Implement a thread-safe rate limiter

Q: Implement a thread-safe rate limiter

This question evaluates a candidate's ability to design and implement a thread-safe per-user API rate limiter, focusing on concurrency control, synchronization techniques, and time-based request accounting in the Coding & Algorithms domain.

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Question

Design and implement a per-user API rate limiter.

Requirements:

Method: boolean allow(String userId, long nowMillis) returns whether the request is allowed at time nowMillis .
Policy: allow at most N requests per W milliseconds per user.
Choose any standard algorithm (e.g., fixed window, sliding window, token bucket).

Follow-ups:

How do you make it safe under concurrency (multiple threads calling allow for the same userId )?
Discuss tradeoffs between using locks vs lock-free/low-lock approaches (e.g., ConcurrentHashMap , atomics) and performance implications.

Implement a thread-safe rate limiter

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