Design a URL Shortener
bit.ly at scale: turn long URLs into short codes, redirect in milliseconds from anywhere, count the clicks.
The Cursor for system design interviews
Stop studying system design. Start practicing it.
Practicing with ChatGPT is like coding by copy-paste: every question means explaining your design again.
Here the interviewer, your whiteboard and the AI Mentor share one room, so you can go straight to the deep dives, where interviews are actually decided.
No sign-up. No credit card. Start in one click.
Chat 1: the interviewer
Interviewer
Traffic spikes 10x and your click queue starts backing up. What happens?
Chat 2: my questions
Chat 2 can’t see the interview. You re-explain your design every time.
Reference designs21
Deep dives112
Questions from 43 companies
Why this exists
The diagram was never the problem. The deep dives were.
- 01
Onsite after onsite, the same round
I started interviewing again and kept making it to onsites. Every time, system design is where it fell apart. Rejection after rejection.
- 02
Prep stopped at the diagram
Most material ends at load balancer, cache, database, done. My high-level designs were fine. I got stuck when the interviewer asked how exactly I would handle a hot partition, or what happens when this queue backs up.
- 03
Two AI windows
So I practiced with AI: one chat interviewed me, and I asked my follow-up questions in a second chat so the interview stayed clean. But the second chat had no idea what we had discussed. I was re-explaining my design every time.
- 04
So I put it all in one room
It felt like pasting code into ChatGPT before the model moved inside the editor, where it could see everything. So I built myself a tool that keeps the interview, the whiteboard and the AI Mentor in one place. It turned into this site, and I still use it.
ChatGPT in a tabSysDesignPrep.com
Your whiteboardCan’t see it. You describe it in text.The AI Mentor reads the board as you draw
The interview so farOnly what you paste in, againEvery turn, without re-explaining
Follow-up questionsDerail the interview, or go to a chat that knows nothingAsked beside the interview, which stays clean
Past the diagramOnly as deep as you know to ask112 written deep dives: hot keys, backpressure, failure modes
What strong looks likeYou would have to know alreadyA strong candidate’s answer, turn by turn, then a scored debrief against it
How it works
Two ways to prepare, in one room
A whiteboard, the conversation between interviewer and candidate, and an AI Mentor beside it. First you walk through it, then you sit it.
Walkthrough
Walk through a strong candidate's answer
- A recorded interview, played a turn at a time
- The whiteboard fills in as they talk
- You answer first at the key decisions
- Ask the AI Mentor anything along the way
Mock interview
Then sit the same question yourself
- An AI interviewer that probes what you say
- It sees what you draw on the board
- Timed, or untimed with the AI Mentor beside you
- A scored debrief, stage by stage
- 01
The interviewer asks
A real system design question, opened the way a real interviewer opens it: the problem in two sentences, then "where would you like to start?"
- 02
A strong candidate answers
Clarifying questions, requirements with numbers, estimates, the API, then the design, drawn on the whiteboard as they talk, one bottleneck at a time. You press Next for each turn.
- 03
You answer first at the key decisions
Which database? How do you generate the codes? 301 or 302? The walkthrough pauses and asks what you would say, then shows what the candidate said.
- 04
Ask the AI Mentor anything
Why Postgres and not Cassandra? What is request coalescing? Would my idea work? The AI Mentor sees the interview and the whiteboard where you are, and answers there and then.
- 05
Then sit it yourself
The same question as a mock interview: an AI interviewer asks, probes what you say and what you draw, and debriefs you, stage by stage, against the walkthrough you went through.
Questions
Each one teaches patterns the others do not
Every question comes three ways: a walkthrough to go through, a mock interview to sit, and a full written reference design to read.
Design Slack
Real-time team messaging: channels, DMs, presence, history and search for millions of concurrent users.
Design a Payment System
Accept card payments for an e-commerce platform, move money correctly exactly once, and be able to prove it.
Design a News Feed
The home timeline of Twitter, Instagram or Facebook: posts from people you follow, ranked, fresh, and fast for hundreds of millions of users.
Design YouTube
Upload, transcode, and stream video to a billion viewers: a processing pipeline on one side, a CDN problem on the other.
Design Google Maps
Map tiles, place search, turn-by-turn routing with live traffic, and ETAs for a billion users.
Design a Matchmaking System
Group online game players into fair, low-latency matches within seconds, at millions of concurrent players.
Design Uber
Match riders with nearby drivers in seconds, track trips live, and price them, for millions of concurrent users worldwide.
Design a Notification System
Deliver push, email, SMS and in-app notifications to hundreds of millions of users: the right channel, once, on time, and never at 3 a.m.
Design Google Docs
Real-time collaborative editing: many cursors in one document, every keystroke converging to the same text, offline edits merged on reconnect.
Design an LLM Inference Service
Serve 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 a Distributed Cache
A 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 Crawler
Fetch billions of pages a month politely, discover new links, skip duplicates, and keep the copy fresh without getting the crawler banned.
Design a Rate Limiter
A distributed service that decides, in under a millisecond, whether a request from a client is allowed under its quota.
