Case studies

Each lab designs a well-known product from a blank page: requirements, numbers, API, data, the hard parts, and what breaks under load.

BitlyFreeURL shortenerShort codes, very fast redirects, and IDs generated without a central bottleneck.easyWhatsAppFreeChatOne-to-one and group messages, delivered in order, to phones that drop offline.mediumRate limiterShared API quotasA shared quota check in front of every endpoint, fast enough to be invisible.mediumLeaderboardLive ranks at game scaleLive rank and nearby players for tens of millions of scores.easyJob schedulerTimed jobs at scaleMillions of timed jobs fired on schedule, with no job lost or run twice by accident.mediumAutocompleteSearch suggestionsPrefix suggestions that keep up with typing speed.mediumNotificationsPush, SMS and emailOne API for every channel, with preferences, retries and no duplicates.mediumGooglebotWeb crawlerA polite crawler that covers a billion pages a month and skips what it has seen.mediumSearch engineWeb searchAn inverted index split across many machines, answering a query in a few hundred milliseconds.hardTwitterHome timelineTimelines assembled for hundreds of millions of readers, including followers of huge accounts.mediumInstagramPhoto feedUploads, a ranked feed, and the cost of building it at write time or read time.mediumRedditRanked feedVotes stream in while hot rankings stay current.mediumFacebookRanked news feedThousands of candidate posts narrowed to the few worth showing, ranked by a model, in under a second.hardDiscordOnline presenceAccurate online status for a huge user base without drowning in heartbeats.mediumCricbuzzLive commentaryEvery ball pushed to millions of fans within a second or two.mediumGmailEmailAccept, store, search and deliver mail without losing any of it.mediumGoogle DocsCollaborative editorMany cursors in one document, and every keystroke kept.hardYouTubeVideo platformUpload, transcode into many renditions, and stream at the right quality.mediumSpotifyMusic streamingNear-instant playback for a catalog of millions of tracks, even on weak networks.mediumDropboxFile syncFiles kept in step across devices, sending only the parts that changed.mediumS3Object storageHuge numbers of objects, stored durably and served from anywhere.hardUberRide hailingLive driver locations and a match for every rider within seconds.hardYelpPlaces near meNearby search over millions of places, with filters, ranking and a map that moves.mediumSwiggyDelivery dispatchAssigning riders to food orders while everyone is on the move.hardGoogle MapsRoutes and ETAsShortest routes and arrival times that react to traffic.hardBookMyShowTicket bookingSeat holds that expire, payments that may fail, and no seat sold twice.mediumFlipkartFlash saleHuge demand for limited stock, without overselling or melting down.hardRazorpayPaymentsCharging, refunding and reconciling money when any network call can fail.hardStock exchangeOrder matchingA matching engine that is fast, fair and deterministic, with every order sequenced exactly once.hardKafkaDistributed message queueA replicated, partitioned log that keeps every acknowledged message.hardDynamoDBKey-value storeA Dynamo-style store that keeps serving while machines fail.hardDatadogMetrics and monitoringHigh-volume time series in, dashboards and alerts out within seconds.hardAd clicksBilling-grade click countsClick counts accurate enough to bill advertisers on.hardChatGPTServing an LLMStreaming answers from a GPU fleet: batching, the KV cache, queues and the cost of a token.hard

Each system is modelled on a well-known product. Names belong to their owners.