum[={OFF|ON}]
  Introduced	8.0.26
  System Variable	replica_sql_verify_checksum
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	ON

  From MySQL 8.0.26, use replica_sql_verify_checksum in place of slave_sql_verify_checksum, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_sql_verify_checksum.

  slave_sql_verify_checksum causes the replication SQL (applier) thread to verify data using the checksums read from the relay log. In the event of a mismatch, the replica stops with an error. Setting this variable takes effect for all replication channels immediately, including running channels.

  Note

  The replication I/O (receiver)thread always reads checksums if possible when accepting events from over the network.

• replica_transaction_retries

  Command-Line Format	--replica-transaction-retries=#
  Introduced	8.0.26
  System Variable	replica_transaction_retries
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	10
  Minimum Value	0
  Maximum Value	18446744073709551615

  From MySQL 8.0.26, use replica_transaction_retries in place of slave_transaction_retries, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_transaction_retries.

  replica_transaction_retries sets the maximum number of times for replication SQL threads on a single-threaded or multithreaded replica to automatically retry failed transactions before stopping. Setting this variable takes effect for all replication channels immediately, including running channels. The default value is 10. Setting the variable to 0 disables automatic retrying of transactions.

  If a replication SQL thread fails to execute a transaction because of an InnoDB deadlock or because the transaction's execution time exceeded InnoDB's innodb_lock_wait_timeout or NDB's TransactionDeadlockDetectionTimeout or TransactionInactiveTimeout, it automatically retries replica_transaction_retries times before stopping with an error. Transactions with a non-temporary error are not retried.

  The Performance Schema table replication_applier_status shows the number of retries that took place on each replication channel, in the COUNT_TRANSACTIONS_RETRIES column. The Performance Schema table replication_applier_status_by_worker shows detailed information on transaction retries by individual applier threads on a single-threaded or multithreaded replica, and identifies the errors that caused the last transaction and the transaction currently in progress to be reattempted.

• replica_type_conversions

  Command-Line Format	--replica-type-conversions=set
  Introduced	8.0.26
  System Variable	replica_type_conversions
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Set
  Default Value
  Valid Values	ALL_LOSSY ALL_NON_LOSSY ALL_SIGNED ALL_UNSIGNED

  From MySQL 8.0.26, use replica_type_conversions in place of slave_type_conversions, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_type_conversions.

  replica_type_conversions controls the type conversion mode in effect on the replica when using row-based replication. Its value is a comma-delimited set of zero or more elements from the list: ALL_LOSSY, ALL_NON_LOSSY, ALL_SIGNED, ALL_UNSIGNED. Set this variable to an empty string to disallow type conversions between the source and the replica. Setting this variable takes effect for all replication channels immediately, including running channels.

  For additional information on type conversion modes applicable to attribute promotion and demotion in row-based replication, see Row-based replication: attribute promotion and demotion.

• replication_optimize_for_static_plugin_config

  Command-Line Format	--replication-optimize-for-static-plugin-config[={OFF|ON}]
  Introduced	8.0.23
  System Variable	replication_optimize_for_static_plugin_config
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  Use shared locks, and avoid unnecessary lock acquisitions, to improve performance for semisynchronous replication. This setting and replication_sender_observe_commit_only help as the number of replicas increases, because contention for locks can slow down performance. While this system variable is enabled, the semisynchronous replication plugin cannot be uninstalled, so you must disable the system variable before the uninstall can complete.

  This system variable can be enabled before or after installing the semisynchronous replication plugin, and can be enabled while replication is running. Semisynchronous replication source servers can also get performance benefits from enabling this system variable, because they use the same locking mechanisms as the replicas.

  replication_optimize_for_static_plugin_config can be enabled when Group Replication is in use on a server. In that scenario, it might benefit performance when there is contention for locks due to high workloads.

• replication_sender_observe_commit_only

  Command-Line Format	--replication-sender-observe-commit-only[={OFF|ON}]
  Introduced	8.0.23
  System Variable	replication_sender_observe_commit_only
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  Limit callbacks to improve performance for semisynchronous replication. This setting and replication_optimize_for_static_plugin_config help as the number of replicas increases, because contention for locks can slow down performance.

  This system variable can be enabled before or after installing the semisynchronous replication plugin, and can be enabled while replication is running. Semisynchronous replication source servers can also get performance benefits from enabling this system variable, because they use the same locking mechanisms as the replicas.

• report_host

  Command-Line Format	--report-host=host_name
  System Variable	report_host
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	String

  The host name or IP address of the replica to be reported to the source during replica registration. This value appears in the output of SHOW REPLICAS on the source server. Leave the value unset if you do not want the replica to register itself with the source.

  Note

  It is not sufficient for the source to simply read the IP address of the replica server from the TCP/IP socket after the replica connects. Due to NAT and other routing issues, that IP may not be valid for connecting to the replica from the source or other hosts.

• report_password

  Command-Line Format	--report-password=name
  System Variable	report_password
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	String

  The account password of the replica to be reported to the source during replica registration. This value appears in the output of SHOW REPLICAS on the source server if the source was started with --show-replica-auth-info or --show-slave-auth-info.

  Although the name of this variable might imply otherwise, report_password is not connected to the MySQL user privilege system and so is not necessarily (or even likely to be) the same as the password for the MySQL replication user account.

• report_port

  Command-Line Format	--report-port=port_num
  System Variable	report_port
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	[slave_port]
  Minimum Value	0
  Maximum Value	65535

  The TCP/IP port number for connecting to the replica, to be reported to the source during replica registration. Set this only if the replica is listening on a nondefault port or if you have a special tunnel from the source or other clients to the replica. If you are not sure, do not use this option.

  The default value for this option is the port number actually used by the replica. This is also the default value displayed by SHOW REPLICAS.

• report_user

  Command-Line Format	--report-user=name
  System Variable	report_user
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	String

  The account user name of the replica to be reported to the source during replica registration. This value appears in the output of SHOW REPLICAS on the source server if the source was started with --show-replica-auth-info or --show-slave-auth-info.

