Art of Codeartofcode
Lesson 14 of 15

33. Design a Notification System

1
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?
6
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?
10
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
13
Variations to practise
14
Sources
15
Code lab
33. Design a Notification System

Sources

3 min read+10 XP

Source notes

  • Gmail and Yahoo bulk sender requirements (from February 2024: SPF, DKIM and DMARC authentication; one-click unsubscribe processed within two days; spam complaint rate below 0.3%; Google defines a bulk sender as more than 5,000 messages a day to Gmail addresses): Google and Yahoo sender guidelines, as summarised by email providers; checked 2026-10-08.
  • SMS/PSTN one-time passcodes classified as a "restricted" authenticator: NIST SP 800-63B (Digital Identity Guidelines), checked 2026-10-08.
  • APNs and FCM payload limits and delivery behaviour: see Chapter 21's source notes.
  • All traffic numbers, peak factors and provider rates are interview assumptions agreed in Step 1, not figures for any real service. Arithmetic computed 2026-10-08.
  • Queue isolation, failover with de-duplication, preference decisions and OTP rules: tested 2026-10-08 (code/ch33_notifications.py, all checks pass).