rollback

Compare rollout capacity, traffic exposure and state compatibility, then work through a payments canary with too little evidence to promote.

Sequence a compatible column migration, reconcile concurrent writes, and identify when returning to the old application starts requiring data repair.

Redeploying the old tag reverses code and nothing else. What separates staff-level answers is a per-change map of what rolls back in seconds, what rolls back in days, and what only moves forward.

The code did not change, the data did. That inverts what triggers the pipeline, what the tests assert, and what a rollback means.

Code rolled back and the database did not. The old version is now running against a schema it has never seen, and that is a worse incident than the one you were trying to end.

Drawing an agent that polls a server is the easy third. The interview is about partial applies, fleet-versus-world state, and why rollback must be a pointer flip, never a reverse edit.