🏗️ System Design — "Design X" Problems (Detailed)¶

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Goal: Classic "Design X" problems asked in FAANG / highload interviews. For each problem: scale, functional requirements, non-functional requirements, and the hard parts to discuss.

Format: Each problem is framed as an interview prompt, sized for a 30–60-minute whiteboard session. Rule: Start with clarifying questions, then back-of-the-envelope estimation, then APIs / data model, HLD diagram, and deep dives.

📑 Contents (by topic)¶

Total: 1000 detailed design problems + 50 OOD problems = 1050.

Birinchi qism — Klassik domenlar (1–202)¶

Ikkinchi qism — Industry-specific & Specialized (203–1000)¶

Bonus — Object-Oriented Design (50 OOD masalasi)¶

🧭 Universal Framework — Har bir masalada ishlatish¶

1️⃣  CLARIFY      → Functional / non-functional / scale (5 daqiqa)
2️⃣  ESTIMATE     → DAU, QPS, storage, bandwidth, memory (5 daqiqa)
3️⃣  API DESIGN   → Endpoints, contracts (5 daqiqa)
4️⃣  DATA MODEL   → Tables / collections / indexes (5 daqiqa)
5️⃣  HLD          → Boxes & arrows: client → LB → service → DB (10 daqiqa)
6️⃣  DEEP DIVE    → Hot parts: caching, sharding, replication, MQ (15 daqiqa)
7️⃣  TRADE-OFFS   → Justify choices, alternatives, scaling next 10x (5 daqiqa)

🌐 1. Social Media & Feed¶

1. Design Twitter (X)¶

• Scale: 500M MAU, 200M DAU, 6K tweets/sec write, 300K timeline reads/sec.
• Core features: Post tweet (≤280 chars), follow user, home timeline, user timeline, like, retweet, reply.
• Hard parts: Fanout (push vs pull vs hybrid), celebrity problem, timeline ranking, search across all tweets, trending topics.
• Discuss: How to deliver tweets to 1M followers? When does a write become 1M writes? How to cache home timelines?

2. Design Instagram¶

• Scale: 2B MAU, 500M daily uploads, average photo 2MB.
• Core features: Photo/video upload, follow, feed, stories, explore, comments, likes, DM (basic).
• Hard parts: Image storage and CDN, feed personalization, stories' 24h TTL, hot creators.
• Discuss: Pre-resize variants vs on-the-fly? How to fan out to 100M followers?

3. Design Facebook News Feed¶

• Scale: 3B MAU, ranking score per post.
• Core features: News feed ranking, friends' posts, ads insertion, reactions.
• Hard parts: Real-time ranking (EdgeRank-like), ads injection, graph queries for "friends of friends".
• Discuss: Push vs pull vs hybrid; how to inject ads without breaking ranking; edge cases (friend just posted but ranked low).

4. Design TikTok / For You Page¶

• Scale: 1B MAU, infinite-scroll video feed, average 30s watched per video.
• Core features: Personalized video feed, like/share/comment, follow, upload.
• Hard parts: Cold start for new users, ML ranking pipeline, deduplication, content moderation in real-time.
• Discuss: How does the FYP serve the next video before user finishes current? How to recommend without explicit follows?

5. Design Reddit¶

• Scale: 500M MAU, 100K subreddits, 50K comments/min on hot threads.
• Core features: Subreddit feed, post submission, threaded comments, voting, hot/top/new ranking.
• Hard parts: Comment tree storage at scale, vote ranking with vote-fuzzing for fairness, brigading detection.
• Discuss: How to render a 100K-comment thread? Materialized view per subreddit?

6. Design Pinterest¶

• Scale: 500M MAU, billions of pins, board-based discovery.
• Core features: Save pin, create board, follow boards, home feed, search by image.
• Hard parts: Image deduplication via perceptual hashing, board-level personalization, related pins via embeddings.
• Discuss: How to serve "more like this" in <100ms? Vector search architecture?

7. Design LinkedIn Feed¶

• Scale: 1B users, 50M DAU, B2B-focused content.
• Core features: Connection feed, recommended posts, "people you may know", articles, jobs.
• Hard parts: Graph distance computation for PYMK, mixed-content ranking, ad placement.
• Discuss: How to compute 2nd-degree connections at scale? Caching strategy for graph queries?

8. Design Tumblr / Medium¶

• Scale: 100M DAU, long-form posts, blogs.
• Core features: Publish article, follow blogs, recommendations, drafts, claps.
• Hard parts: Editor save & autosave, tag-based discovery, SEO-friendly URLs.
• Discuss: Markdown rendering pipeline, cache invalidation when author edits.

9. Design Tinder / Bumble¶

• Scale: 50M DAU, swipe-based matching, geo-filtered.
• Core features: Swipe left/right, match notification, chat after match, geo-radius filter.
• Hard parts: Recommendation engine, double-opt-in matching, undo swipe, fairness (ELO-like rating).
• Discuss: How to avoid showing same profile twice for 30 days? How to scale geo queries?

10. Design Quora / Stack Overflow¶

• Scale: 300M MAU, Q&A platform.
• Core features: Ask question, answer, upvote/downvote, follow topics, search.
• Hard parts: Answer ranking, duplicate detection, expertise scoring, spam filter.
• Discuss: Full-text search with relevance, how to detect duplicate questions?

11. Design Trending Topics (Twitter Trends)¶

• Scale: 100K topics tracked per region, real-time count.
• Core features: Detect rising hashtags, region-specific trends, time-window ranking.
• Hard parts: Heavy hitter detection (Count-Min Sketch), windowing (5min, 1h, 24h), spam/bot filtering.
• Discuss: Why approximate algorithms? How to deal with timezone biases?

12. Design "People You May Know"¶

• Scale: 1B users in social graph.
• Core features: Recommend 50 connections, refresh weekly.
• Hard parts: Friend-of-friend computation, graph storage (adjacency, edge list), feature engineering.
• Discuss: Precompute vs on-demand? Graph DB vs Hadoop batch job?

13. Design Notification Feed (Bell icon)¶

• Scale: 1B users, 10–100 notifications/day each.
• Core features: "X liked your post", "Y commented", real-time push, mark read.
• Hard parts: Aggregation ("3 people liked"), TTL, read state sync across devices.
• Discuss: Push vs pull on bell icon open? Where to aggregate?

14. Design Live Comments / Reactions on a Live Stream¶

• Scale: 1M concurrent viewers per stream, 10K reactions/sec.
• Core features: Comments scroll, reaction floats, real-time delivery.
• Hard parts: Fanout to 1M WebSockets, rate limiting per user, profanity filter.
• Discuss: Sampling at extreme scale (do we deliver every comment?), pub/sub architecture.

15. Design Stories (Instagram/Snapchat Stories)¶

• Scale: 500M DAU, 24h TTL per story.
• Core features: Post photo/video, view stories of friends, "seen by", auto-expire.
• Hard parts: TTL-driven storage (cold after 24h), seen-state sync, batched fanout.
• Discuss: Why immutable per-user story timelines? Background expiration job vs lazy delete?

💬 2. Messaging & Chat¶

16. Design WhatsApp / Messenger¶

• Scale: 2B users, 100B messages/day, 100M concurrent connections.
• Core features: 1-1 chat, group chat (up to 1024), delivery + read receipts, media, E2E encryption.
• Hard parts: WebSocket fanout, offline message queue, E2E key exchange (Signal protocol), multi-device sync.
• Discuss: Per-user inbox vs group inbox? How to handle a user with 10 devices?

17. Design Slack¶

• Scale: 30M DAU, multi-tenant workspaces, channels, threads.
• Core features: Channels, threads, DMs, file uploads, search, mentions.
• Hard parts: Workspace-level data isolation, per-channel scrollback, search per workspace.
• Discuss: How to store messages? Multi-tenant DB? How to render unread indicators?

18. Design Discord¶

• Scale: 200M MAU, voice + text, large guilds (250K members).
• Core features: Servers, channels, voice rooms, screen share, bots.
• Hard parts: Voice with WebRTC, large-server fanout, role-based permissions.
• Discuss: Centralized voice routing vs P2P; how to deliver mention to 250K members?

19. Design iMessage¶

• Scale: 1B+ Apple users, E2E.
• Core features: SMS fallback, multi-device, reactions, threads, edit/delete.
• Hard parts: APNs at scale, key management for new device, sync across devices without server seeing plaintext.
• Discuss: End-to-end vs server-side; how to support "send later"?

20. Design SMS Gateway (Twilio-like)¶

• Scale: 1B SMS/day, multi-carrier routing.
• Core features: Send SMS, receive reply, delivery report, opt-out compliance.
• Hard parts: Carrier rate limits, retry policies, regulatory compliance, webhook callbacks.
• Discuss: Outbound queue with priorities? Idempotency on retries?

21. Design Group Video Conference (Zoom/Meet)¶

• Scale: 500K concurrent meetings, up to 1000 participants.
• Core features: Video + audio, screen share, recording, breakout rooms, chat.
• Hard parts: Selective forwarding unit (SFU) vs MCU vs P2P; bandwidth adaptation; meeting recording pipeline.
• Discuss: How does media routing scale? Recording stored where?

22. Design Read Receipts / Typing Indicators¶

• Scale: Same as 16, but specifically the ephemeral signals.
• Core features: "typing…" indicator, "delivered/read" markers.
• Hard parts: Volume of ephemeral writes, throttling, opt-out users.
• Discuss: Why not store every typing event? Pub/sub vs DB writes.

23. Design Message Search (across own DM history)¶

• Scale: 10M messages per user, search in <500ms.
• Core features: Full-text search, filters by date/contact.
• Hard parts: Per-user inverted index, encrypted search (E2E), index size.
• Discuss: Server-side search vs on-device search; index update on new message.

24. Design Online Presence System¶

• Scale: 1B users, "online/away/offline" status.
• Core features: Presence indicator on chat list, typing, last-seen.
• Hard parts: Heartbeat at scale, push to interested subscribers (friends), TTL for stale heartbeats.
• Discuss: Pull vs push on chat-list open; how big is the heartbeat overhead?

25. Design Push Notification Service (FCM/APNs equivalent)¶

• Scale: 100B notifications/day, 1B devices.
• Core features: Send notification, device registration, topic subscription, ack.
• Hard parts: Device-token management, delivery retries, prioritization.
• Discuss: Persistent connection per device (TCP) — how many on one server? Sharding strategy.

26. Design Telegram-like Bot Platform¶

• Scale: 5M bots, webhooks vs long-polling.
• Core features: Bot API, command handling, inline queries, payments.
• Hard parts: Webhook delivery reliability, rate-limit per bot, bot abuse detection.
• Discuss: Push vs pull for bot updates; how to handle bots that crash repeatedly.

27. Design Comments Service (Reusable, embedded — Disqus)¶

• Scale: 100K sites, 1B comments stored.
• Core features: Embed widget, threaded comments, moderation, voting.
• Hard parts: Multi-tenant data, spam filter, embed performance (don't slow host page).
• Discuss: Loading comments async vs SSR; per-site quotas.

🎬 3. Video & Audio Streaming¶

28. Design YouTube¶

• Scale: 2B MAU, 500h/min uploads, 1B hours/day watched.
• Core features: Upload, transcode, watch, comments, subscriptions, recommendations.
• Hard parts: Transcoding pipeline (multiple resolutions/bitrates), CDN distribution, view-count denormalization, copyright detection (Content ID).
• Discuss: Adaptive bitrate (HLS/DASH), how to count views accurately at scale, anti-fraud on view counts.

29. Design Netflix¶

• Scale: 250M subscribers, 250M concurrent during peak, 15% of global internet traffic.
• Core features: Browse catalog, watch, recommendations, profiles, downloads.
• Hard parts: Open Connect CDN (ISP-embedded), encoding ladder, personalization, cross-device resume.
• Discuss: Why their own CDN? Pre-positioning content based on regional popularity.

30. Design Spotify¶

• Scale: 600M MAU, 100M tracks, personalized playlists.
• Core features: Stream music, playlists, search, recommendations (Discover Weekly), podcasts, offline.
• Hard parts: Audio streaming with low buffer, royalty calculation, collaborative playlists, offline DRM.
• Discuss: How to compute Discover Weekly weekly for 600M users? Storage for offline tracks.

31. Design Twitch (Live Streaming)¶

• Scale: 30M DAU, peak 5M concurrent viewers, low-latency live.
• Core features: Go live, chat, follow, subscriptions, clips.
• Hard parts: Sub-second latency (WebRTC vs LL-HLS), real-time chat fanout, transcoding live ingest.
• Discuss: Ingest → transcode → distribute pipeline; how to scale chat alongside video.

32. Design Audio-Only Live Streaming (Clubhouse-like)¶

• Scale: 1M rooms, 10K listeners per popular room.
• Core features: Audio rooms, raise hand, speaker promotion, recording.
• Hard parts: Audio mixing on server, latency for back-and-forth, capacity per room.
• Discuss: SFU for many speakers vs all-in-one mixing; how to scale viral rooms.

33. Design Podcast Platform (Apple Podcasts-like)¶

• Scale: 100M users, 5M shows, RSS-based ingestion.
• Core features: Subscribe, episode list, download, sync playback position cross-device.
• Hard parts: RSS poller for millions of feeds, dedup of identical episodes, transcript generation.
• Discuss: Pull vs push from RSS publishers; episode storage and CDN.

34. Design Video Upload & Transcoding Pipeline¶

• Scale: 500h/min input, output 6+ resolutions.
• Core features: Resumable upload, queue for transcoding, multi-resolution output, thumbnail generation.
• Hard parts: GPU vs CPU transcoding, parallelism per file, dead-letter for failed transcodes.
• Discuss: Chunked upload (multipart), priority queue (paid vs free creators).

35. Design Adaptive Bitrate Streaming¶

• Scale: Service for 100M concurrent streams.
• Core features: Manifest playlist (HLS/DASH), segment download, quality switch.
• Hard parts: Encoding ladder per content, manifest CDN cache, ABR algorithm on client.
• Discuss: Per-title encoding vs per-genre; CDN cache key strategy.

36. Design Video CDN (like Cloudflare Stream / Mux)¶

• Scale: 10K customers, multi-region delivery.
• Core features: Ingest API, transcoding, delivery URLs, analytics.
• Hard parts: Multi-tenant cost attribution, abuse prevention, custom domains.
• Discuss: Origin shielding, per-tenant TLS certs.

37. Design Live Video Recording (Cloud DVR)¶

• Scale: 10M users, schedule-based recording.
• Core features: Schedule recording, store, playback later.
• Hard parts: Storage tiering (recent vs old), per-user vs shared copy (legal), seek inside long recording.
• Discuss: Why "shared copy" is legally tricky in some jurisdictions.

38. Design Closed Captions / Subtitles Pipeline¶

• Scale: Auto-generate for 500h/min new uploads.
• Core features: Auto-transcribe, sync timing, multi-language, manual edit.
• Hard parts: Speech-to-text pipeline, timing alignment, profanity masking.
• Discuss: Async pipeline with status; how to backfill for old content.

39. Design Watch Party / Synced Playback¶

• Scale: Up to 100 participants per party.
• Core features: Synced play/pause/seek, group chat overlay.
• Hard parts: Drift correction on each client, host vs democratic control, NAT-traversal.
• Discuss: Authoritative server time vs peer-clock; how to recover from network blip.

🛒 4. E-commerce & Marketplace¶

40. Design Amazon (e-commerce)¶

• Scale: 300M users, 200M products, Prime Day 100x spikes.
• Core features: Browse, search, cart, checkout, order tracking, reviews.
• Hard parts: Inventory consistency, payment, recommendations, fraud, search relevance.
• Discuss: Single product page = how many service calls? Cart in Redis vs DB.

41. Design Shopify (multi-tenant store platform)¶

• Scale: 2M merchants, custom domains, themes.
• Core features: Store builder, product mgmt, checkout, order, payments.
• Hard parts: Multi-tenant isolation, custom domain SSL, theme rendering, app marketplace.
• Discuss: Per-tenant DB vs shared schema; webhook delivery to merchants.

42. Design eBay / Online Auctions¶

• Scale: 150M users, real-time bidding on auctions.
• Core features: List item, bid, watch, snipe protection, payment.
• Hard parts: Last-second bid race, atomic bid increment, time extensions.
• Discuss: Why classical OLTP isn't enough for sniping; counter sharding.

43. Design a Shopping Cart Service¶

• Scale: 100M users, anonymous + logged-in carts.
• Core features: Add/remove items, persist across devices, expire after 30 days.
• Hard parts: Anonymous-to-logged-in cart merge, stock check, price drift.
• Discuss: Cart in Redis with periodic flush vs cart in DB; cart-abandonment pipeline.

44. Design a Payment / Checkout Flow¶

• Scale: 10K checkouts/sec at peak.
• Core features: Address, payment method, tax, shipping, place order.
• Hard parts: Idempotency, saga across inventory + payment + shipping + email.
• Discuss: Why eventual consistency is dangerous in checkout; how to recover stuck orders.

45. Design Inventory Management at Amazon Scale¶

• Scale: 100M SKUs, multiple warehouses, Prime Now.
• Core features: Track stock per warehouse, allocate to orders, restock.
• Hard parts: Concurrent decrement (oversell), reservation TTL, near-real-time picture.
• Discuss: Distributed counter for stock; reservation vs commit pattern.

46. Design a Recommendation System (e-commerce)¶

• Scale: 200M products, 300M users.
• Core features: "Customers also bought", "based on history".
• Hard parts: Item-item similarity matrix, real-time personalization, cold start.
• Discuss: Offline batch (Spark) vs online; embedding-based vs collaborative filtering.

47. Design Product Reviews & Ratings¶

• Scale: 1B reviews, 5-star ratings, photos.
• Core features: Submit review, read reviews, sort by helpful, moderation.
• Hard parts: Average rating denormalization, helpful-vote ranking, fake review detection.
• Discuss: How to compute "weighted average" considering reviewer credibility.

48. Design Order History / Tracking¶

• Scale: 1B orders/year, 90-day-active.
• Core features: List orders, status timeline, tracking number, return.
• Hard parts: State machine for order, integration with carrier APIs, return reverse logistics.
• Discuss: Webhook from carriers vs polling; storage strategy for old orders.

49. Design a Coupon / Promo Code Service¶

• Scale: 100K active codes, applied at checkout.
• Core features: Generate code, apply, expire, single-use vs multi-use.
• Hard parts: Atomic decrement of redemption count, abuse detection, stacking rules.
• Discuss: Race condition on last redemption; fraud rings.

50. Design Shopping Cart Abandonment Pipeline¶

• Scale: 10M abandoned carts/day.
• Core features: Detect abandonment, send email after 1h, after 24h.
• Hard parts: Session vs cart vs user, opt-out compliance, A/B testing emails.
• Discuss: Event-driven (Kafka) vs cron sweep.

51. Design Multi-currency / Multi-region Pricing¶

• Scale: 200 currencies, 50 regions, daily FX updates.
• Core features: Show local price, tax, region-specific availability.
• Hard parts: Price freezing in cart for X minutes, FX update propagation.
• Discuss: Pre-compute all prices vs convert on read.

52. Design a Fraud Detection System (e-commerce)¶

• Scale: 100K transactions/sec, decision in <50ms.
• Core features: Score transaction, block/allow, manual review queue.
• Hard parts: Real-time features, ML model serving, feedback loop.
• Discuss: Feature store, online vs offline features, model retraining cadence.

53. Design DoorDash / UberEats¶

• Scale: 30M users, 500K restaurants, peak dinner hours.
• Core features: Browse, order, real-time dispatch to driver, ETA, tracking.
• Hard parts: Driver matching (3-sided marketplace), surge pricing, ETA prediction.
• Discuss: Restaurant capacity, batched delivery, geo-indexing for nearby drivers.

54. Design Marketplace Search (Amazon/eBay)¶

• Scale: 200M items, search in <200ms.
• Core features: Keyword, filter (price, brand), sort, autocomplete.
• Hard parts: Relevance ranking, faceted filters, real-time inventory.
• Discuss: Elasticsearch vs Solr; how to keep index in sync with primary DB.

🏨 5. Booking & Reservations¶

55. Design Airbnb¶

• Scale: 150M users, 7M listings, calendar-based.
• Core features: Search by location/date, book, host calendar, reviews, messaging.
• Hard parts: Geospatial search, calendar consistency (no double-book), pricing engine, trust signals.
• Discuss: S2/H3 cells for geo, locking for booking, hot listings.

56. Design Booking.com / Hotel Reservations¶

• Scale: 28M listings, multi-supplier (chains, OTAs).
• Core features: Search, compare, book, cancel.
• Hard parts: Real-time availability across suppliers, channel manager sync, GDS integration.
• Discuss: Cache stale availability vs always live; how to avoid double-bookings across channels.

57. Design a Movie Ticket Booking System (BookMyShow)¶

• Scale: 50K theaters, 100K concurrent users at release time.
• Core features: Browse shows, pick seats, hold seat for 5 min, pay, ticket QR.
• Hard parts: Seat-hold race, distributed locking on seat, theater-side sync.
• Discuss: Optimistic vs pessimistic locking; graceful unhold on payment failure.

58. Design a Flight Booking System¶

• Scale: Multi-airline, multi-leg, GDS integration (Sabre, Amadeus).
• Core features: Search by O&D, fare calc, hold, book, e-ticket.
• Hard parts: Itinerary combinations, fare rules, hold expiry, currency.
• Discuss: Why caching airline inventory is dangerous; on-demand vs pre-fetched.

59. Design a Restaurant Reservation System (OpenTable)¶

• Scale: 60K restaurants, table-level inventory.
• Core features: Search by cuisine/time, book table, no-show prediction.
• Hard parts: Real-time table availability, party-size matching, restaurant POS integration.
• Discuss: Restaurant-side push vs polling; show "first available time".

60. Design a Doctor / Clinic Appointment System¶

• Scale: 1M doctors, slot-based.
• Core features: Search doctor, book slot, reschedule, reminders.
• Hard parts: Doctor's time-block availability, double-booking, cross-clinic doctors.
• Discuss: Slot vs queue model; SMS reminder pipeline.

61. Design Calendly / Meeting Scheduler¶

• Scale: 10M users, calendar integrations.
• Core features: Public link with availability, multi-calendar sync, time-zone aware.
• Hard parts: Cross-calendar conflict detection, busy/free polling, time-zone DST bugs.
• Discuss: Push notifications from Google/Microsoft vs polling; cache invalidation on edit.

62. Design a Meeting Room / Workspace Booking¶

• Scale: Enterprise: 10K rooms across 100 offices.
• Core features: Find available room, book, recurring, equipment filters.
• Hard parts: Recurring meeting expansion, conflict detection, no-show release.
• Discuss: Materialize occurrences vs compute on demand; smart-room sensor integration.

63. Design a Car-Rental Service (Hertz/Turo)¶

• Scale: 100K cars, location-based.
• Core features: Pick location, dates, vehicle class, book.
• Hard parts: Inventory across locations, one-way drop-off, dynamic pricing.
• Discuss: Vehicle-class vs specific-vehicle; demand-based price adjust.

64. Design Event Ticketing (Eventbrite / Ticketmaster)¶

• Scale: Hot drop = 1M concurrent users at sale start.
• Core features: Browse events, queue at sale start, pick seat, buy.
• Hard parts: Virtual waiting room, anti-bot, fairness, queueing.
• Discuss: Queue-based admission, "you're #5,000 in line", session tokens.

🗺️ 6. Maps & Location¶

65. Design Uber / Lyft (Ride-sharing)¶

• Scale: 100M riders, 5M drivers, 30M rides/day.
• Core features: Request ride, match driver, ETA, route, fare, in-app payment.
• Hard parts: Geo-index for nearest driver (S2/H3), matching algorithm, surge, driver pings.
• Discuss: How does dispatch select 1 driver from 100 nearby? Why grid-based geo > radius search.

