Data and schema migrations have always been an operational challenge for administrators of relational databases, and integrating migrations into a modern CD pipeline is even more painful. We want developers to be able to quickly write and deploy schema changes, but we don't want to take down the site! In addition, a schema change that will take hours or require scheduled downtime in production should be applied transparently in development and testing environments. At Nextdoor, we solved this in a way that allows developers to move fast while keeping customer data safe.