  Although the name of this variable might imply otherwise, report_user is not connected to the MySQL user privilege system and so is not necessarily (or even likely to be) the same as the name of the MySQL replication user account.

• rpl_read_size

  Command-Line Format	--rpl-read-size=#
  System Variable	rpl_read_size
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	8192
  Minimum Value	8192
  Maximum Value	4294959104
  Unit	bytes
  Block Size	8192

  The rpl_read_size system variable controls the minimum amount of data in bytes that is read from the binary log files and relay log files. If heavy disk I/O activity for these files is impeding performance for the database, increasing the read size might reduce file reads and I/O stalls when the file data is not currently cached by the operating system.

  The minimum and default value for rpl_read_size is 8192 bytes. The value must be a multiple of 4KB. Note that a buffer the size of this value is allocated for each thread that reads from the binary log and relay log files, including dump threads on sources and coordinator threads on replicas. Setting a large value might therefore have an impact on memory consumption for servers.

• rpl_semi_sync_replica_enabled

  Command-Line Format	--rpl-semi-sync-replica-enabled[={OFF|ON}]
  Introduced	8.0.26
  System Variable	rpl_semi_sync_replica_enabled
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  rpl_semi_sync_replica_enabled is available when the rpl_semi_sync_replica (semisync_replica.so library) plugin was installed on the replica to set up semisynchronous replication. If the rpl_semi_sync_slave plugin (semisync_slave.so library) was installed, rpl_semi_sync_slave_enabled is available instead.

  rpl_semi_sync_replica_enabled controls whether semisynchronous replication is enabled on the replica server. 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 replica-side semisynchronous replication plugin is installed.

• rpl_semi_sync_replica_trace_level

  Command-Line Format	--rpl-semi-sync-replica-trace-level=#
  Introduced	8.0.26
  System Variable	rpl_semi_sync_replica_trace_level
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	32
  Minimum Value	0
  Maximum Value	4294967295

  rpl_semi_sync_replica_trace_level is available when the rpl_semi_sync_replica (semisync_replica.so library) plugin was installed on the replica to set up semisynchronous replication. If the rpl_semi_sync_slave plugin (semisync_slave.so library) was installed, rpl_semi_sync_slave_trace_level is available instead.

  rpl_semi_sync_replica_trace_level controls the semisynchronous replication debug trace level on the replica server. See rpl_semi_sync_master_trace_level for the permissible values.

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

• rpl_semi_sync_slave_enabled

  Command-Line Format	--rpl-semi-sync-slave-enabled[={OFF|ON}]
  System Variable	rpl_semi_sync_slave_enabled
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  rpl_semi_sync_slave_enabled is available when the rpl_semi_sync_slave (semisync_slave.so library) plugin was installed on the replica to set up semisynchronous replication. If the rpl_semi_sync_replica plugin (semisync_replica.so library) was installed, rpl_semi_sync_replica_enabled is available instead.

  rpl_semi_sync_slave_enabled controls whether semisynchronous replication is enabled on the replica server. 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 replica-side semisynchronous replication plugin is installed.

• rpl_semi_sync_slave_trace_level

  Command-Line Format	--rpl-semi-sync-slave-trace-level=#
  System Variable	rpl_semi_sync_slave_trace_level
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	32
  Minimum Value	0
  Maximum Value	4294967295

  rpl_semi_sync_slave_trace_level is available when the rpl_semi_sync_slave (semisync_slave.so library) plugin was installed on the replica to set up semisynchronous replication. If the rpl_semi_sync_replica plugin (semisync_replica.so library) was installed, rpl_semi_sync_replica_trace_level is available instead.

  rpl_semi_sync_slave_trace_level controls the semisynchronous replication debug trace level on the replica server. See rpl_semi_sync_master_trace_level for the permissible values.

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

• rpl_stop_replica_timeout

  Command-Line Format	--rpl-stop-replica-timeout=#
  Introduced	8.0.26
  System Variable	rpl_stop_replica_timeout
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	31536000
  Minimum Value	2
  Maximum Value	31536000
  Unit	seconds

  From MySQL 8.0.26, use rpl_stop_replica_timeout in place of rpl_stop_slave_timeout, which is deprecated from that release. In releases before MySQL 8.0.26, use rpl_stop_slave_timeout.

  You can control the length of time (in seconds) that STOP REPLICA waits before timing out by setting this variable. This can be used to avoid deadlocks between STOP REPLICA and other SQL statements using different client connections to the replica.

  The maximum and default value of rpl_stop_replica_timeout is 31536000 seconds (1 year). The minimum is 2 seconds. Changes to this variable take effect for subsequent STOP REPLICA statements.

  This variable affects only the client that issues a STOP REPLICA statement. When the timeout is reached, the issuing client returns an error message stating that the command execution is incomplete. The client then stops waiting for the replication I/O (receiver)and SQL (applier) threads to stop, but the replication threads continue to try to stop, and the STOP REPLICA statement remains in effect. Once the replication threads are no longer busy, the STOP REPLICA statement is executed and the replica stops.

• rpl_stop_slave_timeout

  Command-Line Format	--rpl-stop-slave-timeout=#
  Deprecated	8.0.26
  System Variable	rpl_stop_slave_timeout
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	31536000
  Minimum Value	2
  Maximum Value	31536000
  Unit	seconds

  From MySQL 8.0.26, rpl_stop_slave_timeout is deprecated and the alias rpl_stop_replica_timeout should be used instead. In releases before MySQL 8.0.26, use rpl_stop_slave_timeout.

  You can control the length of time (in seconds) that STOP REPLICA waits before timing out by setting this variable. This can be used to avoid deadlocks between STOP REPLICA and other SQL statements using different client connections to the replica.

  The maximum and default value of rpl_stop_slave_timeout is 31536000 seconds (1 year). The minimum is 2 seconds. Changes to this variable take effect for subsequent STOP REPLICA statements.

  This variable affects only the client that issues a STOP REPLICA statement. When the timeout is reached, the issuing client returns an error message stating that the command execution is incomplete. The client then stops waiting for the replication I/O (receiver) and SQL (applier) threads to stop, but the replication threads continue to try to stop, and the STOP REPLICA instruction remains in effect. Once the replication threads are no longer busy, the STOP REPLICA statement is executed and the replica stops.

