← Capital One Interview Insights
The basic CRUD part was fine, I had that sketched out in a few minutes.
Start by clarifying requirements and constraints, then design a thread-safe AccountService using a lock per account or a global lock, ensuring atomic transfers with proper ordering to avoid deadlocks. Implement the operations with checks for insufficient funds and discuss trade-offs between different synchronization strategies.
Pro tip: Mention that you would use a lock ordering strategy (e.g., by account ID) to prevent deadlocks during transfers, and consider using optimistic locking with versioning for higher concurrency if the use case allows.
Ask about expected concurrency level, whether accounts are in-memory or persistent, and if there are performance requirements. This shows you think before coding.
Define an Account class with balance and a lock (e.g., ReentrantLock). For transfers, acquire locks in a consistent order (e.g., by account ID) to prevent deadlocks.
Implement deposit, withdraw, and getBalance with proper locking. For withdraw, check for sufficient funds and throw an exception if not. For transfer, lock both accounts, check balance, then update both atomically.
Ensure that if an exception occurs during transfer, both accounts remain unchanged (use try-finally to release locks). Discuss how to handle partial failures if using a database.
Compare coarse-grained vs fine-grained locking, and mention optimistic concurrency control (e.g., CAS) as an alternative for high-throughput scenarios.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.