First introduced in MySQL 5.5, Semisynchronous Replication offers a healthy alternative to MySQL Replication, which is asynchronous by nature. Here is how they differ:

MySQL Replication

• Master Executes SQL
• Master Records SQL Event in its Binary Logs
• Slave Reads SQL Event from Master Binary Logs
• Slave Stores SQL Event in its Relay Logs via I/O Thread
• Slave Reads Next SQL Event From Relay Log via SQL Thread
• Slave Executes SQL
• Slave Acknowledges Master of the Complete Execution of the SQL Event

Semisynchronous Replication

• Master Executes SQL
• Master Records SQL Event in its Binary Logs
• Slave Reads SQL Event from Master Binary Logs
• Slave Acknowledges Master of the Receipt of the SQL Event
• Slave Stores SQL Event in its Relay Logs via I/O Thread
• Slave Reads Next SQL Event From Relay Log via SQL Thread
• Slave Executes SQL
• Slave Acknowledges Master of the Complete Execution of the SQL Event

Benefits

1. This permits a Slave to be closer sync'd to its Master
2. If there are multiple Slaves, the Slave with the minimum ProcessID on the Master would be closer sync'd to its Master
3. In the event of network latency, MySQL will switch back to standard asynchronous replication until network latency dissipates.