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Complete walkthroughs of interview problems, in the order you would deliver them: requirements, the design step by step, the deep dives an interviewer pushes on, and what is expected at each level. Each one matches a problem you can practise.

Ticket bookingDesign TicketmasterSell reserved seats to live events without selling any seat twice, keep money and tickets in step, and stay up and fair when a hundred thousand fans arrive in one minute.Medium · Core 27 min · 13 diagrams
URL shortenerDesign a URL shortenerTurn long URLs into short codes that open quickly worldwide, while keeping every code unique, removable and hard to abuse.Medium · Core 28 min · 10 diagrams
RAG assistantDesign a RAG assistantBuild an assistant that retrieves only the company passages an employee may open, answers from them with sources, and keeps the corpus fresh as documents and access change.Hard · Core 28 min · 12 diagrams
ChatGPT-style assistantDesign a ChatGPT-style assistantLet tens of millions of people talk to models you run on your own GPUs, with words that appear as they are written, a memory each person controls and nothing unsafe on screen.

Hard · Core 28 min · 12 diagrams

LLM inference serviceDesign an LLM inference serviceRun your company’s own language models for every internal team: answers that begin quickly for people watching, millions of background jobs a day, and a GPU fleet that is the biggest computing cost.Hard · Core 27 min · 12 diagrams

Rate limiterDesign a rate limiterA good limiter is a fast policy decision at the gateway, backed by shared counters that stay correct under concurrency and predictable under outages.Medium · Core 28 min · 12 diagrams

ChatDesign WhatsAppDesign a global encrypted messenger where messages reach every device, stay in one conversation order, and remain unreadable to the service.Hard · Core 28 min · 11 diagrams

News feedDesign a news feedShow half a billion people the posts they care about most from the accounts they follow, fresh within seconds, while some accounts have a hundred million followers and a deleted post has to vanish everywhere at once.Medium · Core 28 min · 12 diagrams

AI inference gatewayDesign an LLM gatewayRoute many companies through model providers without mixing data, overspending their quotas or hiding failures behind a slow stream.Hard · Core 28 min · 11 diagrams

Ride dispatchDesign UberMatch a rider with a nearby driver in seconds while a million phones report where they are, without giving one driver two trips or losing a trip when a region goes dark.Hard · Core 28 min · 12 diagrams

Video streamingDesign YouTubeTake a video from a creator's phone to tens of millions of screens at once, starting in about two seconds on any connection, anywhere in the world.Hard · Core 27 min · 12 diagrams

Notification serviceDesign a notification systemBuild the one service every team uses to reach people by push, email, text and in-app, so that codes arrive in seconds, big promotions go out steadily and every accepted message has a fate someone can see.Medium · Core 27 min · 12 diagrams

Search autocompleteDesign search autocompleteAs someone types, show the searches they are probably making, fast enough to feel instant, drawn from what millions of people look for, with nothing offensive or private in the list.Medium · Core 27 min · 12 diagrams

Payments ledgerDesign a payment systemTake buyers’ money by card or wallet, pay sellers every day, and show an auditor where each cent went, without charging anyone twice or losing a cent along the way.Hard · Core 28 min · 12 diagrams

Distributed cacheDesign a distributed cacheA shared cache the whole company leans on has to find any item in one hop, live within each machine’s memory, and stay full while machines fail, one item gets hot and the cluster grows.Medium · Core 26 min · 12 diagrams

File syncDesign DropboxKeep one folder identical on every laptop, phone and browser a person uses, through weeks offline, shared teams and a lost region, without silently losing a single edit.Hard · Core 27 min · 11 diagrams

Webhook deliveryDesign a webhook delivery systemSend a platform’s events to its customers’ own servers so that every accepted event arrives or visibly fails, receivers can trust what they get, and nobody waits on someone else’s slow endpoint.Medium · Core 28 min · 12 diagrams

Ad-click aggregatorDesign an ad click aggregatorSend people on to the advertiser without delay, show each advertiser their numbers within a minute, and bill every valid click exactly once, however busy the night gets.Hard · Core 26 min · 12 diagrams

Top-K and leaderboardsDesign a leaderboard and top-KTell tens of millions of players exactly where they stand the moment a match ends, and show which levels everyone is playing right now, without letting a retry or a failover change who wins the prizes.Hard · Core 28 min · 12 diagrams

