The following diagram illustrates how Amazon RDS for PostgreSQL performs replication between a source and replica in different Regions. Restoring WAL files from the archived location adds replication lag and hampers replication performance. This makes sure that during any network issues between the Regions, WALs are accumulated at the source instance and not archived.
Replication slots provide an automated way to make sure that the master does not remove WAL segments until the cross-Region replica receives them. Physical replication slots resolve dependency on this parameter. The archive_command parameter determines the archival of any additional WAL files in Amazon S3. In the traditional streaming replication, the parameter wal_keep_segments maintains the number of WAL files at the instance. Unlike streaming physical replication for replicas in the same Region, Amazon RDS for PostgreSQL uses physical streaming replication with slots for cross-Region replicas. You can also use AWS CLI create-db-instance-read-replica to create a cross-Region read replica.
One of the managed service offerings of Amazon RDS for PostgreSQL is cross-Region read replicas.