This one sprawled way more than I expected.
Start by clarifying requirements and scale, then model the transfer as a state machine with explicit states (PENDING, ACCEPTED, CANCELLED, EXPIRED) and balance semantics (available vs. held). Walk through the full lifecycle: creation with hold, acceptance/cancellation, expiration, and failure recovery, emphasizing idempotency, atomicity, and authorization at each step.
Pro tip: Treat the hold as a first-class entity with its own lifecycle and idempotency key, and use a ledger-based approach for auditability and consistency. Explicitly discuss how you'd handle race conditions (e.g., double-spend, concurrent accept/cancel) using optimistic locking or serializable transactions.
Ask about expected throughput, latency, consistency requirements, and failure tolerance. Confirm whether transfers are between users, accounts, or currencies, and whether partial captures are needed.
Enumerate states (e.g., PENDING, ACCEPTED, CANCELLED, EXPIRED) and transitions. Explain how balances are affected: available balance decreases on hold, held balance increases; on capture, held decreases and total decreases; on release, held decreases and available increases.
Propose tables/collections for transfers, holds, and ledger entries. Define APIs for create, accept, cancel, and expire, including idempotency keys and authorization checks.
Explain how to use transactions or two-phase commit to atomically update balances and transfer state. Describe idempotency via unique request IDs and how to handle concurrent accept/cancel with locking or versioning.
Outline a background job or TTL mechanism for expiration, and recovery from crashes using write-ahead logs or event sourcing. Analyze time and space complexity of core operations.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.