AI InterviewGet started

Design a Rate Limiter: System Design Interview Practice

Design a Rate Limiter as a system design interview question: Limit how many requests each client may make to an API, across a fleet of API servers.

How do you answer "Design a Rate Limiter" in a system design interview?

Rate limiting caps how many requests a client may make in a period, such as 100 requests a minute per API key. Requests over the limit are refused, usually with HTTP 429 Too Many Requests. It protects servers from overload and abuse, keeps one client from starving others, and is built with algorithms such as token bucket, leaky bucket, fixed window and sliding window.

  • Level: Beginner
  • Scored out of: 100
  • Cost: Free to read; a free account gets the review

Limit how many requests each client may make to an API, across a fleet of API servers.

The scale

50,000 requests a second across 20 API servers; limits like 100 requests a minute per user.

What a strong answer covers

  1. Requirements (20 points). States the functional requirements in scope and the non-functional ones that drive the design (scale, latency, availability, consistency), and leaves out what is out of scope.
  2. Scale estimates (15 points). Turns the stated scale into requests per second (average and peak), storage and bandwidth, with the arithmetic shown, and uses the numbers to make a decision.
  3. API and data model (20 points). Defines the main API calls with their inputs and outputs, and a data model whose keys and store fit the access patterns.
  4. Architecture (25 points). Chooses components that fit the requirements and explains how a request moves through them, for the main read and write paths.
  5. Bottlenecks and trade-offs (20 points). Names the real bottlenecks and failure points at this scale and how the design handles them, weighing the alternatives (consistency vs availability, push vs pull, cost).

How the practice works

Write your design in sections — functional and non-functional requirements, scale estimates, the API, the data model, the request flow and a deep dive on the bottlenecks — and pick the components to draw the architecture. A review scores it on the five areas above in under a minute, then you can defend it in an AI mock interview.

The reference design

Rate Limiting: the walkthrough, with animated figures.

Common questions

How do you answer "Design a Rate Limiter" in a system design interview?

Rate limiting caps how many requests a client may make in a period, such as 100 requests a minute per API key. Requests over the limit are refused, usually with HTTP 429 Too Many Requests. It protects servers from overload and abuse, keeps one client from starving others, and is built with algorithms such as token bucket, leaky bucket, fixed window and sliding window.

What scale is "Design a Rate Limiter" designed for here?

50,000 requests a second across 20 API servers; limits like 100 requests a minute per user. Your estimates turn that into requests per second, storage and bandwidth before you pick components.

How is a design scored?

Out of 100 on a fixed rubric: requirements (20), scale estimates (15), api and data model (20), architecture (25), bottlenecks and trade-offs (20). The review names what the design covered and what it missed, then points to the reference design.

Is "Design a Rate Limiter" a good first system design problem?

Yes. It is rated beginner: few components, a clear read and write path, and the classic estimates every later problem builds on.

Do I need an account?

Not to read the problem or the reference design. Getting your design reviewed needs a free account.

More design problems