01Ad Click AggregatorDesign an ad click aggregation system that ingests millions of click events per second and serves near-real-time plus historical aggregate metrics to advertisers.MetaStreaming Windowed Aggregation Idempotency Key+1Free guide
02DropboxDesign Dropbox — a cloud file storage and synchronization service where users upload large files and keep them in sync across devices and shared folders.—Chunked Transfer + Content-Hash Dedup Conflict-Free Collaboration (OT/CRDT)+1Free guide
03Facebook MessengerDesign Facebook Messenger — a real-time chat system delivering messages with low latency, ordering guarantees, and multi-device sync at massive scale.MetaPersistent Connection Management Pub/Sub Fan-out+1Free guide
04TicketmasterDesign Ticketmaster — an event ticketing system that sells reserved seats under extreme flash-sale contention without ever double-selling a seat.—Reservation Hold with TTL Virtual Queue / Admission Control+2Free guide
05TinderDesign Tinder — a location-based dating app with swipe feeds, mutual-match detection, and recommendation of nearby profiles.—Geospatial Indexing Probabilistic Data Structures+1Free guide
06Top K Songs (Spotify)Design Spotify's Top-K most-played songs feature — compute top charts over sliding time windows at listening-event scale.Spotify / MetaStreaming Windowed Aggregation Probabilistic Data Structures+2Free guide
07Uber (Ride Hailing)Design Uber — a ride-hailing platform matching riders to nearby drivers in real time, with live location tracking, pricing, and trip lifecycle management.UberGeospatial Indexing Optimistic Concurrency Control+1Free guide
08Web CrawlerDesign a web-scale crawler that politely fetches and indexes billions of pages, deduplicates content, and keeps the corpus fresh.GoogleProbabilistic Data Structures Async Job Queue+2Free guide
09URL ShortenerDesign a scalable URL shortener for 100M daily active users.InstagramCache-Aside and Stampede Control Sharding and Hot-Key MitigationFree guide
10LLM Inference GatewayDesign an LLM inference gateway that sits between internal product teams and multiple model providers/backends, handling routing, streaming, quotas, and safety controls at company scale.OpenAI / AnthropicMulti-Provider Routing / Failover Virtual Queue / Admission Control+2Free guide
11AI Customer Support AgentDesign an AI customer support agent for a SaaS product.—Agentic Tool-Calling Loop RAG Grounding and Citation+2Free guide

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12AI Agent SystemDesign a production AI agent application platform that accepts user goals, manages context and memory, calls tools safely, pauses for human review when needed, and improves through traces and evals.—Agentic Tool-Calling Loop Human-in-the-Loop Review+1Preview
13Auction (eBay)Design an eBay-style auction system — bidders compete on live listings under heavy contention near closing time, and the system must never accept a losing bid as the winner or let an auction close...eBayOptimistic Concurrency Control State Machine + Idempotent Transitions+1Preview
14Chat AppDesign the client-side architecture of a WhatsApp-style mobile messenger — connection handling, message state, offline storage, and encryption all have to survive a flaky mobile network.DiscordPersistent Connection Management State Machine + Idempotent Transitions+1Preview
15Chat ServiceDesign the backend infrastructure behind a chat product — stateful connection servers, message storage, and group fan-out — for hundreds of millions of concurrent users.DiscordPersistent Connection Management Pub/Sub Fan-out+1Preview
16Collaborative ShoppingDesign a collaborative shopping experience where multiple people share a single cart in real time, add and remove items concurrently, and one designated participant checks out on behalf of the group.—Conflict-Free Collaboration (OT/CRDT)Reservation Hold with TTL+2Preview
17Coupons and Deals PlatformDesign a coupons and deals platform where users browse and search a large, frequently changing catalog of deals and redeem coupon codes exactly once per limit, without ever letting stale prices or...—Cache-Aside and Stampede Control Idempotency Key+1Preview
