System Design Interviews

When AI writes the code, system design is the job

AI can emit a working service in seconds. It cannot decide push versus pull fan-out at 500M users, or which consistency you trade away under load, and that judgment is what interviews test now. 27 problems, each worked end to end the way a Staff engineer explains them.

The bar has moved at every level: an entry-level interview and a staff design review both expect you to defend real depth, not generic boxes and arrows.

Start with the classics like the URL shortener, then work through rate limiters, distributed caches, news feeds, chat and messaging, payment systems, notification services, and ride-sharing. Every problem includes architecture diagrams, back-of-envelope math, and the follow-ups interviewers actually ask. New here? Follow the HLD Playbooks for a structured prep path.

Case studies

27 problems

Every problem

Diagrams + math

Depth

Read & write paths

Follow-ups

Real interview Qs

Why strong engineers still get rejected

It is rarely that you cannot name the components. It is these four gaps, and every case study here is built to close them.

They jump straight to boxes and arrows, before pinning the scale.

Every case study opens by clarifying requirements, SLOs, and back-of-envelope numbers, the way the news feed starts at 500M users and 58K reads per second.

They name components but dodge the one decision that actually matters.

Each problem makes the core call out loud and defends the trade-off, like push versus pull fan-out, instead of just adding a cache.

They have no numbers, so 'will it scale?' turns into a guess.

Back-of-envelope math in every problem, so capacity, QPS, and storage are estimates you can defend under pressure.

They freeze on the follow-ups: what breaks, where is the bottleneck, how do you 10x it?

Each case study ends with the bottlenecks, the failure modes, and the exact follow-ups interviewers ask next.

Why strong engineers still get rejected

All system design case studies

Design a Durable Object Runtime Like Cloudflare Durable Objects

System Design: LLM Inference Serving

System Design: LLM Safety Pipeline

System Design: LLM Evaluation Platform

System Design: Distributed 70B Training

System Design: ML Model Serving Platform

System Design: ChatGPT End-to-End (Inference, Memory, RAG, Streaming, RLHF)

System Design: Maps Platform (Navigation, Routing, and Map Rendering)

System Design: GitHub (200M Repos, Git Object Storage, Sparse Trigram Code Search, Per-Job VM CI)

System Design: Online Auction (50K Bids/sec, Effectively-Once Settlement, Anti-Sniping)

System Design: Job Scheduler (10M Jobs/day, DAG Dependencies, Effectively-Once Execution)

System Design: LeetCode (Code Sandbox, Container Isolation, Real-Time Contests)

System Design: URL Shortener (10B Short URLs, 100K Redirects/sec)

System Design: Ad Click Aggregator (10B Clicks/day, Lambda Architecture, Fraud Detection)

System Design: Ad Exchange (Real-Time Bidding, Sub-100ms Auctions, DSP/SSP, Impression Serving)

System Design: E-Commerce Flash Sales (10M Users, Coupon System, One-Per-User Enforcement)

System Design: News Aggregator (100K Sources, Dedup, Personalized Ranking)

System Design: Dropbox (File Sync, Chunking, and Deduplication)

System Design: Object Storage (Erasure Coding, Flat Namespace, and Exabyte Scale)

System Design: AI Software Engineer (From Autocomplete to Autonomous App Builder)

RAG and LLM Platform at Scale: Ingestion, Retrieval, Generation, and Evaluation for 10M Queries/Day

Building a Multi-Tenant AI Agent Platform for Restaurant Intelligence

System Design: Real-Time Collaborative Editor

Observability Platform at Scale - Metrics, Traces, Logs, and Profiles

System Design: Notification Platform, 100M Notifications/Second

Building a Fail-Safe, Scalable Top-K Streaming System

Designing a Multi-Tenant, Globally Scalable Tagging System

System design interview FAQ

01How do I prepare for a system design interview?

02What system design questions are asked most often?

03How is a system design interview scored?

04How long should I spend on each part of the interview?

05What level are these case studies for?

06Are these system design case studies free?

Build the judgment AI can't fake

Why strong engineers still get rejected

All system design case studies

Design a Durable Object Runtime Like Cloudflare Durable Objects

System Design: LLM Inference Serving

System Design: LLM Safety Pipeline

System Design: LLM Evaluation Platform

System Design: Distributed 70B Training

System Design: ML Model Serving Platform

System Design: ChatGPT End-to-End (Inference, Memory, RAG, Streaming, RLHF)

System Design: Maps Platform (Navigation, Routing, and Map Rendering)

System Design: GitHub (200M Repos, Git Object Storage, Sparse Trigram Code Search, Per-Job VM CI)

System Design: Online Auction (50K Bids/sec, Effectively-Once Settlement, Anti-Sniping)

System Design: Job Scheduler (10M Jobs/day, DAG Dependencies, Effectively-Once Execution)

System Design: LeetCode (Code Sandbox, Container Isolation, Real-Time Contests)

System Design: URL Shortener (10B Short URLs, 100K Redirects/sec)

System Design: Ad Click Aggregator (10B Clicks/day, Lambda Architecture, Fraud Detection)

System Design: Ad Exchange (Real-Time Bidding, Sub-100ms Auctions, DSP/SSP, Impression Serving)

System Design: E-Commerce Flash Sales (10M Users, Coupon System, One-Per-User Enforcement)

System Design: News Aggregator (100K Sources, Dedup, Personalized Ranking)

System Design: Dropbox (File Sync, Chunking, and Deduplication)

System Design: Object Storage (Erasure Coding, Flat Namespace, and Exabyte Scale)

System Design: AI Software Engineer (From Autocomplete to Autonomous App Builder)

RAG and LLM Platform at Scale: Ingestion, Retrieval, Generation, and Evaluation for 10M Queries/Day

Building a Multi-Tenant AI Agent Platform for Restaurant Intelligence

System Design: Real-Time Collaborative Editor

Observability Platform at Scale - Metrics, Traces, Logs, and Profiles

System Design: Notification Platform, 100M Notifications/Second

Building a Fail-Safe, Scalable Top-K Streaming System

Designing a Multi-Tenant, Globally Scalable Tagging System

System design interview FAQ

01How do I prepare for a system design interview?

02What system design questions are asked most often?

03How is a system design interview scored?

04How long should I spend on each part of the interview?

05What level are these case studies for?

06Are these system design case studies free?

Build the judgment AI can't fake