PostgreSQL 10: Setting up Logical Replication

2017-12-19 ·

Logical Replication is available in PostgreSQL since version 10. It is based on pgLogical. It does not have all of its features, but still it provides a basic version of this powerful functionality. Logical Replication was primarily developed by Petr Jelinek and Peter Eisentraut from 2ndquadrant.

Traditional replication works by transferring the write ahead log to the standby server which applies the changes. It is a low-level, binary format. Thus, the standby server is an exact copy of the master server.

Logical replication allows replicating at the level of particular tables, columns or rows. Thus, it is a logical change. It can consolidate multiple databases into a single one.

You can enable replication by changing wal_level to logical in postgresql.conf. From now on the write ahead log knows how to translate the binary changes back to logical ones.

On the master server create a publication:

CREATE PUBLICATION a_pub_name FOR TABLE a_table;

You can also set it to all tables:

CREATE PUBLICATION alltables FOR ALL TABLES;

On the slave server create a subscription. host can point to a remote location, you will need to adjust pg_hba.conf in that case.

CREATE SUBSCRIPTION a_sub_name
CONNECTION 'host=localhost port=5432 dbname=source_db'
PUBLICATION a_pub_name;

A subscription must be refreshed when new tables are added to the related publication; otherwise they won't be replicated.

ALTER SUBSCRIPTION testsub REFRESH PUBLICATION;

There are two monitoring views: pg_stat_replication which shows all the replication connections to the current server and pg_stat_subscription which shows status information about the subscription on the downstream server.

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