66. Design Google Maps¶

• Scale: 1B users, real-time traffic, turn-by-turn navigation.
• Core features: Map tiles, search, directions, real-time traffic, street view.
• Hard parts: Map tile generation pyramid, A* / Contraction Hierarchies for routing, traffic ingestion.
• Discuss: Tile pre-rendering vs vector tiles on client; ETA with traffic.

67. Design Yelp / Google Places¶

• Scale: 200M users, 5M businesses, location-based search.
• Core features: Search nearby, filter, reviews, photos, business profile.
• Hard parts: Geospatial search with filters, review moderation, business claim.
• Discuss: Geohash vs S2 vs PostGIS; cold-start ranking for new business.

68. Design a Geo-fenced Notification System¶

• Scale: 100M devices reporting location, 1M geofences.
• Core features: Define geofence, push notification on enter/exit.
• Hard parts: Battery efficiency on device, scaling many geofences, dwell-time triggers.
• Discuss: R-tree on server, on-device fence sharding, throttling overlapping fences.

69. Design a Real-time Location-sharing (Find My Friends)¶

• Scale: 100M users, opt-in friend list.
• Core features: See location of friends on map, history.
• Hard parts: Frequency of updates vs battery, privacy (whom to share with), encryption.
• Discuss: Push to interested subscribers vs poll; pub/sub with location channels.

70. Design Find My Device / Anti-theft Tracker¶

• Scale: 1B devices, low-power offline tracking (AirTag).
• Core features: Device beacons via Bluetooth, crowd-sourced location, mark as lost.
• Hard parts: End-to-end encryption (only owner reads), anti-stalking, dropped packets.
• Discuss: Apple's rotating-key approach; threat model and mitigations.

71. Design Surge Pricing (Uber Surge)¶

• Scale: Per-region per-minute updates.
• Core features: Detect demand vs supply imbalance, raise multiplier, notify riders/drivers.
• Hard parts: Real-time signal aggregation, smoothing (no jitter), fair pricing.
• Discuss: Hexagon-based zones; ML demand forecast.

72. Design a Delivery Dispatch (DoorDash)¶

• Scale: 1M concurrent orders, 1M dashers.
• Core features: Match order to dasher, batched deliveries, ETA.
• Hard parts: Bin-packing for batched delivery, ETA prediction, dasher acceptance rate.
• Discuss: Optimization problem; greedy vs ILP solver.

73. Design Live ETA Updates¶

• Scale: 10M concurrent rides each with live ETA.
• Core features: Push updated ETA every few seconds.
• Hard parts: Many small updates; rate-limiting per ride; battery efficiency.
• Discuss: Push to client vs client polls; ETA with traffic re-route.

74. Design Location History (Google Timeline)¶

• Scale: 1B users, store decades of location.
• Core features: View past trips, places visited, time spent.
• Hard parts: Privacy, storage tiering, place-detection (clustering).
• Discuss: GeoTime-series DB; how to detect "visited a place" from raw GPS.

75. Design a Bike / Scooter Sharing System (Lime/Bird)¶

• Scale: 100K vehicles in 100 cities.
• Core features: Find vehicle, unlock, ride, dock/end-trip, payment.
• Hard parts: Vehicle-state sync, geofence operating area, end-of-day rebalancing.
• Discuss: IoT integration with vehicle; battery monitoring.

76. Design Weather Service (OpenWeatherMap)¶

• Scale: 1B requests/day, location-based.
• Core features: Current weather, hourly, alerts.
• Hard parts: Multi-source data ingestion, geo-grid resolution, push alerts.
• Discuss: Cache by geohash; how to scale with global radar data.

📁 7. Storage & File Sharing¶

77. Design Dropbox / Google Drive¶

• Scale: 1B users, exabyte-scale storage.
• Core features: Upload, sync across devices, share, version history, search.
• Hard parts: Block-level dedup, delta sync, conflict resolution, multi-device sync.
• Discuss: Chunking strategy, presigned URLs, change journal.

78. Design Google Photos / iCloud Photos¶

• Scale: 1B users, 10TB per power user.
• Core features: Auto-upload, search by face/object, albums, sharing.
• Hard parts: ML-based tagging, deduplication of HEIC/JPEG, RAW handling.
• Discuss: Free-tier compression vs original quality; on-device ML vs server-side.

79. Design a S3-like Object Storage¶

• Scale: Trillions of objects, 11 nines durability.
• Core features: PUT/GET/DELETE, versioning, ACLs, lifecycle.
• Hard parts: Erasure coding, geo-replication, metadata index, hot-bucket throttling.
• Discuss: Why erasure coding > 3x replication at scale.

80. Design a Backup Service (Carbonite / Time Machine)¶

• Scale: 10M users, daily incremental backups.
• Core features: Continuous/scheduled backup, restore, encryption.
• Hard parts: Incremental dedup (rsync, content-addressable), restore speed.
• Discuss: Hash-based chunking; ransomware protection.

81. Design a CDN (Cloudflare / Akamai)¶

• Scale: Global PoPs, terabits/sec, 30T req/day.
• Core features: Edge cache, origin shield, purge API, WAF, DDoS mitigation.
• Hard parts: Cache hierarchy, consistent hashing, cache poisoning prevention.
• Discuss: Anycast routing, TLS termination at edge, stale-while-revalidate.

82. Design a File Sharing Link Service (WeTransfer)¶

• Scale: 50M users, files up to 200GB, 7-day TTL.
• Core features: Upload, share link, password protect, expiry.
• Hard parts: Resumable upload of huge files, abuse detection, hot links.
• Discuss: Direct-to-S3 upload; presigned URL strategy.

83. Design a Notes Sync (Apple Notes / Evernote)¶

• Scale: 100M users, multi-device sync.
• Core features: Create note, sync across devices, search, attachments.
• Hard parts: Conflict resolution (CRDT?), offline edits, encrypted notes.
• Discuss: OT vs CRDT; per-user merkle tree for sync.

84. Design Code Hosting (GitHub)¶

• Scale: 100M users, 400M repos, billions of files.
• Core features: Git push/pull, PRs, issues, actions.
• Hard parts: Git protocol at scale, large monorepos, code search.
• Discuss: Sharding by repo, pack files, bare-metal performance.

85. Design a Container Registry (Docker Hub / ECR)¶

• Scale: Petabytes of layers, billions of pulls/day.
• Core features: Push image, pull, tag, sign, scan.
• Hard parts: Layer dedup across users, vulnerability scanning, regional caches.
• Discuss: Content-addressable layers, GC for unreferenced.

86. Design a Content-Addressable Storage (IPFS-like)¶

• Scale: Distributed, peer-to-peer.
• Core features: Hash-addressed blocks, DHT, gateway HTTP access.
• Hard parts: Pinning incentives, NAT traversal, GC of unpinned.
• Discuss: Centralized gateway pragmatism; how to scale DHT lookups.

🔍 8. Search & Discovery¶

87. Design Google Search¶

• Scale: Web-scale crawl (trillions of pages), <300ms response.
• Core features: Crawl, index, rank, query, snippet.
• Hard parts: PageRank, freshness, anti-spam, query understanding.
• Discuss: Inverted index sharding, cache (cached results), query rewriting.

88. Design YouTube Search¶

• Scale: Billions of videos, real-time updates on new uploads.
• Core features: Title/desc/transcript search, filter, sort.
• Hard parts: Multi-modal ranking, freshness vs relevance, channel boost.
• Discuss: Typeahead, query expansion (synonyms).

89. Design Twitter Search¶

• Scale: 500M tweets/day, query historical + real-time.
• Core features: Search by keyword, hashtag, user, timeframe.
• Hard parts: Real-time index (within seconds of tweet), retention policy.
• Discuss: Earlybird-style index sharded by time; hot index in memory.

90. Design Autocomplete / Typeahead¶

• Scale: 100K QPS, <50ms latency.
• Core features: Suggest top-N completions, personalized, multilingual.
• Hard parts: Trie size, popularity update, typo tolerance.
• Discuss: Pre-built suggestion trie; weighted edges.

91. Design "Did You Mean?" / Spell Correction¶

• Scale: Same as search.
• Core features: Suggest corrected query, top results.
• Hard parts: Edit distance at scale, query log mining.
• Discuss: N-gram language model; user click signal as feedback.

92. Design a Document Search Engine (Internal — Confluence)¶

• Scale: 1M documents per workspace.
• Core features: Full-text + permission-aware results.
• Hard parts: ACL filter at index time vs query time.
• Discuss: Per-workspace index vs shared with ACL filter.

93. Design Code Search (GitHub Code Search)¶

• Scale: 1B+ source files, regex queries.
• Core features: Symbol search, regex, language filter.
• Hard parts: Trigram index, regex on 1B docs, line-level ranking.
• Discuss: Why trigram; sparse-grams; regex acceleration.

94. Design a Vector Search Service (Pinecone)¶

• Scale: Billions of vectors, ANN queries.
• Core features: Upsert vector, k-NN query, filter by metadata.
• Hard parts: ANN index (HNSW, IVF), index rebuilds, hybrid search.
• Discuss: Memory vs disk, quantization, multi-tenant isolation.

95. Design a Reverse Image Search (Google / TinEye)¶

• Scale: 1B images, <1s response.
• Core features: Upload image, find similar.
• Hard parts: Perceptual hashing + CNN embeddings, near-duplicate vs semantic similar.
• Discuss: pHash for near-dup, vector ANN for semantic.

96. Design a Job-Search Engine (LinkedIn Jobs)¶

• Scale: 30M active jobs, location + skill filters.
• Core features: Search, filter, alerts, recommendations.
• Hard parts: Skill normalization, recency boost, saved-search alerts.
• Discuss: Async alert pipeline; ranking with personalization.

✏️ 9. Real-time Collaboration¶

97. Design Google Docs (Real-time Collaborative Editor)¶

• Scale: 100M users, up to 100 collaborators per doc.
• Core features: Concurrent edit, presence, comments, suggestions.
• Hard parts: OT vs CRDT, conflict resolution, offline edits.
• Discuss: Why OT in Docs; cursor presence pub/sub; auto-save cadence.

98. Design Figma (Collaborative Vector Editor)¶

• Scale: 4M+ designers, large files.
• Core features: Concurrent shape edits, multi-cursor, comments, plugins.
• Hard parts: CRDT for vector ops, large-scene perf, web-based rendering.
• Discuss: WebGL canvas; selective sync of viewport.

99. Design Notion (Block-based Collaborative Doc)¶

• Scale: 30M users, page tree.
• Core features: Blocks (text, image, embed), nested, real-time edit.
• Hard parts: Block-tree CRDT, permissions on subtree, search.
• Discuss: Block as first-class node; sync engine.

100. Design Miro / Whiteboard¶

• Scale: 30M users, shared infinite canvas.
• Core features: Sticky notes, draw, multi-user cursors, voice.
• Hard parts: Spatial sync (only viewport), CRDT for shapes.
• Discuss: Quad-tree partitioning of canvas; LOD rendering.

101. Design Real-time Code Editor (VS Code Live Share)¶

• Scale: Pair-programming sessions.
• Core features: Shared editor, terminal, language server.
• Hard parts: Cursor sync, language-server multiplexing, latency.
• Discuss: Operational transforms on text; bandwidth for terminal.

102. Design Excel/Sheets Collaborative¶

• Scale: 100M users.
• Core features: Concurrent edit, formulas, comments, history.
• Hard parts: Formula recompute on collaborator's edit, cycles.
• Discuss: Dependency graph; partial recompute.

103. Design Comments + Reactions on a Document¶

• Scale: Like Google Docs comments.
• Core features: Anchored comments, threads, resolve, mention.
• Hard parts: Anchor when text moves, comment migration on big edit.
• Discuss: Anchor as character range vs node-id.

104. Design Real-time Multiplayer Whiteboarding (Excalidraw)¶

• Scale: 1M concurrent rooms.
• Core features: Free-draw, shapes, export.
• Hard parts: P2P vs server-relayed, low-latency strokes.
• Discuss: WebRTC datachannel; server-authoritative for export.

105. Design Version-history / Time Machine for a Doc¶

• Scale: 100M docs, 1000 versions each.
• Core features: Browse versions, restore, diff.
• Hard parts: Storage compression (delta), retention policy.
• Discuss: Snapshot every N edits + delta in between.

106. Design @Mention + Notifications in Collaborative Tools¶

• Scale: 1M users, mentions in comments/chats.
• Core features: Detect mention, route notification, mark read.
• Hard parts: Spam mentions, mute settings.
• Discuss: Notification fanout from collab tool.

💳 10. Payments & Finance¶

107. Design Stripe / Payment Gateway¶

• Scale: 250M API requests/day, 10M merchants.
• Core features: Charge card, subscriptions, refunds, payouts, idempotency.
• Hard parts: Idempotency keys, double-entry ledger, payment routing, PCI scope.
• Discuss: Why ledger > balance update; reconciliation pipelines.

108. Design a Digital Wallet (Venmo / Cash App)¶

• Scale: 80M users, P2P transfers.
• Core features: Add funds, send/receive, social feed, instant transfer.
• Hard parts: ACID across accounts, fraud, AML/KYC.
• Discuss: Account-balance vs ledger model; idempotent transfers.

109. Design a Banking Core Ledger¶

• Scale: Millions of accounts, ACID transfers.
• Core features: Debit, credit, statements, hold, reverse.
• Hard parts: Strong consistency, audit trail, regulatory reporting.
• Discuss: Why an immutable append-only ledger; reconciliation.

110. Design a Trading System (Robinhood)¶

• Scale: 25M users, broker integration.
• Core features: Buy/sell stocks/crypto, watchlist, real-time quotes.
• Hard parts: Order routing, order book, market data fan-out, halts.
• Discuss: Internal matching vs market routing; outage handling.

111. Design a Crypto Exchange (Coinbase / Binance)¶

• Scale: 100M users, real-time order matching.
• Core features: Order book, deposit/withdraw, market data, OHLC charts.
• Hard parts: Single-threaded matching engine, hot/cold wallet, withdrawal review.
• Discuss: Why matching is not horizontally sharded by symbol; risk engine.

112. Design Stock Market Order-Matching Engine¶

• Scale: Microsecond latency, millions of orders/sec.
• Core features: Limit, market, stop orders; cancel; partial fill.
• Hard parts: Determinism, FIFO at price-level, fairness, audit.
• Discuss: Single-threaded for determinism; multicast feeds.

113. Design a Payment Splitting System (Splitwise)¶

• Scale: 10M users, group expenses.
• Core features: Add expense, split among friends, settle up.
• Hard parts: Multi-currency, owe-graph, payback simplification.
• Discuss: Graph algorithms for minimum settlement set.

114. Design Subscription Billing (Stripe Billing)¶

• Scale: 1M merchants, recurring charges.
• Core features: Plans, trials, proration, dunning, invoices.
• Hard parts: Time-zone-aware renewals, failed-charge retry, proration math.
• Discuss: Idempotent renewals; jobs queue per customer.

115. Design a Loyalty / Points System¶

• Scale: 100M users, multi-merchant.
• Core features: Earn, redeem, expiry, tiers.
• Hard parts: Anti-double-redeem, cross-merchant settlement.
• Discuss: Ledger-based points balance; expiry sweeper.

116. Design Tax Calculation Service (Avalara / Stripe Tax)¶

• Scale: Per-checkout, sub-50ms.
• Core features: Compute tax by jurisdiction, rate tables, exemptions.
• Hard parts: 12K US tax jurisdictions, frequent rate changes, B2B exempt cert.
• Discuss: Hot reload of rate tables; address normalization.

117. Design a Refund / Dispute Pipeline¶

• Scale: 1% of transactions disputed.
• Core features: Initiate refund, partial, evidence collection for chargeback.
• Hard parts: Async with bank, time limits, evidence storage.
• Discuss: State machine; SLA tracking.

118. Design a Real-time Risk Scoring Service¶

• Scale: 100K transactions/sec, <50ms decision.
• Core features: Score, decision (approve/review/decline), feedback loop.
• Hard parts: Online features (recent velocity), model versioning.
• Discuss: Feature store, A/B model serving, drift monitoring.

📢 11. Ads & Marketing¶

119. Design Google AdWords / Ad Auction¶

• Scale: Billions of auctions/day, <100ms.
• Core features: Advertiser bid, keyword targeting, ranking, billing.
• Hard parts: Real-time auction (GSP/VCG), quality score, budget pacing.
• Discuss: Why second-price auction; pacing across day.

120. Design Google AdSense (Publisher-side Ads)¶

• Scale: Millions of publisher sites.
• Core features: Ad slot, targeting, click tracking, payout.
• Hard parts: Click fraud, viewability, compliance.
• Discuss: Bot detection signals; revenue share model.

121. Design a Real-time Bidding (RTB) Platform¶

• Scale: 10M QPS bid requests, 50ms timeout.
• Core features: Bid request, response, win notice, billing.
• Hard parts: Timeout budget, candidate filtering, low-latency network.
• Discuss: OpenRTB protocol; geo-distributed edge bidders.

122. Design Pixel / Conversion Tracking¶

• Scale: 1B events/day.
• Core features: Tracking pixel, deduplicate, attribution.
• Hard parts: Cross-device attribution, privacy (cookies, ATT), spam.
• Discuss: Server-side conversion API; identity graph.

123. Design an Email Marketing Platform (Mailchimp)¶

• Scale: 12M users, 10B emails/month.
• Core features: List management, template, send, track opens/clicks, A/B.
• Hard parts: Deliverability (SPF/DKIM/DMARC), bounce handling, unsubscribe compliance.
• Discuss: Sending rate per IP; warming new IPs.

124. Design a Push Marketing Campaign Platform¶

• Scale: Send to 100M users in 1 hour.
• Core features: Audience targeting, schedule, A/B variants, deliverability tracking.
• Hard parts: Throttle to APNs/FCM limits, time-zone-staggered send.
• Discuss: Scheduler + worker model; per-channel rate limits.

125. Design Attribution (Multi-touch)¶

• Scale: 100M users, all touchpoints over 30 days.
• Core features: Track touch, attribute conversion (last-click, linear, U-shape).
• Hard parts: Identity stitching, touch retention, fractional credit.
• Discuss: Session vs touchpoint; Markov-chain attribution.

126. Design A/B Testing / Experiment Platform¶

• Scale: 10K simultaneous experiments.
• Core features: Define experiment, assign user, log exposure, compute metrics, decide winner.
• Hard parts: Bucketing consistency, peeking problem, sequential testing.
• Discuss: Sticky bucketing via hash; SRM checks.

127. Design a Recommendation Email ("Today's deals")¶

• Scale: Daily personalized email to 100M users.
• Core features: Generate per-user candidates, render template, send.
• Hard parts: Throughput of nightly job, candidate diversity.
• Discuss: Offline batch + per-user assembly; opt-out/spam compliance.

128. Design a Coupon Distribution System¶

• Scale: 100K codes per campaign, fair distribution.
• Core features: Generate batch, assign per claim, expiry.
• Hard parts: Race-free claim, fraud (one per user).
• Discuss: Pre-allocate vs claim-on-demand.

🎮 12. Gaming¶

129. Design a Real-time Multiplayer FPS Server (Counter-Strike-like)¶

• Scale: 64-player matches, <50ms latency.
• Core features: Authoritative server, tick-based sim, anti-cheat.
• Hard parts: Lag compensation, client prediction, anti-cheat.
• Discuss: UDP vs TCP; tick rate vs interpolation.

130. Design an MMO Server (World of Warcraft)¶

• Scale: Millions of players, persistent world.
• Core features: Zones/instances, world chat, inventory, raids.
• Hard parts: Zone sharding, cross-zone events, character DB.
• Discuss: Player handoff between zones; auction house.

131. Design Matchmaking System (Skill-based)¶

• Scale: 1M concurrent players, find a match in <60s.
• Core features: Queue, ELO/MMR, party support, region.
• Hard parts: Queue time vs match quality, expansion of skill range over time.
• Discuss: Trueskill / Glicko; queue priority.

132. Design a Game Leaderboard at Global Scale¶

• Scale: 100M players, real-time top-K.
• Core features: Submit score, top-K query, my-rank.
• Hard parts: Approximate rank for 100Mth user; real-time updates.
• Discuss: Redis sorted-set sharding; bucketing for percentile.

133. Design an In-Game Chat (Lobby + Team)¶

• Scale: Same as concurrent players.
• Core features: Lobby chat, team chat, ban / mute.
• Hard parts: Profanity filter, low-latency delivery.
• Discuss: Pub/sub per channel; rate limit per user.

134. Design a Gaming Asset Distribution (CDN for Game Updates)¶

• Scale: 10M downloads of 50GB patch on launch day.
• Core features: Delta patching, parallel CDN, signature verification.
• Hard parts: Bandwidth peak, partial-download resumption.
• Discuss: Pre-position to PoPs; P2P assist.

135. Design an In-game Ad / Sponsorship Platform¶

• Scale: 100M players, dynamic 3D ad slots.
• Core features: Serve ads, track impression, brand safety.
• Hard parts: 3D-rendered ads, viewability, kid-safe filtering.
• Discuss: SDK vs server-side rendering.

136. Design a Turn-based Game Server (Chess.com)¶

• Scale: 10M concurrent games.
• Core features: Move, validate, save game, replay.
• Hard parts: Anti-cheat (engine detection), live spectate.
• Discuss: Server validation per move; ELO.

137. Design an Online Casino / Slot Game¶

• Scale: 1M concurrent players, regulated.
• Core features: RNG, payouts, jurisdiction filtering.
• Hard parts: Provably-fair RNG, audit trails, geofencing for regulation.
• Discuss: Server-side RNG with hashing; immutable bet history.

138. Design a Battle Royale Lobby (100 players per match)¶

• Scale: Millions of concurrent matches.
• Core features: Lobby fill, drop into game, post-game.
• Hard parts: Server provisioning, region matchmaking.
• Discuss: Game-server orchestration via Kubernetes / Agones.

📡 13. IoT & Devices¶

139. Design a Smart Home Hub (Google Home / Alexa)¶

• Scale: 100M devices, voice + control.
• Core features: Voice command, device control, routines.
• Hard parts: Local vs cloud control, vendor integrations, latency.
• Discuss: Local fallback when offline; security model.

140. Design a Connected Car Telemetry Pipeline¶

• Scale: 10M cars, 1Hz telemetry.
• Core features: Ingest signals, real-time alerts, OTA updates.
• Hard parts: Cell-network unreliability, OTA rollback, fleet-wide queries.
• Discuss: MQTT, edge filtering.

141. Design a Wearable Health Tracker Backend¶

• Scale: 50M devices, sub-second heart-rate ingest.
• Core features: Sync, dashboards, alerts (high HR), HIPAA.
• Hard parts: Battery-friendly upload, data retention, HIPAA scope.
• Discuss: Batch sync vs streaming; encryption keys.

142. Design an IoT Device Provisioning System¶

• Scale: Onboard 1M devices/month.
• Core features: Device cert, TLS bootstrap, claim by user.
• Hard parts: Hardware secure element, certificate rotation, anti-spoofing.
• Discuss: PKI; just-in-time provisioning at edge.

143. Design an Industrial Sensor Network (Factory Floor)¶

• Scale: 100K sensors per facility.
• Core features: Real-time dashboards, anomaly detect, historian DB.
• Hard parts: Sub-second alerts, downsampling for archive.
• Discuss: Time-series DB; on-prem ingestion gateway.

144. Design a Smart-Lock Backend¶

• Scale: 5M locks, sub-second unlock latency.
• Core features: Issue/revoke key, audit log, offline unlock.
• Hard parts: Offline-resilient cryptography, anti-relay attacks.
• Discuss: Tokens with short expiry; bluetooth attestation.

145. Design a Maritime AIS Tracking Backend¶

• Scale: 100K vessels, 1 ping/min.
• Core features: Real-time map, route history, port arrival alerts.
• Hard parts: Sparse coverage areas, satellite vs terrestrial AIS.
• Discuss: Geo-bucketed map tiles; replay query.

