Design a Multi-Currency Payment System

FX Rate Management

Multi-provider rate ingestion with staleness detection is worth stating before your interviewer asks what happens when an upstream rate feed goes stale.

Rate ingestion:
  Multiple FX rate providers (redundancy + best rate selection):
    - Reuters/Bloomberg: institutional rates
    - ECB (European Central Bank): reference rates
    - Open Exchange Rates API: retail rates

Rate with markup:
  Raw rate: 1.0845
  Platform markup: 2% (revenue)
  Buy rate (user buys USD with EUR): 1.0845 * 0.98 = 1.0628
  Sell rate (user sells USD for EUR): 1.0845 * 1.02 = 1.1062

Rate triangulation:
  No direct rate for NGN->BRL?
  NGN -> USD -> BRL (via common intermediate currency)
  rate = ngn_usd_rate * usd_brl_rate

FX Rate Locking

Rate locking establishes the checkout user experience contract. Explain who absorbs FX risk during the 15-minute window before diving into the lock token flow.

Without locking, the user sees €92.17, takes 5 minutes to fill payment details, and the rate changes before completion. With locking, the platform absorbs FX risk for 15 minutes. At scale (10M transactions/day), average FX movement in 15 minutes remains < 0.05%.

Checkout flow with rate lock:

1. User sees price: "$100 USD" -> "Show in my currency" -> "€92.17 EUR"
2. User clicks "Pay €92.17":
   FX Service locks rate:
     INSERT INTO rate_locks (lock_id, from_ccy, to_ccy, rate, expires_at)
     VALUES ('lock-uuid', 'EUR', 'USD', 1.0850, NOW() + '15 min');
3. User completes payment within 15 min:
   Payment Service: validate lock_id is not expired
   Charge: €92.17 EUR -> convert at locked rate 1.0850 -> $100.00 USD to merchant
4. If user takes > 15 min:
   Lock expired -> re-quote with current rate

Multi-Currency Ledger

Per-currency ledger accounts keep FX conversions auditable, aligning with the double-entry bookkeeping detailed in our Banking Ledger System, ensuring that every cross-border payment creates balanced entries in both currencies.

Transaction: Buyer pays €92.17, Merchant receives $100.00

EUR ledger:
  Entry 1: { account: buyer_eur,     DEBIT,  €92.17 }
  Entry 2: { account: platform_eur,  CREDIT, €92.17 }

USD ledger:
  Entry 3: { account: platform_usd,  DEBIT, $100.00 }
  Entry 4: { account: merchant_usd, CREDIT, $100.00 }

FX conversion record:
  { from: €92.17 EUR, to: $100.00 USD, rate: 1.0850, lock_id: "lock-uuid" }

Per-currency balancing:
  SUM(EUR debits) = SUM(EUR credits) ?
  SUM(USD debits) = SUM(USD credits) ?

Smallest Currency Units

Different currencies have different decimal places:
  USD: 2 decimals ($1.00 = 100 cents)
  JPY: 0 decimals (¥100 = 100 yen, no fractional yen)
  BHD: 3 decimals (0.001 BHD = 1 fils)

Best practice: store amounts as integers in smallest unit.
  $100.00 -> 10000 (cents)
  ¥100 -> 100
  0.500 BHD -> 500 (fils)

Avoids all decimal arithmetic issues. Integer math is exact.