I started with the obvious read-write lock approach and the interviewer just kind of waited.
Start by clarifying the concurrency model and requirements, then propose a locking strategy that orders locks consistently to prevent deadlocks. Discuss trade-offs between pessimistic and optimistic concurrency, and mention how to handle failures and ensure atomicity.
Pro tip: Mention that you would use a lock ordering based on account IDs (e.g., always lock the lower ID first) to prevent deadlocks, and consider using a database transaction with SELECT ... FOR UPDATE if accounts are persisted. This shows practical experience with real-world systems.
Ask about the environment: in-memory vs. database, expected concurrency level, and whether accounts can be locked individually. Confirm that the transfer must be atomic and consistent.
Decide between pessimistic locking (e.g., mutexes per account) and optimistic locking (e.g., version numbers). For pessimistic, ensure locks are acquired in a consistent global order to avoid deadlocks.
Order locks by a unique identifier (e.g., account ID) so that all threads acquire locks in the same sequence. Alternatively, use a single global lock or lock-free techniques if contention is low.
Wrap the transfer in a transaction or critical section. If using a database, use transactions with appropriate isolation levels. Handle exceptions by rolling back and releasing locks.
Compare performance implications: lock contention, throughput, and scalability. Mention optimistic concurrency for low-conflict scenarios and database-level solutions for distributed systems.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.