How would you design a ledger for a wallet or payments product so balances are always correct?

What the interviewer wants to hear

• Double-entry bookkeeping: every transfer has balanced debit and credit entries
• Entries are immutable and append-only; corrections are new entries
• Integer minor units with currency, never floats
• Idempotency keys and strong transactions or row locking to avoid double spends
• Balances derived from entries, cached with checks, and reconciled with providers

Model answer

I would use double-entry bookkeeping. Every movement of money is a journal transaction with two or more entries, debits and credits, that must sum to zero, so money is never created or lost by a bug. Entries are append-only and never updated; a mistake is fixed with a reversing entry, which gives a full audit trail. Amounts are stored as integers in minor units, like cents, together with a currency code. Each request carries an idempotency key with a unique constraint so retries do not double charge. To stop overspending, I write the entries and update a cached balance in one database transaction, locking the account row or using a version check, and enforcing that the balance cannot go below zero. The cached balance can always be rebuilt from entries. Daily reconciliation compares the ledger with bank and provider reports.

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