• skip_replica_start

  Command-Line Format	--skip-replica-start[={OFF|ON}]
  Introduced	8.0.26
  System Variable	skip_replica_start
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  From MySQL 8.0.26, use skip_replica_start in place of skip_slave_start, which is deprecated from that release. In releases before MySQL 8.0.26, use skip_slave_start.

  skip_replica_start tells the replica server not to start the replication I/O (receiver) and SQL (applier) threads when the server starts. To start the threads later, use a START REPLICA statement.

  This system variable is read-only and can be set by using the PERSIST_ONLY keyword or the @@persist_only qualifier with the SET statement. The --skip-replica-start command line option also sets this system variable. You can use the system variable in place of the command line option to allow access to this feature using MySQL Server’s privilege structure, so that database administrators do not need any privileged access to the operating system.

• skip_slave_start

  Command-Line Format	--skip-slave-start[={OFF|ON}]
  Deprecated	8.0.26
  System Variable	skip_slave_start
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  From MySQL 8.0.26, skip_slave_start is deprecated and the alias skip_replica_start should be used instead. In releases before MySQL 8.0.26, use skip_slave_start.

  Tells the replica server not to start the replication I/O (receiver) and SQL (applier) threads when the server starts. To start the threads later, use a START REPLICA statement.

  This system variable is available from MySQL 8.0.24. It is read-only and can be set by using the PERSIST_ONLY keyword or the @@persist_only qualifier with the SET statement. The --skip-slave-start command line option also sets this system variable. You can use the system variable in place of the command line option to allow access to this feature using MySQL Server’s privilege structure, so that database administrators do not need any privileged access to the operating system.

• slave_checkpoint_group

  Command-Line Format	--slave-checkpoint-group=#
  Deprecated	8.0.26
  System Variable	slave_checkpoint_group
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	512
  Minimum Value	32
  Maximum Value	524280
  Block Size	8

  From MySQL 8.0.26, slave_checkpoint_group is deprecated and the alias replica_checkpoint_group should be used instead. In releases before MySQL 8.0.26, use slave_checkpoint_group.

  slave_checkpoint_group sets the maximum number of transactions that can be processed by a multithreaded replica before a checkpoint operation is called to update its status as shown by SHOW REPLICA STATUS. Setting this variable has no effect on replicas for which multithreading is not enabled. Setting this variable has no immediate effect. The state of the variable applies on all subsequent START REPLICA statements.

  Previously, multithreaded replicas were not supported by NDB Cluster, which silently ignored the setting for this variable. This restriction was lifted in MySQL 8.0.33.

  This variable works in combination with the slave_checkpoint_period system variable in such a way that, when either limit is exceeded, the checkpoint is executed and the counters tracking both the number of transactions and the time elapsed since the last checkpoint are reset.

  The minimum allowed value for this variable is 32, unless the server was built using -DWITH_DEBUG, in which case the minimum value is 1. The effective value is always a multiple of 8; you can set it to a value that is not such a multiple, but the server rounds it down to the next lower multiple of 8 before storing the value. (Exception: No such rounding is performed by the debug server.) Regardless of how the server was built, the default value is 512, and the maximum allowed value is 524280.

• slave_checkpoint_period

  Command-Line Format	--slave-checkpoint-period=#
  Deprecated	8.0.26
  System Variable	slave_checkpoint_period
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	300
  Minimum Value	1
  Maximum Value	4294967295
  Unit	milliseconds

  As of MySQL 8.0.26, slave_checkpoint_period is deprecated, and replica_checkpoint_period should be used instead; prior to MySQL 8.0.26, use slave_checkpoint_period.

  slave_checkpoint_period sets the maximum time (in milliseconds) that is allowed to pass before a checkpoint operation is called to update the status of a multithreaded replica as shown by SHOW REPLICA STATUS. Setting this variable has no effect on replicas for which multithreading is not enabled. Setting this variable takes effect for all replication channels immediately, including running channels.

  Previously, multithreaded replicas were not supported by NDB Cluster, which silently ignored the setting for this variable. This restriction was lifted in MySQL 8.0.33.

  This variable works in combination with the slave_checkpoint_group system variable in such a way that, when either limit is exceeded, the checkpoint is executed and the counters tracking both the number of transactions and the time elapsed since the last checkpoint are reset.

  The minimum allowed value for this variable is 1, unless the server was built using -DWITH_DEBUG, in which case the minimum value is 0. Regardless of how the server was built, the default value is 300 milliseconds, and the maximum possible value is 4294967295 milliseconds (approximately 49.7 days).

• slave_compressed_protocol

  Command-Line Format	--slave-compressed-protocol[={OFF|ON}]
  Deprecated	8.0.18
  System Variable	slave_compressed_protocol
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	OFF

  slave_compressed_protocol is deprecated, and from MySQL 8.0.26, the alias replica_compressed_protocol should be used instead. In releases before MySQL 8.0.26, use slave_compressed_protocol.

  slave_compressed_protocol controls whether to use compression of the source/replica connection protocol if both source and replica support it. If this variable is disabled (the default), connections are uncompressed. Changes to this variable take effect on subsequent connection attempts; this includes after issuing a START REPLICA statement, as well as reconnections made by a running replication I/O (receiver) thread.

  Binary log transaction compression (available as of MySQL 8.0.20), which is activated by the binlog_transaction_compression system variable, can also be used to save bandwidth. If you use binary log transaction compression in combination with protocol compression, protocol compression has less opportunity to act on the data, but can still compress headers and those events and transaction payloads that are uncompressed. For more information on binary log transaction compression, see Binary Log Transaction Compression.

  As of MySQL 8.0.18, if slave_compressed_protocol is enabled, it takes precedence over any SOURCE_COMPRESSION_ALGORITHMS | MASTER_COMPRESSION_ALGORITHMS option specified for the CHANGE REPLICATION SOURCE TO | CHANGE MASTER TO statement. In this case, connections to the source use zlib compression if both the source and replica support that algorithm. If slave_compressed_protocol is disabled, the value of SOURCE_COMPRESSION_ALGORITHMS | MASTER_COMPRESSION_ALGORITHMS applies. For more information, see Connection Compression Control.

