SoFi·Software Engineer·Technical Phone Screen·Senior

Senior

Jul 2026

Summary

SoFi software engineering interview with a concurrency design problem that was more involved than I expected. The question looked like a straightforward semaphore exercise but kept growing in scope as we went.

Questions Asked (1)

Design and implement a thread-safe TaskExecutor that uses a semaphore-like permit system to cap concurrency, where tasks can be either blocking or fire-and-forget, and permits must always be released even on failure.

System DesignTechnical Trade-offsAlgorithms & Data Structures

Author's notes

I started with the semaphore acquire/release loop and got that part okay, but the blocking vs fire-and-forget distinction tripped me up.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then design the TaskExecutor using a Semaphore to control concurrency. Explain how you'll handle blocking and fire-and-forget tasks, and ensure permits are released in a finally block. Finally, discuss trade-offs and potential improvements.

Pro tip: Mention that using a Semaphore with try-finally ensures permits are released even if the task throws an exception, and consider using a bounded queue to prevent resource exhaustion. Also, highlight the importance of testing under high concurrency to catch race conditions.

1. Clarify Requirements

Ask questions to understand expected task types, concurrency limits, error handling, and whether tasks need to return results. Confirm if the executor should support timeouts or cancellation.

2. Design the Core Mechanism

Use a Semaphore initialized with the max concurrency. For each task, acquire a permit before execution and release it in a finally block. For blocking tasks, return a Future; for fire-and-forget, submit to an executor and return void.

3. Handle Task Types and Errors

Differentiate between blocking (Callable) and fire-and-forget (Runnable) tasks. Ensure that exceptions in tasks don't prevent permit release. Consider using a wrapper that always releases the permit.

4. Discuss Trade-offs and Edge Cases

Talk about fairness of the semaphore, potential deadlocks if tasks acquire multiple permits, and the impact of unbounded queues. Mention alternatives like using a ThreadPoolExecutor with a bounded queue and rejection policy.

5. Test and Validate

Describe how you would test the implementation: unit tests for permit release on success/failure, stress tests with many tasks, and verifying concurrency limits are respected.

Key Points to Mention

Use of Semaphore for concurrency control and try-finally for permit release.
Distinction between blocking (Callable/Future) and fire-and-forget (Runnable) tasks.
Thread safety considerations: atomicity of acquire/release, visibility, and potential race conditions.
Handling exceptions and ensuring permits are released even on failure.
Trade-offs: fairness, throughput vs. latency, bounded vs. unbounded queues.
Testing strategies: unit tests, stress tests, and monitoring for permit leaks.

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