18Donation PlatformDesign a donation platform like GoFundMe — a campaign can go viral overnight and receive a massive spike of small donations, and every dollar must be captured, refundable, and reflected in an...—Virtual Queue / Admission Control Async Job Queue+1Preview
19Facebook Post SearchDesign search over a social feed — let users find posts by keyword in near real time while respecting each post’s privacy/visibility settings.MetaInverted Index Search Access Control Filtering (ACL)+1Preview
20Flash SaleDesign a flash sale system where a limited quantity of a hot item goes on sale at a fixed instant and receives a 1000x traffic spike in seconds — inventory must never oversell, and most users should...—Atomic Counter Inventory Virtual Queue / Admission Control+2Preview
21Flight Booking SystemDesign a flight booking system that lets travelers search and book multi-leg itineraries across airlines, reserving fare-class seat inventory and issuing tickets without double-selling a seat or...DuffelReservation Hold with TTL Saga / Compensating Transaction+1Preview
22Game LeaderboardDesign a real-time game leaderboard that ranks millions of players by score, supports regional and time-windowed boards, and serves both top-N and a given player’s rank cheaply.StravaTop-K Ranking Sharding and Hot-Key Mitigation+1Preview
23Google DocDesign Google Docs’ real-time collaborative editing — multiple users typing in the same document simultaneously and seeing each other’s edits converge correctly.GoogleConflict-Free Collaboration (OT/CRDT)Persistent Connection Management+1Preview
24Google DriveDesign Google Drive as a permissioned file platform — folder hierarchies, shared drives, and access control that scale across billions of files and third-party integrations.GoogleAccess Control Filtering (ACL)Metadata/Data Separation+1Preview
25Hotel Reservation (Booking.com)Design Booking.com — a hotel marketplace aggregating live availability and pricing from thousands of independent supplier hotels, where search results must stay fast even though the underlying...Booking.comMulti-Provider Routing / Failover Cache-Aside and Stampede Control+2Preview
26Hotel ReservationDesign a single-chain hotel reservation engine — travelers search date-range availability across properties, place a hold, and confirm a booking without ever overbooking a room type past the chain’s...—Reservation Hold with TTL State Machine + Idempotent TransitionsPreview
27Job SchedulerDesign a distributed job scheduler (a cron-as-a-service system) that lets tenants register recurring and one-off jobs, guarantees each job fires close to its scheduled time, and survives worker and...AirbnbResource Scheduling and Preemption Distributed Lock / Lease Fencing+1Preview
28Metric Monitoring and AlertingDesign the metrics pipeline behind a monitoring platform — the path from an SDK on thousands of hosts emitting numeric measurements, through streaming pre-aggregation, into durable time-series...UberStreaming Windowed Aggregation Sharding and Hot-Key Mitigation+1Preview
29Moderation ServiceDesign a content moderation pipeline that screens user-submitted posts/media for policy violations, balancing posting latency against review accuracy at platform scale.LinkedInHuman-in-the-Loop Review Async Job Queue+1Preview
30Monitoring and AlertingDesign the alerting and observability layer of a monitoring platform — the system that evaluates rules against incoming signals, decides who gets paged, and keeps on-call humans from drowning in...—Streaming Windowed Aggregation Notification Delivery and RoutingPreview
31NetflixDesign Netflix’s video delivery system — stream a licensed, curated catalog to tens of millions of concurrent viewers worldwide with minimal buffering and consistent quality.—Edge Caching / CDN Multi-Provider Routing / Failover+1Preview
32Payment SystemDesign a payment system for an e-commerce product — money must move correctly between buyer, platform, and seller even when networks fail mid-transaction, and every dollar must be traceable after the...StripeDouble-Entry Ledger Idempotency Key+2Preview
33Recommendation SystemDesign a large-scale personalized recommendation system for feed/video/ads.Netflix / MetaMulti-Stage Ranking Funnel Two-Tower Retrieval+2Preview
34Restaurant ReservationDesign an OpenTable-style restaurant reservation system where diners book a time slot and party size at a restaurant, restaurants optimize table assignment, and no-shows are handled without requiring...YelpReservation Hold with TTL Notification Delivery and RoutingPreview
