PostgreSQL 10 Administration Cookbook - Fourth Edition

Data loss

By default, PostgreSQL provides asynchronous replication, where data is streamed out whenever convenient for the server. If replicated data is acknowledged back to the user prior to commit, we refer to that as synchronous replication.

With synchronous replication, the replication delay directly affects the elapsed time of transactions on the master. With asynchronous replication, the master may continue at full speed, though this opens up a possible risk that the standby may not be able to keep pace with the master. All replication must be monitored to ensure that a significant lag does not develop, which is why we must be careful to monitor the replication delay.

Synchronous replication guarantees that data is written to at least ...