  As of MySQL 8.0.18, this system variable is deprecated. You should expect it to be removed in a future version of MySQL. See Configuring Legacy Connection Compression.

• slave_exec_mode

  Command-Line Format	--slave-exec-mode=mode
  System Variable	slave_exec_mode
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Enumeration
  Default Value	IDEMPOTENT (NDB) STRICT (Other)
  Valid Values	STRICT IDEMPOTENT

  From MySQL 8.0.26, slave_exec_mode is deprecated and the alias replica_exec_mode should be used instead. In releases before MySQL 8.0.26, use slave_exec_mode.

  slave_exec_mode controls how a replication thread resolves conflicts and errors during replication. IDEMPOTENT mode causes suppression of duplicate-key and no-key-found errors; STRICT means no such suppression takes place.

  IDEMPOTENT mode is intended for use in multi-source replication, circular replication, and some other special replication scenarios for NDB Cluster Replication. (See NDB Cluster Replication: Bidirectional and Circular Replication, and NDB Cluster Replication Conflict Resolution, for more information.) NDB Cluster ignores any value explicitly set for slave_exec_mode, and always treats it as IDEMPOTENT.

  In MySQL Server 8.0, STRICT mode is the default value.

  Setting this variable takes immediate effect for all replication channels, including running channels.

  For storage engines other than NDB, IDEMPOTENT mode should be used only when you are absolutely sure that duplicate-key errors and key-not-found errors can safely be ignored. It is meant to be used in fail-over scenarios for NDB Cluster where multi-source replication or circular replication is employed, and is not recommended for use in other cases.

• slave_load_tmpdir

  Command-Line Format	--slave-load-tmpdir=dir_name
  Deprecated	8.0.26
  System Variable	slave_load_tmpdir
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	Directory name
  Default Value	Value of --tmpdir

  From MySQL 8.0.26, slave_load_tmpdir is deprecated and the alias replica_load_tmpdir should be used instead. In releases before MySQL 8.0.26, use slave_load_tmpdir.

  slave_load_tmpdir specifies the name of the directory where the replica creates temporary files. Setting this variable takes effect for all replication channels immediately, including running channels. The variable value is by default equal to the value of the tmpdir system variable, or the default that applies when that system variable is not specified.

  When the replication SQL thread replicates a LOAD DATA 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 source is huge, the temporary files on the replica are huge, too. Therefore, it might be advisable to use this option to tell the replica to put temporary files in a directory located in some file system that has a lot of available space. In that case, the relay logs are huge as well, so you might also want to set the relay_log system variable to place the relay logs in that file system.

  The directory specified by this option should be located in a disk-based file system (not a memory-based file system) so that the temporary files used to replicate LOAD DATA statements can survive machine restarts. The directory also should not be one that is cleared by the operating system during the system startup process. However, replication can now continue after a restart if the temporary files have been removed.

• slave_max_allowed_packet

  Command-Line Format	--slave-max-allowed-packet=#
  Deprecated	8.0.26
  System Variable	slave_max_allowed_packet
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	1073741824
  Minimum Value	1024
  Maximum Value	1073741824
  Unit	bytes
  Block Size	1024

  From MySQL 8.0.26, slave_max_allowed_packet is deprecated and the alias replica_max_allowed_packet should be used instead. In releases before MySQL 8.0.26, use slave_max_allowed_packet.

  slave_max_allowed_packet sets the maximum packet size in bytes that the replication SQL (applier) and I/O (receiver) threads can handle. Setting this variable takes effect for all replication channels immediately, including running channels. It is possible for a source to write binary log events longer than its max_allowed_packet setting once the event header is added. The setting for slave_max_allowed_packet must be larger than the max_allowed_packet setting on the source, so that large updates using row-based replication do not cause replication to fail.

  This global variable always has a value that is a positive integer multiple of 1024; if you set it to some value that is not, the value is rounded down to the next highest multiple of 1024 for it is stored or used; setting slave_max_allowed_packet to 0 causes 1024 to be used. (A truncation warning is issued in all such cases.) The default and maximum value is 1073741824 (1 GB); the minimum is 1024.

• slave_net_timeout

  Command-Line Format	--slave-net-timeout=#
  Deprecated	8.0.26
  System Variable	slave_net_timeout
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	60
  Minimum Value	1
  Maximum Value	31536000
  Unit	seconds

  From MySQL 8.0.26, slave_net_timeout is deprecated and the alias replica_net_timeout should be used instead. In releases before MySQL 8.0.26, use slave_net_timeout.

  slave_net_timeout specifies the number of seconds to wait for more data or a heartbeat signal from the source before the replica considers the connection broken, aborts the read, and tries to reconnect. Setting this variable has no immediate effect. The state of the variable applies on all subsequent START REPLICA commands.

  The default value is 60 seconds (one minute). The first retry occurs immediately after the timeout. The interval between retries is controlled by the SOURCE_CONNECT_RETRY | MASTER_CONNECT_RETRY option for the CHANGE REPLICATION SOURCE TO | CHANGE MASTER TO statement, and the number of reconnection attempts is limited by the SOURCE_RETRY_COUNT | MASTER_RETRY_COUNT option.

  The heartbeat interval, which stops the connection timeout occurring in the absence of data if the connection is still good, is controlled by the SOURCE_HEARTBEAT_PERIOD | MASTER_HEARTBEAT_PERIOD option for the CHANGE REPLICATION SOURCE TO | CHANGE MASTER TO statement. The heartbeat interval defaults to half the value of slave_net_timeout, and it is recorded in the replica's connection metadata repository and shown in the replication_connection_configuration Performance Schema table. Note that a change to the value or default setting of slave_net_timeout does not automatically change the heartbeat interval, whether that has been set explicitly or is using a previously calculated default. If the connection timeout is changed, you must also issue CHANGE REPLICATION SOURCE TO | CHANGE MASTER TO to adjust the heartbeat interval to an appropriate value so that it occurs before the connection timeout.