35Robinhood (Stock Trading)Design a retail stock trading platform like Robinhood that lets millions of users place buy/sell orders, tracks cash and positions with regulatory-grade accuracy, and streams live quotes without ever...RobinhoodDouble-Entry Ledger State Machine + Idempotent Transitions+2Preview
36Search Bar AutocompleteDesign the autocomplete/typeahead system behind a search bar that returns the top-K query suggestions for a partial prefix within a sub-50ms budget, at the scale of a major search engine.MetaPrecomputed Read View Top-K Ranking+2Preview
37Spend Management SystemDesign a corporate spend management platform like Brex or Ramp — every card swipe must be authorized or declined against a live policy engine in under two seconds, and every transaction must flow...BrexReal-Time Rules/Policy Engine Reconciliation / Background Repair+2Preview
38StripeDesign Stripe itself — a payments platform that other companies build on top of, exposing charge/webhook APIs to thousands of merchants while routing transactions across multiple underlying card...—Double-Entry Ledger Idempotency Key+2Preview
39Technical Deep DivePresent a technical deep dive of a system you personally built — the format used in senior/staff interview loops where you narrate one project end to end and defend your decisions under questioning.—Latency Budget Decomposition Capacity Estimation and Scale SizingPreview
40Ticketmaster (General Admission)Design Ticketmaster for general-admission events — there is no seat map, only a fixed capacity count per ticket tier, and the system must sell tickets correctly under extreme flash-sale contention...TicketmasterAtomic Counter Inventory Virtual Queue / Admission Control+1Preview
41TikTokDesign TikTok’s For You feed — rank and serve an endless short-video feed personalized per user without relying on who they follow.ByteDanceMulti-Stage Ranking Funnel Precomputed Read View+1Preview
42WebhookDesign a webhook delivery platform (Stripe-webhooks-style) that reliably delivers event notifications to thousands of third-party consumer endpoints under retry, ordering, and security constraints.StripeIdempotency Key Retry with Backoff + Dead-Letter+1Preview
43YelpDesign Yelp — a local business search and review platform that returns nearby, filterable business results ranked by relevance and rating.—Geospatial Indexing Cache-Aside and Stampede Control+1Preview
44YouTubeDesign YouTube’s upload and processing pipeline — accept massive volumes of user-generated video uploads and transform them into a playable, multi-resolution catalog.—Chunked Transfer + Content-Hash Dedup Async Job Queue+2Preview
45RAG Knowledge AssistantDesign an enterprise RAG knowledge assistant that answers employee questions over internal documents (wikis, tickets, contracts) with grounded, cited answers while respecting per-document access...OpenAIRAG Grounding and Citation ANN Vector Index+2Preview
46AI Training ClusterDesign the scheduler for a large GPU/TPU training cluster shared by multiple teams running distributed training jobs, handling gang scheduling, preemption, and failure recovery.MetaResource Scheduling and Preemption Rate Limiting / QuotaPreview
47MCP Tool GatewayDesign an MCP-style tool gateway that lets AI agents discover and invoke enterprise tools (CRM writes, ticketing, internal APIs) safely, with per-user authorization and auditability.AnthropicAgentic Tool-Calling Loop Access Control Filtering (ACL)+2Preview
48Ad Ranking Evaluation FrameworkDesign an evaluation framework for an ad ranking system.MetaOffline/Online Evaluation and A/B Testing Bias Correction (Position/Counterfactual)Preview
49Rate LimiterDesign a distributed API rate limiter for a public API platform.CloudflareRate Limiting / Quota Sharding and Hot-Key MitigationPreview
50Notification SystemDesign a multi-channel notification system that sends email, push, SMS, and in-app notifications on behalf of many internal services, respecting user preferences and delivery guarantees.UberNotification Delivery and Routing Idempotency Key+1Preview
51ML Feature StoreDesign an ML feature store that serves the same features to both offline model training and low-latency online inference for a company like Uber.UberFeature Store / Point-in-Time Correctness Async Job QueuePreview
