Highly available databases often come in pairs for redundancy purposes. These servers can have any number of procedures to keep the data synchronized, and this book suggests direct connections when possible. Direct connections between servers ensure fast communication between redundant servers, and it resembles the following network design:
In some cases, it can be advantageous to connect the database servers to a general network fabric. Depending on the interaction of the upstream network devices, this can significantly increase the network packet's round-trip time (RTT). This is usually fine for PostgreSQL replication, but online transaction processing (OLTP) systems may be more sensitive. Block-level replication systems (such as Distributed Regulated Block Device (DRBD)) that operate beneath the filesystem fare even worse.
Each of our...
