System Design Questions - Interviewers Actually Ask

Here’s the thing about system design resources.

Most of them show you the final answer.

The perfect architecture diagram.
Load balancer. Microservices. Cache. Database.
Clean arrows. Labeled boxes.

And you think:

“Great. I’ll memorize this.”

But when you’re in a system design interview -
or worse, in a production incident -

you realize something uncomfortable.

You memorized what to build,
not why it works.

You don’t know:

• which trade-offs actually matter
• what breaks first under pressure
• why the obvious solution quietly fails at scale

Because no one showed you the obvious solution failing.

This Book Is Different

This book does not give you polished answers.

It gives you failure.

Every chapter starts with:

• a feature that looks simple
• a design that feels obvious
• an approach you’ve probably seen before

And then we break it.

We stress it.
We add time.
We add retries, scale, concurrency, partial failures.

Until you’re forced to answer the real questions:

• Why does this design work?
• When does it stop working?
• What assumption just killed it?

Who This Is For

This book is for you if:

• you already know the basics of system design
• you’re tired of memorizing diagrams
• you want to think like an interviewer or a senior engineer
• you’ve felt stuck on follow-ups you didn’t see coming

This is suitable for beginner’s.
And what comes after that.

What You’ll Learn

• How interviewers actually evaluate system design answers
• Why “good designs” still fail in real systems
• How to reason about trade-offs under uncertainty
• How to defend your design when assumptions are challenged
• How to think beyond boxes and arrows

If you’ve ever walked out of an interview thinking:

“I knew the concepts… so why did I still struggle?”

This book is for you.

These topics will be covered in detail, clearly showing what works and what doesn’t.

Table of Contents

Introduction

1. How would you design a system to send 100,000 notifications?
2. How do you log 1M events per second without slowing the app?
3. How would you compute trending posts every minute?
4. How do you handle 10,000 concurrent users?
5. How would you design search autocomplete for 100M queries per day?
6. How do you handle 10,000 concurrent database connections?
7. How do you ensure exactly-once payment processing?
8. How would you design a recommendation engine for millions of users?
9. How would you design a key-value store for 100M reads per second?
10. How do you migrate 10TB of data with zero downtime?
11. How do you build a live streaming platform for millions of viewers?
12. How would you design a real-time leaderboard for 100K players?
13. How would you design a URL shortener for 500M redirects per day?
14. How do you aggregate logs from 10,000 servers without data loss?
15. How would you design a tagging system for 100 million posts?
16. How would you store and display edit history for millions of posts?
17. How would you design Instagram Stories that expire after 24 hours?
18. How would you design a ride-sharing matching system like Uber?
19. How would you design a load balancer for 1M requests per second?
20. How would you design a real-time collaborative editor like Google Docs?
21. How would you design delayed scheduling for 200K precise reminders?
22. How would you design a news feed for 1 billion users?
23. How would you design a web crawler for 1 billion pages?
24. How would you build a rate limiter for 1M requests per second?
25. How would you design file sync for 100M files across devices?
26. How would you design a ticket booking system for 100K concurrent users?
27. How would you design proximity search for 100M points of interest?
28. How would you design a video upload pipeline for 500 hours per minute?
29. How would you design a distributed cache for 1M hits per second?
30. How would you design a fraud detection system for 10K transactions per second?
31. How do you design systems knowing requirements will change?
32. How do you design systems where data must disappear everywhere?
33. How do you debug a bug that only appears at peak traffic?
34. How do you design systems where correctness is optional but cost is not?
35. How would you design a stock trading system?

Bonus Content

• 10 Questions to Ask Before Designing Any System
• Backend Tech Concepts – One-Liner Explanations
• 25 Concepts You’ll Only Learn After a Year in Production
• System Design Patterns – Quick Reference

35 chapters + bonus content covering notifications, caching, search, real-time systems, payments, architecture patterns, and more.