System design interview question
"Design a ledger service. Every other system in the company — payments, payouts, refunds, fees — records money movements here. It must always balance, it must be auditable years later, and it must never lose or duplicate an entry. Millions of entries per day."
immutability, invariants, and auditability as primary design goals. There is no clever caching or sharding trick to reach for; the question is whether the candidate understands double-entry accounting as a systems discipline and can enforce its invariants under concurrency and partial failure.
On Practick another engineer interviews you live for 60 minutes on a question from this bank. They have the full interviewer guide beside the call: the answers to your clarifying questions, the numbers to hand out, a ladder of follow-ups and a scoring rubric. You get the model solution once the session settles. It is free: interview someone for an hour and you have earned your own.
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