Design an API rate limiter for multiple users
FreeBackend systemEasy
- Beginner
- System design
- Rate limiting
Quick answer
This backend system interview question asks you to design an API rate limiter for multiple users. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Design an API rate limiter that allows each user a specific number of requests per time window and blocks or throttles requests when the limit is exceeded.
What must an API rate limiter for multiple users do?
- Enforce rate limits per user, API key, IP address, or endpoint.
- Support configurable limits such as requests per second, minute, or hour.
- Support different limits for free and premium users.
- Allow controlled bursts when using token bucket style limits.
- Return clear feedback when a request is blocked.
- Provide remaining quota and retry timing when possible.
- Support dynamic updates to rate limit policies.
How well must it do it?
- Rate limit checks should add very low latency.
- The system should work across multiple API servers.
- Limit checks should be accurate enough to prevent abuse.
- The system should avoid race conditions during concurrent requests.
- The system should remain available during partial Redis or network failures.
- The system should scale to many users and high request volume.
What scale should you design for?
Use these numbers in your estimates.
| Assumption | Estimate total users and active users before sizing counters |
|---|---|
| Assumption | Estimate API request QPS from active users and request frequency |
| Assumption | Peak traffic may be 3x to 10x average traffic |
| Assumption | Estimate Redis operations using request QPS and number of counter updates per request |
| Assumption | Estimate memory using active identities x counters x metadata size |
| Assumption | Hot users or API keys may create uneven load on rate limiter storage |
Key concepts
- Beginner
- System design
- Rate limiting
- Security
- Redis
- Token bucket
How you’ll practise
6 short steps · save and come back any time
- Requirements
- Estimate
- API and data
- High-level design
- Deep dives
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any backend system design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a backend system question. You design an API rate limiter for multiple users and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Easy, so it suits engineers who are new to backend system interviews. It is also a good warm-up before harder questions.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- Yes. It is one of the free questions. Sign up to practise it step by step. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team