migration-planning

Plan safe migrations by breaking schema changes into Prepare, Backfill, Switch, and Contract phases, defining reversible steps, data verification queries, and explicit rollback ordering to minimize downtime.

The expand-contract pattern keeps API contracts stable by adding new fields or endpoints alongside old ones, removing legacy compatibility only after monitoring proves the new contract is safely adopted.

Deployment sequencing coordinates staged changes across code and data, ensuring new code reads old data and old code reads new data, with feature flags and validation checks gating each phase transition.

Remove legacy compatibility code only after proof of safety, specifically when monitoring and alert signals confirm all traffic uses the new contract and no rollback to the old schema is anticipated.

Database backfill during migration involves writing data to both old and new schemas simultaneously, running background jobs to update historical records, and adding validation checks to verify data consistency before the final switch.