Starting with MySQL 5.5 it is possible to configure semi-synchronous replication. As you probably know, by default, MySQL replication is asynchronous. Asynchronous replication means that events/transactions applied on one server are not immediately applied on the other one. In the specific context of MySQL such behavior means that the MySQL Master writes transactions/events to the local binary logs, however it has no idea when the slave(s) will get and process them. Of course such configuration can lead, in case of failover, to some transaction loss since there is no guaranty that the transactions have been transmitted on the standby server(s).
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