High-Level Design (HLD)
Systems, architectures, and concepts to scale to millions of users.
| # | Topic | Importance | Status | Asked At | Best Resources |
|---|---|---|---|---|---|
| 1 | URL Shortener Base62 encoding, KV store for redirects, analytics, custom aliases, expiry | critical | AmazonGoogleUberMicrosoft | ||
| 2 | Twitter / Social Feed Fanout on write vs read, celebrity problem, Redis sorted sets for timeline | critical | TwitterMetaLinkedInSnap | ||
| 3 | YouTube / Video Streaming HLS/DASH adaptive bitrate, transcoding pipeline, CDN edge delivery, S3 storage | critical | NetflixYouTubeAmazonDisney+ | ||
| 4 | WhatsApp / Chat System WebSocket, offline delivery, message ordering, E2E encryption, Cassandra | critical | WhatsAppSlackDiscordTelegram | ||
| 5 | Uber / Ride Sharing Geohash geo-indexing, real-time driver location, ETA, matching, surge pricing | critical | UberLyftOlaDoorDash | ||
| 6 | Rate Limiter Token bucket, sliding window log, sliding window counter, Redis + Lua atomic ops | critical | StripeCloudflareGoogleAmazon | ||
| 7 | Distributed Cache (Redis) Eviction (LRU/LFU), consistent hashing for cache nodes, write policies, hot keys | critical | AmazonGoogleMetaNetflix | ||
| 8 | Message Queue (Kafka) Partitions, consumer groups, log compaction, at-least-once vs exactly-once | critical | LinkedInUberNetflixAirbnb | ||
| 9 | Consistent Hashing Hash ring, virtual nodes, minimal data movement on add/remove server | critical | AmazonGoogleCassandraRedis | ||
| 10 | CAP Theorem / BASE / ACID C/A/P tradeoffs, CP vs AP systems, eventual consistency, MongoDB vs Cassandra | critical | All FAANGStripeCloudflare | ||
| 11 | Database Sharding Range vs hash sharding, hot spots, cross-shard queries, resharding strategy | critical | AmazonUberAirbnbPinterest | ||
| 12 | API Gateway / Load Balancer L4 vs L7 LB, round-robin, least connections, health checks, reverse proxy | critical | AmazonNetflixUberCloudflare | ||
| 13 | Instagram / Photo Sharing Object storage (S3), CDN for images, feed generation, follow graph sharding | high | MetaPinterestSnapTwitter | ||
| 14 | Amazon / E-Commerce Product catalog, inventory, cart, order service, payment, search | high | AmazonFlipkartShopifyeBay | ||
| 15 | Notification System APNs/FCM gateways, priority queues, user preferences, retry, fan-out | high | AppleGoogleMetaSlack | ||
| 16 | Google Maps / Location Geohash / S2 cells, routing algorithms, ETA prediction, map tiles | high | GoogleUberDoorDashLyft | ||
| 17 | Distributed File Storage (S3) Object storage, chunking, erasure coding, deduplication, eventual consistency | high | AmazonGoogleDropboxBox | ||
| 18 | Search Autocomplete Trie, top-k suggestions per prefix, Kafka for frequency, Redis sorted sets | high | GoogleAmazonTwitterLinkedIn | ||
| 19 | Ticketmaster / Event Booking Seat reservation, concurrency with locks, payment, waiting room queue | high | TicketmasterBookMyShowAmazon | ||
| 20 | Google Docs / Collaborative Editor Operational Transformation or CRDT, real-time sync, conflict resolution | high | GoogleMicrosoftNotionAtlassian | ||
| 21 | Payment System Idempotency keys, ledger design, saga pattern for rollbacks, PCI-DSS | high | StripePayPalRazorpaySquare | ||
| 22 | Distributed ID Generator Snowflake ID, UUID v4/v7, sonyflake, monotonic IDs, clock skew | high | TwitterInstagramUberDiscord | ||
| 23 | Circuit Breaker / Resiliency Hystrix, retry with backoff, bulkhead, timeout, fallback patterns | high | NetflixAmazonUberMicrosoft | ||
| 24 | CDN Push vs pull CDN, edge PoPs, cache invalidation, origin shield | high | CloudflareAmazonNetflixAkamai | ||
| 25 | Recommendation Engine Collaborative filtering, content-based, matrix factorization, feature pipeline | medium | NetflixSpotifyAmazonYouTube | ||
| 26 | Web Crawler BFS/DFS traversal, politeness, deduplication with Bloom filter, URL frontier | medium | GoogleAmazonBing | ||
| 27 | Metrics & Logging System Time-series DB, aggregation pipeline, alerting, distributed tracing (Jaeger) | medium | DatadogSplunkNetflixUber | ||
| 28 | Distributed Transactions (Saga/2PC) Two-phase commit, saga choreography vs orchestration, compensating transactions | high | AmazonStripeAirbnb |