• slave_parallel_type

  Command-Line Format	--slave-parallel-type=value
  Deprecated	8.0.26
  System Variable	slave_parallel_type
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Enumeration
  Default Value (≥ 8.0.27)	LOGICAL_CLOCK
  Default Value (≤ 8.0.26)	DATABASE
  Valid Values	DATABASE LOGICAL_CLOCK

  From MySQL 8.0.26, slave_parallel_type is deprecated and the alias replica_parallel_type should be used instead. In releases before MySQL 8.0.26, use slave_parallel_type.

  For multithreaded replicas (replicas on which replica_parallel_workers or slave_parallel_workers is set to a value greater than 0), slave_parallel_type specifies the policy used to decide which transactions are allowed to execute in parallel on the replica. The variable has no effect on replicas for which multithreading is not enabled. The possible values are:

  • LOGICAL_CLOCK: Transactions that are part of the same binary log group commit on a source are applied in parallel on a replica. The dependencies between transactions are tracked based on their timestamps to provide additional parallelization where possible. When this value is set, the binlog_transaction_dependency_tracking system variable can be used on the source to specify that write sets are used for parallelization in place of timestamps, if a write set is available for the transaction and gives improved results compared to timestamps.

  • DATABASE: Transactions that update different databases are applied in parallel. This value is only appropriate if data is partitioned into multiple databases which are being updated independently and concurrently on the source. There must be no cross-database constraints, as such constraints may be violated on the replica.

  When replica_preserve_commit_order=ON or slave_preserve_commit_order=ON is set, you can only use LOGICAL_CLOCK. Before MySQL 8.0.27, DATABASE is the default. From MySQL 8.0.27, multithreading is enabled by default for replica servers (replica_parallel_workers=4 by default), so LOGICAL_CLOCK is the default, and the setting replica_preserve_commit_order=ON is also the default.

  When your replication topology uses multiple levels of replicas, LOGICAL_CLOCK may achieve less parallelization for each level the replica is away from the source. You can reduce this effect by using binlog_transaction_dependency_tracking on the source to specify that write sets are used instead of timestamps for parallelization where possible.

  When binary log transaction compression is enabled using the binlog_transaction_compression system variable, if replica_parallel_type or slave_parallel_type is set to DATABASE, all the databases affected by the transaction are mapped before the transaction is scheduled. The use of binary log transaction compression with the DATABASE policy can reduce parallelism compared to uncompressed transactions, which are mapped and scheduled for each event.

• slave_parallel_workers

  Command-Line Format	--slave-parallel-workers=#
  Deprecated	8.0.26
  System Variable	slave_parallel_workers
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value (≥ 8.0.27)	4
  Default Value (≤ 8.0.26)	0
  Minimum Value	0
  Maximum Value	1024

  From MySQL 8.0.26, slave_parallel_workers is deprecated and the alias replica_parallel_workers should be used instead. In releases before MySQL 8.0.26, use slave_parallel_workers.

  slave_parallel_workers enables multithreading on the replica and sets the number of applier threads for executing replication transactions in parallel. When the value is a number greater than 0, the replica is a multithreaded replica with the specified number of applier threads, plus a coordinator thread to manage them. If you are using multiple replication channels, each channel has this number of threads.

  Before MySQL 8.0.27, the default for this system variable is 0, so replicas are not multithreaded by default. From MySQL 8.0.27, the default is 4, so replicas are multithreaded by default.

  Retrying of transactions is supported when multithreading is enabled on a replica. When replica_preserve_commit_order=ON or slave_preserve_commit_order=ON is set, transactions on a replica are externalized on the replica in the same order as they appear in the replica's relay log. The way in which transactions are distributed among applier threads is configured by replica_parallel_type (from MySQL 8.0.26) or slave_parallel_type (before MySQL 8.0.26). From MySQL 8.0.27, these system variables also have appropriate defaults for multithreading.

  To disable parallel execution, set replica_parallel_workers to 0, which gives the replica a single applier thread and no coordinator thread. With this setting, the replica_parallel_type or slave_parallel_type and replica_preserve_commit_order or slave_preserve_commit_order system variables have no effect and are ignored. From MySQL 8.0.27, if parallel execution is disabled when the CHANGE REPLICATION SOURCE TO option GTID_ONLY is enabled on a replica, the replica actually uses one parallel worker to take advantage of the method for retrying transactions without accessing the file positions. With one parallel worker, the replica_preserve_commit_order (slave_preserve_commit_order) system variable also has no effect.

  Setting replica_parallel_workers has no immediate effect. The state of the variable applies on all subsequent START REPLICA statements.

  Previously, multithreaded replicas were not supported by NDB Cluster, which silently ignored the setting for this variable. This restriction was lifted in MySQL 8.0.33.

• slave_pending_jobs_size_max

  Command-Line Format	--slave-pending-jobs-size-max=#
  Deprecated	8.0.26
  System Variable	slave_pending_jobs_size_max
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value (≥ 8.0.12)	128M
  Default Value (8.0.11)	16M
  Minimum Value	1024
  Maximum Value	16EiB
  Unit	bytes
  Block Size	1024

  From MySQL 8.0.26, slave_pending_jobs_size_max is deprecated and the alias replica_pending_jobs_size_max should be used instead. In releases before MySQL 8.0.26, use slave_pending_jobs_size_max.

  For multithreaded replicas, this variable sets the maximum amount of memory (in bytes) available to applier queues holding events not yet applied. Setting this variable has no effect on replicas for which multithreading is not enabled. Setting this variable has no immediate effect. The state of the variable applies on all subsequent START REPLICA commands.

  The minimum possible value for this variable is 1024 bytes; the default is 128MB. The maximum possible value is 18446744073709551615 (16 exbibytes). Values that are not exact multiples of 1024 bytes are rounded down to the next lower multiple of 1024 bytes prior to being stored.

  The value of this variable is a soft limit and can be set to match the normal workload. If an unusually large event exceeds this size, the transaction is held until all the worker threads have empty queues, and then processed. All subsequent transactions are held until the large transaction has been completed.