Job schedulerDesign a distributed job schedulerCustomers trust a scheduler with emails, reports and card charges at a precise minute, so it must create one run per scheduled time, drop none, make repeats harmless, and hold up when a million are due together.Medium · Core 27 min · 12 diagrams

Web crawlerDesign a web crawlerFind and fetch billions of public pages for a search engine without hurting any website, wasting a request on a repeat or losing a single discovered address when machines fail.Hard · Core 27 min · 11 diagrams

Content moderationDesign a content moderation systemStop the worst content before anyone sees it, spend scarce human attention where it counts, and keep a record that can defend each call months later.Hard · Core 27 min · 12 diagrams

Distributed message queueDesign a distributed message queueBuild the shared pipe every team publishes to and reads from, so a confirmed event survives a lost broker or zone, readers go at their own pace, and no team or hot key holds up the rest.Hard · Core 28 min · 12 diagrams

Feature flagsDesign a feature flag systemLet thousands of companies switch features on for some of their users, change course within seconds, and switch off a broken feature in the middle of the night, without ever slowing or breaking their apps.Medium · Core 27 min · 12 diagrams

Distributed key-value storeDesign a distributed key-value storeTeams trust a shared store with carts, sessions and usernames, so every saved write must survive a machine or a zone, every read must say what it promises, and copies must heal after failures.Hard · Core 26 min · 12 diagrams

Collaborative document editorDesign Google DocsLet many people type in one document at the same moment and show every one of them the same words, with nothing anyone typed disappearing, even offline or when a server dies.Hard · Core 27 min · 11 diagrams

Metrics and monitoringDesign a metrics and monitoring systemWatch thousands of services without slowing them down, keep a year of graphs within budget, and wake the right person for the right reasons, even on the night the monitoring breaks too.Hard · Core 26 min · 11 diagrams

Distributed ID generatorDesign a unique ID generatorHundreds of services need ids for orders, users and messages, so every id must stay unique across machines, regions and clock mistakes, cost no network hop, sort by time and fit in 64 bits for decades.Medium · Core 27 min · 12 diagrams

Coding practice platformDesign LeetCodeBuild a place where people practise coding problems and race in weekly contests, where strangers’ code cannot hurt the platform, the hidden tests stay secret, and a first-minute rush still gets every verdict back fast.Hard · Core 27 min · 12 diagrams

Online auctionDesign an online auction system (eBay)Run millions of timed auctions so that the highest bid wins, everyone watching sees the price move within a second, and a last-second rush on a popular item still crowns the right winner.Medium · Core 28 min · 11 diagrams

Flash saleDesign a flash sale systemA few thousand identical units, millions of people and one opening second: sell every unit once, to people rather than bots, while the site stays up.Hard · Core 28 min · 12 diagrams

Nearby placesDesign YelpHelp people find a good place nearby in a busy city or an empty stretch of countryside, in well under a second, while reviews keep arriving and a few businesses try to game their ratings.Medium · Core 27 min · 11 diagrams

PastebinDesign PastebinPeople paste logs, configs and code to share them, so the site must keep unlisted pastes away from strangers, serve every read quickly, honour expiry and stay clear of spam and malware.Easy · Core 26 min · 11 diagrams

Food deliveryDesign a food delivery appTake an order, get a restaurant to cook it and a delivery partner to bring it while it’s hot, and keep the promised time, even on a rainy Saturday.Medium · Core 28 min · 12 diagrams

Payment gatewayDesign a Stripe-style payment gatewayLet online shops take card payments that are charged exactly once, survive banks that go quiet, keep card numbers in one locked-down place, and pay merchants only from money that has arrived.Hard · Core 28 min · 12 diagrams

Stock exchangeDesign a stock exchangeMatch buyers and sellers of thousands of listed stocks within a millisecond, treat every member firm alike, and lose nothing a broker was told when a machine fails.Hard · Core 27 min · 12 diagrams

ChatDesign a Slack-style team chatRun chat for whole companies: channels of a hundred thousand people, threads, badges that stay right on every device, and years of history you can search.Hard · Core 26 min · 11 diagrams

Subscription billingDesign a subscription billing systemTurn millions of subscriptions into correct invoices and charges every month, for years, through trials, plan changes, declined cards and India’s rules for automatic debits.Hard · Core 28 min · 12 diagrams