Design a financial ledger service
Design a general-purpose financial ledger service that records ~20,000 postings/sec across millions of accounts, must be immutable and auditable, and must answer 'what is account X's balance as of timestamp T' for any historical T. Reads (balance lookups) outnumber writes ~10:1. Describe the data model, how balances are computed without scanning the whole journal, and how you keep current balances correct under high write concurrency.
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.
Clarify5:30 left
Estimate5:30 planned
Design16:30 planned
Deep dive13:30 planned
Failure9:00 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.