Every design below has an animated diagram, step-by-step walkthrough, component swap, and in-browser stress simulator. Click any title to read the full walkthrough, or open the live interactive version.
These are the same problems asked at Google, Meta, Amazon, Netflix, Apple, Uber, Stripe, and Airbnb. Every page has a step-by-step walkthrough, capacity estimates, and an in-browser stress simulator — no other free resource combines all three.
New here? Start with URL Shortener (classic warm-up), then WhatsApp Messaging (concurrency and fan-out), then Payment System (ACID + idempotency). Or follow the structured 8-week roadmap →
Problem: Design a real-time ad click event aggregation service that powers advertiser reporting and near-real-time budgeting.
Key components: Ad Client (SDK / Pixel), Edge Ingest Service, Click Kafka, Stream Aggregator (Flink), OLAP Store (ClickHouse)
Problem: Design a distributed KV cluster that spreads keys across N shards, routes requests to the owning shard in O(1), replicates for durability, and rebalances with minimal key movement when nodes are added or removed.
Key components: Client SDK, Coordinator, Consistent-Hash Ring, Shard A (Storage Node), Shard B (Storage Node)
Problem: Design a digital wallet (PayPal/Venmo-style) that supports top-up from bank, ACID peer-to-peer transfers, and fast balance reads.
Key components: Client (Mobile / Web), Load Balancer, Wallet API, Auth Service, Balance Service
Problem: Design a distributed email service like Gmail supporting SMTP send/receive, spam filtering, full-text search, and attachments.
Key components: Client (Web / Mobile / IMAP), Load Balancer, Mail Submission API, Spam / Anti-abuse Filter, Outbound SMTP Relay (MTA)
Problem: Design an always-writable, horizontally scaled key-value store with tunable consistency: N replicas, configurable W/R quorums, vector-clock conflict resolution, gossip membership, and anti-entropy repair.
Key components: Client SDK, Coordinator, Consistent-Hash Ring, Replica 1 (Storage Node), Replica 2 (Storage Node)
Problem: Design a distributed, durable, high-throughput message queue like Apache Kafka.
Key components: Producer Client, Bootstrap Load Balancer, Broker (Partition Leader), Partition Log (on-disk), Replica Broker (Follower)
Problem: Design a distributed rate limiter that protects APIs at 1M+ RPS with per-user / per-IP / per-API quotas.
Key components: Client (3rd-party / internal), Load Balancer, API Gateway, Rate Limiter Middleware, Redis (Token Bucket Store)
Problem: Design a system that generates globally unique, roughly time-sortable 64-bit IDs at very high throughput across many machines, without a central bottleneck on the write path.
Key components: Client (Service or User Request), Application Server, Snowflake Generator (in-process), Coordinator (ZooKeeper / etcd), ID Usage DB (downstream consumer)
Problem: Design a cloud file storage and sync service like Google Drive, Dropbox, or OneDrive.
Key components: Client (Desktop Sync / Web / Mobile), Load Balancer, Drive API (Metadata Service), Metadata DB, Metadata Cache
Problem: Design a web-scale map service that renders tiled maps, searches places, and computes routes from A to B.
Key components: Client (Browser / Mobile SDK), CDN Edge (Tile POP), Load Balancer, Maps API Gateway, Tile Service
Problem: Design a hotel reservation system (Booking.com / Expedia / direct hotel chain) that takes rooms from search, through hold, payment, and confirmed booking, without overselling.
Key components: Client (Web / Mobile), Load Balancer, Reservation API, Search Service, Inventory Service
Problem: Design a metrics monitoring and alerting system for a fleet of ~10,000 servers and thousands of services, Prometheus/Grafana-style.
Key components: Operator Browser, Instrumented App, Push Gateway, Scrape Collector, Metrics Kafka
Problem: Design a system like Facebook Nearby Friends / Snap Map that shows you which of your friends are nearby in near-real-time.
Key components: Client (iOS / Android), Connection Load Balancer, WebSocket Server, Location Publisher, Redis GEO
Problem: Design a news feed system that shows a user the latest posts from people they follow.
Key components: Client (iOS / Android / Web), Load Balancer, News Feed Service, Post Service (Feed Publishing), Post Store
Problem: Design a system that delivers notifications across push (APNs/FCM), SMS, and email channels at scale, with reliable retries, rate limiting, per-user preferences, and minute-scale latency for transactional messages.
Key components: Services / Clients (producers), Notification Servers, Cache (tokens / templates / settings), Notification DB (users / devices / settings / templates), Rate Limiter
Problem: Design a payment system that charges customers via external PSPs (Stripe/Braintree), keeps an auditable ledger, supports refunds, and reconciles with the PSP daily.
Key components: Client (Merchant checkout / mobile app), Load Balancer, Payment Service API, Idempotency Store (Redis), Payment Orchestrator
Problem: Design a proximity / 'find businesses near me' service like Yelp, Google Maps nearby, or DoorDash restaurant search.
Key components: Client (Mobile App / Web), Load Balancer, Proximity API, Location Service, Geohash Index
Problem: Design a real-time global leaderboard for a mobile game: top-100 worldwide, top-K by region, and a given player's rank with surrounding neighbors, all updated in near real-time.
Key components: Game Client, Load Balancer, Score Ingest API, Leaderboard Read API, Redis Sorted Set (ZSET)
Problem: Design a distributed, highly durable object storage service like Amazon S3 supporting PUT / GET / DELETE of arbitrary-size objects with versioning.
Key components: Client (SDK / CLI / Browser), Load Balancer, S3 REST API, Auth Service, Metadata Service (Index)
Problem: Evolve a single-server web app into a multi-tier, multi-region architecture that serves millions of users with low latency and high availability.
Key components: Client (Browser / Mobile), DNS (Route53 / Cloud DNS), CDN (CloudFront / Cloudflare), Load Balancer, Web Tier (Stateless App Servers)
Problem: Design a top-k search autocomplete system that returns the 5 most popular queries for a typed prefix within 100ms.
Key components: Client (Browser / Mobile), Load Balancer, API Servers, Filter Layer, Trie Cache
Problem: Design a stock exchange with a matching engine that processes limit orders, a market-data publisher that broadcasts book updates at 100K ticks/s, and a batched clearing/settlement pipeline.
Key components: Client / Broker, Load Balancer, Order Gateway, Order Validation / Risk, Matching Engine (per-symbol shard)
Problem: Design a real-time trending topics / hashtag system over a 500M-tweets/day firehose.
Key components: Client UI, Tweet Ingest API, Tweet Stream (Kafka), Stream Processor (Flink), CMS + Top-K State
Problem: Design a URL shortener like TinyURL or bit.ly.
Key components: Client (Browser / Mobile), Load Balancer, Shortener API, Unique ID Generator (base62 conversion), Redis Cache
Problem: Design a video upload, transcoding, and streaming platform like YouTube.
Key components: Client (Web / Mobile / TV), CDN Edge, Load Balancer, Video API, Metadata DB
Problem: Design a polite, scalable web crawler that ingests billions of URLs, respects robots.txt, and avoids both URL-level and content-level duplicates.
Key components: Seed URLs, URL Frontier (priority queue), HTML Downloader, DNS Resolver + Cache, Content Parser
Problem: Design a 1-to-1 and small group chat system (up to 100 members) with online presence, multi-device sync, and push notifications.
Key components: Client (iOS / Android / Web), Load Balancer, API Servers (stateless), Service Discovery (ZooKeeper), Chat Servers (stateful WebSocket)