There's a lot of surface area here and I think I spread myself too thin early on.
Start by clarifying requirements (functional and non-functional) and scale, then propose a high-level architecture covering core components like payment processing, ledger, and integrations. Dive into critical areas such as idempotency, consistency, and security, and discuss trade-offs for each design decision.
Pro tip: Emphasize idempotency and exactly-once processing early, as payment systems must handle retries and network failures without double-charging. Also, mention the importance of reconciliation with external providers to ensure data integrity.
Ask about functional requirements (payment methods, currencies, refunds) and non-functional requirements (scale, latency, consistency, compliance). Establish scope and constraints.
Outline main components: API gateway, payment service, ledger, database, message queue, and external integrations (banks, card networks). Sketch data flow for a payment transaction.
Discuss idempotency, consistency (ACID vs BASE), data modeling (transactions, accounts), and security (PCI DSS, encryption). Explain how to handle failures and retries.
Address horizontal scaling, partitioning, replication, and fault tolerance. Discuss monitoring, alerting, and disaster recovery.
Summarize key trade-offs (e.g., consistency vs availability, SQL vs NoSQL) and justify choices. Mention potential future improvements.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.