How we upgraded PostgreSQL at GitLab.com

This post details the successful major version upgrade of GitLab.com's main PostgreSQL cluster from version 9.6 to 11 using pg_upgrade. It outlines the planning, testing, and automation involved, including the use of blueprints and design documents. Key aspects covered are the reasons for the upgrade (EOL of 9.6, new features in 11), the existing architecture (Patroni, Consul, GCS), upgrade requirements (no regressions, automated process, short maintenance window), the phased project execution (isolated environment, staging tests, production upgrade), and the rollback plan. It also explains how pg_upgrade works and the considerations for external modules.