Start by clarifying requirements (e.g., single-process vs distributed, blocking vs non-blocking) and then present a solution that uses fine-grained locking with a global lock ordering to prevent deadlocks. Emphasize atomicity via locks or transactional memory, and discuss trade-offs like lock contention, throughput, and alternative approaches (e.g., optimistic concurrency, STM).
Pro tip: Mention that in real-world systems like Revolut, you'd likely use a database transaction with row-level locks or a distributed lock service, but for an in-memory implementation, lock ordering is key. Also, consider using a lock manager or tryLock with timeout to avoid deadlocks entirely.
Ask about the environment: single JVM or distributed? Are accounts in-memory or in a database? What are the performance and consistency requirements? This shows you think before coding.
Define an Account class with a balance and a lock. Choose a locking strategy: e.g., lock both accounts in a consistent order (by account ID) to prevent deadlocks. Alternatively, use a single global lock for simplicity, but discuss its drawbacks.
Write pseudocode or actual code: acquire locks in order, check sufficient funds, debit one account, credit the other, release locks. Ensure atomicity: either both operations succeed or neither does (e.g., using try-finally to release locks).
Explain how the lock ordering prevents deadlocks (circular wait). Discuss other issues: race conditions, visibility (use synchronized or ReentrantLock), and potential for lock contention. Mention alternatives like optimistic concurrency with retry.
Compare fine-grained vs coarse-grained locking, blocking vs non-blocking, and single-node vs distributed. Mention how you'd handle failures, timeouts, and scalability. This demonstrates depth and awareness of real-world systems.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.