Two MySQL servers, each replicating data to the other. Any INSERT, UPDATE, etc. operations are transmitted over the network to the other instance. The two servers are local to each node and only the primary node gets all the HTTP/s connections. This is done so we can ensure only one web server accepts connections and writes to its local MySQL database.
Any change gets written to active server's binary log and flows through replication to the passive server's relay log. The passive server executes the query and writes the event to its own binary log, because the option log_slave_updates
is enabled. The active server then retrieves the same change via replication into its own relay log, but ignores it because the server ID in the event matches its own.