Question guides

53 System Design questions, each tagged by pattern. Badged questions have the complete write-up—patterns, expected topics, self-check prompts, and common mistakes—free, no account needed. The rest have a quick summary here; open them in the app for the full guide.

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Ad Click Aggregator
Design Ad Click Aggregator.
- Streaming Aggregation Pattern
- Rate Limiter
AI Agent System
Design an AI agent platform that can answer user tasks by planning steps, calling tools, using short-term memory, and stopping safely when confidence is low.
- Agent loop
- Tool calling
- Memory
- Guardrails
Auction (eBay)
Design Auction (eBay).
Chat App
Design Chat App.
- Fan-out
Chat Service
Design Chat Service.
- Fan-out
- Pub-Sub
Collaborative Shopping
Design Collaborative Shopping.
- Real-time Updates Pattern
Coupons and Deals Platform
Design Coupons and Deals Platform.
- Read-Heavy Search Pattern
- Idempotent Redemption Pattern
Donation Platform
Design Donation Platform.
- Payment System
- Reconciliation
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Dropbox
Design Dropbox.
- File Sync Pattern
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Facebook Messenger
Design Facebook Messenger.
- Fan-out
- WebSocket Pattern
- Presence Pattern
Facebook Post Search
Design Facebook Post Search.
- Search Pattern
Flash Sale
Design Flash Sale.
- Reservation Pattern
Flight Booking System
Design Flight Booking System.
- Reservation Pattern
- Saga
Leaderboard
Design a game leaderboard that supports score updates, global and regional rankings, top-K reads, user rank lookup, season resets, and near real-time freshness.
- Top-K ranking
- Sorted set
- Hot leaderboard
- Seasonal aggregation
Google Doc
Design Google Doc.
- Collaborative Editing Pattern
Google Drive
Design Google Drive.
- File Sync Pattern
Hotel Reservation (Booking.com)
Design Hotel Reservation (Booking.com).
- Reservation Pattern
Hotel Reservation
Design Hotel Reservation.
- Reservation Pattern
Job Scheduler
Design Job Scheduler.
- Scheduler Pattern
- Rate Limiter
Leaderboard
Design a game leaderboard that supports score updates, global and regional rankings, top-K reads, user rank lookup, season resets, and near real-time freshness.
- Top-K ranking
- Sorted set
- Hot leaderboard
- Seasonal aggregation
Metric Monitoring and Alerting
Design Metric Monitoring and Alerting.
- Streaming Aggregation Pattern
- Rate Limiter
Moderation Service
Design Moderation Service.
- Moderation Pipeline Pattern
Monitoring and Alerting
Design Monitoring and Alerting.
- Pull vs Push
- Time Series Pattern
- Fan-out
Netflix
Design Netflix.
- Video Streaming Pattern
Payment System
Design Payment System.
- Ledger Pattern
- Idempotency Pattern
- Saga
- Reconciliation
Recommendation System
Design a large-scale personalized recommendation system for feed/video/ads.
- Multi-stage ranking
- Candidate retrieval
- Feature store
- Sequence features
- A/B testing
Restaurant Reservation
Design Restaurant Reservation.
- Reservation Pattern
Robinhood (Stock Trading)
Design Robinhood (Stock Trading).
- Ledger Pattern
Search Bar Autocomplete
Design Search Bar Autocomplete.
- Top-K Pattern
Spend Management System
Design Spend Management System.
- Ingestion Pipeline Pattern
- CDC-Outbox
Stripe
Design Stripe.
- Payment System
- Reconciliation
Technical Deep Dive
Design Technical Deep Dive.
- Project Deep Dive Pattern
Ticketmaster (General Admission)
Design Ticketmaster (General Admission).
- Reservation Pattern
- Inventory Pattern
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Ticketmaster
Design Ticketmaster.
- Reservation Pattern
TikTok
Design TikTok.
- Feed Pattern
- Video Pipeline Pattern
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Tinder
Design Tinder.
- Geo-Proximity Pattern
- Recommendation Pattern
- Pub-Sub Fanout Pattern
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Top K Songs (Spotify)
Design Top K Songs (Spotify).
- Top K Pattern
- Stream Processing Pattern
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Uber (Ride Hailing)
Design Uber (Ride Hailing).
- Geo-Proximity Pattern
- Reservation Pattern
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Web Crawler
Design Web Crawler.
- Crawler Pattern
Webhook
Design Webhook.
- Idempotency Pattern
Yelp
Design Yelp.
- Proximity Search Pattern
YouTube
Design YouTube.
- Video Pipeline Pattern
- Async Two-Phase Upload
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URL Shortener
Design a scalable URL shortener for 100M daily active users.
- Unique ID Generation
- Read-Heavy Caching
Uber
Design a ride-hailing system like Uber.
- Geo-Proximity Pattern
- Trip State Machine
- Real-Time Fan-Out
RAG Knowledge Assistant
Design an enterprise RAG knowledge assistant.
- RAG
- Hybrid search
- Vector index
- Grounding
- Reranking
LLM Inference Gateway
Design an LLM inference gateway for product teams.
- Model gateway
- Streaming inference
- Provider fallback
- Prompt guardrails
- Cost attribution
AI Training Cluster
Design an AI training and inference cluster scheduler for GPU/TPU capacity.
- GPU/TPU scheduling
- AI infrastructure
- Placement
- Checkpointing
- Power/cooling
MCP Tool Gateway
Design an MCP-style tool gateway that lets AI agents safely connect to enterprise tools.
- MCP
- Tool gateway
- OAuth consent
- Protected resource metadata
- Human-in-the-loop
- Prompt injection defense
Ad Ranking Evaluation Framework
Design an evaluation framework for an ad ranking system.
- Offline/online evaluation
- A/B testing
- Counterfactual evaluation
Rate Limiter
Design a distributed API rate limiter for a public API platform.
Search Autocomplete
Design search autocomplete.
Metric Monitoring
Design a metric monitoring and alerting system.
Notification System
Design a notification system that can send email, push, SMS, and in-app notifications.
- Fan-out queue
- Preference filtering
- Retry / DLQ
- Provider failover