Library

System Design Library

102 real-world system design breakdowns for products like feeds, chat, payments, search, storage, analytics, and collaboration.

2pc

Distributed Transaction CoordinatorCommit a transaction spanning multiple services/databases atomically.

Aggregation

Flight/Hotel Aggregator (Kayak)Search many providers in parallel and merge results fast.

Ann

Vector DatabaseFind nearest-neighbor vectors among billions for semantic search/RAG.

Auction

Google Ads / Ad ServingRun a realtime auction to pick the best ad for each impression in <100ms.

Availability

Scheduling (Calendly)Let people book time on someone's calendar without double-booking.

Batch

Batch Processing (MapReduce/Spark)Process petabytes across a cluster with automatic parallelism and fault tolerance.

Bloom-filter

Probabilistic Structures (Bloom / HLL)Answer "have I seen this?" and "how many unique?" at huge scale with tiny memory.

Caching

Build a CDNCache and serve content from edge locations worldwide with smart invalidation.DNS ResolverResolve domain names to IPs fast, globally, with heavy caching.Link Preview / UnfurlGenerate rich previews (title/image) for pasted URLs, safely and fast.URL Shortener (TinyURL)Map billions of long URLs to short codes and redirect in <50ms.

Catalog

E-commerce (Amazon)Browse a huge catalog, manage carts, and check out with consistent inventory.

Cdn

Spotify / Music StreamingStream audio to millions and recommend playlists.YouTube / Netflix (video)Upload, store, transcode and stream video to millions globally on any device/network.

Chunking

Distributed File System (GFS/HDFS)Store huge files across commodity machines, optimized for large sequential reads.

Config

Feature Flag ServiceToggle features per user/segment in realtime with near-zero read latency.

Consensus

Distributed Lock / Coordination (Chubby/ZK)Provide distributed locks, leader election and config — correctly, through failures.Globally-Consistent DB (Spanner)A horizontally-scalable SQL database with external (strong) consistency across regions.

Consistency

Flash Sale / Limited InventorySell N limited items to a massive simultaneous crowd without overselling.Inventory ManagementTrack stock across warehouses with no oversell under high concurrency.Ticketmaster / BookingSell event seats with no double-booking, under flash-sale spikes.

Consistent-hashing

Distributed Cache (Redis)A horizontally-scaled in-memory cache with sharding, replication and eviction.Key-Value Store (DynamoDB)A highly-available, partitioned, replicated key-value store with tunable consistency.

Counting

Polls / VotingCollect votes and show live tallies without a write hotspot or double-voting.View / Like CounterCount views/likes on hot items accurately enough, without a write hotspot on one row.

Crdt

Collaborative IDEMultiple users editing and running code together in a shared cloud workspace.Collaborative SpreadsheetRealtime multi-user spreadsheet with formulas that recalculate.Collaborative Whiteboard (Miro)Realtime multi-user canvas of shapes/strokes that converges.CRDT / Conflict-Free StoreLet many replicas update concurrently and converge automatically, even offline.Google Docs (collaborative editor)Many users editing one document simultaneously, seeing each other's keystrokes live and converging.

Data-modeling

Google CalendarEvents, recurrence, invitations, reminders and free/busy across time zones.Issue Tracker (Jira)Track issues with custom workflows, fields, search and permissions.

Edge

API GatewayA single entry point that authenticates, routes, rate-limits and observes all API traffic.

Experimentation

A/B Testing PlatformAssign users to experiment variants and measure impact correctly.

Failure-detection

Health Monitor / HeartbeatDetect when servers/services go down, fast and without false positives.

Fan-out

Facebook News FeedA ranked, personalized feed mixing friends' posts, pages and ads.Live Comments (Twitch/YouTube)Stream live comments to millions of concurrent viewers of one stream.PinterestSave and discover images on boards, with a recommendation-heavy feed.Push Notification GatewayDeliver mobile push to billions of devices via APNs/FCM reliably.Stock Price FeedPush live price ticks to millions of subscribed clients with minimal latency.Twitter / News FeedDeliver each user a timeline of recent posts from people they follow — instantly, at 300M users.

Geo

Airbnb / BookingSearch listings by location/dates and book without double-booking.Food Delivery (DoorDash)Connect customers, restaurants and couriers with live order tracking.Google MapsRender maps and compute fast routes over a planet-scale road graph.Tinder / MatchingShow nearby profiles and create matches on mutual likes.Uber / Ride DispatchMatch riders to nearby drivers in realtime as millions of locations update every few seconds.Yelp / Nearby PlacesFind businesses near a location with filters, reviews and ratings.

Gossip

Gossip Membership (SWIM)Let thousands of nodes know who's alive without a central coordinator.

Graph

LinkedIn (connections / PYMK)Model a professional graph and suggest "People You May Know" at scale.

Id-generation

Unique ID GeneratorGenerate unique, roughly time-ordered 64-bit IDs at massive scale without a single bottleneck.

