17.4.1.18. Replication and MEMORY Tables

When a master server shuts down and restarts, its MEMORY tables become empty. To replicate this effect to slaves, the first time that the master uses a given MEMORY table after startup, it logs an event that notifies slaves that the table must to be emptied by writing a DELETE statement for that table to the binary log.

When a slave server shuts down and restarts, its MEMORY tables become empty. This causes the slave to be out of synchrony with the master and may lead to other failures or cause the slave to stop:

  • Row-format updates and deletes received from the master may fail with Can't find record in 'memory_table'.

  • Statements such as INSERT INTO ... SELECT FROM memory_table may insert a different set of rows on the master and slave.

The safe way to restart a slave that is replicating MEMORY tables is to first drop or delete all rows from the MEMORY tables on the master and wait until those changes have replicated to the slave. Then it is safe to restart the slave.

An alternative restart method may apply in some cases. When binlog_format=ROW, you can prevent the slave from stopping if you set slave_exec_mode=IDEMPOTENT before you start the slave again. This allows the slave to continue to replicate, but its MEMORY tables will still be different from those on the master. This can be okay if the application logic is such that the contents of MEMORY tables can be safely lost (for example, if the MEMORY tables are used for caching). slave_exec_mode=IDEMPOTENT applies globally to all tables, so it may hide other replication errors in non-MEMORY tables.

See Section 13.9, “The MEMORY Storage Engine”, for more information about MEMORY tables.

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