This is part 3 of a series about Postgres logical replication use-cases, and about how the feature set has evolved over the past ten years and ten releases (Postgres 10 through the Postgres 19 beta), one architecture at a time. Part 1 built a hub-and-workers system for write scalability and has the table of what each release added, which this post assumes. Part 2 consolidated several application databases into one warehouse. This one is the most common reason to touch logical replication at all: a major-version upgrade with no downtime, and a way back if it goes wrong.