Backfill race corrupts denormalized field
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?
What a strong answer looks like
Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.
0:00 of about 40 min
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.