How would you implement a thread-safe rate limiter?
Company: Palo
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Technical Screen
Design and implement a rate limiter for a backend service.
Requirements / follow-ups:
- The limiter must be **thread-safe** when accessed concurrently by many threads.
- Discuss how you would handle **very high QPS** (e.g., millions of checks/sec), including performance bottlenecks and scaling strategies.
Clarify in your answer what algorithm you choose (e.g., token bucket, leaky bucket, fixed/sliding window), what guarantees it provides (burstiness, fairness, accuracy), and what the behavior is when a request is rejected.
Quick Answer: This question evaluates competency in concurrency control, thread-safe design, rate-limiting concepts, and high-throughput system scalability. It is commonly asked in system design interviews to assess reasoning about performance bottlenecks, correctness under concurrent access, trade-offs between accuracy and burst handling, and practical implementation and architectural choices; category: System Design, level of abstraction: practical implementation and architectural reasoning.