Horizontally sharded database
Design a horizontally sharded relational database for a payments ledger handling 80k writes/sec and 200k reads/sec, with 12 TB of account data growing 1 TB/month. Each transaction must be strongly consistent within a single account, and you must support online resharding without a maintenance window. How do you choose the shard key, route queries, and split a hot shard while it serves traffic?
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.
Clarify5:00 left
Estimate5:00 planned
Design15:00 planned
Deep dive12:00 planned
Failure8:00 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.