sky_postgresql_cluster is a PostgreSQL HA module write in shell, HA via three host, include two postgresql
(primary and stream replication standby) and one vote host. Applicatoin connect to sky_postgresql_cluster via a virtual
the two postgresql host must have fence device.
the two PostgreSQL host, one is master, another is standby. standby build with PostgreSQL's stream replication.
sky_postgresql_cluster's status monitor use nagios.