Incremental materialized view maintenance
Design the storage and maintenance of an incrementally-maintained materialized view that powers a real-time merchant dashboard: 'per-merchant, per-day count and sum(amount) of completed orders', over a base orders table taking 100k writes/sec (inserts, plus status updates that flip an order to/from 'completed', plus refunds/deletes). A full recompute is far too expensive and too stale. Design how the view is stored, how each base-table change updates it incrementally and correctly (including updates and deletes, not just inserts), and the consistency/staleness trade-off the merchant sees.
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.