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.
21 of 21 side quests have a sheet
- 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-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-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-04 · 7 diagramsVisa/Mastercard Global Payment NetworkA card network that routes ~434B authorizations a year between merchants' banks and ~14,500 card-issuing banks in a sliver of a 2-second checkout, moving messages in real time and money later in netted batches.
- 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-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-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-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-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-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-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-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-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-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-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-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-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-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-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.
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