MySQL replication is asynchronous by default. With semisynchronous replication, a commit performed on the master side blocks before returning to the session that performed the transaction until at least one slave acknowledges that it has received and logged the events for the transaction. Semisynchronous replication is implemented through complementary master and client plugins. See Section 17.3.8, “Semisynchronous Replication”.
For more information about semisynchronous replication plugins, see Section 18.104.22.168, “Writing Semisynchronous Replication Plugins”.