Fresh system design problems every day to keep you sharp. Then timed duels against other engineers to prove it.
Coming soonAndroidWhy we built it
Each year, less of an engineer's job is typing code. Agents now write a growing share of it, and they write it quickly.
What they are still poor at is deciding. Ask one how to build a system and it hands back several designs that all sound reasonable. Which one fits your load, your data and the ways it will fail is still your call, and you can only make it if you know the strategies and what each one costs.
So system design is becoming more of the job, not less. The trouble is that studying it is usually a slog: long books, long videos, and nothing to tell you whether any of it stuck.
We built System Design Duel to make that study a game. A few minutes of practice every day, real systems to draw, and another engineer to beat. It is a way to stay sharp on how systems that scale are really built.
The app
The day's practice, your streak and what is due for review.
Your rank, your duels under way, and a way into the next one.
Your track, design practice, every area and the catalogue.
Level, activity, mastery by area, your account and settings.
Learn
A practice set is five new questions on one topic, with up to two due for review. Each question is explained once you answer, option by option, with a link to the source it was checked against.
The plan carries on from where you stopped. Finish a topic and the next one on your track is tomorrow's practice.
A track is an order through the topics. Switch whenever you like.
Then there are platforms: the same ideas, as one provider's services.
Say where you are in your career, and the plan begins there.
Not everything worth knowing fits four options.
Every topic opens with three short cards that teach the idea before you are asked about it.
A question you get wrong returns the next day, then at widening gaps of 3, 7, 16 and 35 days, until it has stuck.
Ten questions mixed from across an area, no more than two from any topic, to check the whole of it holds together.
The app rates your skill in every topic from your first-try answers, and asks at the tier you are nearest: harder as you improve.
What it covers
From how a request crosses a network to how a payment system survives a retry. Every question is reviewed against its source.
100+ questions
Networking, APIs, estimation, consistency.
80+ questions
Indexes, replication, sharding, transactions.
110+ questions
Caches, queues, load balancers, CDNs.
140+ questions
Redis, Kafka, PostgreSQL and friends.
70+ questions
Contention, real-time, scaling reads and writes.
80+ questions
Failure modes, SLOs, deployments, postmortems.
150+ questions
URL shorteners, chat, feeds, ride sharing.
80+ questions
Geo search, time series, vectors, CDC.
Platforms
A platform takes what the areas teach and shows where it lives in a real cloud: which service is the queue, which is the cache, and what each one will and will not do for you. AWS is the first.
180+ questions
S3, SQS, Lambda, Aurora and the services around them.
Case studies
30+ case studies, each a short run of linked questions that walks through one real system decision by decision.
They are drawn from write-ups published by teams at:
Design boards
Whole systems to build on a whiteboard from parts and lines: clients, services, caches, queues, databases. Each board says what the system must do, and checks your drawing against it.
Stuck on a requirement? Ask for a hint. There are three for each, and each says a little more than the last. Give up and the board shows designs that work, usually more than one, because there is rarely a single right answer.
Once the drawing works, the board asks about the choices a drawing cannot show, such as how many replicas a part needs.
01
Most of the system is drawn. Add the pieces that are missing.
02
A design that looks finished and is not. Find what is wrong with it and put it right.
03
A client and a blank board, as in an interview.
The boards
Duels
A duel is one engineer against another over the same questions. Every round is fought over one component of a system. Score more in the round and it is yours. The first to three components wins.
How a duel works
Nobody chooses the area, and you both get the same questions. The wheel covers every general area. From round 2 it can land on a scenario: one system, settled one decision at a time.
Every question gives you 10 to 60 seconds to read, by its length, and that costs nothing. Then a right answer is worth 60, plus up to 40 more the sooner it comes. Out of time counts as wrong.
| 30s | True or false, single choice, number |
|---|---|
| 60s | Select all, ordering, matching, diagram |
No need to be online together. You each play a round when you have a moment, and the app tells you when it is your turn. Take longer than 48 hours over a turn and you forfeit.
The ladder
Your first five duels are placement matches. After the last one you are shown your rank. From then on a win moves you up and a loss moves you down, and beating a stronger player moves you further.
Five ranks, three divisions each. Division 3 is the highest.
Every placed player, in order of rating: the top of the ladder, and the players just above and below you.
Cross into a new division and the result screen stops to show you the badge you have just earned.
Progress
The point is to come back tomorrow. The app keeps count so you do not have to.
Free and Pro
Your daily practice, your streak and every explanation stay free. Nothing in Pro changes who wins a duel.
| Feature | Free | Pro |
|---|---|---|
| Daily practice | One set a day, on the Core curriculum | Unlimited, on any topic, any time |
| Explanations and sources in practice | ● | ● |
| Streak, XP and levels | ● | ● |
| Tracks | Core curriculum | Every track |
| Platforms | - | AWS |
| Primers | For the day's topic | Every primer |
| Case studies | One | All of them |
| Design boards | One, the link shortener | All of them |
| Review | Inside the daily practice | Review sessions whenever you like |
| Area checkpoints | - | ● |
| Practice on your weakest topics | - | ● |
| Duels, rank and the leaderboard | ● | ● |
| Duels under way at once | Up to 3 | No limit |
| After a duel answer | Right or wrong, and the correct answer | Every option's rationale, the explanation and the source |
| Rating history | - | ● |
| Streak freeze | - | One a week |
Pro is a subscription through the App Store. The app shows the plans and their prices before you buy.
Questions
Yes. One practice set a day with every explanation, your streak, duels, your rank and the leaderboard are free and stay free.
Depth, choice and volume: unlimited practice on any topic, every track, case study and design board, review sessions, the full explanation after each duel answer, your rating history and a streak freeze. It never changes who wins a duel.
Anyone who designs software or wants to. You say where you are when you start, from student to staff engineer, and the plan begins there.
No. There is an Interview prep track, and it suits that well. The app is built for staying sharp in the job, which lasts longer than any interview.
A player near your rank, or a friend by invite link. If nobody is free within a minute, a computer takes the match.
No. Duels are played in turns, and each turn has 48 hours. The app tells you when it is yours.
Not for daily practice or design boards. Duels and the leaderboard ask you to sign in with Apple, which also backs up your progress.
Each one is written from a published source, such as official documentation or an engineering team's own write-up, and links to it.
Report it from the question. A person reviews every report, and a question reported by enough players is pulled from play until it has been checked.
Yes, as a platform of its own, with each topic tied to the general idea it is an instance of. Platform questions stay out of duels, which are played on the general areas.
Yes, from the You tab. Everything we hold goes with it.
Something we did not answer? Ask us.
Coming soonAndroid