146. Design an OTA (Over-the-Air) Firmware Update Platform¶

• Scale: 10M devices, staged rollout.
• Core features: Build, sign, distribute, percent-rollout, rollback.
• Hard parts: Atomic update, A/B partition, signature verification.
• Discuss: TUF (The Update Framework); device fleet sharding.

📊 14. Analytics & Logging¶

147. Design a Web Analytics Service (Google Analytics)¶

• Scale: 50M sites, 100B events/day.
• Core features: Pageview tracking, funnels, real-time, retention.
• Hard parts: Cardinality of dimensions, privacy (GDPR), real-time aggregates.
• Discuss: Sampling for big sites; columnar OLAP.

148. Design a Logging / Log-aggregation Platform (Splunk / Datadog Logs)¶

• Scale: 10PB ingest/day.
• Core features: Ingest, parse, search, alert.
• Hard parts: Hot-cold tiering, query on petabytes, multi-tenant cost.
• Discuss: Inverted index per time-bucket; Lucene-based.

149. Design a Metrics Platform (Prometheus / Datadog Metrics)¶

• Scale: 100M time-series, sub-second writes.
• Core features: Ingest gauge/counter, query (PromQL), alert.
• Hard parts: Cardinality explosion, downsampling, long-term storage.
• Discuss: Pull vs push (Prom is pull); remote-write.

150. Design Distributed Tracing (Jaeger / Honeycomb)¶

• Scale: 1B spans/day, query <2s.
• Core features: Span ingest, trace assembly, query by trace-id, latency-tail dashboards.
• Hard parts: Sampling strategy, span volume, trace stitching across services.
• Discuss: Head vs tail sampling; OpenTelemetry.

151. Design a Real-time Dashboard (Grafana-like)¶

• Scale: 10K concurrent dashboards.
• Core features: Multi-source query, panels, alerts.
• Hard parts: Query backend across data sources, cache freshness.
• Discuss: Live mode via streams; query coalescing.

152. Design a Click-stream Pipeline (Snowplow)¶

• Scale: 10B events/day.
• Core features: Ingest event, validate schema, enrich, sink to warehouse.
• Hard parts: Schema evolution, late-arriving data, dedup.
• Discuss: Streaming SQL; self-describing JSON.

153. Design a Funnel Analysis Engine¶

• Scale: Compute conversion across 7-day funnel.
• Core features: Define steps, compute counts at each step.
• Hard parts: Joinless multi-step (sessionize), order matters.
• Discuss: Bitmap per user; pre-computed funnels.

154. Design a Cohort Analysis Engine¶

• Scale: Cohort by signup-week, retention curve.
• Core features: Cohort definition, retention chart.
• Hard parts: Compute on event store at warehouse scale.
• Discuss: Materialized views; OLAP cube.

155. Design Real-time Anomaly Detection¶

• Scale: 100K series, alert in seconds.
• Core features: Stream, baseline, alert.
• Hard parts: Seasonality, bursts, false-positive rate.
• Discuss: EWMA, Prophet, isolation forest.

156. Design a Centralized Audit Log (SOC2)¶

• Scale: Immutable, 7-year retention.
• Core features: Append-only, search, tamper-evident.
• Hard parts: Immutability guarantee, fast point-in-time search.
• Discuss: Hash-chained log; WORM storage.

⚙️ 15. Infrastructure & DevOps¶

157. Design a CI/CD Platform (Jenkins / CircleCI)¶

• Scale: 1M jobs/day, 100K concurrent.
• Core features: Trigger build, run pipeline, parallel jobs, artifacts.
• Hard parts: Worker autoscale, build cache, secret injection.
• Discuss: Job queue, build isolation (containers vs VMs).

158. Design a Feature-Flag Service (LaunchDarkly)¶

• Scale: 10K customers, sub-50ms flag evaluation.
• Core features: Define flag, target rules, percentage rollout, audit.
• Hard parts: Edge-evaluation (no roundtrip), instant propagation.
• Discuss: SSE / streaming for flag updates; SDK with local cache.

159. Design a Configuration Service (Etcd / Consul)¶

• Scale: Strong-consistent KV for cluster.
• Core features: Get/Put with watch, leases, distributed lock.
• Hard parts: Linearizable reads, lease expiration, leader election.
• Discuss: Raft; how watches handle disconnects.

160. Design a Container Orchestration (Kubernetes-lite)¶

• Scale: 5K nodes, 100K pods.
• Core features: Schedule pods, health-check, autoscale, networking.
• Hard parts: Scheduler bin-packing, networking (CNI), service discovery.
• Discuss: etcd for state; controller pattern.

161. Design a Function-as-a-Service Platform (Lambda)¶

• Scale: Cold start in <100ms.
• Core features: Upload code, trigger, scale, billing per ms.
• Hard parts: Sandbox (Firecracker), warm-pool, cold-start mitigation.
• Discuss: MicroVM vs containers; pre-warming.

162. Design a Secret-Management System (Vault)¶

• Scale: 1M secrets, audit.
• Core features: Store secret, lease, rotate, audit log.
• Hard parts: Encryption at rest with HSM, dynamic secrets, ACLs.
• Discuss: Master key sealing; auto-unseal.

163. Design a DNS Provider (Route53)¶

• Scale: Anycast, billions of queries/day.
• Core features: Zone management, geolocation routing, health checks.
• Hard parts: Anycast routing, fast propagation, DDoS resilience.
• Discuss: Why edge resolvers + auth servers split; cache TTL.

164. Design a Load Balancer (L7)¶

• Scale: 1M RPS per LB instance.
• Core features: TLS termination, routing, health checks, sticky sessions.
• Hard parts: Connection pool to backends, graceful drain.
• Discuss: L4 vs L7 trade-offs; consistent hashing for cache-affinity.

165. Design a Service Mesh (Istio / Linkerd)¶

• Scale: 10K services, mTLS everywhere.
• Core features: Sidecar proxy, retries, traffic shifting, telemetry.
• Hard parts: Sidecar overhead, control-plane scale, certificate rotation.
• Discuss: Envoy proxy; xDS API.

166. Design a Build-cache / Remote-build Execution (Bazel RBE)¶

• Scale: 1B build actions/day.
• Core features: Cache hit lookup, remote execution, deterministic build.
• Hard parts: Hash-action correctness, distributed cache, hermetic builds.
• Discuss: Action input hashing; cache eviction.

167. Design a Package Registry (npm / PyPI)¶

• Scale: 200B downloads/year.
• Core features: Publish package, version, dependency, search.
• Hard parts: Squatting, supply-chain attacks, mirror.
• Discuss: Immutable publishes; signing.

168. Design a Multi-tenant SaaS Database¶

• Scale: 10K tenants, isolation + shared cost.
• Core features: Per-tenant data, query, backup.
• Hard parts: Noisy neighbor, schema migration, per-tenant restore.
• Discuss: Shared vs siloed vs hybrid; tenant_id in row.

🤖 16. AI / ML Systems¶

169. Design an LLM Serving Platform (Like Anthropic API)¶

• Scale: 1M req/sec, multiple model sizes.
• Core features: API endpoint, streaming, batching, billing.
• Hard parts: GPU scheduling, KV-cache reuse, queueing.
• Discuss: Continuous batching; speculative decoding.

170. Design an LLM Gateway / Cost-Routing Layer¶

• Scale: Multi-model (OpenAI, Anthropic, OSS).
• Core features: Route by cost/latency, cache, fallback.
• Hard parts: Cache key for prompts, fallback during outage, cost attribution.
• Discuss: Semantic vs exact-match cache; circuit breaker per provider.

171. Design a Vector DB-Backed RAG System¶

• Scale: 1B documents indexed.
• Core features: Ingest, embed, retrieve, generate.
• Hard parts: Chunking, hybrid search, freshness.
• Discuss: Re-ranker, latency budget, eval pipeline.

172. Design a Recommendation Pipeline (Two-tower Model)¶

• Scale: 1B users, 100M items.
• Core features: Train, deploy, serve, online features.
• Hard parts: ANN for retrieval, online + offline features, ranker.
• Discuss: Candidate gen vs ranker; cold start.

173. Design an ML Feature Store (Feast)¶

• Scale: 10K features, online + offline.
• Core features: Define feature, materialize, serve online (Redis), offline (warehouse).
• Hard parts: Train-serve skew, online TTL, point-in-time correctness.
• Discuss: Why two stores; unified read API.

174. Design an ML Model Registry + CI/CD for Models¶

• Scale: 1000 models, multi-team.
• Core features: Register, version, promote, rollback, A/B.
• Hard parts: Reproducibility, lineage, shadow deploy.
• Discuss: Why model registry; canarying for ML.

175. Design a Federated Learning Platform¶

• Scale: 1B mobile devices.
• Core features: Train on-device, aggregate, deploy.
• Hard parts: Differential privacy, secure aggregation, client selection.
• Discuss: FedAvg; opt-in/out.

176. Design a Speech-to-Text Service¶

• Scale: Millions of streams.
• Core features: Real-time + batch transcription, multi-language.
• Hard parts: Streaming inference, GPU pooling, language detection.
• Discuss: Word-by-word streaming; punctuation post-process.

177. Design a Content-Moderation Pipeline (CSAM / Toxicity)¶

• Scale: 1B images/day.
• Core features: Auto-detect (ML), human review queue, action.
• Hard parts: Latency for posting flow, escalation, appeals.
• Discuss: Sync block vs async post-publish; reviewer tooling.

178. Design an OCR Pipeline (Receipts, IDs)¶

• Scale: 100M docs/day.
• Core features: Upload doc, extract structured fields, redact PII.
• Hard parts: Multi-format, language support, accuracy thresholds.
• Discuss: Two-stage (detect → recognize); confidence-driven human review.

🔐 17. Security & Identity¶

179. Design SSO / OIDC Identity Provider¶

• Scale: 100M users.
• Core features: Login, refresh, logout, MFA, SAML/OIDC.
• Hard parts: Token revocation, device trust, session lifetimes.
• Discuss: Rotating refresh tokens; risk-based step-up.

180. Design a Permission System (Google Zanzibar)¶

• Scale: 10B objects, sub-50ms check.
• Core features: Define relation, check access, list permitted users.
• Hard parts: Graph traversal latency, consistent reads, write throughput.
• Discuss: Zookies for consistency; relation-tuple storage.

181. Design a 2FA / OTP Service¶

• Scale: 100M users.
• Core features: TOTP, SMS, push, backup codes.
• Hard parts: Replay protection, sub-30s validity, lockout.
• Discuss: TOTP secret storage; backup-code one-time use.

182. Design a Password-Reset Flow¶

• Scale: Common flow with security gotchas.
• Core features: Email link, single-use, expire.
• Hard parts: Email enumeration, reset-token leakage, race condition.
• Discuss: Generic response messages; signed tokens.

183. Design a Bot-Detection / CAPTCHA System¶

• Scale: 10M req/sec.
• Core features: Score request, challenge if low.
• Hard parts: Adversarial, accessibility, mobile.
• Discuss: Behavioral signals; risk-engine + challenge.

184. Design a Web Application Firewall (WAF)¶

• Scale: 1M RPS.
• Core features: Rule engine, allow/block, log, virtual patching.
• Hard parts: Performance of regex rules, false positive, rule rollouts.
• Discuss: Edge deployment; managed rule sets.

185. Design a DDoS Mitigation System¶

• Scale: Tbps attacks.
• Core features: Detect, scrub, allow-list legit traffic.
• Hard parts: Volumetric vs L7, BGP rerouting, false positive.
• Discuss: Scrubbing centers; rate limiting at edge.

186. Design a Secret-Scanning Service (GitHub Secret Scanning)¶

• Scale: Scan billions of commits.
• Core features: Detect leaked secrets, notify token providers, revoke.
• Hard parts: Pattern matching at scale, partner integrations.
• Discuss: Streaming scan vs batch; token-provider partnerships.

🛠️ 18. Communication Tools¶

187. Design Email Service (Gmail-lite)¶

• Scale: 1B users, IMAP/SMTP.
• Core features: Send, receive, search, filter, spam.
• Hard parts: Spam (Bayesian/ML), search, threading, attachment.
• Discuss: Per-user inverted index; conversation grouping.

188. Design a Calendar Service (Google Calendar)¶

• Scale: 1B users, recurring events.
• Core features: Event CRUD, recurring rules (RRULE), invite, free-busy.
• Hard parts: Recurring expansion, time zones with DST, free/busy across calendars.
• Discuss: Materialize occurrences vs compute; conflict detection.

189. Design a Contact / Address Book Sync¶

• Scale: 1B contacts.
• Core features: CRUD, multi-device sync, dedup.
• Hard parts: Merge from multiple sources, conflict on edit.
• Discuss: vCard standard; sync token.

190. Design a To-Do / Task App with Sync (Todoist)¶

• Scale: 30M users, multi-device.
• Core features: Tasks, projects, reminders, sync.
• Hard parts: Offline edits, conflict on simultaneous edits.
• Discuss: CRDT for tasks; reminder scheduler.

191. Design a Voicemail Transcription Service¶

• Scale: 100M users.
• Core features: Receive call, record, transcribe, deliver.
• Hard parts: Speech-to-text accuracy, multi-language, async pipeline.
• Discuss: Telephony integration; cost per minute.

192. Design Survey / Form Builder (Google Forms / Typeform)¶

• Scale: 10M forms, 1B submissions.
• Core features: Form builder, submission, response export, analytics.
• Hard parts: Anti-spam, conditional logic, integrations.
• Discuss: Form schema; per-form quotas.

193. Design a Screen-Sharing System (TeamViewer / RustDesk)¶

• Scale: 1M concurrent sessions.
• Core features: Screen capture, remote control, file transfer.
• Hard parts: Low-latency video, NAT traversal, security.
• Discuss: Direct P2P with relay fallback.

194. Design Webinar Platform (Zoom Webinars)¶

• Scale: 10K attendees per webinar.
• Core features: Presenter video, attendees view-only, Q&A, polls.
• Hard parts: One-to-many fanout, large-scale chat.
• Discuss: RTMP ingest + HLS delivery; scaling to 10K.

🔗 19. URL / Identifier / Misc¶

195. Design a URL Shortener (Bitly)¶

• Scale: 1B shortens, 10B clicks/month, 200:1 read:write.
• Core features: Shorten URL, redirect, click tracking, custom alias.
• Hard parts: ID generation (base62 of counter / hash), 301 vs 302, click analytics.
• Discuss: Counter-based vs random vs hash; cache for hot links.

196. Design a Distributed Unique-ID Generator (Snowflake)¶

• Scale: 1M IDs/sec, cluster-wide unique.
• Core features: 64-bit ID, sortable.
• Hard parts: Clock skew, machine ID assignment.
• Discuss: Twitter Snowflake bit layout; ULID alternative.

197. Design a Distributed Rate Limiter¶

• Scale: Limit per-user per-API per-minute.
• Core features: Allow/deny, per-key bucket.
• Hard parts: Cross-region consistency, sliding window vs token bucket.
• Discuss: Redis + Lua; sticky vs distributed counter.

198. Design a QR-Code Service (Generate + Scan-tracked)¶

• Scale: 100M QR codes.
• Core features: Generate, host short URL, track scans.
• Hard parts: High-volume redirect, dynamic QR, fraud.
• Discuss: CDN for QR images; rate limit per endpoint.

199. Design a Pastebin / Code Snippet Sharing¶

• Scale: 100M snippets.
• Core features: Upload, expiry, syntax highlight, public/private.
• Hard parts: Spam, abuse, malicious payloads.
• Discuss: Append-only store; expire job.

200. Design a Geocoding / Reverse-Geocoding Service¶

• Scale: 100K QPS.
• Core features: Address → lat/long, lat/long → address.
• Hard parts: Address normalization, multi-language, autocomplete.
• Discuss: Multi-tier cache; geohash bucketing.

201. Design a Translation Service (Google Translate)¶

• Scale: 1B requests/day, 100+ languages.
• Core features: Translate text/document, language detect, cache.
• Hard parts: Cache for repeated phrases, latency, batch large docs.
• Discuss: Sentence-level cache; ML inference fleet.

202. Design a Webhook Delivery Service (Stripe Webhooks)¶

• Scale: Reliable delivery to customer endpoints.
• Core features: Send event, retry on failure, signature.
• Hard parts: Slow customer endpoints, retry budget, replay attack protection.
• Discuss: Per-customer queue; signed payloads.

🏥 20. Healthcare & Medical¶

203. Design an Electronic Health Record (EHR) System¶

• Scale: 100M patient records, HIPAA-compliant, 50K hospitals.
• Core features: Patient profile, medical history, prescriptions, lab results, multi-provider access.
• Hard parts: HL7/FHIR integration, audit trail per access, break-glass emergency access, decades retention.
• Discuss: Multi-tenant DB strategy; field-level encryption for sensitive data.

204. Design a Telemedicine Video-Visit Platform¶

• Scale: 10M consultations/month, peak times.
• Core features: Schedule visit, waiting room, video call, e-prescription, payment.
• Hard parts: HIPAA-compliant WebRTC, recording for legal, e-Rx integration with pharmacies.
• Discuss: Where TURN servers sit; record-everything vs no-record policy.

205. Design a Hospital Patient Admission System¶

• Scale: 5K beds across multi-site hospital network.
• Core features: Admission, bed assignment, transfer, discharge, insurance verification.
• Hard parts: Bed availability across departments, transfer chain, insurance authorization async.
• Discuss: Bed-state machine; race condition on simultaneous admissions.

206. Design a Pharmacy Prescription System¶

• Scale: 50M Rx/year, multi-pharmacy.
• Core features: Rx receive, fill, refill, controlled-substance tracking, drug-interaction check.
• Hard parts: DEA controlled-substance audit, drug-interaction DB, fill-time SLA.
• Discuss: Why a real-time interaction engine; supplier integration.

207. Design a Medical Imaging (PACS) Storage System¶

• Scale: Petabyte-scale DICOM images.
• Core features: Upload imaging study, viewer integration, multi-site replication.
• Hard parts: DICOM compliance, on-demand decompression, radiologist viewer SLA.
• Discuss: Archive tiering; instant-load thumbnail generation.

208. Design a Clinical Trial Data Platform¶

• Scale: Multi-site trials, FDA-compliant.
• Core features: Patient enrollment, eCRF (case report forms), adverse event reporting, audit.
• Hard parts: 21 CFR Part 11 compliance, data lock for analysis, blind/unblind.
• Discuss: Why immutability is key; CDISC standards.

209. Design a Pulse-Oximeter / Wearable Health Aggregator¶

• Scale: 10M devices, second-by-second data.
• Core features: Ingest signals, anomaly detection, alert clinician.
• Hard parts: Battery efficiency, false-positive alerts, HIPAA encryption end-to-end.
• Discuss: Edge filter at phone vs full-fidelity stream.

210. Design a Mental-Health Therapy Booking Platform (BetterHelp)¶

• Scale: 4M users, therapist matching.
• Core features: Onboarding questionnaire, match therapist, chat, video.
• Hard parts: Anonymity with safety, escalation for self-harm, therapist licensing per state.
• Discuss: PII redaction in transcripts; emergency-contact policy.

211. Design a Vaccine Inventory & Cold-Chain Tracker¶

• Scale: Country-scale, 1B doses.
• Core features: Track vials, temperature, administer dose, audit.
• Hard parts: IoT sensor reliability, gap detection in cold chain, allocation fairness.
• Discuss: Edge buffering; central reconciliation.

212. Design a Genomic Sequencing Pipeline¶

• Scale: 100K samples/year, ~100GB per genome.
• Core features: Upload reads, alignment, variant call, store + query.
• Hard parts: Compute-heavy aligners, genomic DB, privacy of DNA data.
• Discuss: Why container-based pipeline (Cromwell/Nextflow); sample-level encryption.

213. Design an Insurance Claims-Adjudication System (Healthcare)¶

• Scale: 1B claims/year.
• Core features: Claim submission, eligibility check, adjudicate, pay/deny.
• Hard parts: Coding (ICD-10, CPT), prior-auth, appeals, fraud.
• Discuss: Rule-engine vs ML; clearinghouse integration.

214. Design a Hospital Radiology Workflow System¶

• Scale: 100K studies/day per hospital network.
• Core features: Order imaging, route to radiologist, report, sign-off.
• Hard parts: Worklist routing by sub-specialty, urgent vs routine, second-opinion.
• Discuss: Priority queue; tele-radiology routing across timezones.

215. Design a Personal Health Record (PHR) — Apple Health-like¶

• Scale: 1B users, multi-source aggregation.
• Core features: Aggregate from devices, hospitals, manual; trends; share.
• Hard parts: Source-of-truth conflicts, FHIR ingest, export to provider.
• Discuss: On-device storage vs cloud; HealthKit-like model.

216. Design a Clinical Decision Support (CDS) Engine¶

• Scale: Real-time alerts at every chart-open.
• Core features: Ingest patient state, run rule sets, surface alerts to clinician.
• Hard parts: Alert fatigue, real-time perf, evidence updates.
• Discuss: Tiered severity; rule authoring UI.

217. Design a Medical Device IoT Platform (Hospital-grade)¶

• Scale: 10K beds × 20 devices each.
• Core features: Stream vitals, alarms, archive.
• Hard parts: FDA SaMD classification, clock-sync, alarm storm.
• Discuss: Edge gateway per ward; alarm de-duplication.

218. Design a Dental Practice Management System¶

• Scale: 100K practices.
• Core features: Appointment, charting (tooth-by-tooth), imaging, billing.
• Hard parts: Tooth-state model, X-ray storage, multi-doctor schedule.
• Discuss: Per-tenant isolation; insurance eligibility integration.

219. Design a Donor-Match (Blood / Organ) System¶

• Scale: National registry, 100K donors.
• Core features: Match recipient to donor by blood type, HLA, geography.
• Hard parts: Multi-criteria match, life-or-death SLA, fairness algorithm.
• Discuss: Why matching is not horizontally sharded; ranking ethics.

220. Design a Health Insurance Provider-Network Search¶

• Scale: 5M providers, in-network filter.
• Core features: Search by specialty, location, plan, accepting-patients.
• Hard parts: Provider data freshness ("ghost networks"), plan-network mapping.
• Discuss: Provider-data ingest pipeline; periodic outreach.

221. Design a Patient-Portal Messaging System¶

• Scale: 50M users, HIPAA-secured.
• Core features: Patient ↔ care team chat, attachments, response SLA.
• Hard parts: Triage routing, auto-acknowledge, encryption.
• Discuss: Provider inbox vs patient inbox model.

222. Design a Lab-Result Reporting Pipeline¶

• Scale: 100M results/day.
• Core features: Lab → EHR delivery, abnormal flagging, patient portal release.
• Hard parts: Critical-value escalation, delayed-release for sensitive results.
• Discuss: HL7 v2 vs FHIR R4; delivery confirmation.

223. Design a Prior-Authorization Service¶

• Scale: 200M annual auth requests.
• Core features: Submit auth, attach docs, decision, appeal.
• Hard parts: Async insurer response (days), status updates, document OCR.
• Discuss: Workflow engine; supplemental info loop.

224. Design a Care-Coordination / Case-Management Platform¶

• Scale: Multi-disciplinary team for chronic patients.
• Core features: Care plan, tasks, cross-org messaging, outcomes.
• Hard parts: Different EHR sources, plan-level vs visit-level consent.
• Discuss: Federated identity across organizations.

225. Design a Health-Risk Assessment Engine¶

• Scale: Per-member real-time risk score.
• Core features: Ingest claims + clinical, score, surface high-risk for outreach.
• Hard parts: Feature freshness, claims latency (90 days), explainability.
• Discuss: Rolling features; SHAP for explanations.

226. Design a Telehealth Triage Bot¶

• Scale: 1M assessments/day.
• Core features: Symptom intake, triage to ER/urgent/PCP/self-care.
• Hard parts: Clinical safety, multi-language, escalation.
• Discuss: Decision tree vs LLM; physician audit.

