17.1.2.2. Safe and Unsafe Statements in Logging and Replication

Major changes in the replication environment and in the behavior of applications can result from using row-based logging (RBL) or row-based replication (RBR) rather than statement-based logging or replication. This section describes a number of issues known to exist when using row-based logging or replication, and discusses some best practices for taking advantage of row-based logging and replication.

For additional information, see Section 17.1.2, “Replication Formats”, and Section 17.1.2.1, “Comparison of Statement-Based and Row-Based Replication”.

  • RBL, RBR, and temporary tables.  As noted in Section 17.4.1.19, “Replication and Temporary Tables”, temporary tables are not replicated when using row-based format. When mixed format is in effect, “safe” statements involving temporary tables are logged using statement-based format. For more information, see Section 17.1.2.1, “Comparison of Statement-Based and Row-Based Replication”.

    Note

    Temporary tables are not replicated when using row-based format because there is no need. In addition, because temporary tables can be read only from the thread which created them, there is seldom if ever any benefit obtained from replicating them, even when using statement-based format.

  • RBL and synchronization of nontransactional tables.  When many rows are affected, the set of changes is split into several events; when the statement commits, all of these events are written to the binary log. When executing on the slave, a table lock is taken on all tables involved, and then the rows are applied in batch mode. (This may or may not be effective, depending on the engine used for the slave's copy of the table.)

  • Latency and binary log size.  Because RBL writes changes for each row to the binary log, its size can increase quite rapidly. In a replication environment, this can significantly increase the time required to make changes on the slave that match those on the master. You should be aware of the potential for this delay in your applications.

  • Reading the binary log.  mysqlbinlog displays row-based events in the binary log using the BINLOG statement (see Section 12.4.6.1, “BINLOG Syntax”). This statement displays an event in printable form, but as a base 64-encoded string the meaning of which is not evident. When invoked with the --base64-output=DECODE-ROWS and --verbose options, mysqlbinlog formats the contents of the binary log in a manner that is easily human readable. This is helpful when binary log events were written in row-based format if you want to read or recover from a replication or database failure using the contents of the binary log. For more information, see Section 4.6.7.2, “mysqlbinlog Row Event Display”.

  • Binary log execution errors and slave_exec_mode If slave_exec_mode is IDEMPOTENT, a failure to apply changes from RBL because the original row cannot be found does not trigger an error or cause replication to fail. This means that it is possible that updates are not applied on the slave, so that the master and slave are no longer synchronized. Latency issues and use of nontransactional tables with RBR when slave_exec_mode is IDEMPOTENT can cause the master and slave to diverge even further. For more information about slave_exec_mode, see Section 5.1.4, “Server System Variables”.

    Note

    slave_exec_mode=IDEMPOTENT is generally useful only for circular replication or multi-master replication with MySQL Cluster, for which IDEMPOTENT is the default value.

    For other scenarios, setting slave_exec_mode to STRICT is normally sufficient; this is the default value for storage engines other than NDB.

    The NDBCLUSTER storage engine is currently not supported in MySQL 5.5. MySQL Cluster users wishing to upgrade from MySQL 5.0 should instead migrate to MySQL Cluster NDB 7.0 or later; these are based on MySQL 5.1 but contain the latest improvements and fixes for NDBCLUSTER. For more information, see MySQL Cluster NDB 6.X/7.X.

  • Lack of binary log checksums.  RBL uses no checksums. This means that network, disk, and other errors may not be identified when processing the binary log. To ensure that data is transmitted without network corruption, you may want to consider using SSL, which adds another layer of checksumming, for replication connections. The CHANGE MASTER TO statement has options to enable replication over SSL. See also Section 12.5.2.1, “CHANGE MASTER TO Syntax”, for general information about setting up MySQL with SSL.

  • Filtering based on server ID not supported.  A common practice is to filter out changes on some slaves by using a WHERE clause that includes the relation @@server_id <> id_value clause with UPDATE and DELETE statements, a simple example of such a clause being WHERE @@server_id <> 1. However, this does not work correctly with row-based logging. If you must use the server_id system variable for statement filtering, you must also use --binlog_format=STATEMENT.

    In MySQL 5.5, you can do filtering based on server ID by using the IGNORE_SERVER_IDS option for the CHANGE MASTER TO statement. This option works with the statement-based and row-based logging formats.

  • Database-level replication options.  The effects of the --replicate-do-db, --replicate-ignore-db, and --replicate-rewrite-db options differ considerably depending on whether row-based or statement-based logging is used. Because of this, it is recommended to avoid database-level options and instead use table-level options such as --replicate-do-table and --replicate-ignore-table. For more information about these options and the impact that your choice of replication format has on how they operate, see Section 17.1.3, “Replication and Binary Logging Options and Variables”.

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