MySQL 5.7.3 added the rpl_semi_sync_master_wait_for_slave_count option for semisynchronous replication:
The number of slave acknowledgments the master must receive per transaction before proceeding. By default rpl_semi_sync_master_wait_for_slave_count is 1, meaning that semisynchronous replication proceeds after receiving a single slave acknowledgment. Performance is best for small values of this variable.
For example, if rpl_semi_sync_master_wait_for_slave_count is 2, then 2 slaves must acknowledge receipt of the transaction before the timeout period configured by rpl_semi_sync_master_timeout for semisynchronous replication to proceed. If less slaves acknowledge receipt of the transaction during the timeout period, the master reverts to normal replication.
Note
This behavior also depends on rpl_semi_sync_master_wait_no_slave
This variable is available only if the master-side semisynchronous replication plugin is installed.