Mock system design interview at Google

A live 45-minute round on a question reported in Google interviews, run by an AI interviewer that asks one thing at a time, probes what you say, looks at your whiteboard, and weighs what that round is reported to weigh. It ends with a debrief: a score, what was strong, what was missing.

your question

Design a URL Shortener

bit.ly at scale: turn long URLs into short codes, redirect in milliseconds from anywhere, count the clicks.

You will not see the requirements, the numbers or the design before you start. That is the point.

Walk through it first

What the Google round is reported to look like

45 minutes, whiteboard or shared document, usually one open-ended design with the interviewer steering into depth. Now asked at more levels than it used to be.

What it weighs

Scale: the numbers behind the design, and what changes at 10x
Technical depth on whichever component the interviewer picks
Data modelling and storage choices, defended against the access patterns
Clear tradeoffs rather than a single asserted answer

The common mistake

Depth is sampled, not announced. Any component you name can become the next fifteen minutes, so do not put something on the board you cannot open up.

From public interview guides and candidate reports, and Google’s own interview guidance where it publishes any. Not endorsed by Google. The interviewer is an AI running in that style; it is practice, not a prediction.

Start the round

Timed · 45 minutesInterview conditions. The AI interviewer owns the clock; the AI Mentor is off.With the AI Mentor · untimedNo clock, and an AI Mentor beside you to ask when you are stuck.

The interviewer’s opening question is free. Answering it is part of Pro. See an example round and its debrief →

The 60 questions this round draws from

Each is reported in Google interviews. Pick one to sit, or read its reference design and walk through a strong answer first.

