What Annihilation Teaches About Uncontrolled Refactors

A refactor without tests is a rewrite the running system did not agree to. The damage is not the new names. It is the lost path back.

I watched this happen on the Rails demo that fed accessibility-rails-components. One PR renamed tables, collapsed callbacks, and “simplified” the request path. Staging passed the happy path. Production still had polymorphic attachments and a cron job writing the previous schema. We spent a week reconstructing behavior from git history because nobody had pinned what “done” meant before the rewrite started. Characterization tests would have been a fence. We built a corridor instead and called it cleanup. Someone will call it bold. It is just irreversible.¹

Containment is a border you draw first: tests that lock current behavior, a flag that can still speak the old contract, a canary with a blast radius you can live with. Some legacy cannot be saved in place. Isolate it, replace it, and keep the old path until the new one has real traffic, not a checklist.

Do not start a rewrite you cannot stop. Draw the border before you touch the code.

— JV · Dark Heart Labs.

Michael Feathers, Working Effectively with Legacy Code (Prentice Hall, 2004) — characterization tests as the fence around behavior you do not yet understand. ↩