Requirements
You can create this setup with as little as 3 machines, by running 2 instances of PgPool II in 2 of the machines running PostgreSQL. You can increase performance by moving the PgPool instances to their own machines and by adding extra PostgreSQL slaves (specially if the load on the database is mostly read queries).
I recommend using at least 5 machines so each program runs on it's own, simplifying the troubleshooting process.
PgPool II at the moment (09/2015) only supports Linux Machines.
I used CentOS 7 so if you're on a different GNU/Linux distribution some things might work differently. (I also used CentOS 6.5 on some of the PostgreSQL machines to avoid some re-installations)
- On my PostgreSQL machines I used both CentOS 6.5 and 7.1.1 with PostgreSQL 9.4.4
- For load balancing I used two machines with CentOS 7.1.1 and installed PgPool II 3.4.3
- Automated failover is handled by a custom script that PgPool II runs when a node goes down
