Immutable ledger corrections
Design how an append-only, immutable financial ledger handles MISTAKES: a posting was made with the wrong amount, to the wrong account, or duplicated. You can never edit or delete a posted entry (audit + regulatory requirement), yet the balances must end up correct. ~10M postings/day. How do you model corrections, keep the audit trail intact, and make sure a correction is itself idempotent so a retried 'fix' doesn't over-correct?
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify4:30 left
Estimate4:30 planned
Design13:30 planned
Deep dive10:30 planned
Failure7:00 planned
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.