Upgrading high load PostgreSQL databases is a challenge on its own. When having customers around the globe with tight SLAs, the requirement arises to execute these upgrades with minimal or even no downtime at all. This talk shares GitLab's journey from multi-hour maintenance windows to truly zero-downtime upgrades for our PostgreSQL infrastructure. You'll learn the battle-tested techniques we've developed over the last 4 years, like how we execute PostgreSQL major upgrades and OS (glibc) upgrades at the same time, prevent data corruption, as well as always keeping a rollback path via reverse replication. We'll walk through real production examples, the gotchas we discovered, and the tooling we built. Whether you're managing a single HA cluster or a global fleet, you'll leave with actionable strategies to minimize (or eliminate) downtime during your next major upgrade.