Design a multi-level API rate limiter

## Scenario You are building a backend for an “insight platform”. The platform exposes HTTP APIs that are called by many tenants and many end-consumers. You need to design a **rate-limiting layer** with **two concurrent limits**: 1. **API-level limit:** max **100 requests/second per API key** (tenant/application). 2. **Consumer-level limit:** max **10 requests/second per consumer** (end user / device / client id). A request should be **allowed only if it satisfies both limits**. ## Requirements - **Enforcement point:** rate limiter sits in front of multiple stateless API servers (e.g., gateway/middleware). - **Correctness target:** practical accuracy under high concurrency; avoid letting traffic exceed limits by large margins. - **Latency:** add minimal overhead (single-digit milliseconds typical). - **Scale:** handle many unique API keys and consumers; traffic can spike. - **Operability:** metrics and logs for throttling decisions. - **Behavior:** when over limit, return HTTP **429** with a helpful response (e.g., retry-after). ## Deliverables 1. Choose a rate-limiting algorithm and explain why (token bucket / leaky bucket / sliding window / fixed window, etc.). 2. Propose a distributed design (single node vs multi node) that works with multiple API servers. 3. Show how you would enforce **both** limits atomically (or explain acceptable approximations). 4. Discuss data model, keying, expiration, and failure modes.