Payment System — Mock Interview

You're designing a Stripe-like payment system for a live system-design interview. Merchants charge customers' cards; funds flow through external card networks and banks; merchants are paid out later. The defining challenge is that this system moves real money, where two failure modes are unacceptable: double-charging a customer on a retry, and losing or inventing money so the books don't balance.

Scale assumptions:

• ~100M payments/day → ~1,000 charges/s average, ~5,000/s peak.
• ~4 double-entry ledger postings per payment → ~20,000 ledger writes/s peak.
• ~$5B/day moved; the append-only ledger is retained 7+ years for audit.
• Targets: authorize < 500 ms p99, synchronous ledger commit < 50 ms p99, 99.99% on the charge API, zero double-charges or lost postings.

We'll move through requirements, estimation and the ledger schema, architecture, the idempotency + double-entry deep dive, and trade-offs. Think out loud and state your assumptions.