Design sheets.
Each side quest's whole design on a few pages: the numbers, the architecture with real tech icons, the key flows as sequence diagrams, the data model, the state machine, the decisions and what breaks. Read the quest once; revise it here in ten minutes, or download the PDF.
36 of 36 side quests have a sheet
read the guide →- SQ-10 · 7 diagramsTwitter/XA timeline system that puts ~500 million posts a day into followers' home timelines within 5 seconds and serves ~300,000 timeline reads a second from memory, by pushing ordinary posts at write time and pulling celebrity posts at read time.
- SQ-06 · 7 diagramsNetflix StreamingA video platform that starts any title in under 2 seconds for ~10 million concurrent streams, because the cloud only decides what to play and appliances inside ISPs deliver the bytes.
- SQ-07 · 7 diagramsSpotifyA music service that starts any of 100M+ tracks in under 200 ms for 777M monthly users, because most taps are served from the device or a CDN edge and nothing slow sits on the play path.
- SQ-21 · 6 diagramsInstagram Feed and StoriesA ranked feed and a stories tray for 3 billion monthly users, by storing every photo once, moving only small time-sortable IDs, and spending the compute on choosing which twenty posts to show.
- SQ-05 · 7 diagramsGoogle SearchA web search engine that answers 100K queries a second over 50B+ pages in under 500 ms, because the index is sharded by document, every query fans out to all shards in parallel, and crawling stays off the clock.
- SQ-02 · 7 diagramsWhatsApp MessagingA chat backend that routes 100B+ end-to-end encrypted messages a day to ~50M connected phones, kept small because the server is a delivery queue, not an archive.
- SQ-19 · 6 diagramsYouTube Video PlatformTurning 20 million+ uploads a day into playable video within minutes and counting billions of views honestly, by keeping every original forever and treating everything derived from it as a rebuildable pipeline.
- SQ-03 · 7 diagramsAadhaar (UIDAI)A national identity system that has issued 1.45B numbers and answers 90M+ authentications a day in under 200 ms, because checking one person (1:1) and proving nobody enrols twice (1:N) are built as two separate paths.
- SQ-09 · 7 diagramsUberA ride-hailing marketplace that matches a rider to a driver in under 3 seconds while ~500K driver pings a second land at peak, by keeping live locations as soft state in an H3 hexagon index and only trips in a strongly consistent store.
- SQ-08 · 7 diagramsAWS S3An object store that keeps 500 trillion+ objects at 11 nines of durability and 200M+ requests a second, by splitting a small partitioned index from erasure-coded bytes spread across three AZs and repairing lost shards fast.
- SQ-01 · 6 diagramsIndia's UPIA national switch that routes 800 million payments a day between 740+ banks in under 2 seconds, without ever holding the money.
- SQ-04 · 7 diagramsVisa/Mastercard Global Payment NetworkA card network that routes ~434B transactions a year (2025) between merchants' banks and card-issuing banks, ~14,500 financial institutions on Visa's network alone, in a sliver of a 2-second checkout, moving messages in real time and money later in netted batches.
- SQ-26 · 7 diagramsTruecaller Caller ID and Spam DetectionCaller ID and spam blocking for about 411 million daily phones, by answering from the phone first and then from one precomputed record within a single ring, and by keeping a number's spam verdict (shared, cached everywhere) apart from a person's name (sourced, kept in its jurisdiction, removable within a day).
- SQ-17 · 6 diagramsSwiggy Food DeliveryA three-sided marketplace that gets food from kitchens it doesn't run to customers across the city, about 2 million times a day, by dispatching riders just in time for the predicted ready time and promising a calibrated quantile, not an average.
- SQ-11 · 7 diagramsIRCTC Tatkal Ticket Booking SystemOpens a ~500-seat Tatkal quota to ~2 million people at 10:00:00 and sells each seat exactly once, by saying "no" to 99.975 % of them at the edge in milliseconds so only traffic the size of the inventory reaches the database.
- SQ-12 · 7 diagramsBlinkit Quick CommerceGets a basket from a dark store ~2 km away to the door in about ten minutes, a few million times a day across ~2,500 stores, by winning the minutes inside the store and showing each customer an estimate the store can actually keep.
- SQ-16 · 7 diagramsBookMyShow Seat BookingA ticketing platform where people pick exact seats from a live map, for cinemas every day and for on-sales of 13 million people, and no seat is ever sold twice because one writer per show holds a booking's seats all together or not at all.
- SQ-14 · 7 diagramsChatGPT LLM ServingA serving stack that streams ~50 million output tokens a second to over a billion weekly users, by scheduling on tokens and GPU memory so each replica's every decode step serves a couple of hundred replies at once.
- SQ-18 · 6 diagramsRazorpay Payment GatewayA gateway that takes over a billion payments a quarter for online businesses across cards, UPI and banks, and gives every one exactly one final outcome by treating silence as UNKNOWN and reconciling it, never guessing.