227. Design a Hospital Bed-Management Dashboard¶

• Scale: 10K beds across system.
• Core features: Real-time occupancy, predicted discharges, surge plan.
• Hard parts: Discharge prediction, ED-to-floor handoff, cleaning sync.
• Discuss: Forecasting model; integration with EVS housekeeping.

228. Design a Surgical-Scheduling System¶

• Scale: 500 ORs across network.
• Core features: Schedule case, OR/team allocation, supply, anesthesia.
• Hard parts: Multi-resource constraints, last-minute cancellations, on-call.
• Discuss: Constraint solver; turnover minimization.

229. Design a Patient-Generated Health Data (PGHD) Pipeline¶

• Scale: Wearables + manual entries → EHR.
• Core features: Ingest, transform, surface to clinician dashboard.
• Hard parts: Data validation, signal-to-noise, clinician burden.
• Discuss: Threshold filtering; physician opt-in by metric.

230. Design a Pandemic / Disease-Surveillance System¶

• Scale: Country-wide, multi-source.
• Core features: Aggregate cases, geographic clustering, alerts.
• Hard parts: Data lag, deduplication across labs, privacy.
• Discuss: k-anonymity for public release; case-definition versioning.

231. Design a Maternal/Fetal Monitoring System¶

• Scale: Hospital labor & delivery floor.
• Core features: Continuous CTG capture, archive, AI analysis.
• Hard parts: Sub-second waveform, false-alarm tuning, archive 18 years.
• Discuss: Stream + archive duality; AI-assisted dot-plot.

232. Design a Fitness-to-Drive Health Check Service (DOT / DMV)¶

• Scale: Truck driver medical certification.
• Core features: Schedule exam, certify, expiry tracking, registry submit.
• Hard parts: Federal registry sync, expiry alerts, fraud.
• Discuss: Certificate signing chain.

233. Design a Veterinary EHR¶

• Scale: Pet clinics, 10K practices.
• Core features: Patient = pet, owner accounts, vaccinations, surgery, billing.
• Hard parts: Multi-pet households, breed-specific norms, prescription drugs.
• Discuss: Why species matters in schema.

234. Design a Pharmacy Drug-Recall Notification System¶

• Scale: 70K pharmacies, FDA recall feeds.
• Core features: Detect recall, identify dispensed-to patients, notify pharmacy + patient.
• Hard parts: Lot-level tracking, patient reach, time-bound urgency.
• Discuss: NDC + lot number index; recall severity tiers.

235. Design a Healthcare Provider-Credentialing Platform¶

• Scale: 1M providers, multi-payor.
• Core features: Collect docs, verify license, primary-source verify, expirations.
• Hard parts: Multi-source verification, expiration sweep, payer-specific rules.
• Discuss: Per-state board APIs; document evidence vault.

🚚 21. Logistics, Supply Chain & Shipping¶

236. Design Amazon Warehouse Management System (WMS)¶

• Scale: 200 fulfillment centers, millions of SKUs.
• Core features: Receive, stow, pick, pack, ship; bin location tracking.
• Hard parts: Robot orchestration, optimal pick path, real-time inventory.
• Discuss: Slotting algorithms; pick-to-light vs voice-pick.

237. Design a Last-Mile Delivery Routing System¶

• Scale: 1M packages/day, 10K drivers.
• Core features: Daily route generation, dynamic re-route, proof of delivery.
• Hard parts: VRP at scale, traffic, customer time windows.
• Discuss: Heuristic vs solver; driver-acceptance constraints.

238. Design FedEx/UPS Package Tracking¶

• Scale: Billions of packages/year, multi-leg routing.
• Core features: Scan events, ETA, exception alerts, delivery confirmation.
• Hard parts: Hub-and-spoke routing, scan-event ingestion at scale, ETA prediction.
• Discuss: Why pre-compute ETA per leg; exception policies.

239. Design a Container-Shipping (Maritime) Tracking¶

• Scale: 10M containers in transit.
• Core features: Booking, vessel ETA, port handover, customs status.
• Hard parts: Vessel sparse data, port system integrations, rolled cargo.
• Discuss: AIS + carrier API + EDI 315; ETA recompute cadence.

240. Design a Cold-Chain Logistics System¶

• Scale: Vaccines, perishables.
• Core features: Temp tracking per pallet, alert on excursion, chain-of-custody.
• Hard parts: IoT sensor offline buffering, gap-fill, regulatory audit.
• Discuss: Why immutable temp log; spike rules.

241. Design Uber Freight (Trucking Marketplace)¶

• Scale: 100K carriers, 1M loads/year.
• Core features: Post load, match carrier, dispatch, paperwork.
• Hard parts: Lane pricing, ELD integration, detention/demurrage.
• Discuss: Why instant-book; insurance overlays.

242. Design a Returns Management System¶

• Scale: Reverse logistics for e-commerce.
• Core features: Initiate return, label, receive, inspect, refund.
• Hard parts: Restock vs liquidation, fraud, ASN to warehouse.
• Discuss: State machine; return policy enforcement.

243. Design a Demand-Forecasting Engine for Retail¶

• Scale: 10M SKU-store-day predictions.
• Core features: Ingest history, generate forecast, promotion-aware.
• Hard parts: Seasonality, new-product cold start, promo lift.
• Discuss: Hierarchical forecasting; reconciliation across product/store.

244. Design a Inventory-Replenishment / Auto-Reorder¶

• Scale: Multi-warehouse, 100M SKUs.
• Core features: Stock level monitoring, reorder point, supplier PO.
• Hard parts: Lead time variance, MOQ, multi-supplier.
• Discuss: Continuous review (s,Q) vs periodic (R,S).

245. Design a Multi-Carrier Shipping API (ShipStation)¶

• Scale: 10M small merchants.
• Core features: Quote rates, generate label, track, returns.
• Hard parts: Each carrier's API quirks, address validation, rate-shopping.
• Discuss: Adapter pattern; rate-cache TTL.

246. Design a Dock-Scheduling / Yard-Management System¶

• Scale: Distribution center receiving.
• Core features: Appointment booking, gate check-in, dock assignment.
• Hard parts: Trailer-status sync, dock conflicts, detention billing.
• Discuss: Yard-map state; OCR for trailer numbers.

247. Design a Customs / Import-Compliance Platform¶

• Scale: Cross-border B2C and B2B.
• Core features: HS-code classify, duties calc, declaration.
• Hard parts: HS classification ML, country-specific rules, restricted items.
• Discuss: Rule engine + ML; broker integration.

248. Design Pallet/Tote-level Tracking with RFID¶

• Scale: 100K reads/sec at warehouse.
• Core features: RFID scan, location tracking, dwell time.
• Hard parts: Phantom reads, real-time aggregation, hardware diversity.
• Discuss: Edge filtering; reader topology.

249. Design a Manifest System for Airlines (Cargo)¶

• Scale: Major hub airport.
• Core features: AWB creation, ULD pack, weight & balance, security screen.
• Hard parts: Hazmat rules, dangerous-goods, partial unloads.
• Discuss: IATA messaging; deadline-driven workflow.

250. Design a Locker-Pickup Network (Amazon Hub)¶

• Scale: 100K lockers.
• Core features: Carrier drop, customer pickup with code, expiry.
• Hard parts: Locker-state sync, theft, return.
• Discuss: IoT lock + per-locker keys; capacity rotation.

251. Design a Drone-Delivery Dispatch System¶

• Scale: Suburban delivery, 10K drones.
• Core features: Order → drone → route → land → return.
• Hard parts: Airspace coordination, weather routing, no-fly zones.
• Discuss: UTM integration; battery & swap planning.

252. Design a Trucking Telematics Platform (Samsara-like)¶

• Scale: 1M trucks.
• Core features: Engine data, GPS, driver behavior, video.
• Hard parts: Cell coverage gaps, video upload at scale, ELD compliance.
• Discuss: Edge buffer + compress upload.

253. Design a Warehouse Robotics Orchestrator¶

• Scale: 1000 robots in one facility.
• Core features: Task assignment, path planning, charging schedule.
• Hard parts: Collision avoidance, deadlock-free traffic, robot-failure recovery.
• Discuss: Centralized vs distributed planner.

254. Design a Crowd-Sourced Delivery (Doordash Drive / Roadie)¶

• Scale: Driver acquisition + fleet match.
• Core features: Post task, claim by driver, navigate, complete.
• Hard parts: Independent contractors, surge pricing, fraud (fake deliveries).
• Discuss: Geofence at pickup/drop; signature verification.

255. Design a Cross-Dock System¶

• Scale: Inbound trailer → outbound trailer < 24h.
• Core features: Receive, sort, stage, ship.
• Hard parts: No-storage flow, sorter routing, delay propagation.
• Discuss: Wave planning; dynamic dock assignment.

256. Design a Procurement (Purchase Order) System¶

• Scale: 100K POs/day at large enterprise.
• Core features: Requisition, approval, PO, receiving, 3-way match.
• Hard parts: Approval workflow, partial receipts, invoice mismatch.
• Discuss: Workflow engine; ERP integration.

257. Design a Vendor / Supplier-Risk Management Platform¶

• Scale: 10K vendors.
• Core features: Onboarding, risk score (financial, geo, cyber), monitoring.
• Hard parts: Continuous data ingest, news monitoring, periodic reassessment.
• Discuss: Score weighting; alert routing.

258. Design a Lot/Batch Traceability System (Food Safety)¶

• Scale: Global supply chain.
• Core features: Track lot from farm → store, recall trace, certificate.
• Hard parts: Multi-tier supplier visibility, immutable trace, partial info.
• Discuss: GS1 standards; blockchain vs DB.

259. Design a Toll-Booth / Open-Road Tolling System¶

• Scale: Highway-wide ANPR.
• Core features: ALPR plate read, account match, charge.
• Hard parts: Read accuracy, plate-not-recognized escalation, billing.
• Discuss: Async OCR; reconciliation with transponder.

260. Design a Reverse-Auction Procurement Platform¶

• Scale: Buyers post RFQ, suppliers bid.
• Core features: RFQ, sealed/open bid, award.
• Hard parts: Bid sniping, blind vs visible, multi-currency.
• Discuss: Auction-end extension rules.

261. Design a Fuel-Card / Fleet-Card System¶

• Scale: 10M cards.
• Core features: Authorize at pump, limit by driver/vehicle, fraud.
• Hard parts: Sub-second auth, geo-fraud detection, chargeback.
• Discuss: Card-network integration; offline limits.

262. Design an EDI Translation / B2B Integration Platform¶

• Scale: Thousands of trading-partner connections.
• Core features: Inbound/outbound EDI mapping, ACK, monitoring.
• Hard parts: Per-partner schema, AS2, retry/resend.
• Discuss: Why event-sourced; partner onboarding workflow.

263. Design a Last-Mile Locker-Network for Apartments¶

• Scale: Building-level smart lockers.
• Core features: Resident notification, code-based pickup, carrier interface.
• Hard parts: Multi-carrier auth, package size mismatch, expiry.
• Discuss: API for carrier "drop"; bin-size matching.

264. Design an Auto-Delivery / Subscribe & Save System¶

• Scale: Predictable replenishment for 10M households.
• Core features: Schedule cadence, skip, swap.
• Hard parts: Reschedule cascading, inventory commit, price drift.
• Discuss: Forecasting demand; lead-time aware ordering.

265. Design a Dispatching Optimizer for Snowplow / City Services¶

• Scale: City-wide fleet.
• Core features: Plan routes per snow event, real-time adapt.
• Hard parts: Priority streets, equipment match, weather feedback.
• Discuss: GIS overlay; replan on accident.

266. Design a Container-Yard Stacking System¶

• Scale: Port terminal.
• Core features: Where to stack; how to retrieve fastest.
• Hard parts: Stacking depth, retrieval shuffles, ship-call schedule.
• Discuss: ML for predicted retrieval time.

267. Design a Blockchain-based Provenance for Diamonds (Everledger)¶

• Scale: 1M stones.
• Core features: Cert-of-origin, ownership chain, transfer.
• Hard parts: Off-chain → on-chain bridging, certificate issuer trust.
• Discuss: Why permissioned chain; data minimization.

268. Design a Customs Bonded-Warehouse Inventory¶

• Scale: Multi-importer goods, duty-deferred.
• Core features: Track duty status, withdraw, re-export.
• Hard parts: Complex customs reporting, partial withdrawals.
• Discuss: Bond status state machine; auditor view.

269. Design a Fleet Vehicle-Maintenance Platform¶

• Scale: 100K vehicles.
• Core features: PM schedule, repair orders, parts, technician.
• Hard parts: Telematics-driven PM, downtime impact, parts ETA.
• Discuss: Telematics → fault → work-order automation.

270. Design a Supply-Chain Control Tower (Visibility Dashboard)¶

• Scale: Enterprise-wide.
• Core features: End-to-end visibility, exception detection, scenario plan.
• Hard parts: Heterogeneous data, real-time alerts, what-if simulation.
• Discuss: Event hub; graph data model for supply chain.

🎓 22. Education / E-Learning¶

271. Design Coursera / Udemy (MOOC Platform)¶

• Scale: 100M learners, video courses.
• Core features: Enroll, video, quizzes, certificate.
• Hard parts: Video DRM, progress tracking cross-device, multi-language.
• Discuss: Quiz autograder; certificate signing.

272. Design Khan Academy / Duolingo¶

• Scale: 500M learners, gamified, adaptive.
• Core features: Lessons, streaks, leaderboards, mastery.
• Hard parts: Adaptive curriculum, A/B tests, retention.
• Discuss: Skill graph; spaced repetition.

273. Design an Online Coding Practice Platform (LeetCode)¶

• Scale: 10M users, 100K problems, sandboxed runs.
• Core features: Submit code, run tests, leaderboard, contests.
• Hard parts: Sandboxed execution, time/memory limits, judge consistency.
• Discuss: Container per submission; test-case caching.

274. Design a Live Tutoring Marketplace (Wyzant / VIPKid)¶

• Scale: 1M tutors, video lessons.
• Core features: Search tutor, schedule, video, payment.
• Hard parts: Cross-timezone scheduling, no-show handling, payouts.
• Discuss: Video room provisioning; rating fraud.

275. Design a School LMS (Moodle / Canvas)¶

• Scale: Multi-school district.
• Core features: Course content, assignments, grading, gradebook.
• Hard parts: SIS integration, parent access, accessibility.
• Discuss: Per-school multitenancy; LTI tools.

276. Design an Online Proctoring System (ExamSoft)¶

• Scale: Bar exam, 10K simultaneous.
• Core features: Lockdown browser, webcam, AI flagging, human review.
• Hard parts: Cheating detection, false positives, accessibility.
• Discuss: Local capture + upload; chain-of-custody.

277. Design Quiz-Based Game (Kahoot!)¶

• Scale: Classroom + viral, 100K simultaneous rooms.
• Core features: Host quiz, join code, real-time scoring.
• Hard parts: Sub-second scoring at viral scale, anti-cheat.
• Discuss: Pub/sub per room; latency normalization.

278. Design a Plagiarism-Detection Service (Turnitin)¶

• Scale: 50M papers/year.
• Core features: Submit paper, compare to corpus, similarity report.
• Hard parts: Big corpus, fingerprint hashing, citations.
• Discuss: N-gram fingerprints; cluster lookup.

279. Design a Student-Information System (PowerSchool)¶

• Scale: Districts of 100K students.
• Core features: Enroll, schedule, attendance, grades, transcripts.
• Hard parts: Year rollover, transcript archive, parent portal.
• Discuss: Fiscal year vs school year; data retention.

280. Design an Adaptive Learning Engine¶

• Scale: Per-student dynamic difficulty.
• Core features: Item bank, IRT model, next-item selection.
• Hard parts: Real-time scoring, item drift, exposure control.
• Discuss: Bayesian Knowledge Tracing.

281. Design a School Bus-Route Planning System¶

• Scale: Per-district daily routes.
• Core features: Stops, capacity, time windows, special needs.
• Hard parts: Constraint solver, last-minute address change, multi-tier schools.
• Discuss: Walking-radius rules; transfer routes.

282. Design a Homework-Help Q&A Platform (Chegg)¶

• Scale: 100M questions.
• Core features: Post Q, expert answers, search archive.
• Hard parts: Honor-code policing, auto-suggest similar Qs, cheating risk.
• Discuss: Image OCR for problem photos.

283. Design a Live-Streaming Lecture System (Zoom for Edu)¶

• Scale: 1M concurrent classrooms.
• Core features: Lecture stream, breakout, recording, captions.
• Hard parts: Multi-camera (whiteboard + face), low-bandwidth fallback.
• Discuss: SVC encoding; transcription pipeline.

284. Design a Course-Recommendation Engine¶

• Scale: 100K courses, 100M users.
• Core features: "Based on your progress" suggest next.
• Hard parts: Cold start, knowledge dependency graph, interest evolution.
• Discuss: Embedding-based; skill-gap detection.

285. Design a Code-Review Educational Tool (Replit Pair Programming)¶

• Scale: Student groups.
• Core features: Shared editor, instructor view, comments.
• Hard parts: Real-time collaboration, instructor monitoring.
• Discuss: OT for editor; scoped code execution.

286. Design a Test-Prep / Practice-Test Platform (Khan SAT)¶

• Scale: Millions of practice tests.
• Core features: Timed tests, scoring, review.
• Hard parts: Item exposure control, cheating, score reliability.
• Discuss: Item bank rotation; timer enforcement.

287. Design a Research-Paper Repository (arXiv)¶

• Scale: 2M papers, daily preprints.
• Core features: Submit, version, search, alert.
• Hard parts: PDF rendering, LaTeX build, citation graph.
• Discuss: Build pipeline; DOI assignment.

288. Design a Citation-Manager (Zotero)¶

• Scale: 10M users, sync.
• Core features: Collect, annotate, generate bibliography.
• Hard parts: Browser-extension capture, attachment storage, multi-style output.
• Discuss: CSL styles; PDF annotation sync.

289. Design a University Course-Registration System¶

• Scale: 50K students, registration window.
• Core features: Browse, register, drop, waitlist.
• Hard parts: Last-second rush, capacity caps, prereq enforcement.
• Discuss: Queue at registration open; race conditions.

290. Design an Online Whiteboard Tutoring Tool¶

• Scale: Live tutor & student session.
• Core features: Whiteboard, audio, screen share, math input.
• Hard parts: Math equation rendering, low-latency drawing.
• Discuss: Vector vs raster sync.

291. Design an Education Standards (CCSS) Mapping System¶

• Scale: Map content → standards across states.
• Core features: Tag content, align lessons, report coverage.
• Hard parts: Standards versioning, multi-state mapping.
• Discuss: Graph DB; cross-walk algorithm.

292. Design a Special-Education IEP System¶

• Scale: District-level, K-12.
• Core features: IEP draft, meeting workflow, progress reporting.
• Hard parts: Compliance with IDEA, signatures, audit.
• Discuss: Document templates; e-sign.

293. Design an Educational AI-Tutor (Khanmigo)¶

• Scale: 1M students, LLM-backed.
• Core features: Conversational tutor, hint not answer, math.
• Hard parts: Avoid giving away answer, math correctness, safety.
• Discuss: RAG with curriculum; guardrails.

294. Design a College Application Platform (Common App)¶

• Scale: 1.5M applicants.
• Core features: Common essay, school-specific supplements, recommend letters.
• Hard parts: Submission deadline spike, file uploads, PII.
• Discuss: Deadline-day capacity.

295. Design an Alumni Network / Career Services Portal¶

• Scale: Per-university, multi-decade alumni.
• Core features: Directory, mentorship, events, donations.
• Hard parts: Privacy controls, mentor matching, opt-in.
• Discuss: Graph of alum-to-student.

296. Design a Library Borrowing / E-book System¶

• Scale: Public-library e-book lending.
• Core features: Catalog, hold queue, lend (DRM-limited copies).
• Hard parts: Concurrent-copy DRM, hold-position transparency.
• Discuss: Why "1 copy = 1 lend"; OverDrive-style.

297. Design Learning Analytics Dashboard for Teachers¶

• Scale: Per-class, per-student.
• Core features: Engagement, mastery, at-risk alerts.
• Hard parts: Real-time signal, FERPA privacy, false-positive risk.
• Discuss: Aggregate vs individual views.

298. Design a STEM Lab-Simulation Platform (PhET / Labster)¶

• Scale: Browser-based simulations at scale.
• Core features: Interactive labs, save state, instructor view.
• Hard parts: WebGL physics simulation, low-bandwidth, accessibility.
• Discuss: Headless simulator for grading.

299. Design a School-Communication Platform (Remind / ClassDojo)¶

• Scale: Teacher ↔ parent messaging.
• Core features: Group messages, translations, opt-out.
• Hard parts: PII safety, multi-language auto-translate, school-day rules.
• Discuss: Non-real-time delivery; quiet hours.

300. Design a Hackathon / Coding-Competition Platform¶

• Scale: 100K simultaneous participants in finals.
• Core features: Register, submit, judge, leaderboard.
• Hard parts: Submission spike, fair judging, plagiarism.
• Discuss: Container-based judging; queue saturation.

🏠 23. Real Estate¶

301. Design Zillow / Realtor.com Listing Search¶

• Scale: 130M homes, 250M MAU.
• Core features: Search by location/price/beds, photos, Zestimate, save search.
• Hard parts: Geo+facet search, Zestimate ML pipeline, MLS data ingest.
• Discuss: Per-MLS feed sync; price-history append.

302. Design a Rental Listing Platform (Apartments.com)¶

• Scale: 1M units, 50M searches/day.
• Core features: Search, lease application, screening, contact.
• Hard parts: Fake listings, screening provider integration, fair-housing compliance.
• Discuss: Application data retention; phone-anonymization.

303. Design a Property-Management System (Yardi)¶

• Scale: Multi-property landlord, 10M units.
• Core features: Tenant CRUD, lease, rent, maintenance, accounting.
• Hard parts: Multi-entity accounting, maintenance dispatch, lease renewal.
• Discuss: GL integration; per-property isolation.

304. Design a Tenant-Screening Service¶

• Scale: Background + credit pulls.
• Core features: Credit, eviction, criminal, income verification.
• Hard parts: Bureau integration, dispute, FCRA compliance.
• Discuss: Soft-pull vs hard-pull; consent capture.

305. Design Real-Estate Agent CRM¶

• Scale: 1M agents, leads pipeline.
• Core features: Lead capture, drip campaigns, MLS sync, contracts.
• Hard parts: MLS feed merge, e-sign, document templates.
• Discuss: RESO Web API; document versioning.

306. Design a Mortgage-Origination System¶

• Scale: Lender-side, 1M apps/year.
• Core features: Application, doc upload, underwrite, e-close.
• Hard parts: Compliance (TRID, RESPA), credit pull, appraisal vendor.
• Discuss: State machine; document OCR.

307. Design a Home-Insurance Platform¶

• Scale: Quote → policy → renewal.
• Core features: Quote engine, bind, claims.
• Hard parts: Property data sources, peril rating, cancellations.
• Discuss: Aerial imagery underwriting.

308. Design a Home-Showing Scheduling System (ShowingTime)¶

• Scale: 1M showings/month.
• Core features: Request showing, instant approve, lockbox unlock.
• Hard parts: Listing-agent approval window, conflicts, lockbox IoT.
• Discuss: Time-bounded keys; agent calendar sync.

309. Design a 3D Virtual-Tour Platform (Matterport)¶

• Scale: Million homes scanned.
• Core features: Capture from 3D camera, view in browser, embed.
• Hard parts: Mesh storage, viewer perf, link sharing.
• Discuss: Tile-based 3D streaming.

310. Design a Real-Estate Investing / iBuyer (Opendoor)¶

• Scale: Auto-offers on homes.
• Core features: Offer engine, walk-through, close, list.
• Hard parts: Pricing model, repair-cost estimate, hold inventory cost.
• Discuss: Fast-decision pipeline.

