Payment System

Phase 1 — Clarify & Scope

Interviewer: Design a payment system — think Stripe, PayPal, or an internal payment service for an e-commerce platform. What do you want to clarify?

Candidate: Critical questions:

1. What's the payment flow — are we processing cards directly (acquirer integration), or acting as a payment orchestrator on top of existing PSPs (Stripe, Braintree)?
2. What payment methods — credit/debit cards, ACH, digital wallets, BNPL?
3. What's the transaction volume and average value?
4. Synchronous (user waits for confirmation) or async (bank transfer)?
5. Which compliance requirements — PCI-DSS, SOC2, geographic regulations?

Interviewer: Payment orchestrator on top of PSPs (Stripe primary, Braintree as fallback). Credit/debit cards and digital wallets. 100,000 transactions/day, average $50. Synchronous confirmation required — user waits for payment success/failure. PCI-DSS compliance required.

Candidate: Let me size this:

$\text{Transaction rate} = \frac{100{,}000}{86{,}400} \approx 1.16 \text{ txns/sec average}$

$\text{Peak (10× average)} = 11.6 \text{ txns/sec}$

$\text{Daily volume} = 100{,}000 \times \$50 = \$5\text{M/day}$

$\text{Annual volume} = \$5\text{M} \times 365 = \$1.825\text{B/year}$

At typical PSP fees of 2.9% + $0.30 per transaction:

$\text{Processing cost} = 100{,}000 \times (0.029 \times \$50 + \$0.30) = \$175{,}000/\text{day}$

Interviewer: What are the core requirements?

Candidate:

• Accept payment (card tokenization → charge via PSP)
• Route to best PSP (lowest cost, highest approval rate, geographic availability)
• Handle PSP failure with automatic retry and failover
• Idempotent payment operations — never double-charge
• Full audit trail for…