Design a URL Shortenerbit.ly at scale: turn long URLs into short codes, redirect in milliseconds from anywhere, count the clicks.
drawn
Design SlackReal-time team messaging: channels, DMs, presence, history and search for millions of concurrent users.
Design YouTubeUpload, transcode, and stream video to a billion viewers: a processing pipeline on one side, a CDN problem on the other.
Design Google MapsMap tiles, place search, turn-by-turn routing with live traffic, and ETAs for a billion users.
Design Google DocsReal-time collaborative editing: many cursors in one document, every keystroke converging to the same text, offline edits merged on reconnect.
Design an LLM Inference ServiceServe a large language model behind an API: queue the requests, batch them onto scarce GPUs, stream tokens back, and keep every accelerator busy without blowing the latency budget.
Design ChatGPTAn AI chat assistant for 100 M weekly users whose streamed answers survive dropped connections, call tools mid-answer, and share scarce GPUs across tiers.
Design a Distributed CacheA Redis-like cache as a service: hundreds of nodes, terabytes of memory, sub-millisecond gets, and a cluster that survives losing a machine mid-request.
Design a Web CrawlerFetch billions of pages a month politely, discover new links, skip duplicates, and keep the copy fresh without getting the crawler banned.
Design a Rate LimiterA distributed service that decides, in under a millisecond, whether a request from a client is allowed under its quota.
Design DropboxKeep a folder identical on every device: upload huge files in resumable chunks, push only what changed, and reconcile two machines that both edited the same file offline.
Design Search AutocompleteSuggest the top ten completions after every keystroke, in under fifty milliseconds, from what the world searched for in the last hour, without suggesting anything that gets you in the newspaper.
Design an Ad Click AggregatorCount a million ad clicks a second, show advertisers a dashboard that is seconds behind, stop a campaign the moment its budget runs out, and be exactly right when the invoice is cut.
Design a Distributed Job SchedulerRun millions of cron jobs and one-off tasks on time across a fleet, exactly once each, and keep going when a worker dies halfway through one.
Design a Stock ExchangeMatch buy and sell orders in microseconds, in a strict and provable order, never lose a fill, and stay correct when the matching engine dies mid-trade.
Design InstagramUpload photos, publish posts and stories, and load a feed of them for 500 M daily users, with every image served from a CDN in milliseconds.
Design WhatsAppDeliver 50 billion end-to-end encrypted messages a day to phones that are often offline, with sent, delivered and read ticks, and a server that cannot read any of it.
Design NetflixStream a fixed catalog to 40 M people at once, 200 Tbps at peak, starting in under two seconds, from cache servers placed inside the viewers’ own internet providers.
Design YelpFind the best places near you, filtered by what is open now, from 200 M businesses, in under 200 ms, and keep ratings honest as reviews arrive.
Design a Distributed Key-Value StoreA Dynamo-style store that keeps 10 billion keys on dozens of machines, survives node and rack failures, and lets each caller trade consistency for latency with N, R and W.
Design a Unique ID GeneratorHand out a million 64-bit ids a second across many hosts and regions, never twice, roughly in time order, with no central bottleneck and no trust in the wall clock.
Design LeetCodeRun untrusted code from strangers against hidden tests in a dozen languages, return a fair verdict in seconds, and survive 50,000 people submitting in the last minutes of a contest.
Design a Metrics and Monitoring SystemIngest 10 million samples a second from 100,000 hosts, keep them queryable for a year at shrinking resolution, draw dashboards in under a second, and page the right person within a minute.
Design TinderShow each person a deck of nearby profiles that fit both people’s preferences, never repeat one, take 2 billion swipes a day, and announce a match the instant two people like each other.
Design RedditServe communities of posts with deeply nested comment threads, take tens of thousands of votes a second, rank by hot and best, and keep a 50,000-comment thread fast while it is still growing.
Design an Online AuctionRun 50 million live eBay-style auctions: order every bid strictly per auction, push the price to watchers live, and close each auction exactly once.
Design Live CommentsComments and reactions on live video that reach viewers within two seconds, at 50 M concurrent viewers and 5 M on one stream.
Design a Top-K Heavy Hitters ServiceRank the top 1,000 videos by views over the last minute, hour, day and all time, from a million views a second.
Design ZoomReal-time audio, video and screen share for 2 to 1,000 people per meeting, 300 M participants a day, under 150 ms mouth to ear.
Design Google SearchSearch 50 billion pages at 100 k queries a second: the ten best results with snippets in under 200 ms, and news within minutes.
Design SpotifyStream 100 M tracks to 600 M listeners with sound in 200 ms, offline downloads, regional licensing, and every play counted for royalties.
Design GmailReceive, filter, store and search email for 1.5 billion users: 200 billion inbound messages a day, mostly spam, kept for 20 years.
Design a Large File Distribution SystemGet a 200 GB checkpoint from one origin onto 10,000 machines in five data centres within minutes, under capped links, verifying every byte.
Design a Recommendation SystemThe “recommended for you” video home feed: 50 personalised videos picked from billions for 500 M daily users in under 200 ms.
Design a Distributed Lock ServiceA Chubby-style lock service for 50 k processes: leases, leader election, fencing tokens and watched config, never letting two holders act at once.
Design an Object Storage ServiceAn S3-like store for objects from bytes to terabytes, with eleven nines of durability and strong read-after-write consistency at exabyte scale.
Design Google CalendarRecurring events, invitations, free/busy and reminders for 500 million users, correct in every time zone and synced to every device within seconds.
Design a Code Deployment SystemBuild each merge once into a signed artifact, roll it out in stages to 50,000 hosts in 10 regions, and roll back automatically on regression.
Design a Distributed Logging SystemCollect 10 million log lines a second from 100,000 hosts, make them searchable within seconds, stream them live, and keep a year affordably.
Design TwitchIngest 100 k live broadcasts, transcode them in real time, and deliver each about three seconds behind live, to three viewers or 500 k.
Design a Price TrackerTrack 500 M products' price history and alert 50 M users when a price drops below their target, in time to buy.
Design StravaRecord runs and rides offline, process each upload into a map, stats and segment efforts, and rank them for 100 M athletes.
Design Figma500 k design files open at once, some with 200 people: objects converge live, cursors stream, and 300 MB files open in seconds.
Design a Load BalancerSpread 5 M requests a second over 50 k backends, terminate TLS, route by host and path, survive losing any balancer or data centre.
Design a RAG SystemAn assistant that answers employees’ questions from their company’s documents, with citations, only from what each user may open, minutes after edits.
Design a CDNA multi-tenant CDN for 30 M customer domains: 300 PoPs, 50 M requests a second served mostly from cache, and global purges in seconds.
Design an API GatewayOne front door for public and partner APIs: authenticate, limit and route 1 M requests a second to 300 services, adding under 10 ms.
Design an Ad Serving SystemPick the best of 10 M active ads for 5 M requests a second in under 100 ms, within every campaign’s targeting, budget and frequency caps.
Design GitHubHost 400 M git repositories on replicated file servers, serve 100 k fetches a second, and run pull requests, forks and webhooks on top.
Design Venmo80 M users paying friends instantly: 15 M transfers a day on a sharded double-entry ledger, funded and cashed out over slow bank rails.
Design TikTokA full-screen feed of short videos for 1 B daily users, where the next clip starts instantly and every swipe reshapes what comes next.
Design Google NewsCollect 2 M articles a day from 50,000 publishers, group them into stories, rank them per edition, and serve 300 M daily readers.
Design GitHub CopilotInline AI code suggestions for 10 M developers, shown within 300 ms of a pause in typing, when most requests are never used.
Design Nearby FriendsShow each user which friends are within 5 miles, refreshed every 30 s, for 10 M concurrent users, without leaking anyone’s location.
Design PastebinStore and serve 10 M text pastes a day behind unguessable links: expiry that is never late, burn after reading, and abuse takedown.
Design PinterestSave images to boards, browse a personalised grid and search by picture: 100 M daily users, 5 B distinct images behind 100 B saves.
Design a Vector DatabaseA managed vector database: 10,000 tenants, 20 B vectors, filtered top-k in under 50 ms at 0.95 recall, and writes searchable in seconds.
Design a Content Moderation SystemCatch spam, nudity, hate and illegal media in 4 billion daily posts and comments before they spread, with 15,000 reviewers for the hard cases.
Design a Flight Booking SystemAnswer 50 M searches a day from 20 slow, metered suppliers through a fare cache, then book seats you do not own, exactly once.
Design a Container OrchestratorPlace 300,000 containers on 10,000 machines from declared desired state, keep them running through failures, and give every service a stable address.

Other companies: Anthropic · Meta · Amazon · OpenAI · Microsoft · Uber · DoorDash · Netflix