PGCluster是一个为Postgresql设计的多主机数据同步备份系统。从PGCluster的官方介绍来看，它最大的两个特点就是：多主机和同步备份。
1. Since a replication system is a synchronous replication,delay does not occur with the data duplicate between the Cluster DBs.
2. Since a server is multi-master composition,two or more the Cluster DBs can receive access from a user simultaneously.
系统架构如下图：
PGCluster还有2个功能：
1. Load sharing function
2. High availability function
PGCluster实质上是一种Master-Master备份方案，其数据备份方案有normal mode 和 reliable mode两种，并能保证事务的一致性。我觉得它的亮点是做到了数据的实时同步性，但是换来的却是牺牲了性能。