311. Design a Construction-Project Management Platform (Procore)¶

• Scale: Per-project, multi-stakeholder.
• Core features: RFI, submittals, daily logs, drawings.
• Hard parts: Drawing version control, photo geo-tagging.
• Discuss: Mobile-first offline; drawing diffs.

312. Design HOA / Condo Management System¶

• Scale: Per-association, dues + voting.
• Core features: Dues collection, violation tracking, voting.
• Hard parts: Compliance, electronic vote integrity.
• Discuss: Audit trail per resident.

313. Design a Roommate-Matching Platform¶

• Scale: University + city-wide.
• Core features: Profile, match preferences, chat.
• Hard parts: Safety, verification, location-aware.
• Discuss: Match score algorithm.

314. Design a Vacation-Rental Calendar Sync (iCal across Airbnb/VRBO)¶

• Scale: Multi-platform host calendars.
• Core features: Sync availability across listing sites.
• Hard parts: Polling cadence, two-way conflicts, time-zone DST.
• Discuss: ICS feed merging; lock window during sync.

315. Design a Smart-Home Real-Estate Walkthrough (IoT)¶

• Scale: Home full of sensors during showing.
• Core features: Door access logs, motion, video.
• Hard parts: Privacy, consent, retention.
• Discuss: Per-showing data isolation.

316. Design a Real-Estate Comp / CMA Tool¶

• Scale: Comparable-market analysis on demand.
• Core features: Pull comps, adjust, generate report.
• Hard parts: Geo + feature similarity, data freshness.
• Discuss: Vector search for comps.

317. Design an Eviction-Workflow / Court-Filing Platform¶

• Scale: Multi-jurisdiction.
• Core features: Notice, file, court-date sync, judgment.
• Hard parts: Per-state procedural differences, e-file integration.
• Discuss: Workflow per jurisdiction.

318. Design a Land-Records / Title-Search Platform¶

• Scale: County recorder data ingestion.
• Core features: Search title chain, lien check, plat maps.
• Hard parts: Heterogeneous county systems, OCR of historic deeds.
• Discuss: Document-image pipeline; chain-of-title model.

✈️ 24. Travel & Hospitality (Advanced)¶

319. Design a Global Distribution System (GDS) (Sabre / Amadeus)¶

• Scale: Trillion-dollar industry, microsecond auth on inventory.
• Core features: Multi-airline inventory, fare quote, ticket issue.
• Hard parts: Schema across airlines, inventory locks, IATA settlement.
• Discuss: Cache-stale risk; PNR storage.

320. Design a Loyalty / Frequent-Flyer Program¶

• Scale: 100M members, multi-airline alliance.
• Core features: Earn, burn, status tiers, partner accrual.
• Hard parts: Multi-currency points, expiration sweeps, fraud.
• Discuss: Append-only ledger; reciprocity rules.

321. Design a Dynamic-Pricing Engine for Hotels (Revenue Mgmt)¶

• Scale: Per-room-night pricing.
• Core features: Forecast demand, adjust price, channel-distribute.
• Hard parts: Multi-channel rate parity, last-minute spikes.
• Discuss: Reinforcement learning; competitor scrape.

322. Design a Travel Insurance Platform¶

• Scale: Quote at booking.
• Core features: Quote, policy, claim flight-delay payout.
• Hard parts: Real-time flight-delay data, fraud, automated payout.
• Discuss: Parametric vs traditional claims.

323. Design a Tour / Experience Booking (GetYourGuide)¶

• Scale: 50K activities, scheduled departures.
• Core features: Search by city/date, capacity, instant confirm.
• Hard parts: Supplier inventory sync, multi-language ops.
• Discuss: Push vs pull from supplier.

324. Design a Travel-Itinerary Planner (TripIt)¶

• Scale: Email-parsing of confirmations.
• Core features: Aggregate flights/hotels/cars into itinerary.
• Hard parts: Email parsing reliability, ICS export, alerts.
• Discuss: ML for confirmation extraction.

325. Design a Concierge / Room-Service System (Hotel)¶

• Scale: Per-hotel, in-room ordering.
• Core features: Menu, order, dispatch staff, billing to room.
• Hard parts: PMS integration, kitchen routing.
• Discuss: OPERA integration; tablet-app provisioning.

326. Design a Hotel Front-Desk PMS (Property Mgmt System)¶

• Scale: Multi-property, 100K rooms.
• Core features: Reservation, check-in, folio, housekeeping.
• Hard parts: Channel manager, group blocks, night audit.
• Discuss: Night-audit batch; rate plan model.

327. Design a Cruise-Booking System¶

• Scale: Multi-ship, multi-cabin.
• Core features: Cabin selection, dining, excursions.
• Hard parts: Cabin-level inventory, group bookings, port operations.
• Discuss: Deck-plan model; itinerary changes.

328. Design an Airline Crew-Scheduling System¶

• Scale: 100K flight crew, regulatory.
• Core features: Pair flights to crew, FAA rest rules, swap.
• Hard parts: FAR 117 compliance, fatigue model, swap board.
• Discuss: Constraint solver; bid-line model.

329. Design a Visa / e-Visa Application Service¶

• Scale: Country-government scale.
• Core features: Apply, biometrics, decision, e-visa.
• Hard parts: Cross-government data, fraud, identity verify.
• Discuss: Workflow tied to gov system; rate-limit per nationality.

330. Design an Airport Self-Service Kiosk Platform¶

• Scale: 1000 kiosks per airport.
• Core features: Check-in, bag tag, boarding pass, payment.
• Hard parts: Carrier multi-DCS integration, kiosk health, queue analytics.
• Discuss: Common-use platform standards (CUPPS).

331. Design a Traveler-Disruption / Re-accommodation System¶

• Scale: Mass cancellation, rebook 100K passengers in 1h.
• Core features: Identify affected, propose options, auto-rebook.
• Hard parts: Inventory race, multi-leg recovery, lodging vouchers.
• Discuss: Why pre-computed swap candidates.

332. Design a Hotel Loyalty-Free-Night Calendar¶

• Scale: Member booking with points.
• Core features: Filter availability, point-cost calc, blackout.
• Hard parts: Rate-vs-points fairness, dynamic point cost.
• Discuss: Optimizer for point value.

333. Design a Travel Expense-Report System (Concur)¶

• Scale: Enterprise, 1M employees.
• Core features: Receipts capture, mileage, approval workflow.
• Hard parts: Card-feed reconciliation, OCR, policy enforcement.
• Discuss: Per-policy rule engine.

334. Design an Airline Operations Control Center (OCC)¶

• Scale: Real-time fleet status.
• Core features: Track flights, ATC delays, swap aircraft.
• Hard parts: Decisions in minutes, multi-system integration, what-if.
• Discuss: Event-stream + simulator.

335. Design a Boarding-Pass / Mobile-Wallet Pass System¶

• Scale: Apple/Google Wallet integration.
• Core features: Issue pass, push update, gate scan.
• Hard parts: Real-time updates (gate change), offline scan.
• Discuss: Push vs pull update; barcode redundancy.

336. Design an On-Demand Helicopter / Charter App (Blade)¶

• Scale: Niche, premium.
• Core features: Routes, seats, dynamic pricing.
• Hard parts: Aircraft availability, weather cancellation.
• Discuss: Weather-API ingestion; auto-rebook.

337. Design Restaurant Tip-Pool / Tip-Out System¶

• Scale: Per-restaurant.
• Core features: Capture tips by shift, allocate to roles.
• Hard parts: Policy customization, payroll integration.
• Discuss: Audit trail; dispute handling.

338. Design a Loyalty-Card Stamp System (Coffee shop punch)¶

• Scale: Independent merchant base.
• Core features: Earn stamp via QR, reward redeem.
• Hard parts: Anti-stamp-spam, multi-merchant.
• Discuss: Server-issued stamp tokens.

339. Design a Resort All-Inclusive Wristband System¶

• Scale: Resort-wide spending tracking.
• Core features: RFID wristband, charge to room.
• Hard parts: POS integration, lost wristband, family limits.
• Discuss: Edge POS w/ central reconciliation.

340. Design a Theme-Park Queue Reservation (Disney Genie+)¶

• Scale: Park-wide, virtual queue.
• Core features: Reserve return time, walk on, ride.
• Hard parts: Capacity allocation, lottery vs FCFS.
• Discuss: Lightning-Lane fairness; demand forecasting.

👔 25. HR & Recruiting¶

341. Design Workday / HRIS (Human Resources Info System)¶

• Scale: Enterprise, 100K employees.
• Core features: Employee record, compensation, time-off, reports.
• Hard parts: Effective-dated changes, multi-country payroll, security.
• Discuss: Bi-temporal model; org-tree changes.

342. Design an Applicant-Tracking System (Greenhouse)¶

• Scale: 10K companies, multi-stage funnels.
• Core features: Job posts, application intake, interview kit, offer.
• Hard parts: Job-board fanout, interview-team coord, EEOC reporting.
• Discuss: Pipeline state machine.

343. Design a Resume Parsing / Search Service¶

• Scale: 100M resumes.
• Core features: Parse PDF, structured fields, semantic search.
• Hard parts: Format diversity, name dedup, skills taxonomy.
• Discuss: ML extraction + human-in-loop.

344. Design a Background-Check Platform (Checkr)¶

• Scale: Multi-source, county-level.
• Core features: SSN trace, county criminal, MVR, education.
• Hard parts: County system integration, FCRA dispute, turnaround.
• Discuss: Per-source SLAs; dispute workflow.

345. Design a Payroll System (Gusto / ADP)¶

• Scale: Multi-state, garnishments.
• Core features: Pay run, tax calc, direct deposit, W-2.
• Hard parts: State + local tax, garnishments, retro.
• Discuss: Tax-engine updates; ACH timing.

346. Design a Time-Tracking / Timesheet System¶

• Scale: Hourly workers, biometric clock.
• Core features: Clock in/out, breaks, approvals.
• Hard parts: Biometric privacy, geofence verify, overtime calc.
• Discuss: State labor laws; meal-break compliance.

347. Design an Employee Performance-Review System¶

• Scale: Annual + quarterly cycles.
• Core features: Self-review, peer-review, manager calibration.
• Hard parts: Multi-rater workflow, calibration, comp link.
• Discuss: Anonymity; 9-box grid.

348. Design an Onboarding-Workflow Engine¶

• Scale: New-hire pipelines.
• Core features: Tasks, e-sign, IT provisioning, training.
• Hard parts: Cross-system provisioning, role-based templates.
• Discuss: SaaS connectors; deprovisioning.

349. Design a Benefits-Enrollment Platform¶

• Scale: Open enrollment annual rush.
• Core features: Plan compare, elect, dependents, deductions.
• Hard parts: Carrier feed, EDI 834, qualifying events.
• Discuss: Plan-design model; deduction split.

350. Design an Internal Job-Board / Mobility Platform¶

• Scale: Internal candidate matching.
• Core features: Skill profile, internal jobs, manager approval.
• Hard parts: Skill graph, privacy, manager visibility.
• Discuss: Two-sided match algorithm.

351. Design an Engineering Levels / Career-Ladder Tool¶

• Scale: Company-wide framework.
• Core features: Ladder docs, self-assess, promo packet.
• Hard parts: Cross-org calibration, evidence collection.
• Discuss: Document templates; promo committee workflow.

352. Design a Recruiting-Ad / Job Distribution Platform¶

• Scale: Post job to 100+ boards.
• Core features: Sponsor jobs, attribution, budget.
• Hard parts: Indeed-style scrape rules, attribution, budget pacing.
• Discuss: Bidder per board.

353. Design a Skills-Assessment Test Platform (HackerRank)¶

• Scale: Coding tests at hire.
• Core features: Test, sandbox run, score, anti-cheat.
• Hard parts: Container isolation, plagiarism, IDE features.
• Discuss: Submission queue; comparison to peers.

354. Design an Interview Scheduling Tool (GoodTime)¶

• Scale: Multi-interviewer panel.
• Core features: Find slots across interviewers, candidate timezone.
• Hard parts: Calendar integration, load-balancing across interviewers.
• Discuss: Calendar polling; debounce holds.

355. Design a Reference-Check Platform¶

• Scale: Confidential surveys.
• Core features: Send survey to references, collect, summarize.
• Hard parts: Anonymity, deliverability.
• Discuss: Anti-bias question framework.

356. Design an Org-Chart Tool¶

• Scale: Real-time updates.
• Core features: Tree view, search, reporting line history.
• Hard parts: Effective-dated reporting changes, matrix orgs.
• Discuss: Graph DB; bitemporal time travel.

357. Design an Employee Surveys & Pulse-Check Platform (Culture Amp)¶

• Scale: Quarterly, 100K employees.
• Core features: Send pulse, anonymize, manager dashboards.
• Hard parts: Anonymity threshold, sentiment trending.
• Discuss: Min-cell suppression.

358. Design a Learning Management System for L&D (Workday Learning)¶

• Scale: Required compliance training.
• Core features: Assign, track, certify, expire.
• Hard parts: Cert renewal, role-based curriculum.
• Discuss: Deadline reminders.

359. Design a Mentorship-Matching Platform¶

• Scale: Internal mentor program.
• Core features: Profile, match, sessions, feedback.
• Hard parts: Match algorithm, capacity per mentor.
• Discuss: Bandit-based match.

360. Design an Employee Ticketing System (HR Helpdesk)¶

• Scale: 100K tickets/year.
• Core features: Submit ticket, route, SLA, knowledge base.
• Hard parts: Routing rules, escalation, multi-region.
• Discuss: Routing engine; KB feedback loop.

361. Design a Compensation Planning Tool (Cycle)¶

• Scale: Annual merit cycle.
• Core features: Manager allocations, budget, equity refresh.
• Hard parts: Budget enforcement, calibration, currency.
• Discuss: Lock periods; dual-approval.

362. Design an Employee Recognition Platform (Bonusly)¶

• Scale: Peer-to-peer points.
• Core features: Send points, monthly budget, redeem.
• Hard parts: Budget reset, abuse, redemption catalog.
• Discuss: Per-tenant catalog.

363. Design an Internal Job-Referral Bonus System¶

• Scale: Refer + bonus payout.
• Core features: Refer, track, payout on milestone.
• Hard parts: Attribution, tax, eligibility windows.
• Discuss: Source-of-truth ATS link.

364. Design a Workforce Planning / Headcount Tool¶

• Scale: Quarterly planning.
• Core features: Approved positions, hires, attrition.
• Hard parts: Position-vs-headcount sync, frozen positions.
• Discuss: Approval state machine.

365. Design a Corporate Training-Compliance Tracker (OSHA / HIPAA)¶

• Scale: Required annual cert.
• Core features: Assign, complete, audit reporting.
• Hard parts: Role-mapped courses, expiry, regulatory audit.
• Discuss: Audit-export pipeline.

⚖️ 26. Legal Tech¶

366. Design an E-Discovery Platform (Relativity)¶

• Scale: Petabyte-scale legal review.
• Core features: Ingest emails/files, dedup, tag, produce.
• Hard parts: Predictive coding, privilege review, custodian-level chain.
• Discuss: Full-text + ML; chain-of-custody.

367. Design a Contract-Lifecycle Mgmt (CLM) System (Ironclad)¶

• Scale: Enterprise contracts.
• Core features: Draft, redline, approve, sign, repository.
• Hard parts: Version diff, clause library, expiration alerts.
• Discuss: Word/track-changes integration; OCR'd legacy docs.

368. Design a Legal-Research Platform (Westlaw / LexisNexis)¶

• Scale: Cases, statutes, secondary sources.
• Core features: Full-text search, citation graph, alerts.
• Hard parts: KeyCite-style red flags, paragraph-level perma-links.
• Discuss: Citation graph DB; real-time citator updates.

369. Design a Court Case-Management System¶

• Scale: State court system.
• Core features: Filing, docket, hearings, sealed access.
• Hard parts: Public-records compliance, sealed/redacted, e-filing.
• Discuss: Per-court rule customization.

370. Design an Online Notary / E-Sign Service (DocuSign)¶

• Scale: Billions of envelopes/year.
• Core features: Tags, route, sign, audit.
• Hard parts: Signer authentication, audit trail integrity, ESIGN/UETA.
• Discuss: Hash-chain audit; KBA + biometric.

371. Design a Patent-Search & Filing Tool¶

• Scale: USPTO + EPO + WIPO.
• Core features: Prior-art search, claim drafting, filing.
• Hard parts: Multi-jurisdiction filing, family-tree, IDS submissions.
• Discuss: Patent classification ML.

372. Design a Regulatory-Compliance Tracker (Thomson Reuters)¶

• Scale: Multi-jurisdiction reg watch.
• Core features: Track regs, alert changes, mapping to controls.
• Hard parts: Multi-language, classification, change-impact analysis.
• Discuss: NLP for regulatory text.

373. Design a Time-Tracking / Billing for Lawyers (Clio)¶

• Scale: Per-firm SaaS.
• Core features: Timer, invoice, trust account, conflict check.
• Hard parts: IOLTA trust, conflict of interest, billing rules.
• Discuss: Per-bar-association compliance.

374. Design a Whistleblower / Hotline Platform¶

• Scale: Anonymous intake.
• Core features: Submit anonymously, follow up via case ID.
• Hard parts: Anonymity preservation, EU vs US rules.
• Discuss: Tor-friendly intake; encrypted at rest.

375. Design a Will / Estate-Planning Platform (Trust & Will)¶

• Scale: Self-service docs.
• Core features: Q&A → doc generation, sign, vault.
• Hard parts: State-specific templates, witness req, secure storage.
• Discuss: Document-generation engine.

376. Design a GDPR Subject-Access-Request (SAR) Platform¶

• Scale: Multi-system data discovery.
• Core features: Receive request, find data across systems, package.
• Hard parts: Connector library, deadline (30 days), redaction.
• Discuss: Data-mapping inventory.

377. Design an Online Mediation / Arbitration Service¶

• Scale: Small-claims dispute resolution.
• Core features: File case, exchange, video hearing, decision.
• Hard parts: Asynchronous workflow, evidence storage, enforceability.
• Discuss: State-machine per dispute.

378. Design a Litigation Hold System¶

• Scale: Enterprise legal hold.
• Core features: Issue hold to custodians, pause deletion across systems.
• Hard parts: Connector to email/docs, custodian acknowledgement.
• Discuss: Universal "do-not-delete" tag.

379. Design a Trademark / IP-Watch Service¶

• Scale: Global watch by class.
• Core features: Subscribe to mark, alert filings, oppose deadline.
• Hard parts: Multi-registry ingest, fuzzy match, multi-language.
• Discuss: Phonetic + semantic match.

380. Design an Online Court Filing (eFile)¶

• Scale: Per-state, multi-court.
• Core features: Submit pleading, fee pay, stamp filing.
• Hard parts: Per-court schemas, attachment limits, clerk approval.
• Discuss: Adapter per court system; signed receipts.

🏛️ 27. Government / Civic Tech¶

381. Design a Voting / Election System¶

• Scale: Country-wide, polling places + mail-in.
• Core features: Voter check-in, ballot issue, count, audit.
• Hard parts: Voter privacy + auditability, attack surface, paper trail.
• Discuss: Why E2E-verifiable; risk-limiting audits.

382. Design an Online Tax-Filing Platform (TurboTax)¶

• Scale: 100M filings on April 15.
• Core features: Q&A, federal+state, e-file, refund track.
• Hard parts: Tax-engine updates, multi-state, audit defense.
• Discuss: Tax-form abstraction; April surge capacity.

383. Design a DMV Vehicle-Registration System¶

• Scale: Per-state, multi-million records.
• Core features: Title transfer, registration, plates, fees.
• Hard parts: Inter-state title transfers, lien holders, special plates.
• Discuss: AAMVA NMVTIS integration.

384. Design a National ID / Aadhaar-style Identity System¶

• Scale: 1B+ identities, biometric.
• Core features: Enroll, biometric match, e-KYC API.
• Hard parts: Dedup at billion scale, privacy, federation.
• Discuss: ABIS architecture; tokenization.

385. Design a Public-Records Search (Sunshine Law)¶

• Scale: State/county records portal.
• Core features: Search, request copy, redact, deliver.
• Hard parts: Redaction at scale, public-records exemptions.
• Discuss: ML-assisted PII redaction.

386. Design a Census Enumeration Platform¶

• Scale: Once-a-decade national headcount.
• Core features: Self-response online, field collection, dedup.
• Hard parts: Confidentiality (Title 13), disclosure-avoidance.
• Discuss: Differential privacy.

387. Design a Government Permit-Issuance System¶

• Scale: Local building/business permits.
• Core features: Apply, review, inspect, issue, renew.
• Hard parts: Multi-department review, GIS overlay, code references.
• Discuss: Workflow per permit type.

388. Design a 911 / Emergency Dispatch (CAD)¶

• Scale: PSAP-wide.
• Core features: Call intake, location, dispatch units, status.
• Hard parts: Sub-second response, multi-agency, redundancy.
• Discuss: Why active-active across data centers.

389. Design a Mass-Notification / Public-Alert System¶

• Scale: State/national, AMBER/Wireless Emergency Alerts.
• Core features: Author alert, target geography, deliver.
• Hard parts: Carrier integration, geo-targeting accuracy.
• Discuss: WEA gateway, IPAWS protocol.

390. Design a Court Date-Reminder / Pretrial-Service Platform¶

• Scale: Reduce FTA via reminders.
• Core features: Receive court calendar, send SMS reminders.
• Hard parts: Court-data ingest, multi-channel comms, opt-in.
• Discuss: Reminder cadence A/B test.

391. Design a SNAP / Government-Benefits Disbursement Platform¶

• Scale: State EBT system.
• Core features: Eligibility determination, EBT load, transactions.
• Hard parts: Recertification, fraud, EBT card network.
• Discuss: Caseworker workflow; fraud heuristics.

392. Design an Open-Data Portal (data.gov)¶

• Scale: Thousands of datasets.
• Core features: Catalog, download, API, schema.
• Hard parts: Heterogeneous datasets, versioning, citation.
• Discuss: CKAN-style metadata; DOI assignment.

393. Design a Driver-License Issuance System¶

• Scale: State-wide.
• Core features: Apply, test, photo, issue, renewal.
• Hard parts: REAL-ID compliance, biometric capture, fraud.
• Discuss: Document chain; appointment system.

394. Design a Toll-Road Billing System¶

• Scale: State/region tollway.
• Core features: Toll capture, account match, pay-by-plate billing.
• Hard parts: ALPR accuracy, dispute resolution, interoperability.
• Discuss: Inter-state transponder reciprocity.

395. Design a Public-Transit Real-Time Arrival System¶

• Scale: City-wide buses + trains.
• Core features: Live vehicle position, ETA, alerts.
• Hard parts: GTFS-RT ingest, coverage gaps, congestion.
• Discuss: Hybrid ETA models.

396. Design a City-Service 311 Reporting Platform (NYC 311)¶

• Scale: Millions of complaints/year.
• Core features: Report (photo+geo), route to agency, status.
• Hard parts: Dedup, agency routing rules, SLA tracking.
• Discuss: Geo-clustering of similar reports.

397. Design a Voter-Registration & Roll Maintenance System¶

• Scale: State-level.
• Core features: Register, verify, age-out, deceased remove.
• Hard parts: Inter-state list-maintenance, address standards, audit.
• Discuss: ERIC interstate comparison.

398. Design a Customs Border Pre-Clearance System (Global Entry)¶

• Scale: Trusted-traveler enrollment.
• Core features: Apply, biometric enroll, kiosk on entry.
• Hard parts: Multi-agency vetting, kiosk fleet management.
• Discuss: Biometric-on-entry vs biometric-on-exit.

399. Design a Court Bail / Pretrial Risk-Assessment Tool¶

• Scale: Per-arrest assessment.
• Core features: Compute risk score, recommend bail.
• Hard parts: Bias mitigation, transparency, jurisdictional rules.
• Discuss: Why explainability mandatory.

