The user application must be connected to a master to issue transactions that write data. The default level of durability is defined by the synchronous_commit parameter. That parameter is user settable, so it can be set for different applications, sessions, or even individual transactions. For now, ensure that the user application is using this level:
SET synchronous_commit = on;
We must decide which standbys should take over from the master in the event of a failover. We do this by setting a parameter called synchronous_standby_names.
You will need to configure at least three nodes to use sync rep correctly.