expand contract

Blue-green works for stateless apps because instances are disposable. Databases are shared state, so safe migrations ride on backward compatibility and an explicit rollback boundary.

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

Application redeployment and module downgrades both require current-state checks. The asymmetry that forces expand-contract discipline and plan-reviewed upgrades instead of promote-and-rollback pipelines.

Announcing a deadline is not a migration plan. The teams that miss it are the ones who were never going to read the announcement.