400. Design a Gov-to-Citizen Notification Service (e.g. tax due, license renewal)¶

• Scale: State residents.
• Core features: Multi-channel delivery, opt-in, multi-language.
• Hard parts: Address-of-record sync, accessibility.
• Discuss: USPS NCOA integration.

📰 28. News & Media¶

401. Design The New York Times CMS / Publishing Platform¶

• Scale: Hundreds of stories/day, paywall.
• Core features: Edit, embargo, publish, breaking news, push.
• Hard parts: Embargo enforcement, multi-channel distribution, comment moderation.
• Discuss: Editorial workflow; near-real-time push.

402. Design a News Aggregator (Google News / Apple News)¶

• Scale: 100K publishers, billions of articles.
• Core features: Crawl, dedup, cluster, personalize.
• Hard parts: Story clustering across sources, freshness, paywall handling.
• Discuss: TF-IDF clustering vs embeddings.

403. Design a Paywall / Subscription Engine for News¶

• Scale: 10M subscribers.
• Core features: Metered limits, hard paywall, gift links.
• Hard parts: Cookie-based metering vs login, share-link abuse.
• Discuss: Edge counter; cohort-based limit.

404. Design a Live Election-Night Reporting Platform¶

• Scale: Spike on Tuesday night.
• Core features: Precinct results, projections, maps.
• Hard parts: Result-feed ingest, multi-state schemas, error correction.
• Discuss: Read-heavy CDN strategy.

405. Design a Comments-Moderation System for News¶

• Scale: Millions of comments.
• Core features: Auto-classify, queue, ban, appeal.
• Hard parts: Multi-language, auto-ban evaders, false positives.
• Discuss: ML + community flag.

406. Design a Newsletter-Publishing Platform (Substack)¶

• Scale: 1M writers, 50M readers.
• Core features: Post, paid, email, web archive.
• Hard parts: Email deliverability, payouts, abuse.
• Discuss: Per-writer subdomain.

407. Design a Live Blog / Live Updates Page¶

• Scale: Breaking news, 1M concurrent readers.
• Core features: Streaming updates, pin, push.
• Hard parts: SSE/WebSocket fanout, CDN caching with live updates.
• Discuss: Edge stream vs polling.

408. Design a Fact-Check Database¶

• Scale: Cross-publisher.
• Core features: Claim CRUD, verdict, citations.
• Hard parts: ClaimReview schema, fuzzy claim match.
• Discuss: Cross-org dedup.

409. Design a Photo-Wire / Press-Photo Distribution (Getty / AP)¶

• Scale: Photographers worldwide → newsrooms.
• Core features: Upload, caption, license, search.
• Hard parts: IPTC metadata, embargo, watermarking.
• Discuss: Per-customer license bundling.

410. Design a Real-time Sports-Score Service (ESPN ScoreCenter)¶

• Scale: Live scores, push.
• Core features: Game state, scoreboard widgets, push.
• Hard parts: Multi-data-feed reliability, latency, clock sync.
• Discuss: Authoritative data feed; CDN cache busting.

411. Design a Podcast Hosting + Analytics¶

• Scale: Millions of shows.
• Core features: Upload, RSS, analytics on downloads.
• Hard parts: IAB-certified analytics, prefix tracking, ads insert.
• Discuss: Server-side ad insertion.

412. Design a Magazine PDF Reader (Zinio)¶

• Scale: Subscription of magazines.
• Core features: Download, page-flip viewer, annotations.
• Hard parts: DRM, search across PDFs.
• Discuss: Page-level DRM keys.

413. Design a Book-Publishing Editorial Platform¶

• Scale: Authors → editors → press.
• Core features: Manuscript ingest, track changes, ARC distribution.
• Hard parts: Versioning, multi-format export (EPUB/MOBI).
• Discuss: Pandoc-based pipeline.

414. Design a Book Reader (Kindle Cloud)¶

• Scale: Sync library + position across devices.
• Core features: Library, sync, highlights, dictionary.
• Hard parts: DRM, position sync, offline caching.
• Discuss: Per-book license; whisper-sync.

415. Design a Translation-Memory / CAT Tool (SDL Trados)¶

• Scale: Global translators.
• Core features: Translation memory, glossary, MT assist.
• Hard parts: TMX standard, fuzzy match, customer-specific TMs.
• Discuss: Per-tenant TM isolation.

416. Design an Audio-Book Platform (Audible)¶

• Scale: 1M books, 100M users.
• Core features: Download, sync position with text, sleep timer.
• Hard parts: Whisper-sync between book + audio, DRM.
• Discuss: Multi-narrator chapters; speed-vs-quality encoding.

417. Design a Newsroom CMS for TV Broadcast¶

• Scale: Broadcast control room.
• Core features: Rundown, prompter, lower-thirds, video clip.
• Hard parts: Real-time rundown changes, MOS protocol.
• Discuss: Studio integration; redundancy.

418. Design a Crowdsourced Wiki (Wikipedia)¶

• Scale: 60M articles, multi-language.
• Core features: Edit, version, revert, talk pages.
• Hard parts: Vandalism detection, translation parity, bot edits.
• Discuss: Diff storage; WikiText rendering.

419. Design a Comments Threading Engine (Disqus-tier 2)¶

• Scale: Threaded, vote-sorted.
• Core features: Reply, vote, sort by best.
• Hard parts: Best-sort across millions, near-real-time score.
• Discuss: Confidence-bound (Wilson) vs hot.

420. Design a Media-Embargo / Pressroom Platform¶

• Scale: PR companies → journalists.
• Core features: Distribute embargoed news, lift at time.
• Hard parts: Time-locked release, watermarked drafts.
• Discuss: Per-journalist watermarking; auto-lift.

421. Design a Magazine Subscription Box (Print + Digital)¶

• Scale: Recurring shipments.
• Core features: Auto-renewal, shipping cycles, address mgmt.
• Hard parts: Issue-based vs date-based, donations, gift subs.
• Discuss: Subscription state machine.

422. Design a Live Translation / Captioning for News Streams¶

• Scale: Multi-language broadcast.
• Core features: Translate live audio, render captions.
• Hard parts: Latency, accuracy, profanity.
• Discuss: ML pipeline + human-correction.

423. Design a User-Generated Content Submission Pipeline (CNN iReport)¶

• Scale: Photos/video from public.
• Core features: Upload, verify, publish.
• Hard parts: Authenticity verification, deepfake detection.
• Discuss: Provenance tracking (C2PA).

424. Design a Ratings / Review Aggregator (Rotten Tomatoes)¶

• Scale: Movie/TV reviews.
• Core features: Aggregate critic + audience scores.
• Hard parts: Critic-eligibility curation, review parsing.
• Discuss: Tomatometer threshold; verified-audience.

425. Design a Media Asset Management (MAM)¶

• Scale: Studio video archive.
• Core features: Ingest, transcode, metadata, search, retrieve.
• Hard parts: LTO archive integration, proxy generation.
• Discuss: Hierarchical storage; AI-assisted tagging.

🏃 29. Sports & Fitness¶

426. Design Strava (Activity Tracker)¶

• Scale: 100M athletes, GPS uploads.
• Core features: Record activity, upload, segments, leaderboards.
• Hard parts: GPS smoothing, segment matching, leaderboard scaling.
• Discuss: Geo-line matching; segment leaderboard re-rank.

427. Design a Fitness-Tracking Wearable Backend (Fitbit)¶

• Scale: 30M devices.
• Core features: Sync steps, sleep, HR; trends; challenges.
• Hard parts: Sub-second HR ingest, battery, data backfill on offline.
• Discuss: Streaming vs batched sync.

428. Design a Workout-Plan / Personal Trainer App (Peloton)¶

• Scale: Live classes + on-demand.
• Core features: Live class stream, leaderboard, metric capture (cadence/HR).
• Hard parts: Live-class concurrent metrics, low-latency stream.
• Discuss: Pub/sub per class.

429. Design a Real-time Fantasy-Sports Platform (DraftKings)¶

• Scale: Millions of contests.
• Core features: Lineup, real-time scoring, payout.
• Hard parts: Stat-feed ingest, late-swap rules, regulation per state.
• Discuss: Stream processing for scoring.

430. Design a Sports-Betting Sportsbook¶

• Scale: Global, multi-market.
• Core features: Pre-match + live betting, in-play, settle.
• Hard parts: Line management, risk engine, instant settle.
• Discuss: Cash-out engine; throttling on hot lines.

431. Design a Tournament Bracket Generator (March Madness)¶

• Scale: Office pool style.
• Core features: Bracket entry, scoring as games conclude.
• Hard parts: Score recompute as upsets happen.
• Discuss: Recompute on each game finish.

432. Design a Live Sports Score Broadcasting (ESPN-tier 2)¶

• Scale: Concurrent live games globally.
• Core features: Push score updates to apps.
• Hard parts: Delivery to 50M devices in seconds.
• Discuss: SSE/WebSocket vs push notification.

433. Design a Sports Streaming Platform (Disney+/ESPN)¶

• Scale: Live + DVR.
• Core features: Live stream with auth, multi-camera angles.
• Hard parts: Geo-blackout rules, low-latency live, DVR storage.
• Discuss: Per-team blackout maps.

434. Design a Running-Coach App (Couch to 5K)¶

• Scale: Plan-driven workouts.
• Core features: Weekly plan, audio cues, progress.
• Hard parts: Plan adaptation, GPS-based run capture.
• Discuss: Adaptive coaching algorithm.

435. Design a Gym-Membership Management (Mindbody)¶

• Scale: Per-gym SaaS.
• Core features: Class booking, check-in, billing, waitlist.
• Hard parts: Capacity caps, no-show fees, multi-location.
• Discuss: Time-zone handling.

436. Design a Live Marathon-Tracker¶

• Scale: 50K runners with chip times.
• Core features: Real-time mat timing, push to subscribers.
• Hard parts: Mat-event ingest, runner search, ETA at next mat.
• Discuss: Event mat → runner → ETA pipeline.

437. Design a League-Management Platform (Soccer/Hockey)¶

• Scale: Local leagues.
• Core features: Schedule, refs, standings, stats.
• Hard parts: Round-robin generator, ref availability.
• Discuss: Constraint solver.

438. Design a Coaching Video-Analysis Tool (Hudl)¶

• Scale: Game-film with telestrator.
• Core features: Upload, tag plays, draw, share.
• Hard parts: Frame-accurate seek, tag schema, multi-team perms.
• Discuss: Web-based video annotation.

439. Design a Live ESports Platform¶

• Scale: Tournaments with millions concurrent.
• Core features: Bracket, live stream, chat.
• Hard parts: Stream alongside game state, anti-cheat.
• Discuss: Game-server feed integration.

440. Design a Step-Challenge / Workplace Wellness¶

• Scale: Corporate, 100K employees.
• Core features: Aggregate steps, team leaderboards, prizes.
• Hard parts: Multi-device source, anti-cheat.
• Discuss: HealthKit/GoogleFit ingest.

441. Design a Yoga / Meditation App Backend (Calm)¶

• Scale: 5M subscribers.
• Core features: Session library, downloads, streaks, sleep stories.
• Hard parts: Audio CDN, recommendation, offline.
• Discuss: Per-user playlist personalization.

442. Design a Sports Ticket Resale (StubHub)¶

• Scale: Secondary market.
• Core features: List, bid, transfer, e-ticket.
• Hard parts: Fraud, ticket verification, dynamic pricing.
• Discuss: Barcode reissue protocols.

443. Design a Sports Nutrition / Meal-Plan App¶

• Scale: Per-athlete meal plans.
• Core features: Plan, track macros, recipe.
• Hard parts: Food DB at scale, photo recognition.
• Discuss: USDA + crowdsourced food DB.

444. Design a Climbing-Gym Route-Tracker¶

• Scale: Per-gym route library.
• Core features: Mark routes done, grade, photos.
• Hard parts: Route-set turnover, crowdsourced grades.
• Discuss: Per-gym admin tool.

445. Design a Group Fitness Class Booking (ClassPass)¶

• Scale: Multi-studio aggregator.
• Core features: Search, book, credits, no-show.
• Hard parts: Studio-side inventory, late-cancel fees.
• Discuss: Studio API standardization.

446. Design a Live-Stream Workout (Mirror / Tonal)¶

• Scale: Connected device + mobile.
• Core features: Live class, on-device sensors, leaderboard.
• Hard parts: Form-correction ML, sensor calibration.
• Discuss: On-device CV; cloud aggregation.

447. Design a Sport League Player-Tracking (RFID)¶

• Scale: NBA/NFL player tracking.
• Core features: Real-time positions, derived stats.
• Hard parts: Sub-cm position, multi-camera fusion.
• Discuss: Sensor + CV fusion.

448. Design a Sports Event Photography Distribution (Marathonfoto)¶

• Scale: Photographers tag bib numbers.
• Core features: Bib OCR, runner search, purchase prints.
• Hard parts: OCR accuracy, multi-photographer dedup.
• Discuss: Bib detection ML.

449. Design a Pickleball / Local-League Match-Finder¶

• Scale: Local matchmaking.
• Core features: Find partner, courts, ELO.
• Hard parts: Pickup-game vs scheduled, no-shows.
• Discuss: Geo-radius search; rating ladder.

450. Design a Surf-Forecast / Tide Service¶

• Scale: Per-spot forecasts.
• Core features: Wave height, wind, tide, alerts.
• Hard parts: Weather model ingest, spot-specific local effects.
• Discuss: Buoy data + model fusion.

🎵 30. Music & Audio Tech¶

451. Design a Music-Recognition Service (Shazam)¶

• Scale: Billions of fingerprints.
• Core features: Capture audio, match, return song.
• Hard parts: Robust audio fingerprint, sub-second match, noise.
• Discuss: Constellation map; hash-based lookup.

452. Design a Spotify Discover Weekly Pipeline¶

• Scale: 600M users, weekly playlist.
• Core features: Per-user 30 tracks, fresh weekly.
• Hard parts: Compute at scale, freshness, exposure fairness.
• Discuss: ALS + audio embeddings; offline batch.

453. Design a SoundCloud-Style UGC Audio Platform¶

• Scale: 100M tracks, free upload.
• Core features: Upload, follow, comments at timestamp, plays.
• Hard parts: Copyright detection, pitch-shifted reuploads, CDN.
• Discuss: Audio fingerprint at upload.

454. Design a Music Royalty / Settlement System¶

• Scale: PRO + label payouts.
• Core features: Track plays, calc royalty, distribute.
• Hard parts: Splits across writers, mechanical vs performance, ISRC.
• Discuss: Append-only ledger; periodic statements.

455. Design a Music DAW Cloud Sync (BandLab)¶

• Scale: Multi-track project sync.
• Core features: Cloud projects, collaborator invite, mix.
• Hard parts: Multi-GB project sync, plugin compatibility.
• Discuss: Delta sync.

456. Design a Live-Concert Streaming Platform¶

• Scale: Single-event 1M concurrent.
• Core features: Pay-per-view, multi-cam, chat.
• Hard parts: Spike scaling, geo-blackouts, recording rights.
• Discuss: Pre-warm + autoscale.

457. Design a Music Karaoke App (Smule)¶

• Scale: Sing along + share.
• Core features: Sync lyrics, pitch detection, share recordings.
• Hard parts: Latency for duet, real-time pitch overlay.
• Discuss: Echo cancellation; pitch ML.

458. Design a Beat-Maker / Sample Marketplace (Splice)¶

• Scale: Subscription sample library.
• Core features: Browse, download samples, license.
• Hard parts: License tracking, audio search.
• Discuss: Per-sample license metadata.

459. Design a Music Download Store (iTunes Music Store legacy)¶

• Scale: 30M tracks, pay-per-track.
• Core features: Buy, download, library re-download.
• Hard parts: DRM (or DRM-free), region rights.
• Discuss: Region-rights matrix.

460. Design a Concert Setlist / Tour Tracker (Setlist.fm)¶

• Scale: Crowdsourced setlists.
• Core features: Tour calendar, setlist per show, statistics.
• Hard parts: Crowdsource accuracy, song equivalence (covers, intros).
• Discuss: Edit-history; community moderation.

461. Design a Music Lyrics + Chord Service (Genius / Ultimate Guitar)¶

• Scale: Crowdsourced.
• Core features: Crowd-edited lyrics, annotations, tabs.
• Hard parts: Copyright, edit conflicts, transposition.
• Discuss: License agreements with publishers.

462. Design a Music Notation Editor (MuseScore)¶

• Scale: Browser-based score editor.
• Core features: Note input, playback, share.
• Hard parts: Real-time sync, MIDI import, MusicXML.
• Discuss: Score-data model.

463. Design a DJ Mix Streaming (Mixcloud)¶

• Scale: DJ uploads + listener streams.
• Core features: Continuous mixes, copyright track ID.
• Hard parts: ID tracks within mix, proper licensing.
• Discuss: ACR (auto content recognition).

464. Design a Voice-Memo / Recorder Sync Service¶

• Scale: Apple Voice Memos at scale.
• Core features: Record, sync across devices.
• Hard parts: Variable file size, transcript on demand.
• Discuss: Local-first sync; iCloud-style.

465. Design a Live Audio Description (Accessibility) Service¶

• Scale: TV + film accessibility.
• Core features: Insert audio description in gaps.
• Hard parts: Mixing in real-time, sync to original audio.
• Discuss: Live blender + cue-points.

466. Design a Music Streaming with Spatial Audio¶

• Scale: Apple Spatial Audio.
• Core features: Atmos delivery, head-tracking.
• Hard parts: Bandwidth, format support, fallback.
• Discuss: ABR with Atmos vs stereo.

467. Design a Concert Wristband / RFID Cashless System¶

• Scale: Festival cashless payments.
• Core features: Top-up, tap-to-pay, refund.
• Hard parts: Offline POS, end-of-event reconciliation.
• Discuss: Edge POS sync; offline allowed.

468. Design a Radio Streaming Aggregator (TuneIn)¶

• Scale: Thousands of stations.
• Core features: Live radio, recordings, podcasts.
• Hard parts: Geo-rights, station feed reliability.
• Discuss: Stream proxy + caching.

469. Design a Music Concert Discovery (Bandsintown)¶

• Scale: Match user listening to nearby concerts.
• Core features: Sync library, location, alert.
• Hard parts: Artist disambiguation, geo-radius.
• Discuss: Push notification cadence.

470. Design a Lossless Music Streaming Service (Tidal)¶

• Scale: FLAC + MQA delivery.
• Core features: High-bandwidth audio, offline.
• Hard parts: Bandwidth, DRM, device support.
• Discuss: ABR for audio quality.

📷 31. Photography & Video Editing¶

471. Design a Cloud Photo Editor (Pixlr / Photopea)¶

• Scale: Browser-based image editing.
• Core features: Filters, layers, save/share.
• Hard parts: WebGL/Wasm rendering, large-file handling.
• Discuss: On-device vs cloud; export pipeline.

472. Design a Stock-Photo Marketplace (Shutterstock)¶

• Scale: 500M assets.
• Core features: Upload, license, search, payout.
• Hard parts: Reverse-image dedup, model release, watermark on preview.
• Discuss: Per-license SKU; royalty splits.

473. Design a Photo-Print / Photobook Service (Shutterfly)¶

• Scale: Holiday spike.
• Core features: Upload photos, design book, ship.
• Hard parts: Print-shop queue, print-quality validation.
• Discuss: Vendor routing; preview rendering.

474. Design a Wedding Photography Portfolio Platform (Pixieset)¶

• Scale: Per-photographer galleries.
• Core features: Client gallery, downloads, watermark, sales.
• Hard parts: Right-click protection, sales upsells.
• Discuss: Per-client password; print sales.

475. Design Adobe Creative Cloud File-Sync¶

• Scale: Multi-GB design files.
• Core features: Sync across devices, version, share.
• Hard parts: Delta sync, multi-app file types, attached fonts.
• Discuss: Block-level dedup; font sync.

476. Design a Mobile Photo-Editing App (VSCO / Lightroom Mobile)¶

• Scale: Edit + cloud sync.
• Core features: Filters, presets, RAW edit, sync.
• Hard parts: Non-destructive edits, multi-device sync.
• Discuss: Preset XMP sync; RAW thumbnail.

477. Design a Live Photo / Burst Photo Sync¶

• Scale: Apple Live Photo behavior.
• Core features: Sync image + 1.5s video.
• Hard parts: Pair handling, dedup across devices.
• Discuss: Asset pairing IDs.

478. Design a Video Editing Cloud (Adobe Premiere Cloud)¶

• Scale: Multi-collaborator video editing.
• Core features: Project sync, asset library, render.
• Hard parts: Multi-TB project, proxy generation, render farm.
• Discuss: Proxy editing; render queue.

479. Design a Photo-Recognition / Search by Image¶

• Scale: Within own library, or web-wide.
• Core features: "Search photos with cats", reverse search.
• Hard parts: Embedding model, ANN index, on-device for privacy.
• Discuss: Hybrid metadata + vector search.

480. Design a 3D-Render Farm / Cloud Render¶

• Scale: Animation studio rendering.
• Core features: Submit job, distribute frames, retrieve.
• Hard parts: Frame distribution, retry, asset sync.
• Discuss: Coordinator + worker pool.

481. Design an AI Photo-Enhance Service (Topaz / Adobe Enhance)¶

• Scale: Upload, ML upscale, return.
• Core features: Async pipeline, GPU pool.
• Hard parts: GPU sched, cost per inference.
• Discuss: Batch vs real-time.

482. Design a Real-Time Video Filter (Snapchat lenses)¶

• Scale: On-device AR filters.
• Core features: AR filter, server-distributed, capture share.
• Hard parts: Real-time on-device ML, filter package distribution.
• Discuss: Edge-package pipeline.

483. Design a Video Compression Service for User Uploads¶

• Scale: Pre-upload compress on client + server transcode.
• Core features: Client compress, multi-resolution server.
• Hard parts: Quality vs size, on-device GPU.
• Discuss: Two-stage encoding.

484. Design a 360°/VR Photo Service (Google Street View Photos)¶

• Scale: Crowdsourced street imagery.
• Core features: Upload, stitch, geotag, viewer.
• Hard parts: Equirectangular handling, blur faces/plates.
• Discuss: Auto-blur ML.

485. Design a TikTok-Style Video Editor (CapCut)¶

• Scale: On-device editor.
• Core features: Trim, overlay, effects, music.
• Hard parts: Real-time preview, asset library.
• Discuss: Project-file format.

486. Design a Live-Event Photo-Sharing App (Festival photo wall)¶

• Scale: Event-scoped, ephemeral.
• Core features: Group photo wall, geo-fenced upload.
• Hard parts: Auto-curate, moderation, takedown.
• Discuss: Event-scoped DB; auto-expiry.

487. Design a Document Scan App (Office Lens / CamScanner)¶

• Scale: Document scan + OCR.
• Core features: Detect edges, perspective correct, OCR, PDF.
• Hard parts: Edge detection, multi-page binding, OCR accuracy.
• Discuss: On-device CV; OCR cloud.

488. Design a Stop-Motion / Time-Lapse Pipeline¶

• Scale: Long-running capture.
• Core features: Capture, assemble, render.
• Hard parts: Storage of frames, render assembly.
• Discuss: Frame storage in object store.

489. Design a Family-Photo Sharing App (FamilyAlbum / Tinybeans)¶

• Scale: Closed family group.
• Core features: Upload, comment, monthly book.
• Hard parts: Parental controls, ad-free, offline.
• Discuss: Closed-group ACL.

490. Design a Long-Term Photo Archive Service (Libraries / Museums)¶

• Scale: Decades of preservation.
• Core features: Ingest, metadata, preservation, access.
• Hard parts: Format migration, fixity checks, OAIS.
• Discuss: Tape archive; metadata schema.

🛡️ 32. Insurance Tech¶

491. Design an Auto-Insurance Quote Engine¶

• Scale: Real-time quotes.
• Core features: Driver+vehicle inputs, MVR, quote.
• Hard parts: External data joins (MVR, CLUE), per-state filings.
• Discuss: Rating engine; rate-filing versioning.