Idempotency

Payment Gateway (Stripe)Process card payments via external networks reliably and idempotently.

Ingestion

IoT Telemetry IngestionIngest sensor data from millions of devices reliably and store it cheaply.Log Aggregation (ELK)Collect, index and search logs from thousands of services in realtime.

Inverted-index

Google SearchIndex the web and answer queries in <200ms with ranked results.Search Engine (Elasticsearch)Full-text search over huge document sets with relevance ranking.

Ledger

Venmo / P2P PaymentsSend money between users instantly, with a social feed of transactions.

Log

Distributed Log (Kafka)A durable, partitioned, replicated append-only log delivering high-throughput streams.

Lsm

Sorted Store (BigTable)A sorted, sparse, distributed map from (row, column, time) → value at petabyte scale.Storage Engine (LSM vs B-Tree)Design the on-disk structure a database uses to store and retrieve data.Wide-Column Store (Cassandra)A write-optimized, masterless, partitioned column store with tunable consistency.

Matching

Stock Exchange (Matching Engine)Match buy/sell orders fairly with microsecond latency and strict ordering.

Ml

Recommendation System (Netflix)Recommend relevant items to each user from a huge catalog, fast.

Ml-pipeline

Content Moderation PipelineDetect and act on harmful content (text/image/video) at scale.Voice Assistant (Alexa)Turn speech into an action and a spoken reply, fast.

Ml-serving

ChatGPT / LLM ServingServe LLM completions with streaming tokens, at scale, on scarce GPUs.

Nested

Comment System (Disqus)Embeddable threaded comments across millions of sites.

Object-storage

Dropbox / File SyncSync files across a user's devices reliably and efficiently, including huge files.Image Hosting (Imgur)Upload, store, and serve images globally with multiple sizes.InstagramPhoto/video sharing with feeds, at billions of media objects.Object Storage (Amazon S3)Store exabytes of objects with 11 nines of durability and high availability.PastebinStore and serve text/code snippets by short URL, some large, some private.

Observability

Distributed Tracing (Jaeger)Follow one request across dozens of services to find the slow/broken hop.

Olap

Data Warehouse (Snowflake)Run analytical queries over petabytes with elastic compute.

Proxy

Service Mesh (Envoy/Istio)Manage service-to-service traffic: routing, retries, mTLS, observability — without app changes.

Queues

Code Deployment (CI/CD)Build, test and safely roll out code to thousands of servers.Distributed Job Scheduler (cron)Run scheduled & one-off jobs reliably across a cluster, exactly-once-ish, at scale.Notification SystemSend push/email/SMS notifications reliably across channels, at scale, without spamming.Online Judge (LeetCode)Run untrusted user code against test cases safely, at scale.Web CrawlerCrawl billions of pages: fetch, parse, dedupe, and feed an index, politely.Webhook DeliveryReliably deliver event callbacks to customer HTTP endpoints, with retries.

Raft

Raft Key-Value Store (etcd)A small, strongly-consistent KV store for config, locks and coordination.

Ranking

Quora / Q&AQuestions, ranked answers, topics, and a personalized feed.RedditCommunities of posts and nested comments, ranked by a hot/score algorithm.

Rate-limiting

Distributed Rate LimiterCap each client to N requests/window across a fleet of servers.

Realtime

Live Polling (Slido/Kahoot)Run live audience polls/quizzes with realtime results to thousands.Multiplayer Game ServerAuthoritative realtime state for fast-paced multiplayer games.Presence ServiceTrack who is online/away/typing across millions of connections.

Security

OTP / 2FA ServiceGenerate and verify one-time codes for login/2FA securely.

Sorted-set

Realtime LeaderboardMaintain top-N rankings and a player's rank among millions, updated live.

Storage

Gmail / EmailStore, search, send and receive email at billions-of-mailboxes scale.

Streaming

Ad Click AggregatorCount billions of ad clicks in near-real-time for dashboards & billing, accurately.Fraud DetectionFlag fraudulent transactions in realtime without blocking good users.Live Streaming (Twitch)Ingest a streamer's feed and broadcast to millions with low latency.Realtime Analytics DashboardLive dashboards over high-volume event streams (clicks, metrics).

Time-series

Metrics & Monitoring (Datadog)Ingest billions of metric points, store time-series cheaply, alert in realtime.Time-Series DB (Prometheus)Store and query metrics efficiently with high cardinality.

Transactions

Payment System / WalletMove money between accounts with zero double-spends, ever.

Trie

Typeahead / AutocompleteSuggest top completions as the user types, in <100ms.

Webrtc

Video Calling (Zoom)Low-latency multi-party audio/video calls.

Websockets

Discord / Live ChatRealtime servers/channels with huge rooms and presence, low latency.SlackTeam chat with channels, threads, search, and presence.WhatsApp / MessengerRealtime 1:1 and group chat with ordering, delivery receipts and offline delivery.
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