Design a Rate Limiter: Requirements & Scope

Turn the open-ended prompt into a concrete contract. Separate functional requirements (what the system does) from non-functional ones (latency, availability, consistency, scale), and state non-goals so you do not overbuild. Ask one or two clarifying questions that actually change the design — read/write ratio, scale, and consistency needs are usually the highest-leverage.

• Allow N requests per window per client
• Return 429 when over the limit
• Work across many app servers
• Configurable limits per route/tier

• Adds < 5ms to each request
• Accurate enough under bursts
• Fails open or closed by policy
• Horizontally scalable

• Billing/metering
• Per-user analytics dashboards