Design Payment System (Stripe-style)

Stripe-style payment system case: API gateway with WAF and Redis idempotency cache, payment core with charge service, saga orchestrator, HSM tokenization, Postgres double-entry ledger and outbox, external Visa/Mastercard/SEPA rails via processor adapter, async fan-out via Kafka to webhook dispatcher, reconciliation cron, and payouts. Five animated scenarios: happy-path card charge with idempotency key, retry duplicate caught at idempotency layer, refund with compensating ledger entry, subscription rebill via cron with saved token, and nightly reconciliation cron catching webhook-lost mismatches. Two ADRs: idempotency-key dual-storage strategy (Redis fast path + Postgres unique-index backstop) and ledger storage choice (Postgres double-entry with outbox vs append-only event store). Capacity hints throughout.