The original title is "Designing a payment system that never loses a cent"

Short summary

A correct payment system prioritizes correctness over speed by solving four core problems: duplicate charges via idempotency keys, auditable balances via append-only double-entry ledgers, payment lifecycle management via explicit state machines, and cross-system consistency via asynchronous reconciliation. Real platforms like Stripe and PayPal use exactly this combination—favoring eventual correctness and auditability over fast but occasionally wrong operations. The key mindset is making every step safe to retry, every movement immutable, and every uncertain outcome resolvable.

• •Idempotency keys prevent duplicate charges on network retries
• •Append-only double-entry ledgers provide auditable, derivable balances
• •State machines and reconciliation handle multi-system payment lifecycles safely

Generated with AI, which can make mistakes.