Design quota enforcement for high concurrency
Company: Google
Role: Software Engineer
Category: System Design
Difficulty: hard
Interview Round: Technical Screen
Design a quota enforcement service used by many services for API or storage quotas under very high request rates. Define the APIs to check and increment usage, and discuss strategies for strong versus eventual consistency, hot-key sharding, rate limiting, and correctness when concurrent requests arrive. Explain how to reconcile temporary overage under eventual consistency for billing, and how to ensure low-latency, high-availability enforcement.
Quick Answer: This question evaluates a candidate's understanding of distributed system design and engineering competencies related to quota enforcement, high-throughput rate limiting, consistency models, sharding, and concurrency control under very high QPS.