This is basically a full payments platform crammed into one question.
Start by clarifying requirements and scope, then design an append-only ledger as the source of truth with double-entry accounting. Define the schema, APIs, and core classes to handle idempotency, concurrency, and multi-currency, and finally analyze complexity and trade-offs.
Pro tip: Emphasize idempotency and auditability from the start—use idempotency keys for all mutating operations and ensure every transaction is recorded in an append-only ledger. This demonstrates maturity in handling financial systems.
Ask questions to understand expected scale, consistency requirements, supported currencies, and regulatory constraints. Confirm the need for idempotency, concurrency safety, and auditability.
Propose an append-only ledger with double-entry accounting. Define tables for accounts, transactions, ledger entries, holds, and refunds, including fields for idempotency keys, currency, and timestamps.
Outline RESTful APIs for deposits, holds, captures, releases, refunds, payouts, and FX conversion. Describe core classes like Wallet, Ledger, Transaction, and IdempotencyManager.
Explain how to use database transactions, optimistic locking, and idempotency keys to ensure consistency and prevent duplicate operations. Discuss handling of concurrent updates to balances.
Discuss time and space complexity of key operations, scalability considerations, and trade-offs between consistency and availability. Mention potential bottlenecks and mitigation strategies.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.