492. Design a Home-Insurance Claims Platform¶

• Scale: Catastrophe surge (hurricane).
• Core features: FNOL, adjuster dispatch, payout.
• Hard parts: Catastrophe spike, catastrophe modeling, fraud.
• Discuss: Surge capacity for adjusters.

493. Design a Telematics Auto-Insurance (Progressive Snapshot)¶

• Scale: Driving-behavior monitor.
• Core features: Capture trips, scoring, premium adjust.
• Hard parts: Crash detection, privacy, opt-out.
• Discuss: Edge ML; data minimization.

494. Design a Pet-Insurance Claims App (Trupanion)¶

• Scale: Vet bill submission.
• Core features: Submit invoice, OCR, adjudicate, pay vet directly.
• Hard parts: Vet integration, pre-existing condition rules.
• Discuss: Vet-portal direct integration.

495. Design an Embedded-Insurance API (Lemonade)¶

• Scale: Insurance at checkout.
• Core features: Quote in-flow, bind, manage.
• Hard parts: Underwriting in <1s, partner-merchant SDK.
• Discuss: Decision-engine for instant bind.

496. Design a Health-Insurance Claims Adjudication¶

• Scale: B2B with providers.
• Core features: Claim ingest, COB, adjudicate, EOB.
• Hard parts: Rules per plan, prior auth, COB across payers.
• Discuss: Pricer + bundler engines.

497. Design a Workers-Comp Claims System¶

• Scale: State-regulated.
• Core features: Injury intake, medical payments, indemnity.
• Hard parts: Multi-state regulations, employer experience-mod.
• Discuss: Per-state forms.

498. Design a Life-Insurance Underwriting Pipeline¶

• Scale: App → exam → decision.
• Core features: App, MIB pull, exam orders, decision.
• Hard parts: Multi-vendor data, accelerated UW, mortality model.
• Discuss: Decision-engine + manual review.

499. Design a Reinsurance Treaty Tracking System¶

• Scale: B2B reinsurer.
• Core features: Treaty terms, cession, reporting.
• Hard parts: Complex slip terms, non-standard contracts.
• Discuss: Domain-specific DSL.

500. Design an Insurance Producer (Agent) Platform¶

• Scale: 100K agents.
• Core features: Quote, bind, commissions, licensing.
• Hard parts: State licensing tracking, commission calc.
• Discuss: NIPR sync; carrier integration.

501. Design a Catastrophe-Modeling Backend (RMS / AIR)¶

• Scale: Per-hurricane simulation.
• Core features: Event set, vulnerability, exposure → loss.
• Hard parts: Compute-intensive, simulation determinism.
• Discuss: GPU sim grid.

502. Design a Health-Insurance Open-Enrollment Marketplace (Healthcare.gov)¶

• Scale: Annual surge.
• Core features: Plan compare, enroll, subsidy calc.
• Hard parts: Federal/state hub integration, subsidy compute.
• Discuss: Eligibility hub orchestration.

503. Design an EOB (Explanation of Benefits) Engine¶

• Scale: Per-claim EOB to member.
• Core features: Adjudication detail rendering, mail/portal.
• Hard parts: Multi-language, accessibility, privacy.
• Discuss: Per-claim audit trail.

504. Design a Group-Insurance Enrollment Platform¶

• Scale: Employer group coverage.
• Core features: EOI, dependents, evidence of insurability.
• Hard parts: Group census reconciliation, life events.
• Discuss: EDI 834 ingest/output.

505. Design an Insurance Fraud Detection System¶

• Scale: SIU caseload.
• Core features: Score claims, refer to investigator.
• Hard parts: Multi-claim graph, ring detection.
• Discuss: Graph DB; SIU workflow.

506. Design a Risk-Engineering Visit Scheduler¶

• Scale: Commercial insurance loss-control.
• Core features: Schedule visit, report, recommendations.
• Hard parts: Engineer routing, doc capture, finding tracking.
• Discuss: Mobile app; offline capture.

507. Design an Annuity Servicing Platform¶

• Scale: Long-tail policies.
• Core features: Premium, withdrawal, surrender, RMD.
• Hard parts: Decades of service, regulatory rules.
• Discuss: Effective-dated everything.

508. Design an Insurance Self-Service Portal¶

• Scale: Member self-care.
• Core features: Pay premium, view docs, file claim, change beneficiary.
• Hard parts: Identity proof, POA, MFA.
• Discuss: Service-now style ticketing.

509. Design a Drone-Based Roof Inspection¶

• Scale: Claim adjust via drone.
• Core features: Schedule flight, capture, report.
• Hard parts: Drone fleet, FAA Part 107, ML for damage detect.
• Discuss: Pilot dispatch + AI report.

510. Design a Dental-Insurance Plan-Pricing Tool¶

• Scale: Network procedure pricing.
• Core features: Procedure code price by network.
• Hard parts: Provider contracts, fee schedule updates.
• Discuss: Per-network fee table.

⚡ 33. Energy & Utilities¶

511. Design a Smart-Meter Data Pipeline¶

• Scale: 50M meters, 15-min reads.
• Core features: Ingest, billing, outage detect.
• Hard parts: AMI head-end integration, missing reads, theft detect.
• Discuss: Time-series at scale; gap-fill.

512. Design an Outage-Management System for Utility¶

• Scale: Storm response.
• Core features: Customer reports, predictive map, crew dispatch.
• Hard parts: Outage prediction from meter loss, ETR calc.
• Discuss: Geo-clustering of reports.

513. Design an EV Charging Network (ChargePoint)¶

• Scale: 500K stations.
• Core features: Find/charge/pay, idle fees, fleet billing.
• Hard parts: OCPP, station offline, demand response.
• Discuss: Station-state sync.

514. Design a Demand-Response Aggregator¶

• Scale: Grid-scale curtailment.
• Core features: Enroll devices, dispatch reduction, settle.
• Hard parts: Sub-minute dispatch, baseline computation.
• Discuss: OpenADR; settlement math.

515. Design a Solar-Panel Performance Monitor (Enphase)¶

• Scale: Per-microinverter.
• Core features: Production, alerts, monetary value.
• Hard parts: Long-tail device support.
• Discuss: Edge gateway → cloud.

516. Design a Utility Customer-Billing System (CIS)¶

• Scale: Tens of millions of customers.
• Core features: Billing cycle, payments, arrears, deposit.
• Hard parts: Regulated rate plans, prorations, disconnects.
• Discuss: Rate-engine versioning.

517. Design a Wholesale Energy-Market Trading¶

• Scale: ISO/RTO market.
• Core features: Day-ahead, real-time, ancillary.
• Hard parts: 5-min scheduling, transmission constraints.
• Discuss: SCED algorithm.

518. Design a Smart-Grid SCADA Platform¶

• Scale: Substation telemetry.
• Core features: Real-time SCADA, alarms, control.
• Hard parts: ICS protocols, latency, NERC CIP.
• Discuss: Edge gateway + cyber-segmentation.

519. Design a Pipeline Leak-Detection (Oil/Gas)¶

• Scale: Continuous SCADA.
• Core features: Pressure/flow analysis, alarm, dispatch.
• Hard parts: False positive tuning, regulatory notification.
• Discuss: Anomaly detection methods.

520. Design a Renewable Forecasting Service (Wind/Solar)¶

• Scale: Per-farm forecast.
• Core features: Weather model + on-site, day-ahead curve.
• Hard parts: Multi-model ensemble, real-time sensor fusion.
• Discuss: Forecast skill metrics.

521. Design a Building Management System (BMS) Cloud¶

• Scale: Commercial buildings.
• Core features: HVAC, lighting, occupancy, schedules.
• Hard parts: BACnet integration, cyber-isolation.
• Discuss: Edge gateway; cloud command queue.

522. Design an EV Battery-Telemetry Platform¶

• Scale: Connected vehicles.
• Core features: Battery state, range estimation, OTA.
• Hard parts: Sub-second data, V2G readiness.
• Discuss: Chargeport-side communication.

523. Design a Net-Metering / Solar Settlement¶

• Scale: Customer-generation.
• Core features: Bidirectional kWh, monthly settle.
• Hard parts: Tariff complexity, true-up.
• Discuss: Per-period netting.

524. Design a Water-Utility Leak Detector¶

• Scale: District metering.
• Core features: Monitor flow, detect leaks, dispatch.
• Hard parts: Sensor noise, district modeling.
• Discuss: Bayesian leak detection.

525. Design a Smart-Home Energy Optimizer (Sense)¶

• Scale: Home circuit-level energy.
• Core features: Disaggregate appliances, savings tips.
• Hard parts: Signal disaggregation ML.
• Discuss: Edge ML model.

🌾 34. Agriculture & Food Tech¶

526. Design a Precision-Agriculture Platform (John Deere Operations)¶

• Scale: Farm-level data.
• Core features: Field maps, sensor data, yield, prescriptions.
• Hard parts: Equipment integration, satellite data, offline-rural.
• Discuss: Edge sync at the farm.

527. Design a Drone-Based Crop-Monitoring Service¶

• Scale: Per-field flights.
• Core features: NDVI, anomaly detect, prescription map.
• Hard parts: Image stitching, ML for crop health.
• Discuss: Tile-based map UI.

528. Design a Farm-to-Restaurant Marketplace¶

• Scale: Local sourcing.
• Core features: Browse, order, route delivery.
• Hard parts: Perishable inventory, logistics.
• Discuss: Per-region inventory.

529. Design a Food-Traceability System (FSMA 204)¶

• Scale: Critical-tracking events.
• Core features: Capture KDE/CTE, recall trace.
• Hard parts: Multi-tier supplier data sharing.
• Discuss: GS1 EPCIS standard.

530. Design a Restaurant POS + Kitchen Display¶

• Scale: Per-restaurant ops.
• Core features: Order, kitchen ticket, payment, inventory.
• Hard parts: Offline operations, multi-location franchise.
• Discuss: Edge POS + cloud sync.

531. Design a Recipe-Personalization Service¶

• Scale: User dietary preferences.
• Core features: Recipe DB, filter by dietary, generate meal plan.
• Hard parts: Ingredient substitutions, nutritional calc.
• Discuss: USDA food DB; substitution graph.

532. Design a Grocery-Delivery (Instacart) Picker App¶

• Scale: Picker side of marketplace.
• Core features: Pick list, substitutions, customer chat.
• Hard parts: Real-time inventory, sub policy, payment at register.
• Discuss: In-store inventory uncertainty.

533. Design a Vertical-Farming Control System¶

• Scale: Indoor controlled environment.
• Core features: Climate control, lighting, irrigation.
• Hard parts: Multi-zone control, recipe optimization.
• Discuss: PLC + cloud orchestration.

534. Design a Livestock-Tracking System (RFID Tags)¶

• Scale: Cattle traceability.
• Core features: Birth → slaughter chain, health events.
• Hard parts: Rural connectivity, data ownership.
• Discuss: Edge gateway in barn.

535. Design a Crop-Insurance Platform (USDA RMA)¶

• Scale: Federal crop insurance.
• Core features: Acreage report, yield, indemnity.
• Hard parts: Multi-stage reporting deadlines, weather indexing.
• Discuss: RMA data exchange.

536. Design a Fertilizer-Recommendation Engine¶

• Scale: Per-field prescription.
• Core features: Soil samples, yield goal, recommend rate.
• Hard parts: Sample-to-prescription ML, equipment compatibility.
• Discuss: Variable-rate map export.

537. Design a Direct-to-Consumer Meal-Kit Service (Blue Apron)¶

• Scale: Subscription, ingredient pack.
• Core features: Choose recipes, weekly ship.
• Hard parts: Inventory + perishable, kitting at fulfillment.
• Discuss: Demand-driven kitting.

538. Design a Food-Waste-Tracking System (Restaurants)¶

• Scale: Per-restaurant.
• Core features: Log waste, source by type, reports.
• Hard parts: Camera-based waste detection.
• Discuss: ML on waste-bin camera.

539. Design a Greenhouse Climate-Control Cloud¶

• Scale: Multi-greenhouse operator.
• Core features: Climate setpoints, override, alarms.
• Hard parts: Failsafe controls, multi-vendor sensors.
• Discuss: Edge controller authority.

540. Design a Beekeeping Hive-Monitor Service¶

• Scale: Beehive sensors.
• Core features: Temp, weight, queen alerts.
• Hard parts: Battery life, rural cell.
• Discuss: LPWAN + cloud.

🏗️ 35. Construction & AEC¶

541. Design a BIM (Building Info Modeling) Cloud Platform¶

• Scale: Multi-GB models.
• Core features: Upload, view, federate models, clash detect.
• Hard parts: IFC/Revit ingest, cloud rendering.
• Discuss: Tile-based 3D streaming.

542. Design a Construction-Bid Marketplace¶

• Scale: Public bids.
• Core features: Post project, sub-bid, award.
• Hard parts: Trade-package routing, document control.
• Discuss: Bid-leveling spreadsheet.

543. Design a Field-Service Daily-Log App¶

• Scale: Construction site logs.
• Core features: Daily log, photos, weather, manpower.
• Hard parts: Offline capture, geo-tag, audit.
• Discuss: Sync conflict resolution.

544. Design a Submittal / RFI Workflow¶

• Scale: Per-project routing.
• Core features: Submit, review, approve, distribute.
• Hard parts: Multi-party routing, deadlines.
• Discuss: Workflow engine; reminder schedule.

545. Design a Punch-List Management Tool¶

• Scale: End-of-project items.
• Core features: Capture punches, assign, close, photo.
• Hard parts: Plan-pin location, assignment routing.
• Discuss: Plan-PDF overlay.

546. Design a Subcontractor Compliance / COI Tracker¶

• Scale: Insurance certs from subs.
• Core features: Track expiration, request renewal.
• Hard parts: Document parsing, multi-state requirements.
• Discuss: OCR + structured extraction.

547. Design a Heavy-Equipment Telematics (Caterpillar)¶

• Scale: Construction fleet.
• Core features: Hours, idle, maintenance, location.
• Hard parts: Multi-OEM, satellite when no cell.
• Discuss: ISO 15143-3.

548. Design a Site-Safety Incident-Reporting System¶

• Scale: Daily JHA + incidents.
• Core features: Report, OSHA report, training compliance.
• Hard parts: OSHA log, lagging vs leading indicators.
• Discuss: Mobile-first capture.

549. Design a Concrete-Pour Scheduling System¶

• Scale: Multi-site batch plant.
• Core features: Order, batch, dispatch trucks, ticket.
• Hard parts: Pour-window timing, mix design.
• Discuss: Plant-side scheduling.

550. Design a Modular-Construction Factory Workflow¶

• Scale: Module assembly line.
• Core features: Track modules, station completion, ship.
• Hard parts: Assembly-line throughput, QC.
• Discuss: MES-style integration.

551. Design a Geotech Borehole Data Platform¶

• Scale: Soil samples.
• Core features: Capture borehole logs, lab tests, reports.
• Hard parts: Standard data formats (AGS).
• Discuss: Spatial visualization.

552. Design an Architecture Drawing Markup Tool (Bluebeam Revu)¶

• Scale: Multi-drawing review.
• Core features: PDF markup, sessions, takeoff.
• Hard parts: Real-time multi-user markup.
• Discuss: OT-based markup.

553. Design a Construction-Loan Draw-Schedule Manager¶

• Scale: Multi-project lender.
• Core features: Draw request, lien waivers, inspector verify.
• Hard parts: Lien tracking, mechanic's lien laws.
• Discuss: Document workflow.

554. Design a Smart Hard-Hat Worker Tracker¶

• Scale: Site personnel safety.
• Core features: Locate, fall detection, alert.
• Hard parts: Battery, indoor positioning.
• Discuss: UWB anchors.

555. Design an Interior-Design / Mood-Board App (Houzz)¶

• Scale: Design inspiration sharing.
• Core features: Save photos, build boards, hire pro.
• Hard parts: Image annotation for products.
• Discuss: Product-tagging in photos.

🏭 36. Manufacturing & Industrial IoT¶

556. Design an MES (Manufacturing Execution System)¶

• Scale: Factory floor.
• Core features: Work order, station, traceability.
• Hard parts: OEE metrics, real-time, ERP sync.
• Discuss: ISA-95 levels.

557. Design a Predictive-Maintenance Platform¶

• Scale: Industrial equipment.
• Core features: Vibration/temp, RUL, alert.
• Hard parts: Edge ML, false-alarm cost, retraining.
• Discuss: Edge inference + cloud retrain.

558. Design a Digital-Twin Platform¶

• Scale: Asset-level twins.
• Core features: Live state, sim, what-if.
• Hard parts: Sync with physical, simulation perf.
• Discuss: Stream-driven state.

559. Design a Quality-Inspection / SPC Platform¶

• Scale: Statistical process control.
• Core features: Capture measurements, control charts, alerts.
• Hard parts: Real-time SPC, multivariate.
• Discuss: Control-chart math.

560. Design a Robotic Process Automation (UiPath)¶

• Scale: Bot fleet.
• Core features: Build bot, deploy, schedule, monitor.
• Hard parts: UI-flake handling, credentials, logs.
• Discuss: Orchestrator + worker.

561. Design a Production-Scheduling APS System¶

• Scale: Multi-line factory.
• Core features: Plan jobs, bottleneck, change-overs.
• Hard parts: Constraint solver, demand variability.
• Discuss: APS solver patterns.

562. Design a Material-Requirements-Planning (MRP)¶

• Scale: BOM explosion.
• Core features: Demand, BOM, plan.
• Hard parts: Multi-level BOM, lot sizing.
• Discuss: MRP runs cadence.

563. Design a Plant Energy-Monitoring System¶

• Scale: Energy submetering.
• Core features: Aggregate, baseline, savings.
• Hard parts: Per-line allocation, EnPI.
• Discuss: ISO 50001 alignment.

564. Design a Tool-Calibration Tracking System¶

• Scale: Per-tool cal cert.
• Core features: Schedule cal, cert capture, due alerts.
• Hard parts: Audit trail, traceability.
• Discuss: ISO 17025 cert.

565. Design a Defect-Image AI Classifier¶

• Scale: Inline camera-based QC.
• Core features: Capture, classify, accept/reject.
• Hard parts: Edge inference latency, drift retraining.
• Discuss: Edge GPU + cloud retrain.

566. Design a Plant-Floor Andon System¶

• Scale: Real-time call-for-help.
• Core features: Pull cord, escalation, response time.
• Hard parts: Real-time push to leads, escalations.
• Discuss: Pub/sub by station.

567. Design a SCADA / DCS Modernization (Cloud Connect)¶

• Scale: Legacy plant data online.
• Core features: OPC-UA bridge, cloud time-series.
• Hard parts: ICS cyber-isolation.
• Discuss: Air-gap broker pattern.

568. Design a Spare-Parts Inventory Optimizer¶

• Scale: Per-MRO warehouse.
• Core features: Reorder, criticality, supplier.
• Hard parts: Long-tail SKU forecast, criticality scoring.
• Discuss: ABC/XYZ analysis.

569. Design an OEE Dashboard¶

• Scale: Real-time per-line.
• Core features: Avail, perf, quality, drilldown.
• Hard parts: Reason-code capture, micro-stoppage.
• Discuss: Stop-reason taxonomy.

570. Design a Recipe-Management System for Process Industry¶

• Scale: Pharma/Food batch.
• Core features: Recipe versioning, eBR, regulator audit.
• Hard parts: 21 CFR Part 11.
• Discuss: Electronic batch records.

571. Design a Plant Floor Asset-Tracking (RFID)¶

• Scale: WIP location.
• Core features: Read events, dwell, throughput.
• Hard parts: Reader topology, phantom reads.
• Discuss: Edge filter rules.

572. Design a Worker-Productivity Time-Standard System¶

• Scale: Manufacturing time studies.
• Core features: Standard time DB, measure, variance.
• Hard parts: MTM library, learning curve.
• Discuss: Per-operation rates.

573. Design a Plant Dispatch Routing for AGVs¶

• Scale: AGV fleet.
• Core features: Task assign, traffic mgmt.
• Hard parts: Deadlock-free routing.
• Discuss: Centralized planner.

574. Design an OT Cybersecurity Monitor¶

• Scale: ICS network monitoring.
• Core features: Asset discovery, anomaly, alerts.
• Hard parts: Passive monitoring, ICS protocols.
• Discuss: SPAN ports + DPI.

575. Design a Supply-Chain Track-and-Trace via QR¶

• Scale: Per-item serial.
• Core features: Scan events, chain, customer verify.
• Hard parts: Anti-counterfeit, scan capacity.
• Discuss: Public verify portal.

🏪 37. Retail & POS Systems¶

576. Design a Modern Retail POS (Square / Shopify POS)¶

• Scale: Multi-store.
• Core features: Sale, returns, inventory, tax, tender.
• Hard parts: Offline mode, multi-tax, peripheral support.
• Discuss: Edge DB + sync.

577. Design a Self-Checkout Kiosk Backend¶

• Scale: Grocery scale.
• Core features: Scan, weigh, pay, theft prevention.
• Hard parts: Theft detection ML, age verification.
• Discuss: Computer-vision-assisted.

578. Design a Cashierless Store (Amazon Go)¶

• Scale: Per-store sensor fusion.
• Core features: Detect items taken, charge on exit.
• Hard parts: CV reliability, customer ID, returns.
• Discuss: Multi-camera tracking.

579. Design a Loyalty Program Backend (Starbucks Stars)¶

• Scale: 30M loyalty members.
• Core features: Earn, redeem, tier, partner integration.
• Hard parts: Real-time POS link.
• Discuss: Mobile-app integration.

580. Design a Retail Promo / Pricing Engine¶

• Scale: Multi-promo stacking.
• Core features: Promo eligibility, stacking rules.
• Hard parts: Rules engine, conflict resolution, last-mile pricing.
• Discuss: Drools-style rules.

581. Design a Buy-Online-Pickup-In-Store (BOPIS)¶

• Scale: Multi-store fulfillment.
• Core features: Reserve, pick, notify ready.
• Hard parts: In-store inventory accuracy, pick SLA.
• Discuss: Pick-task allocation.

582. Design a Returns-Management for Retail¶

• Scale: Multi-channel returns.
• Core features: Initiate, accept, refund, restock.
• Hard parts: Channel parity, ASN, fraud.
• Discuss: State machine.

583. Design a Markdown / Clearance Optimizer¶

• Scale: Seasonal markdowns.
• Core features: Recommend markdown timing/depth.
• Hard parts: Demand model, forward-looking inventory.
• Discuss: Per-SKU model.

584. Design a Retail Heatmap from Cameras¶

• Scale: Store traffic analytics.
• Core features: Foot traffic, dwell, conversion.
• Hard parts: Privacy, fixed-camera analytics.
• Discuss: Edge person-detection.

585. Design a Retail Inventory-Audit App (Cycle Count)¶

• Scale: Daily cycle counts.
• Core features: Scan task, variance, adjust.
• Hard parts: Wave planning, variance threshold.
• Discuss: Mobile scanner workflow.

586. Design a Shelf-Out-of-Stock Detector (Camera)¶

• Scale: Per-aisle camera.
• Core features: Detect OOS, alert.
• Hard parts: Multi-product taxonomy, occlusion.
• Discuss: Edge ML, periodic retrain.

587. Design a Smart Shopping-Cart (Caper)¶

• Scale: Cart-with-cameras.
• Core features: Auto-detect items dropped in.
• Hard parts: CV accuracy, payment in-cart.
• Discuss: Edge inference + cloud reconciliation.

588. Design an In-Store Wayfinding Map App¶

• Scale: Per-store map.
• Core features: Item search → aisle, indoor positioning.
• Hard parts: BLE beacons, planogram updates.
• Discuss: Per-store map authoring tool.

589. Design a Retail Endless-Aisle Kiosk¶

• Scale: In-store online catalog.
• Core features: Browse online catalog, ship-to-home.
• Hard parts: Sync with online inventory.
• Discuss: Order-routing decision.

