System designMediumsd-g387

Subject LedgerLevel Mid–Senior~40 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

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.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.