• slave_preserve_commit_order

  Command-Line Format	--slave-preserve-commit-order[={OFF|ON}]
  Deprecated	8.0.26
  System Variable	slave_preserve_commit_order
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value (≥ 8.0.27)	ON
  Default Value (≤ 8.0.26)	OFF

  From MySQL 8.0.26, slave_preserve_commit_order is deprecated and the alias replica_preserve_commit_order should be used instead. In releases before MySQL 8.0.26, use slave_preserve_commit_order.

  For multithreaded replicas (replicas on which replica_parallel_workers or slave_parallel_workers is set to a value greater than 0), setting slave_preserve_commit_order=1 ensures that transactions are executed and committed on the replica in the same order as they appear in the replica's relay log. This prevents gaps in the sequence of transactions that have been executed from the replica's relay log, and preserves the same transaction history on the replica as on the source (with the limitations listed below). This variable has no effect on replicas for which multithreading is not enabled.

  Before MySQL 8.0.27, the default for this system variable is OFF, meaning that transactions may be committed out of order. From MySQL 8.0.27, multithreading is enabled by default for replica servers (replica_parallel_workers=4 by default), so slave_preserve_commit_order=ON is the default, and the setting slave_parallel_type=LOGICAL_CLOCK is also the default. Also from MySQL 8.0.27, the setting for slave_preserve_commit_order is ignored if slave_parallel_workers is set to 1, because in that situation the order of transactions is preserved anyway.

  Up to and including MySQL 8.0.18, setting slave_preserve_commit_order=ON requires that binary logging (log_bin) and replica update logging (log_slave_updates) are enabled on the replica, which are the default settings from MySQL 8.0. From MySQL 8.0.19, binary logging and replica update logging are not required on the replica to set slave_preserve_commit_order=ON, and can be disabled if wanted. In all releases, setting slave_preserve_commit_order=ON requires that slave_parallel_type is set to LOGICAL_CLOCK, which is not the default setting before MySQL 8.0.27. Before changing the value of slave_preserve_commit_order and slave_parallel_type, the replication SQL thread (for all replication channels if you are using multiple replication channels) must be stopped.

  When slave_preserve_commit_order=OFF is set, which is the default, the transactions that a multithreaded replica applies in parallel may commit out of order. Therefore, checking for the most recently executed transaction does not guarantee that all previous transactions from the source have been executed on the replica. There is a chance of gaps in the sequence of transactions that have been executed from the replica's relay log. This has implications for logging and recovery when using a multithreaded replica. See Section 4.1.34, “Replication and Transaction Inconsistencies” for more information.

  When slave_preserve_commit_order=ON is set, the executing worker thread waits until all previous transactions are committed before committing. While a given thread is waiting for other worker threads to commit their transactions, it reports its status as Waiting for preceding transaction to commit. With this mode, a multithreaded replica never enters a state that the source was not in. This supports the use of replication for read scale-out. See Section 3.5, “Using Replication for Scale-Out”.

  Note

  • slave_preserve_commit_order=ON does not prevent source binary log position lag, where Exec_master_log_pos is behind the position up to which transactions have been executed. See Section 4.1.34, “Replication and Transaction Inconsistencies”.

  • slave_preserve_commit_order=ON does not preserve the commit order and transaction history if the replica uses filters on its binary log, such as --binlog-do-db.

  • slave_preserve_commit_order=ON does not preserve the order of non-transactional DML updates. These might commit before transactions that precede them in the relay log, which might result in gaps in the sequence of transactions that have been executed from the replica's relay log.

  • In releases before MySQL 8.0.19, slave_preserve_commit_order=ON does not preserve the order of statements with an IF EXISTS clause when the object concerned does not exist. These might commit before transactions that precede them in the relay log, which might result in gaps in the sequence of transactions that have been executed from the replica's relay log.

  • A limitation to preserving the commit order on the replica can occur if statement-based replication is in use, and both transactional and non-transactional storage engines participate in a non-XA transaction that is rolled back on the source. Normally, non-XA transactions that are rolled back on the source are not replicated to the replica, but in this particular situation, the transaction might be replicated to the replica. If this does happen, a multithreaded replica without binary logging does not handle the transaction rollback, so the commit order on the replica diverges from the relay log order of the transactions in that case.

• slave_rows_search_algorithms

  Command-Line Format	--slave-rows-search-algorithms=value
  Deprecated	8.0.18
  System Variable	slave_rows_search_algorithms
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Set
  Default Value	INDEX_SCAN,HASH_SCAN
  Valid Values	TABLE_SCAN,INDEX_SCAN INDEX_SCAN,HASH_SCAN TABLE_SCAN,HASH_SCAN TABLE_SCAN,INDEX_SCAN,HASH_SCAN (equivalent to INDEX_SCAN,HASH_SCAN)

  When preparing batches of rows for row-based logging and replication, this system variable controls how the rows are searched for matches, in particular whether hash scans are used. The use of this system variable is now deprecated. The default setting INDEX_SCAN,HASH_SCAN is optimal for performance and works correctly in all scenarios. See Section 4.1.27, “Replication and Row Searches”.

• slave_skip_errors

  Command-Line Format	--slave-skip-errors=name
  Deprecated	8.0.26
  System Variable	slave_skip_errors
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	String
  Default Value	OFF
  Valid Values	OFF [list of error codes] all ddl_exist_errors

  From MySQL 8.0.26, slave_skip_errors is deprecated and the alias replica_skip_errors should be used instead. In releases before MySQL 8.0.26, use slave_skip_errors.

  Normally, replication stops when an error occurs on the replica, which gives you the opportunity to resolve the inconsistency in the data manually. This variable causes the replication SQL thread to continue replication when a statement returns any of the errors listed in the variable value.

• replica_skip_errors

  Command-Line Format	--replica-skip-errors=name
  Introduced	8.0.26
  System Variable	replica_skip_errors
  Scope	Global
  Dynamic	No
  SET_VAR Hint Applies	No
  Type	String
  Default Value	OFF
  Valid Values	OFF [list of error codes] all ddl_exist_errors

  From MySQL 8.0.26, use replica_skip_errors in place of slave_skip_errors, which is deprecated from that release. In releases before MySQL 8.0.26, use slave_skip_errors.