590. Design a Grocery-Inventory Expiration Tracker¶

• Scale: Perishable management.
• Core features: Date code capture, FEFO rotation, markdown.
• Hard parts: Capture method (label scan).
• Discuss: OCR for date codes.

591. Design a Restaurant Online-Ordering (Toast / Olo)¶

• Scale: Per-restaurant online.
• Core features: Menu, cart, kitchen send, payment.
• Hard parts: POS integration, item availability.
• Discuss: Menu sync from POS.

592. Design a Drive-Thru Order-Capture System¶

• Scale: QSR.
• Core features: Speaker-post, screen, AI order taker.
• Hard parts: Voice recognition, modifier mapping.
• Discuss: ASR + menu mapping.

593. Design a Loyalty-Linked Receipt OCR App (Fetch)¶

• Scale: Submit receipt for points.
• Core features: Photo, OCR, dedupe, points.
• Hard parts: Accuracy, fraud (reused receipts).
• Discuss: Hash-chain dedup.

594. Design a Vendor Compliance Portal (Retailer)¶

• Scale: Suppliers ship to retailer DCs.
• Core features: ASN, routing guide, chargeback.
• Hard parts: EDI 856/940, compliance scoring.
• Discuss: Chargeback automation.

595. Design a Retail Workforce-Management (Kronos)¶

• Scale: Hourly staff scheduling.
• Core features: Forecast demand, schedule, swap.
• Hard parts: Forecast accuracy, fair-week-week laws.
• Discuss: Predictive scheduling laws.

596. Design a Retail Mystery-Shopper Platform¶

• Scale: Crowd shoppers.
• Core features: Assignment, capture, payout.
• Hard parts: Assignment routing, fraud.
• Discuss: Geo-verification.

597. Design a Convenience Store Fuel-Pump Integration¶

• Scale: Forecourt pumps.
• Core features: Authorize, pump, payment.
• Hard parts: Pre-auth release, payment terminals.
• Discuss: Conexxus standards.

598. Design a Curbside Pickup Real-time Coord¶

• Scale: Customer arrival → bring out.
• Core features: Geofence arrival, notify staff.
• Hard parts: Spot identification, no-show.
• Discuss: Push notifications and GPS triggers.

599. Design a Retail Theft-Detection / EAS System¶

• Scale: Per-store EAS.
• Core features: Tag detection, video link, alert.
• Hard parts: Tag-detection false positive.
• Discuss: Camera-link automation.

600. Design a Retail Personalization Engine (1:1 offers)¶

• Scale: Per-customer dynamic offers.
• Core features: Profile, offer pool, deliver via channel.
• Hard parts: Eligibility logic, frequency caps.
• Discuss: Decision engine.

🏦 38. Banking & FinTech (Advanced)¶

601. Design a Real-Time Payments Network (FedNow / RTP)¶

• Scale: Sub-second settlement.
• Core features: Send, receive, request-to-pay, 24/7.
• Hard parts: Liquidity, fraud, ISO 20022.
• Discuss: Liquidity throttling.

602. Design a Cross-Border Payment System (Wise/SWIFT)¶

• Scale: Multi-currency, multi-corridor.
• Core features: Quote FX, send, settle, recipient.
• Hard parts: Compliance per country, FX exposure, settlement risk.
• Discuss: Pre-funding model.

603. Design a Cards-Issuing Platform (Marqeta-style)¶

• Scale: Virtual + physical card issuance.
• Core features: Issue card, JIT funding, transaction auth.
• Hard parts: Auth in <100ms, network rules, BIN sponsorship.
• Discuss: Real-time JIT funding hooks.

604. Design a Cards-Acquiring / Merchant Acquirer¶

• Scale: Process merchant transactions.
• Core features: Auth, capture, refund, settlement.
• Hard parts: PCI scope, network fees, chargebacks.
• Discuss: Tokenization vault.

605. Design an ACH Origination Platform¶

• Scale: Bulk debit/credit.
• Core features: Submit batch, NACHA file, returns.
• Hard parts: Same-day vs next-day, returns processing.
• Discuss: NACHA file generation.

606. Design an ATM Network¶

• Scale: Bank ATM fleet.
• Core features: Cash dispense, deposit, fraud.
• Hard parts: Reconciliation, cash forecasting, network outages.
• Discuss: Standin processing.

607. Design a Loan-Origination System (LOS)¶

• Scale: Auto/personal/student loans.
• Core features: App, credit pull, decision, fund.
• Hard parts: Decision rules, doc collection, post-fund servicing.
• Discuss: Decision engine; e-sign.

608. Design a Loan-Servicing Platform¶

• Scale: Long-tail loans.
• Core features: Amortize, payments, escrow, payoff.
• Hard parts: Default servicing, escrow analysis.
• Discuss: Calc engine for amortization.

609. Design a Mortgage-Servicing Pipeline¶

• Scale: Long-life loans.
• Core features: Payment, escrow, PMI, modification.
• Hard parts: Reg compliance, loss mitigation.
• Discuss: Workflow engine.

610. Design a Wealth-Management / Robo-Advisor (Wealthfront)¶

• Scale: Algorithmic portfolios.
• Core features: Risk profile, rebalance, tax-loss harvest.
• Hard parts: Trade timing, fractional shares, tax lots.
• Discuss: Cash drag minimize.

611. Design a Brokerage Account-Opening Platform¶

• Scale: Self-service onboarding.
• Core features: KYC, AML, account funding.
• Hard parts: OFAC, identity, name screening.
• Discuss: OCR + ID verification.

612. Design a Margin / Risk Engine for Brokerage¶

• Scale: Real-time margin calc.
• Core features: Initial, maintenance, calls.
• Hard parts: Real-time NAV, margin call timing.
• Discuss: Reg-T calculations.

613. Design a 401(k) / Retirement Plan System¶

• Scale: Plan participants.
• Core features: Contributions, vesting, loans, distributions.
• Hard parts: ERISA compliance, blackout, true-up.
• Discuss: Employer match logic.

614. Design a Tax-Reporting (1099) Generation¶

• Scale: Year-end massive output.
• Core features: Generate 1099s, deliver, IRS file.
• Hard parts: Per-form logic, corrections, FATCA.
• Discuss: Per-form pipeline.

615. Design a Bank-Statement / Statement-Aggregator API (Plaid)¶

• Scale: Account aggregation.
• Core features: Connect bank, pull transactions, balance.
• Hard parts: OFX vs scraping vs OAuth, freshness, MFA.
• Discuss: Open Banking (FDX).

616. Design a Personal Finance Manager (Mint)¶

• Scale: Aggregate accounts, categorize.
• Core features: Auto-categorize, budgets, alerts.
• Hard parts: Category accuracy, dedup.
• Discuss: ML categorization.

617. Design a Budgeting / Envelope App (YNAB)¶

• Scale: Multi-device sync.
• Core features: Categories, monthly assign, rollover.
• Hard parts: Conflict resolution, family-shared.
• Discuss: CRDT-style sync.

618. Design a BNPL (Buy Now, Pay Later) Platform (Affirm)¶

• Scale: At checkout decisions.
• Core features: Underwrite, pay over time, merchant.
• Hard parts: Real-time underwriting, collections.
• Discuss: Decision in <500ms.

619. Design a Credit-Score Free-View Service (Credit Karma)¶

• Scale: Consumer-side credit info.
• Core features: Free score, recommendations, monitoring.
• Hard parts: Bureau pull, partner offers.
• Discuss: Soft-pull vs hard.

620. Design a Stock-Borrowing / Securities-Lending Platform¶

• Scale: B2B short locate.
• Core features: Locate request, lend, recall.
• Hard parts: Inventory across desks, daily mark.
• Discuss: End-of-day allocation.

621. Design a Treasury-Management Platform for Corp¶

• Scale: Multi-bank visibility.
• Core features: Cash position, sweep, pay-by-FX.
• Hard parts: Multi-bank API integration.
• Discuss: SWIFT MT940 ingest.

622. Design a Wire-Transfer Fraud-Prevention System¶

• Scale: High-value detection.
• Core features: Rules + ML scoring, hold, callback.
• Hard parts: Velocity vs friction trade-off.
• Discuss: Step-up auth heuristics.

623. Design a Sanctions-Screening Platform (OFAC)¶

• Scale: Screen wires + customers.
• Core features: Name screening, alert, dispose.
• Hard parts: Fuzzy match, low FP, timeliness.
• Discuss: Phonetic + edit-distance.

624. Design a Fraud Case-Management System¶

• Scale: Investigator workflow.
• Core features: Cases, evidence, dispositions.
• Hard parts: SLA, regulatory reporting (SAR).
• Discuss: Workflow + reporting.

625. Design a Statement-Rendering Platform (Bank statements)¶

• Scale: PDF + electronic.
• Core features: Generate per cycle, deliver.
• Hard parts: Template engine, multi-language.
• Discuss: PDF rendering at scale.

626. Design a Wire-Routing System (SWIFT MT103)¶

• Scale: Inter-bank routing.
• Core features: Validate, route, settlement.
• Hard parts: Compliance, BIC routing, currency cutoffs.
• Discuss: SWIFT GPI.

627. Design a Bank-Branch / Teller App¶

• Scale: Frontline software.
• Core features: Lookup, transactions, holds, audit.
• Hard parts: Latency to core, role-based perms.
• Discuss: Core banking integration.

628. Design a Card-Tokenization Service (Apple Pay)¶

• Scale: Network token vault.
• Core features: Provision, replace, deprovision.
• Hard parts: Network token specs, device-binding.
• Discuss: EMV tokenization.

629. Design a Disputes / Chargeback Platform¶

• Scale: Card disputes.
• Core features: Initiate, evidence, network submit.
• Hard parts: Visa/MC reason codes, timeline.
• Discuss: State machine per dispute.

630. Design a Bank-Account Switching Service¶

• Scale: Switch direct deposit + recurring.
• Core features: ID payees, redirect, close old.
• Hard parts: Payee discovery, settlement window.
• Discuss: ACH redirection.

⛓️ 39. Crypto & DeFi¶

631. Design a Crypto Custodial Wallet¶

• Scale: Custody at scale.
• Core features: Store keys, sign, withdraw.
• Hard parts: HSM design, hot/warm/cold split, MPC.
• Discuss: Threshold signature schemes.

632. Design a Decentralized Exchange (Uniswap)¶

• Scale: AMM-based.
• Core features: Swap, LP, fees, route.
• Hard parts: Slippage, MEV, gas optimization.
• Discuss: AMM math.

633. Design a Centralized Crypto Exchange (Coinbase)¶

• Scale: Spot + derivatives.
• Core features: Order book, deposit, withdrawal.
• Hard parts: Cold-wallet flows, regulation.
• Discuss: Internal matching engine.

634. Design a Stablecoin Issuance Platform¶

• Scale: Mint/burn pegged token.
• Core features: Mint, redeem, attestation.
• Hard parts: Reserves audit, peg defense.
• Discuss: Reserve breakdown reporting.

635. Design a Crypto Tax-Reporting Tool (CoinTracker)¶

• Scale: Multi-exchange aggregation.
• Core features: Import, cost basis, gain/loss.
• Hard parts: FIFO/LIFO/specific-id, multi-chain.
• Discuss: Lot accounting.

636. Design a Blockchain Indexer (The Graph)¶

• Scale: All chain data → queryable.
• Core features: Subscribe to contracts, index events.
• Hard parts: Reorgs, re-index on schema, scale.
• Discuss: Subgraph design.

637. Design a Decentralized Storage System (Arweave / Filecoin)¶

• Scale: Storage marketplace.
• Core features: Store data, prove storage.
• Hard parts: Proof of replication, durability incentives.
• Discuss: Sealing pipeline.

638. Design an NFT Marketplace (OpenSea)¶

• Scale: Multi-chain trading.
• Core features: Mint, list, buy, royalties.
• Hard parts: Metadata permanence, indexing across chains.
• Discuss: Off-chain order books.

639. Design a Crypto On-Ramp / Off-Ramp (MoonPay)¶

• Scale: Fiat ↔ crypto.
• Core features: KYC, fund with card/ACH, deliver.
• Hard parts: Card fraud, settlement risk.
• Discuss: Fraud risk scoring.

640. Design a DeFi Lending Protocol Frontend (Aave-like)¶

• Scale: Lend/borrow.
• Core features: Supply, borrow, liquidate.
• Hard parts: Oracle reliance, liquidation incentives.
• Discuss: Oracle design.

641. Design a Crypto Block Explorer (Etherscan)¶

• Scale: Index all blocks.
• Core features: Search tx/address, contract verify.
• Hard parts: Reorgs, source-code verification.
• Discuss: Bytecode → source matching.

642. Design a Bitcoin Mining Pool Backend¶

• Scale: Coordinate miners.
• Core features: Distribute jobs, account shares, payout.
• Hard parts: Stratum protocol, share validation.
• Discuss: PPLNS payout.

643. Design a Crypto Mempool Monitor¶

• Scale: Watch unconfirmed tx.
• Core features: Stream mempool, alert.
• Hard parts: Multi-node aggregation, eviction.
• Discuss: Peer-network monitoring.

644. Design an MEV Auction System (Flashbots)¶

• Scale: Block-builder market.
• Core features: Bundles, sealed bids, builder.
• Hard parts: Fairness, latency, anti-frontrun.
• Discuss: PBS architecture.

645. Design a Token-Launch / IDO Platform¶

• Scale: Launchpad for tokens.
• Core features: Whitelist, vesting, distribute.
• Hard parts: Sybil resistance, vesting cliffs.
• Discuss: Merkle distributor.

646. Design a Wallet-as-a-Service API (Embedded wallets)¶

• Scale: SDK for apps.
• Core features: Create wallet, sign, recover.
• Hard parts: Custody model, key recovery.
• Discuss: MPC vs Shamir.

647. Design a Bridge Between Chains (cross-chain)¶

• Scale: Move assets between chains.
• Core features: Lock+mint, burn+release.
• Hard parts: Bridge security, reorgs, validators.
• Discuss: Light-client bridges.

648. Design a Decentralized Identity (DID) System¶

• Scale: Self-sovereign identity.
• Core features: DID resolution, credentials.
• Hard parts: Revocation, privacy, recovery.
• Discuss: W3C DID methods.

649. Design a DAO Governance Platform (Snapshot)¶

• Scale: Off-chain voting.
• Core features: Proposal, vote weight by token, execute.
• Hard parts: Snapshot block, signature verification.
• Discuss: EIP-712 signing.

650. Design a Yield-Aggregator (Yearn-like)¶

• Scale: Auto-yield strategies.
• Core features: Strategy switch, perf reporting.
• Hard parts: Strategy risk, gas-cost optimization.
• Discuss: Strategist whitelist.

651. Design a Crypto Payment Processor (BitPay)¶

• Scale: Merchant accept crypto.
• Core features: Quote, invoice, settle to fiat.
• Hard parts: Volatility hedging, refund.
• Discuss: Hedge to USD instantly.

652. Design an Oracle Network (Chainlink)¶

• Scale: Off-chain → on-chain.
• Core features: Aggregate sources, on-chain delivery.
• Hard parts: Source quality, attack resistance.
• Discuss: Median + reputation.

653. Design a Decentralized Rollup Sequencer¶

• Scale: L2 sequencer.
• Core features: Order tx, post to L1, fraud-proof.
• Hard parts: Censorship resistance, fee market.
• Discuss: Shared sequencer designs.

654. Design a Tokenized RWA (Real-World Asset) Platform¶

• Scale: Tokenize bonds/real estate.
• Core features: Onboarding, mint, transfer with KYC.
• Hard parts: Compliance overlay, allowlists.
• Discuss: ERC-3643.

655. Design a Non-Custodial Mobile Wallet (MetaMask Mobile)¶

• Scale: End-user wallet.
• Core features: Manage keys, dapp browser, swaps.
• Hard parts: Secure key storage, phishing.
• Discuss: Secure enclave usage.

656. Design a CryptoTwitter Streaming Bot Platform¶

• Scale: Track addresses + alerts.
• Core features: Watch lists, alerts on tx, post.
• Hard parts: Reorg correctness, dedupe.
• Discuss: Confirmations vs speed.

657. Design a Crypto Insurance Market (Nexus Mutual)¶

• Scale: Cover smart-contract risk.
• Core features: Cover purchase, claim assess.
• Hard parts: Claim assessment governance.
• Discuss: Mutual + governance token model.

658. Design a DAO Treasury Mgmt¶

• Scale: Multisig + budgeting.
• Core features: Multisig, payment streams, budgets.
• Hard parts: Approval workflow on-chain.
• Discuss: Safe + Zodiac modules.

659. Design a Privacy Mixer / Coin-Join (Privacy considerations)¶

• Scale: Privacy by mixing.
• Core features: Pool deposits, mix, withdraw.
• Hard parts: Compliance, anonymity-set size.
• Discuss: ZK-based mixers.

660. Design a Layer-2 ZK-Rollup Prover¶

• Scale: Proof generation.
• Core features: Witness, prove, batch.
• Hard parts: Prover resource, parallelism.
• Discuss: Recursive proofs.

🎲 40. Gambling & Sports Betting¶

661. Design an Online Casino Backend¶

• Scale: Many simultaneous players.
• Core features: Slot/poker/blackjack engines, RNG, payouts.
• Hard parts: Provably fair, regulatory audits.
• Discuss: RNG cert; per-jurisdiction.

662. Design a Sports-Betting Trading Engine¶

• Scale: Real-time odds management.
• Core features: Open lines, accept bets, hedge.
• Hard parts: Odds compiler, in-play data feed.
• Discuss: Risk-managed line moves.

663. Design an In-Play Bet Acceptance System¶

• Scale: Live game betting.
• Core features: Sub-second odds + accept.
• Hard parts: Latency, market suspend, auto-settle.
• Discuss: Pause windows on event.

664. Design a Daily Fantasy Sports Engine (DraftKings DFS)¶

• Scale: Contest entry to payout.
• Core features: Lineup, salary cap, scoring.
• Hard parts: Late swap, scoring lag.
• Discuss: Per-game scoring pipeline.

665. Design a Tote / Pari-Mutuel System (Horse racing)¶

• Scale: Pool wagering.
• Core features: Pool by track, combine pools, dividends.
• Hard parts: Hub host integration.
• Discuss: Inter-tote pooling.

666. Design a Bingo / Lottery Platform¶

• Scale: State lottery scale.
• Core features: Tickets, draws, claims.
• Hard parts: RNG integrity, claim verification.
• Discuss: Sealed-randomness commit/reveal.

667. Design a Poker Tournament Engine¶

• Scale: MTT with thousands.
• Core features: Seating, blinds up, balance tables.
• Hard parts: Player movement, anti-collusion.
• Discuss: Table-balance algorithm.

668. Design a Cashout / Bet-Edit System¶

• Scale: Live partial cashout.
• Core features: Real-time price for cashout.
• Hard parts: Risk-managed offer, sub-second.
• Discuss: Pricing engine throughput.

669. Design a Responsible-Gambling Self-Exclusion¶

• Scale: Multi-operator registries.
• Core features: Register, share between operators, enforce.
• Hard parts: Cross-operator data, identity matching.
• Discuss: State registry integration.

670. Design a KYC / Geo-Compliance Engine for Betting¶

• Scale: Per-state compliance.
• Core features: Geofence, KYC docs, age check.
• Hard parts: Geofence accuracy, VPN detection.
• Discuss: Wi-Fi triangulation.

671. Design a Slot-Game Math / RTP Engine¶

• Scale: Game RNG + math model.
• Core features: Reels, paylines, RTP.
• Hard parts: Cert by labs (GLI), seed mgmt.
• Discuss: Seed and audit.

672. Design a Live-Dealer Casino Platform¶

• Scale: Live blackjack/roulette.
• Core features: Video stream, side-bet timing.
• Hard parts: Latency, dealer station UI.
• Discuss: Bet-window timing.

673. Design a Bonus / Wagering-Requirement Engine¶

• Scale: Promotions across products.
• Core features: Issue bonus, track wagering.
• Hard parts: Game weighting, abuse.
• Discuss: Wagering ledger.

674. Design a Sports Data-Feed Aggregator¶

• Scale: Multiple data providers.
• Core features: Normalize feeds, fill gaps.
• Hard parts: Conflicting data, latency.
• Discuss: Source rank + arbitration.

675. Design a Bet-Surveillance / Suspicious Pattern Detector¶

• Scale: Integrity monitoring.
• Core features: Bet pattern anomaly, alert.
• Hard parts: Cross-account collusion.
• Discuss: Graph DB for accounts.

676. Design an Esports-Betting Live Lines¶

• Scale: Game-data based.
• Core features: Lines from in-game state.
• Hard parts: Game-API access, latency.
• Discuss: Direct game-data feeds.

677. Design a Loyalty/VIP Program for Casino¶

• Scale: Comp points, tiers.
• Core features: Earn, comp, tier.
• Hard parts: Player value calc.
• Discuss: Theoretical hold.

678. Design a Free-to-Play Coin Game (Slotomania)¶

• Scale: F2P scale.
• Core features: Coins, IAP, daily bonus.
• Hard parts: Anti-cheat, loot-box compliance.
• Discuss: F2P monetization.

679. Design a Sweepstakes / Promo Sports Platform¶

• Scale: US states with sweepstakes model.
• Core features: Coins, prize redemption, AMOE.
• Hard parts: Compliance per state.
• Discuss: AMOE flow.

680. Design a Sports Player-Prop Pricing Engine¶

• Scale: Markets per player.
• Core features: Models per stat, lines, hedging.
• Hard parts: Model accuracy, in-play update.
• Discuss: Model-driven price moves.

📦 41. Subscription Commerce¶

681. Design a Subscription Box (Birchbox)¶

• Scale: Curated monthly box.
• Core features: Sub plan, curation, ship.
• Hard parts: Curation engine, inventory matching.
• Discuss: Per-cycle inventory.

682. Design a Curated Wine-Subscription¶

• Scale: Age-verified.
• Core features: Profile, recommend, ship.
• Hard parts: State alcohol law.
• Discuss: Age-verify at delivery.

683. Design a Diaper Auto-Replenish Service¶

• Scale: Predict timing.
• Core features: Recurring ship by predicted use.
• Hard parts: Use-rate estimation.
• Discuss: Pause + accelerate UI.

684. Design a Pet-Food Subscription (Chewy AutoShip)¶

• Scale: Pet-specific cadence.
• Core features: Per-pet schedule, dosing.
• Hard parts: Switching food, vet Rx.
• Discuss: Vet-Rx flow.

685. Design a Razor Subscription (Dollar Shave Club)¶

• Scale: Quarterly ship.
• Core features: Plan, swap, skip.
• Hard parts: SKU-level cadence.
• Discuss: Plan flexibility UI.

686. Design a Magazine + Newspaper Sub Mgmt¶

• Scale: Print + digital combo.
• Core features: Print logistics, digital access.
• Hard parts: Issue-based vs date-based.
• Discuss: Subscription state machine.

687. Design a Subscription Pause / Resume Engine¶

• Scale: Common across services.
• Core features: Pause, resume, refund proration.
• Hard parts: Proration math.
• Discuss: Per-cycle accounting.

688. Design a Family Plan / Sharing for Subscriptions¶

• Scale: Apple Family-style.
• Core features: Share with family, individual profiles.
• Hard parts: Entitlement per-member, privacy.
• Discuss: Group entitlements.

689. Design a Subscription Analytics for SaaS (ChartMogul)¶

• Scale: MRR, churn, LTV.
• Core features: Cohort, churn, expansion.
• Hard parts: Multi-currency MRR, accuracy.
• Discuss: Movement classification.

690. Design an Annual Plan + Monthly Plan Switcher¶

• Scale: Plan upgrade/downgrade.
• Core features: Switch with proration.
• Hard parts: Refund vs credit logic.
• Discuss: Pricing models.