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19.3. Stored Procedures, Functions, Triggers, and Replication: Frequently Asked Questions

  • Do MySQL 5.1 stored procedures and functions work with replication?

    Yes, standard actions carried out in stored procedures and functions are replicated from a master MySQL server to a slave server. There are a few limitations that are described in detail in Section 19.4, “Binary Logging of Stored Routines and Triggers”.

  • Are stored procedures and functions created on a master server replicated to a slave?

    Yes, creation of stored procedures and functions carried out through normal DDL statements on a master server are replicated to a slave, so the objects will exist on both servers. ALTER and DROP statements for stored procedures and functions are also replicated.

  • How are actions that take place inside stored procedures and functions replicated?

    MySQL records each DML event that occurs in a stored procedure and replicates those individual actions to a slave server. The actual calls made to execute stored procedures are not replicated.

    Stored functions that change data are logged as function invocations, not as the DML events that occur inside each function.

  • Are there special security requirements for using stored procedures and functions together with replication?

    Yes. Because a slave server has authority to execute any statement read from a master's binary log, special security constraints exist for using stored functions with replication. If replication or binary logging in general (for the purpose of point-in-time recovery) is active, then MySQL DBAs have two security options open to them:

    • Any user wishing to create stored functions must be granted the SUPER privilege.

    • Alternatively, a DBA can set the log_bin_trust_function_creators system variable to 1, which enables anyone with the standard CREATE ROUTINE privilege to create stored functions.

  • What limitations exist for replicating stored procedure and function actions?

    Non-deterministic (random) or time-based actions embedded in stored procedures may not replicate properly. By their very nature, randomly produced results are not predictable and cannot be exactly reproduced, and therefore, random actions replicated to a slave will not mirror those performed on a master. Note that declaring stored functions to be DETERMINISTIC or setting the log_bin_trust_function_creators system variable to 0 will not allow random-valued operations to be invoked.

    In addition, time-based actions cannot be reproduced on a slave because the timing of such actions in a stored procedure is not reproducible through the binary log used for replication. It records only DML events and does not factor in timing constraints.

    Finally, non-transactional tables for which errors occur during large DML actions (such as bulk inserts) may experience replication issues in that a master may be partially updated from DML activity, but no updates are done to the slave because of the errors that occurred. A workaround is for a function's DML actions to be carried out with the IGNORE keyword so that updates on the master that cause errors are ignored and updates that do not cause errors are replicated to the slave.

  • Do the preceding limitations affect MySQL's ability to do point-in-time recovery?

    The same limitations that affect replication do affect point-in-time recovery.

  • What will MySQL do to correct the aforementioned limitations?

    As of MySQL 5.1.5, you can choose either statement-based replication, or row-based replication. The original replication implement is based on statement-based binary logging. Row-based binary logging resolves the limitations mentioned earlier. For additional information, see Section 6.3, “Row-Based Replication”.

  • Do triggers work with replication?

    Triggers and replication in MySQL 5.1 work the same as in most other database engines: Actions carried out through triggers on a master are not replicated to a slave server. Instead, triggers that exist on tables that reside on a MySQL master server need to be created on the corresponding tables on any MySQL slave servers so that the triggers activate on the slaves as well as the master.

  • How are actions carried out through triggers on a master replicated to a slave?

    First, the triggers that exist on a master must be re-created on the slave server. Once this is done, the replication flow works as any other standard DML statement that participates in replication. For example, consider a table EMP that has an AFTER insert trigger, which exists on a master MySQL server. The same EMP table and AFTER insert trigger exist on the slave server as well. The replication flow would be:

  1. An INSERT statement is made to EMP.

  2. The AFTER trigger on EMP activates.

  3. The INSERT statement is written to the binary log.

  4. The replication slave picks up the INSERT statement to EMP and executes it.

  5. The AFTER trigger on EMP that exists on the slave activates.


 
 
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