Leave comments at: https://elixirforum.com/t/elixirconf-2023-tyler-young-rebuilding-the-plane-while-it-s-still-flying/59772 It is insidiously easy to underestimate the difficulty of running a large-scale, zero-downtime data migration. For those with production experience, it can be rated the most challenging problem of a career; without experience, it’s likely to be scoffed at. After all, we’re “just” talking about moving data (or protocols or systems) from an old format to a new one, right? This talk will present a case study from our work at Felt, where we migrated both large swathes of our Elixir codebase and many Ecto schemas atop our Postgres database over a period of months. Motivated by that, we’ll explore what makes data migrations so thorny and why success looks very different from fixing a bug or shipping a new feature. From there, we’ll turn our attention to the practice of executing migrations: patterns for successful management and how you can build a culture that makes zero-downtime migrations low-stress and low-risk.