Digital Wallet

Phase 1 — Clarify & Scope

Interviewer: Design a digital wallet — think PayPal, Venmo, or WeChat Pay. What do you want to clarify?

Candidate: A few key questions:

1. What operations does the wallet support — add money, send to another user, pay merchants, withdraw?
2. Are balances real money or points/credits?
3. What's the transaction volume and value?
4. Multi-currency or single currency?
5. Consistency model — can we tolerate brief balance inconsistency, or must every transfer be instantly consistent?

Interviewer: Peer-to-peer transfers (send to friend), merchant payments, add money from bank (ACH), withdraw to bank. Real money — USD only. 1 million users, 10 million transactions/day, average $25. Strong consistency — balance must always reflect completed transactions accurately. No overdrafts allowed.

Candidate: Let me size this:

$\text{Transaction rate} = \frac{10\text{M txns/day}}{86{,}400} \approx 116 \text{ txns/sec average}$

$\text{Peak (5× average)} = 580 \text{ txns/sec}$

$\text{Total wallet balance} \approx 1\text{M users} \times \$100 \text{ avg balance} = \$100\text{M}$

This is real money — correctness is more important than performance. Even one incorrect balance update causes regulatory and trust issues.

Interviewer: What are the core requirements?

Candidate:

• Debit/credit wallet balance with ACID guarantees
• P2P transfer: atomic — sender debited iff receiver credited
• No negative balances (insufficient funds check)
• Idempotent operations — duplicate requests must not double-debit
• Complete audit trail of every balance change
• Reconciliation — total system balance must equal sum of all wallet balances

Non-functional:

• Transfer latency: < 500 ms p99
• Exactly-once transfer semantics
• Balance accuracy:…