- SQ-27 · 7 diagramsFlipkart Big Billion DaysA national sale that opens to millions in the same minute, by switching prices and deals on as a pre-staged version, queuing only each deal's button with tokens at the edge, holding stock and delivery slots at checkout before taking money, and treating every unanswered payment as unknown until it is resolved.
- SQ-35 · 6 diagramsFASTag Electronic TollingIndia's electronic toll network (~120 lakh crossings a day through 40 issuer and 12 acquirer banks) works because the lane decides with what it already holds, signed tags and a local list of bad tags, while one transaction number per crossing and written liability windows let the banks settle correctly afterwards, even when a plaza was offline for hours.
- SQ-34 · 7 diagramsAmazon Prime Day CheckoutA cart, checkout and order pipeline for ~5,000 orders and ~100,000 cart operations a second at the Prime Day peak (assumed), where the cart never refuses an add, a purchase id makes every order happen once, and customer cells keep any one failure to a small share of shoppers.
- SQ-15 · 7 diagramsFigma Multiplayer EditingReal-time editing for 13 million+ users and up to 500 people in one file, by giving every file one owner server that holds it in memory, orders every property change, and writes a fenced journal every half second.
- SQ-13 · 7 diagramsZerodha Kite Trading PlatformA broker platform that takes up to 40 million orders a day and streams prices to 1.5 million screens, by giving orders, prices and reads three separate paths sized for what each can afford to lose.
- SQ-28 · 7 diagramsGmailAn email service for 1.5 billion mailboxes, by answering 250 OK only after a quorum write and 4xx when it can't, filtering cheap signals first and models last, judging senders by DMARC-aligned identity, storing immutable blobs in hot then erasure-coded tiers with a copy software can't reach, and partitioning metadata, search index and limits by mailbox.
- SQ-24 · 7 diagramsAirbnb Stays MarketplaceA marketplace of 9 million+ one-of-a-kind homes, by searching a fast in-memory copy of every listing's place, free nights and prices, deciding exactly only at Reserve, and moving guests' money through a double-entry ledger that holds it until check-in.
- SQ-22 · 7 diagramsGoogle Maps Routing, Traffic and TilesDirections, live traffic and a map for 2 billion monthly users, by precomputing what depends on the map, streaming what depends on traffic, and timing every road for when the driver will reach it.
- SQ-20 · 6 diagramsGoogle Docs CollaborationUp to 100 people typing into the same text at once, kept identical by one server per document that transforms late changes past newer revisions and appends each one to a revision log that is the document, its fence and its history.
- SQ-31 · 7 diagramsDiscordA real-time community platform for 90 million+ daily users, with servers of up to a million online, that keeps one ordered process per server and pushes the fan-out outwards (relays, passive sessions, lazy member lists), stores messages by channel and time bucket, and places each drop-in voice call on one SFU host.
- SQ-36 · 7 diagramsSlackA workplace messaging platform for tens of millions of connected clients that routes and stores by channel (channel IDs hashed to channel servers, messages sharded by channel in Vitess) while keeping the organisation as the unit of policy, and puts everything a message causes on a Kafka-fronted job queue.
- SQ-25 · 7 diagramsDream11 Fantasy SportsA fantasy platform at IPL scale, by forecasting the toss-to-deadline spike and pre-scaling for it, taking contest spots from sharded counters under one join id, freezing every team with a single match-lock flip that every write checks, and ranking tens of millions of entries after every ball from points histograms.
- SQ-23 · 7 diagramsZoom Video Meetings at ScaleVideo meetings for 300 million daily meeting participants, by keeping a small, ordered control plane apart from a media plane of routers that forward packets without decoding them, pick a layer for every receiver, and cascade across regions.
- SQ-29 · 7 diagramsGitHubA code host for a billion repositories, by making each reference update a majority vote across five replicas, turning every accepted push into one event that isolated consumers fan out to pull requests, webhooks, CI and search, and running untrusted CI jobs on fresh VMs scheduled fairly per account.
- SQ-30 · 7 diagramsTikTok's For You FeedA short-video feed for a billion monthly users that turns every impression into a training example, trains online, and syncs only the touched embedding rows to serving every minute, so it learns from a swipe in seconds through the session and in minutes through the model.
- SQ-32 · 7 diagramsONDCAn open commerce network where buyer apps and seller apps run by different companies trade through one signed, asynchronous protocol (218 million transactions in FY 2025–26), kept consistent without a shared database by rules both sides follow: the buyer app names the order, the seller app holds it until its confirmation is acknowledged, and money moves only when both books match at a settlement agency.
- SQ-33 · 7 diagramsNSE Stock ExchangeAn exchange core for ~1 million messages a second on a record day (21.89 billion), where one sequencer and one deterministic matching thread per partition decide "first", multicast gives every subscriber each tick at once, and nothing is acknowledged until a surviving copy can replay it.
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