The PGCluster is constituted by Load Balance Server, Cluster DB Server, and Replication Server. With each server's combination, PGCluster can build two kinds of systems, a load-balancing system and a highly available system.

(1).Load Balancing System

(2) Highly available system

Furthermore, PGCluster can make a highly available system from adding loader balancer. ．

Cluster DB is the server which receives the query to DB from a user and actually processes. Cluster DB applies the patch for replications to the back-end of PostgreSQL. An access load can be distributed by increasing the number of Clusters DB. (However, even if it increases Cluster DB, the capacity of storable data does not increase, because the data of every Cluster DB is the same.)

Cluster DB will be immediately executed, when the query of referring to the data is received. The other query is passed to thereplication server and requires a duplicate.

The replication server sends the query from Cluster DB to each cluster DB. The replication server saves the received order and sends a query to each cluster DB.

The replication server is checking the problem of Cluster DB at the time of communication with Cluster DB. When a problem is detected, replication server separates Cluster DB from subsequent replications. If the status of Cluster DB has change, it will write out to a log.

When returning the separated cluster DB to a system, or when adding Cluster DB newly, thereplication server synchronizes data to the Cluster DB.

The load balance server receives the query from client, and sends a query to the cluster DB with the lowest load rate. The load factor to Cluster DB is calculated with the number of sessions under continuation.

Loader balancer checks a problem at the time of communication with Cluster DB. When the problem of Cluster DB is detected, loader balancer separates Cluster DB from a system.

Although it is not load balance server necessity at construction of a load-balancing system, it is required for construction of a highly available system.