Question 1

Token bucket vs sliding window vs fixed window — which rate limiter when?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what an HTTP request is; no rate limiting background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output).

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All behaviour shown comes from running the example; the **Ver

Question 2

How would you design a rate limiter (token bucket vs leaky bucket)?

Accepted Answer

## Step 1: Use Cases, Constraints, Assumptions, Back-of-Envelope

### Use cases
We'll scope to handle only the core flows that expose the bottlenecks; everything else is deferred.

- **User** calls API — each request checks limiter (user_id / IP / API key) against budget (e.g., 100/min), 429 if over.
- **Service** refills — token bucket adds tokens at fixed rate, allows bursts up to capacity; leaky bucket drains at constant rate (smooth).
- **Service** windows — fixed vs sliding window counters

Question 3

How do you build a sliding-window rate limiter with Redis + Lua?

Accepted Answer

**Target Audience:** Backend developers preparing for system design interviews who have used Redis as a cache. No Lua knowledge assumed.
**Difficulty:** Hard
**Verified with:** Redis 7.0.15 (the Lua script, run with `redis-cli`), plus the simulation in Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output).

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer ex

Question 4

How would you design a rate limiter?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what an API and a load balancer are; the algorithms are covered on sibling pages and only summarised here.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). Memory figures were measured on Redis 7.0.15.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact wo

SYSTEM DESIGNDECODED.

Medium

Token bucket vs sliding window vs fixed window — which rate limiter when?

How would you design a rate limiter (token bucket vs leaky bucket)?

Hard

How do you build a sliding-window rate limiter with Redis + Lua?

How would you design a rate limiter?