This is a monster of a question and I did not pace myself well.
Start by clarifying requirements and scope, then design a high-level architecture that separates concerns: API layer, payment orchestration, ledger, and reconciliation. Walk through the payment lifecycle (initiation to refund) while highlighting idempotency, consistency, and failure handling at each stage. Finally, discuss trade-offs, observability, and fraud hooks to demonstrate depth.
Pro tip: Emphasize idempotency and exactly-once semantics early, as they are critical in payment systems and often overlooked. Use concrete examples of how you'd handle duplicate requests and partial failures to show practical experience.
Ask questions to understand expected scale, consistency needs, supported payment methods, and regulatory constraints. Define key entities like Payment, Transaction, LedgerEntry, and their relationships.
Outline the end-to-end flow: initiation (API), authorization (with external processor), capture, and refund. Describe how idempotency keys prevent duplicate operations and how state transitions are managed.
Explain double-entry ledger design for accurate accounting, with append-only entries and balances derived from entries. Discuss consistency guarantees (e.g., ACID for ledger updates) and how to handle concurrent updates.
Detail retry logic with exponential backoff and jitter, partial failure handling (e.g., authorization succeeds but capture fails), and reconciliation with external processors to detect and resolve discrepancies.
Incorporate logging, metrics, and tracing for monitoring payment flows. Describe fraud detection hooks (e.g., pre-authorization checks, risk scoring) and how to integrate them without blocking the main flow.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.