1. The Concurrency Test in Senior Rounds
In senior LLD interviews, the interviewer will intentionally ask: 'What happens if two users try to book the same seat simultaneously?' or 'How does your cache handle concurrent writers?'
Slapping `synchronized` on every method destroys throughput. You must demonstrate granular locking strategies.
2. Choosing the Right Concurrency Primitive
ReentrantLock: When you need explicit lock timeouts (`tryLock(5, TimeUnit.SECONDS)`) or fair queuing.
ReadWriteLock: When read operations vastly outnumber writes. Multiple threads can read concurrently, while writes acquire exclusive locks.
Semaphore: For resource rate limiting (e.g. limiting database connection pools to 20 concurrent threads).
Atomic Variables (AtomicInteger, AtomicReference): For non-blocking, lock-free counter updates using hardware compare-and-swap (CAS).
3. The Four Coffman Deadlock Conditions and How to Break Them
Deadlock occurs if and only if four conditions hold simultaneously: Mutual Exclusion, Hold and Wait, No Preemption, and Circular Wait.
The most practical way to eliminate deadlocks is eliminating Circular Wait by enforcing a strict global lock ordering (e.g. always acquire locks in ascending order of Account ID).
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Written by HLDForge Architecture Team
Lead Systems Software Engineer
The HLDForge Architecture Team publishes practical engineering guides, real-world case studies, and high-concurrency blueprints to help engineers build scalable and fault-tolerant software.