Wednesday, July 24, 2019

When primary server is far away from standby server

Sometimes we want to create a cluster configuration in which the primary server is physically located far away from standby servers. For example, the primary server is located in an AWS region A, while the standby server is located in an AWS region B. This type of configuration is suited for a DR (Disaster Recovery) or a company which has a branch in different countries. In the figure below, a company's head quarter is located in region A,  and people in the HQ office access the primary server database through Pgpool-II.

People in a branch office located in region B access standby database in mostly read only manner. In a few occasion they need to write the database but they understand that it needs to access HQ database which would be slow. Problem for them is, even if they just do a read access to the database, Pgpool-II needs to access system catalog in the primary server to obtain meta info of the tables involved in their query.

From Pgpool-II 4.1,  which is under development, user could eliminate the slow system catalog access in from region B to region A by setting relcache_query_target = load_balance_node and backend weight of primary server to 0 in Pgpool-II running in region B.

Now the system catalog access is redirected to the standby server, rather than to the primary server and the access speed should faster.

Please note that , however, there may be replication delay and that could cause a trouble if the target table is pretty new because the system catalog in the standby server may not the have the new table info. So this configuration is best suitable for a system the table creation is rare, or they are created while users do not access the system.

By the way, Pgpool-II developers define the target date for release 4.1 of Pgpool-II in this October. Please stay tuned.

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