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Lesson 1 of 15

33. Design a Notification System

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The interview prompt
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33.1 What do you ask first?
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33.2 How big is this, really?
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33.3 What are the doors?
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33.4 What do we store?
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33.5 What does the whole system look like?
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33.6 Why one queue per channel?
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33.7 How do retries and de-duplication work?
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33.8 How do user preferences work?
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33.9 What makes OTPs special?
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33.10 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
15
Code lab
33. Design a Notification System

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.

The running example throughout is the one-time password (OTP): the six digits you wait for when you log in. It's the notification that can't be late, can't be sent twice, and can't leak.

The prompt

"Design a notification system for our app. Other teams should be able to send push notifications, SMS and emails to users: order updates, marketing, and login OTPs."

Stop here. Start your timer.