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Lesson 1 of 15

32. Design a Rate Limiter as a Service

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The interview prompt
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32.1 What do you ask first?
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32.2 How big is this, really?
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32.3 What are the doors?
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32.4 What do we store?
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32.5 What does the whole system look like?
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32.6 How do you pick the algorithm?
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32.7 How do counters work in Redis?
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32.8 What race conditions do you have to handle?
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32.9 How do limits work across regions?
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32.10 What would you say in the last five minutes?
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Score yourself
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Variations to practise
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Sources
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Code lab
32. Design a Rate Limiter as a Service

The interview prompt

3 min read+10 XP

This chapter is a full interview round, played out step by step. The interviewer's lines are in quotes. At each step you'll see what a strong candidate says, the pushback the interviewer throws, and how to answer it. At the end, grade yourself with the rubric.

How to use this chapter

  1. Read only the prompt below. Then stop.
  2. Set a 45-minute timer and do the whole round yourself, on paper or a whiteboard, out loud. Use the framework from Chapter 1: READ, then Draw, Dive, Defend.
  3. Then read the walkthrough and compare. Score yourself with the rubric at the end.

Chapter 25 taught the algorithms. This round tests whether you can turn them into a shared service that many teams depend on, at high traffic, without becoming the thing that takes everyone down.

The prompt

"Our company has dozens of services. Every team builds its own rate limiting, badly. Design a rate limiter that any team can use: they say what to limit and how much, and your system enforces it."

Stop here. Start your timer.