Zero-downtime Schema Changes

Schema changes on live systems are hard. They are inherently disruptive in Mysql. We at Etsy have devised a way to make schema changes in a non-disruptive way, and we routinely perform them without any user-facing impact. I will give a brief description of our Mysql architecture, followed by a fairly detailed walk-through of our schema changes procedures. Finally, I will present the tooling we've built around our schema changes procedures: our Schemanator tool.