52Vector DatabaseDesign the storage and query engine of a vector database that indexes billions of embeddings for approximate nearest-neighbor search.Meta (Faiss)ANN Vector Index Sharding and Hot-Key MitigationPreview
53ML Model Serving PlatformDesign a general-purpose ML model serving platform for non-LLM prediction serving used by many teams.NVIDIAFeature Store / Point-in-Time Correctness Elastic Worker AutoscalingPreview
54News Feed RankingDesign the ranking system for a social news feed like Instagram or Facebook.Meta (Instagram)Multi-Stage Ranking Funnel Two-Tower Retrieval+2Preview
55Fraud DetectionDesign a real-time ML fraud/risk scoring system that scores payment transactions in under 100 ms.StripeReal-Time Rules/Policy Engine Streaming Windowed Aggregation+2Preview
56ETA PredictionDesign an ETA prediction service that returns arrival-time estimates for millions of trips per second.UberFeature Store / Point-in-Time Correctness Latency Budget Decomposition+1Preview
57Visual / Multimodal SearchDesign a visual and multimodal search system that retrieves visually similar images (and text-to-image matches) from a catalog of billions.PinterestTwo-Tower Retrieval ANN Vector Index+1Preview
58LLM Guardrails PipelineDesign a guardrails pipeline that wraps an LLM application with layered input and output safety.NVIDIA / MetaLLM Safety Guardrails Real-Time Rules/Policy EnginePreview
59IoT Device PlatformDesign a platform managing ~10 million connected devices over MQTT: connection lifecycle and disconnect detection, telemetry ingest at fleet scale, and — the signature problem — reliably delivering...—Persistent Connection Management Notification Delivery and Routing+2Preview
60Distributed Key-Value StoreDesign a Dynamo-style highly available key-value store: partition keys across nodes with consistent hashing, replicate each key to N nodes, and let every operation tune its own consistency with...—Quorum Replication and Failover Sharding and Hot-Key Mitigation+1Preview
61LLM Evaluation PlatformDesign the evaluation platform for an organization shipping LLM features: manage a versioned golden set (stratified production samples, an adversarial library, constructed edge cases, and replays of...—Offline/Online Evaluation and A/B Testing Bias Correction (Position/Counterfactual)+1Preview
62Real-time Voice AI AgentDesign a real-time voice AI agent platform for phone support and voice assistants: the complete loop from the user speaking to the agent speaking back.—Latency Budget Decomposition Persistent Connection Management+1Preview
63Stock ExchangeDesign the core trading system of a stock exchange: accept orders into a central limit order book, match at price-time priority, and when your own book cannot complete an order, finish the job across...—State Machine + Idempotent Transitions Idempotency Key+2Preview
64Distributed CacheDesign a distributed in-memory cache service at Redis/Memcached scale that serves roughly 1M QPS at p99 under 5ms.MetaCache-Aside and Stampede Control Sharding and Hot-Key Mitigation+1Preview
65Distributed Message QueueDesign a Kafka-style durable, partitioned log broker that ingests high-throughput event streams.Apache Kafka (LinkedIn)Sharding and Hot-Key Mitigation Quorum Replication and FailoverPreview
66Object StorageDesign an S3-like object storage service that stores billions of immutable blobs durably and cheaply.Amazon / BackblazeMetadata/Data Separation Quorum Replication and Failover+1Preview
67Food Delivery MarketplaceDesign a DoorDash-style food delivery marketplace connecting consumers, Dashers, and merchants.UberState Machine + Idempotent Transitions Geospatial Indexing+1Preview
68Real-time Multiplayer GameDesign an authoritative server for a real-time multiplayer game.Riot GamesPersistent Connection Management Optimistic Local Update + ReconciliationPreview
69Live CommentsDesign a live comments system that fans out comments from one live event to millions of concurrent viewers in real time.MetaPub/Sub Fan-out Sharding and Hot-Key Mitigation+2Preview
70Online JudgeDesign a LeetCode-style online judge that safely runs untrusted user code at scale.Google / AmazonAsync Job Queue Sandboxed Execution+1Preview