  Normally, replication stops when an error occurs on the replica, which gives you the opportunity to resolve the inconsistency in the data manually. This variable causes the replication SQL thread to continue replication when a statement returns any of the errors listed in the variable value.

• slave_sql_verify_checksum

  Command-Line Format	--slave-sql-verify-checksum[={OFF|ON}]
  Deprecated	8.0.26
  System Variable	slave_sql_verify_checksum
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Boolean
  Default Value	ON

  From MySQL 8.0.26, slave_sql_verify_checksum is deprecated and the alias replica_sql_verify_checksum should be used instead. In releases before MySQL 8.0.26, use slave_sql_verify_checksum.

  slave_sql_verify_checksum causes the replication SQL thread to verify data using the checksums read from the relay log. In the event of a mismatch, the replica stops with an error. Setting this variable takes effect for all replication channels immediately, including running channels.

  Note

  The replication I/O (receiver) thread always reads checksums if possible when accepting events from over the network.

• slave_transaction_retries

  Command-Line Format	--slave-transaction-retries=#
  Deprecated	8.0.26
  System Variable	slave_transaction_retries
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	10
  Minimum Value	0
  Maximum Value (64-bit platforms)	18446744073709551615
  Maximum Value (32-bit platforms)	4294967295

  From MySQL 8.0.26, slave_transaction_retries is deprecated and the alias replica_transaction_retries should be used instead. In releases before MySQL 8.0.26, use slave_transaction_retries.

  slave_transaction_retries sets the maximum number of times for replication SQL threads on a single-threaded or multithreaded replica to automatically retry failed transactions before stopping. Setting this variable takes effect for all replication channels immediately, including running channels. The default value is 10. Setting the variable to 0 disables automatic retrying of transactions.

  If a replication SQL thread fails to execute a transaction because of an InnoDB deadlock or because the transaction's execution time exceeded InnoDB's innodb_lock_wait_timeout or NDB's TransactionDeadlockDetectionTimeout or TransactionInactiveTimeout, it automatically retries slave_transaction_retries times before stopping with an error. Transactions with a non-temporary error are not retried.

  The Performance Schema table replication_applier_status shows the number of retries that took place on each replication channel, in the COUNT_TRANSACTIONS_RETRIES column. The Performance Schema table replication_applier_status_by_worker shows detailed information on transaction retries by individual applier threads on a single-threaded or multithreaded replica, and identifies the errors that caused the last transaction and the transaction currently in progress to be reattempted.

• slave_type_conversions

  Command-Line Format	--slave-type-conversions=set
  Deprecated	8.0.26
  System Variable	slave_type_conversions
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Set
  Default Value
  Valid Values	ALL_LOSSY ALL_NON_LOSSY ALL_SIGNED ALL_UNSIGNED

  From MySQL 8.0.26, slave_type_conversions is deprecated and the alias replica_type_conversions should be used instead. In releases before MySQL 8.0.26, use slave_type_conversions.

  slave_type_conversions controls the type conversion mode in effect on the replica when using row-based replication. Its value is a comma-delimited set of zero or more elements from the list: ALL_LOSSY, ALL_NON_LOSSY, ALL_SIGNED, ALL_UNSIGNED. Set this variable to an empty string to disallow type conversions between the source and the replica. Setting this variable takes effect for all replication channels immediately, including running channels.

  For additional information on type conversion modes applicable to attribute promotion and demotion in row-based replication, see Row-based replication: attribute promotion and demotion.

• sql_replica_skip_counter

  Introduced	8.0.26
  System Variable	sql_replica_skip_counter
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	0
  Minimum Value	0
  Maximum Value	4294967295

  From MySQL 8.0.26, use sql_replica_skip_counter in place of sql_slave_skip_counter, which is deprecated from that release. In releases before MySQL 8.0.26, use sql_slave_skip_counter.

  sql_replica_skip_counter specifies the number of events from the source that a replica should skip. Setting the option has no immediate effect. The variable applies to the next START REPLICA statement; the next START REPLICA statement also changes the value back to 0. When this variable is set to a nonzero value and there are multiple replication channels configured, the START REPLICA statement can only be used with the FOR CHANNEL channel clause.

  This option is incompatible with GTID-based replication, and must not be set to a nonzero value when gtid_mode=ON is set. If you need to skip transactions when employing GTIDs, use gtid_executed from the source instead. If you have enabled GTID assignment on a replication channel using the ASSIGN_GTIDS_TO_ANONYMOUS_TRANSACTIONS option of the CHANGE REPLICATION SOURCE TO statement, sql_replica_skip_counter is available. See Section 2.7.3, “Skipping Transactions”.

  Important

  If skipping the number of events specified by setting this variable would cause the replica to begin in the middle of an event group, the replica continues to skip until it finds the beginning of the next event group and begins from that point. For more information, see Section 2.7.3, “Skipping Transactions”.

• sql_slave_skip_counter

  Deprecated	8.0.26
  System Variable	sql_slave_skip_counter
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	0
  Minimum Value	0
  Maximum Value	4294967295

  From MySQL 8.0.26, sql_slave_skip_counter is deprecated and the alias sql_replica_skip_counter should be used instead. In releases before MySQL 8.0.26, use sql_slave_skip_counter.

  sql_slave_skip_counter specifies the number of events from the source that a replica should skip. Setting the option has no immediate effect. The variable applies to the next START REPLICA statement; the next START REPLICA statement also changes the value back to 0. When this variable is set to a nonzero value and there are multiple replication channels configured, the START REPLICA statement can only be used with the FOR CHANNEL channel clause.

  This option is incompatible with GTID-based replication, and must not be set to a nonzero value when gtid_mode=ON is set. If you need to skip transactions when employing GTIDs, use gtid_executed from the source instead. If you have enabled GTID assignment on a replication channel using the ASSIGN_GTIDS_TO_ANONYMOUS_TRANSACTIONS option of the CHANGE REPLICATION SOURCE TO statement, sql_slave_skip_counter is available. See Section 2.7.3, “Skipping Transactions”.

