This chapter covers payment system requirements: transactions per second, idempotency guarantee, audit trail, consistency SLA.

This chapter covers key questions: payment rails (card, bank transfer, wallet), chargebacks, compliance (PCI DSS), reconciliation.

This chapter covers direct payment processor call without idempotency — the double-charge failure mode.

This chapter covers idempotency keys, outbox pattern for reliable event publishing, two-phase ledger update.

This chapter covers two approaches: synchronous payment confirmation vs async with reconciliation — trade-offs in UX and reliability.

This chapter covers how Stripe, PayPal, and Square handle idempotency, refunds, and cross-border payments.

This chapter covers idempotency keys as a contract, double-entry bookkeeping for ledger integrity, async reconciliation.

This chapter covers double charges from retry without idempotency, ledger inconsistency, PCI scope creep.