Rate Limiter System Design Interview Question (Solved)

This question was asked to me on multiple interviews

System design interviews often test your ability to solve problems that balance performance, scalability, and correctness. One of the most common questions I’ve encountered is:

“How would you design a Rate Limiter?”

I’ve been asked this exact question multiple times, and each time the interviewer wanted to see how I approached it systematically.

The rate limiter is not just an academic problem—it’s at the heart of many real systems. APIs, login attempts, payment systems, and messaging platforms all use rate limiting to prevent abuse, control costs, and ensure fairness among users.

In this article, I’ll walk you through the problem, the key requirements, different design approaches, and show you code examples (including the simple timestamp array method I used in interviews).

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What is a Rate Limiter?

A Rate Limiter is a system component that restricts the number of actions a user (or client) can perform in a given timeframe.

Examples:

• API Gateway: Only allow 100 requests per user per minute.

• Login System: Allow only 5 failed attempts in 10 minutes.

• Messaging App: Prevent users from sending more than 20 messages per second.

If users exceed these limits, the system should block their requests (often returning an HTTP status code 429 Too Many Requests).

Here is a nice diagram from ByteByteGo which shows Rate Limiter in action:

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Key Requirements in Interviews

When designing a rate limiter, interviewers usually want to see if you can handle: