Knock, a company specializing in notification systems, successfully upgraded its Postgres database from version 11.9 to 15.3 with zero downtime by employing logical replication, support scripts, and tools from Elixir and Erlang's BEAM virtual machine. Due to the impending retirement of Postgres 11.9 by Amazon RDS, Knock prioritized an upgrade that avoided any significant downtime, as their service relies heavily on seamless operation. The process involved careful planning, risk assessment, and mitigation strategies to ensure that the large-scale migration did not disrupt service or customer experience. The approach focused on replication-based upgrades, allowing for gradual migration and testing of the new database while maintaining operational continuity. Challenges such as managing large tables, replication slots, and application-level changes were systematically addressed, culminating in a swift and smooth database transition that ensured Knock's systems remained reliable and performant for its users.