The Database Migration Playbook

1. Three Things That Exist Before Step One

A migration plan that starts with a schema change has started in the wrong place. Three pieces of machinery need to exist first, and building them is part of the work rather than preparation for it.

• A comparison mechanism. Something that reads both representations and reports whether they agree, as a count over time. Without it, every later gate is a guess.
• A flag for each switchable step. Dual writing, read source, and turning off the old write are three separate controls. Combining them into one is how a rollback becomes a deploy.