  Important

  If skipping the number of events specified by setting this variable would cause the replica to begin in the middle of an event group, the replica continues to skip until it finds the beginning of the next event group and begins from that point. For more information, see Section 2.7.3, “Skipping Transactions”.

• sync_master_info

  Command-Line Format	--sync-master-info=#
  Deprecated	8.0.26
  System Variable	sync_master_info
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	10000
  Minimum Value	0
  Maximum Value	4294967295

  From MySQL 8.0.26, sync_master_info is deprecated and the alias sync_source_info should be used instead. In releases before MySQL 8.0.26, use sync_master_info.

  sync_master_info specifies the number of events after which the replica updates the connection metadata repository. When the connection metadata repository is stored as an InnoDB table, which is the default from MySQL 8.0, it is updated after this number of events. If the connection metadata repository is stored as a file, which is deprecated from MySQL 8.0, the replica synchronizes its master.info file to disk (using fdatasync()) after this number of events. The default value is 10000, and a zero value means that the repository is never updated. Setting this variable takes effect for all replication channels immediately, including running channels.

• sync_relay_log

  Command-Line Format	--sync-relay-log=#
  System Variable	sync_relay_log
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	10000
  Minimum Value	0
  Maximum Value	4294967295

  If the value of this variable is greater than 0, the MySQL server synchronizes its relay log to disk (using fdatasync()) after every sync_relay_log events are written to the relay log. Setting this variable takes effect for all replication channels immediately, including running channels.

  Setting sync_relay_log to 0 causes no synchronization to be done to disk; in this case, the server relies on the operating system to flush the relay log's contents from time to time as for any other file.

  A value of 1 is the safest choice because in the event of an unexpected halt you lose at most one event from the relay log. However, it is also the slowest choice (unless the disk has a battery-backed cache, which makes synchronization very fast). For information on the combination of settings on a replica that is most resilient to unexpected halts, see Section 3.2, “Handling an Unexpected Halt of a Replica”.

• sync_relay_log_info

  Command-Line Format	--sync-relay-log-info=#
  Deprecated	8.0.34
  System Variable	sync_relay_log_info
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	10000
  Minimum Value	0
  Maximum Value	4294967295

  The number of transactions after which the replica updates the applier metadata repository. When the applier metadata repository is stored as an InnoDB table (the default in MySQL 8.0 and later), it is updated after every transaction and this system variable is ignored. If the applier metadata repository is stored as a file (deprecated in MySQL 8.0), the replica synchronizes its relay-log.info file to disk (using fdatasync()) after this many transactions. 0 (zero) means that the file contents are flushed by the operating system only. Setting this variable takes effect for all replication channels immediately, including running channels.

  Since storing applier metadata as a file is deprecated, this variable is also deprecated; as of MySQL 8.0.34, the server raises a warning whenever you set it or read its value. You should expect sync_relay_log_info to be removed in a future version of MySQL, and migrate applications now that may depend on it.

• sync_source_info

  Command-Line Format	--sync-source-info=#
  Introduced	8.0.26
  System Variable	sync_source_info
  Scope	Global
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Integer
  Default Value	10000
  Minimum Value	0
  Maximum Value	4294967295

  From MySQL 8.0.26, use sync_source_info in place of sync_master_info, which is deprecated from that release. In releases before MySQL 8.0.26, use sync_source_info.

  sync_source_info specifies the number of events after which the replica updates the connection metadata repository. When the connection metadata repository is stored as an InnoDB table, which is the default from MySQL 8.0, it is updated after this number of events. If the connection metadata repository is stored as a file, which is deprecated from MySQL 8.0, the replica synchronizes its master.info file to disk (using fdatasync()) after this number of events. The default value is 10000, and a zero value means that the repository is never updated. Setting this variable takes effect for all replication channels immediately, including running channels.

• terminology_use_previous

  Command-Line Format	--terminology-use-previous=#
  Introduced	8.0.26
  System Variable	terminology_use_previous
  Scope	Global, Session
  Dynamic	Yes
  SET_VAR Hint Applies	No
  Type	Enumeration
  Default Value	NONE
  Valid Values	NONE BEFORE_8_0_26

  In MySQL 8.0.26, incompatible changes were made to instrumentation names containing the terms master, slave, and mts (for “Multi-Threaded Slave”), which were changed respectively to source, replica, and mta (for “Multi-Threaded Applier”). If these incompatible changes impact your applications, set the terminology_use_previous system variable to BEFORE_8_0_26 to make MySQL Server use the old versions of the names for the objects specified in the previous list. This enables monitoring tools that rely on the old names to continue working until they can be updated to use the new names.

  Set the terminology_use_previous system variable with session scope to support individual users, or with global scope to be the default for all new sessions. When global scope is used, the slow query log contains the old versions of the names.

  The affected instrumentation names are given in the following list. The terminology_use_previous system variable only affects these items. It does not affect the new aliases for system variables, status variables, and command-line options that were also introduced in MySQL 8.0.26, and these can still be used when it is set.

  • Instrumented locks (mutexes), visible in the mutex_instances and events_waits_* Performance Schema tables with the prefix wait/synch/mutex/

  • Read/write locks, visible in the rwlock_instances and events_waits_* Performance Schema tables with the prefix wait/synch/rwlock/

  • Instrumented condition variables, visible in the cond_instances and events_waits_* Performance Schema tables with the prefix wait/synch/cond/

  • Instrumented memory allocations, visible in the memory_summary_* Performance Schema tables with the prefix memory/sql/

  • Thread names, visible in the threads Performance Schema table with the prefix thread/sql/

  • Thread stages, visible in the events_stages_* Performance Schema tables with the prefix stage/sql/, and without the prefix in the threads and processlist Performance Schema tables, the output from the SHOW PROCESSLIST statement, the Information Schema processlist table, and the slow query log

  • Thread commands, visible in the events_statements_history* and events_statements_summary_*_by_event_name Performance Schema tables with the prefix statement/com/, and without the prefix in the threads and processlist Performance Schema tables, the output from the SHOW PROCESSLIST statement, the Information Schema processlist table, and the output from the SHOW REPLICA STATUS statement