System Design: Trade-offs, Not Templates

I put this together while prepping for system design rounds myself. Most material out there hands you one diagram and calls it the answer, but real interviews push on the trade-offs: what breaks when traffic goes 10x, and why you picked this approach over the others.

So I wrote it the way I actually wanted to study. Every question is answered as a set of competing trade-offs with a clear reason for the choice, and every choice is backed by real, buildable code instead of just boxes and arrows.

What is inside. 15 questions, each with the same structure:

• A trade-off matrix of the real options and when each one wins
• A clear reason for the chosen approach, so you can defend a different one when the constraints change
• A "what breaks at 10x" section for the scaling failure modes most resources skip
• A strong-answer versus weak-answer breakdown of what actually separates a hire from a no-hire
• Real code: Kafka listeners, partition keys, SQL schemas, Redis and Lua scripts, and AWS infra

The 15 questions: Rate Limiter, URL Shortener, Notification Fan-out, Distributed Cache, Chat and Messaging, News Feed, Search Autocomplete, Distributed Job Scheduler, Payment and Ledger, API Gateway with OAuth2 and JWT, Video Streaming, Web Crawler, Ride-Sharing Match, Dynamo-style Key-Value Store, and File Storage and Sync.

Who it is for: backend engineers prepping for higher-bar system design rounds who would rather reason from first principles than memorize a template.

How to use it: pick a question, write your own answer first, then compare against the matrix. The value is in seeing where your reasoning diverges.

If it helps you prep, that is exactly what I hoped for. Feedback and ideas for new questions are always welcome.