The master server’s binary log is the basis for communication between the master and its slaves:
*When the master server makes modifications to its databases, it records the changes in its binary log files. Statements are stored in the log as “events.”
*Events recorded in the binary log are sent to each connected slave server, which makes the changes to its copy of the replicated databases. A slave server that isn’t connected to the master when an event is recorded will receive the event when it connects later.
When a slave receives an update from the master, it doesn’t change its own databases immediately. Instead, it records the event in its relay log. The relay log is stored on disk and processed later.