PostgreSQL, Second Edition

Overview

Slony works by adding an AFTER UPDATE/INSERT/DELETE trigger to the master copy (that is, the origin) of every table that you want to replicate. An AFTER UPDATE/INSERT/DELETE trigger is a function that executes after an UPDATE, INSERT, or DELETE command modifies a table. The new trigger pushes each change from the origin into a history table. When a subscriber comes online, it pulls recent changes from the history table. The history table contains a record of every modification. If you UPDATE a row in a replicated table, and then DELETE that row, the history table contains an UPDATE command followed by a DELETE command. When a subscriber pulls the modification history from the provider, it applies each change, in order from earliest change ...