Digital Wallet — Notes

Functional

• Top-up, withdraw.
• Send to friend, request money.
• Pay merchants.
• Bill pay, recurring subscriptions.
• Multi-currency.

Non-functional

• Strict consistency on balance.
• p99 transfer < 1 s for in-network; minutes for external rails.
• 99.99% uptime.

Capacity

• Big platforms: 100M+ active wallets, billions of txns/yr.
• Peak: tens of thousands of txns/s.

Schema

• accounts(id, user_id, currency, status)
• journal_entries(id, account_id, debit, credit, ref, ts) immutable
• transfers(id, from_acc, to_acc, amount, status, idem_key, version)
• link_methods(id, user_id, type, last4, ext_ref)

Trade-offs

• Ledger as source of truth, balances are projections.
• External rails are slow + flaky: pending state mandatory.
• PCI / banking compliance: tokenize cards, isolate scope.
• Strong consistency in-DC, eventual across-region for projections.

Refs

• "Building Square's Money Movement Platform" talks.
• PayPal architecture posts; Paytm engineering.
• "Distributed Transactions Done Right" (Stripe blog).
• ByteByteGo "Design a wallet".