Optiver·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Optiver software engineer interview with a concurrency-focused coding problem. The question was about building a thread-safe stock inventory class where sells have to be atomic check-and-deduct operations. Pretty classic for a trading firm but the details matter a lot here.

Questions Asked (1)

Design and implement a thread-safe StockInventory class that supports concurrent buy and sell operations, where sell is all-or-nothing and the available share count must never go negative.

System DesignTechnical Trade-offsAlgorithms & Data Structures

Author's notes

The single-threaded version took me about two minutes.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then propose a design using a single lock or atomic operations to ensure thread safety and all-or-nothing semantics. Discuss trade-offs between coarse-grained and fine-grained locking, and consider optimistic concurrency for scalability. Finally, outline implementation details and test cases for concurrency.

Pro tip: Mention that you would use a lock-free approach with atomic compare-and-swap (CAS) for high contention, but acknowledge that a simple mutex is often sufficient and less error-prone. This shows you understand both performance and maintainability.

1. Clarify Requirements

Ask about expected concurrency level, performance requirements, and whether buy/sell operations need to be fair or can be starved. Confirm that sell must be all-or-nothing and inventory cannot go negative.

2. Choose Synchronization Strategy

Decide between coarse-grained locking (e.g., a single mutex) and fine-grained/optimistic concurrency (e.g., atomic CAS). Discuss trade-offs: simplicity vs. scalability, and potential for contention.

3. Design the Class Interface

Define methods like buy(int quantity) and sell(int quantity) that return success/failure or throw exceptions. Ensure sell checks available shares atomically before decrementing.

4. Implement Thread-Safe Operations

Use the chosen synchronization to protect the share count. For sell, perform a check-and-decrement atomically. For buy, simply increment atomically. Consider using a ReentrantLock or synchronized block.

5. Test and Validate

Write unit tests with multiple threads to verify no negative inventory and all-or-nothing sell. Use stress tests to uncover race conditions.

Key Points to Mention

Atomicity of check-and-decrement for sell operations
Use of mutex/lock vs. lock-free atomic operations (e.g., AtomicInteger with CAS)
Potential for deadlock and how to avoid it (e.g., single lock, no nested locks)
Performance implications: contention, scalability, and fairness
Exception handling and return values for failed operations
Testing strategies: multithreaded stress tests, race condition detection tools

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.