Design Dropbox
Keep 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 Ticketmaster
Sell 60 000 seats in ten minutes to a million people at once: hold a seat while someone pays, never sell it twice, and keep the site up when demand is a hundred times supply.
Design Search Autocomplete
Suggest 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 Aggregator
Count 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 Scheduler
Run 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 Exchange
Match 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 a Project-to-Contractor Matching System
Match posted projects with qualified, available contractors across a 500 k-person talent pool, and learn from who actually gets hired.
Study guides
All 23 guides The theory behind the questions
One page per building block, written to be said out loud in an interview. Free to read.
01How to run a system design interview02Back-of-envelope estimation03Scalability fundamentals04Load balancing05Caching06CDNs and edge computing07Object storage and large files08Choosing a database09Data modelling for reads10Sharding and partitioning11Geospatial indexing and proximity search12Replication and consistency13Search, indexing and autocomplete14Consensus, leases and coordination15Message queues and event streams16Batch and stream processing17API design18Authentication, authorisation and data protection19Real-time systems: WebSockets, SSE and push20Rate limiting and resilience21Distributed transactions and idempotency22Unique ids, ordering and time23Observability, operations and rollouts
Pricing
Start free. Go Pro when it helps.
The URL Shortener walkthrough is free with no account, and signing in opens every walkthrough. Every plan adds the AI mock interviews and the AI Mentor, including what is added after you buy.
Monthly
$49 / month
billed monthly, cancel any time
For one focused interview loop: every mock and the AI Mentor while it lasts.
Best value
Annual
Save 75 %$12 / month
$147 billed once a year
A year for the price of three months. For prep that runs over months.
Lifetime
$441 once
one payment, about 3 years of annual
Every mock and the AI Mentor, and every question added later, for good.
Payments by Stripe. Cancel subscriptions any time from the billing portal; 7-day refund on a first purchase, under the refund policy.
What is a walkthrough?
A recorded system design interview between an interviewer and a strong candidate, played one turn at a time. You press Next, the candidate answers, and the whiteboard fills in as they talk: requirements, numbers, the API, then a design that gets fixed one bottleneck at a time, then the deep dives. At the key decisions it pauses and asks what you would say before showing what the candidate said.
What is the AI Mentor?
A chat beside the interview, never part of it. Ask it anything about what you have seen so far (why this database, what a term means, whether your own idea would work) and it answers with the interview and the whiteboard in front of it. Each turn also comes with suggested questions whose answers are written in advance; those are free for everyone. Live questions are part of Pro.
What is the mock interview?
You are the candidate. An AI interviewer runs the round the way a real one does: it states the problem, asks one thing at a time, probes what you said and what you drew on the whiteboard, and ends with a debrief (a score out of 100, what was strong, what was missing, and a note on each stage with a link to the same stage of the walkthrough).
Timed or with the AI Mentor?
Your choice at the start. Timed is interview conditions: a 45-minute clock the interviewer owns, and the AI Mentor is off. With the AI Mentor there is no clock and the AI Mentor sits beside you; it gives hints rather than answers unless you ask it outright, and the debrief notes that you had the AI Mentor.
What is free?
The Design a URL Shortener walkthrough, with no account, including the pauses and the suggested questions. Sign in with Google and every other walkthrough is free too. On a mock interview, anyone can hear the interviewer open the round. Which companies ask each question is free too. Pro adds live questions to the AI Mentor and answering the interviewer.
How is this different from reading a system design book or blog?
Reading gives you the final answer. A walkthrough shows you how a strong candidate gets there out loud (the order, the pacing, the numbers, and the design being fixed one problem at a time) and lets you stop and ask why. The mock then makes you produce it yourself, under pressure, with someone following up on the weak part.
What does Pro include?
Live questions to the AI Mentor, including comparing your answers with the candidate’s; AI mock interviews, timed or with the AI Mentor; and the company-style mock interviews. All three plans include the same thing; they differ only in how you pay.
Does the subscription renew automatically?
Monthly and annual plans renew until you cancel. Cancel from the billing portal at any time and you keep access until the end of the period you paid for. Lifetime is a single payment with no renewal.
Can I upgrade from monthly to annual or lifetime?
Yes. Open the billing portal from the pricing page and switch plans; Stripe prorates the unused part of the current period.
Is there a refund policy?
Yes, on a first purchase. Email us within 7 days of it, having started fewer than 3 mock interviews, and it is refunded in full. Renewals are not refundable, so cancel before the renewal date if you are done. The Terms have the details.
Do I need an account?
Not for the free walkthrough, and not to hear the AI interviewer open a round. Signing in with Google opens every walkthrough, and is needed to buy Pro.
Where do the "asked at" company logos come from?
Public interview guides and candidate reports. They indicate where a question has been reported, not a verified dataset, and a company name never implies endorsement.
What level is this for?
Senior and staff loops at large tech companies, where the interview is 45 to 60 minutes, expects numbers, and spends most of its time on deep dives. Mid-level candidates use it too; the walkthroughs explain every term as they go, and the AI Mentor will explain anything they do not.
Walk through one tonight. Sit one tomorrow.
One walkthrough is free with no account, and signing in opens all of them. Pause at any turn and pick it up later.