While there are a few logical asynchronous replication systems for PostgreSQL, Slony-I (Slony, in short) was the first to gain wide adoption. Why would we use Slony when PostgreSQL already has physical and logical replication? PostgreSQL versions prior to 10 could only copy the entire installation.
The only option for those systems is to copy every database, schema, table, and user at the binary level. In effect, streaming replication creates perfect clones of PostgreSQL servers. So what happens if we want to upgrade from one of these antiquated versions to something more modern without shutting down PostgreSQL itself? Sometimes this requires tooling designed back when those versions were more predominant.
Slony accomplishes exactly that goal. It is designed to copy tables only, capturing changes on a provider server and sending them to one or more subscribers...
