Expand-contract: reversible until the last step

Expand-contract: reversible until the last step A workflow diagram generated by Archify. 01 / Deploys EX / Rollback 1. Expand · add new column · Deploys 1. Expand add new column 2. Dual-write · write old + new · Deploys 2. Dual-write write old + new 3. Backfill · batches + verify · Deploys 3. Backfill batches + verify 4. Switch reads · behind a flag · Deploys 4. Switch reads behind a flag 5. Contract · drop old, one-way · Deploys 5. Contract drop old, one-way Flag off · read old column · Rollback Flag off read old column 0 mismatches weeks later problem still dual-writing Legend Service step Check / gate Data step