Question

A backfill recomputes a denormalized `lifetime_spend` on each `account` by reading the account, computing the new value, and writing it back (read-modify-write, no locking). It ran during business hours at ~3k accounts/sec. Afterward, finance finds that for accounts that placed an order *while the backfill was running*, `lifetime_spend` is wrong — sometimes missing the new order, sometimes missing the backfill's correction. Dashboards: write success 100%, no errors, no lag. How do you triage, stop the divergence, and correct the affected accounts?