The image shows three situations: normal operation, failure of master, recovered master.
During normal operation, users are connected to applications that update Database A, the master. The master replicates to Database B, the subscriber.
Failure of the master: When Database A fails, users are still connected to the application, but it cannot update Database A. No replication to the subscriber occurs.
Recovered master: Users are connected to the application, which sends updates to Database A. Database A replicates to Database B, the subscriber.
End of description.