A free course of 11 worlds, 66 core lessons, 10 interview bosses and 11 side quests, built like a game. Reading never needs an account. This page explains every screen, what each number means and where to find things. Here is the whole idea in thirty seconds:
Everything you save while reading: highlights, notes and bookmarks, in one searchable place.
The stage bar in every lesson, and the Progress tab for the whole course.
Practice that feels like a game: a daily challenge, weekly leagues, quick games and your review queue.
The course hub shows the whole campaign as a skill tree. Every row is a world; every square is something you can open right now.
A world is one week of the course: 11 of them, from Week 00 Foundations to Week 10. A typical world has a Preview and 5 day lessons, then a boss (a timed mock interview, the diamond) and, for Week 00, Week 01, Week 02, a trial: the week's multiple-choice questions split into parts (the T boxes). Week 00 is different: six Foundations parts instead of days.
A lesson is split into stages (Part I, Part II… or Phase 1, 2… in a boss). Stages are how progress is saved: you resume by stage, not by scroll position.
Click a world to see its lessons with real reading times, the boss, the trial parts and the world reward: clear every lesson, the boss and every trial part of a world for +300 XP and that world's badge. On a phone, tap a world to open the same panel as a sheet.
Side quests (11 of them) are real systems designed end to end: UPI, WhatsApp, Netflix, Uber and more. They are bonus problems, open any time, worth +120 XP each. The dashed card at the bottom of the hub, The Plan, is the 10-week roadmap.
Go world by world: Preview → Day 1 … Day 5 → each lesson's trial part right after you finish it → the boss to close the week. New to system design? Start with Week 00 Foundations. Already comfortable? Jump to any world, nothing is locked; side quests make good weekend practice once a world or two is done.
where to find itthe course tab (top of every Learn page; bottom dock on phones). The Continue card on the hub always points at your next stage.
Everything around the text is there to tell you where you are and to get out of the way while you read.
Header and course tabs. On every Learn page the site header turns into the course header: course · arena · notebook · progress, a search box (⌘K), your level chip and the light/dark switch. On phones the same tabs live in the dock at the bottom, with a way back to the rest of the site.
Stage rail. One card per stage with its minutes. ✓ means read, the orange outline is where you are, dashed is ahead. Optional stages (exercises, appendix, further reading, summary) sit in a separate box: they never block completion. Save checkpoint bookmarks your place (more in progress rules).
Left and right columns. The course path on the left lists this week's lessons, the boss and the trial with ticks; “On this page” on the right is the lesson's contents. On phones, tap the stage strip for the same contents as a sheet.
The chrome hides while you read. Scroll down and the header slides away; a thin stage bar stays pinned with the stage, minutes left and Save checkpoint. Scroll up a little and everything comes back.
Reading tools (bottom right on desktop, the bottom toolbar on phones): Listen reads the lesson aloud with your browser's voice; Focus hides everything but the article and goes full screen where the browser allows it (Esc to leave); Aa cycles the text size 17 → 19 → 21 px and remembers it. Phones also get Contents and a shortcut to the Notebook.
Quick checks are short multiple-choice blocks inside lessons. Pick an answer to see whether you were right and why. A right answer on the first try is worth +5 XP; get every quick check in a stage right first time for +10 more; your first fully aced block earns the “First blood” quest (+25). Retry is always allowed but only the first try pays.
Diagrams are drawn in the site's colours and follow the light/dark theme; open any of them full screen. Code blocks have Copy and collapse long listings; interview questions have a reveal card where you can mark “knew it” or “review again”.
Practice this cards appear at the end of lessons that have a matching game, for example the Latency Ladder after Networking. They open an Arena game or a Playground board.
where to find itany lesson from the course hub. Each lesson ends with the Finish card, what's up next (across weeks) and the trial part that tests it.
Progress is earned by actually reading, not by scrolling to the bottom. Here are the rules in plain words.
Save checkpoint remembers the stage you are on so the hub's Continue card and “Resume at …” take you back there. If you have already spent enough time in that stage, it also clears it (it trusts you on the “seen” part). Otherwise it tells you how many more minutes that stage needs. The reader also moves your resume point on its own as you read.
| Guest (no account) | Signed in | |
|---|---|---|
| Lesson & stage progress, resume point | This browser only | Your account, on every device. Anything you read as a guest here is merged in when you first sign in |
| XP, quests, badges | This browser only | Your account (merged the same way) |
| Review cards, trial attempts | This browser only | Your account, with this browser as a backup if saving fails |
| Highlights, notes, bookmarks | Need an account | Your account |
| Weekly goal | This browser only | Your account |
| Leaderboard rank | Local points only (not ranked) | Ranked every week |
where to find itthe stage rail and Finish card in every lesson; the Continue card on the hub; the progress tab for everything at once.
Select any sentence in a lesson (long-press on a phone) and a toolbar appears. Everything you save lands in the Notebook, grouped by lesson.
Highlight it in one of four colours, each with a meaning: Key idea (orange), Definition, Numbers (grey) and Interview line (teal). Note attaches your own words to the passage. Bookmark saves the exact passage with an optional title. These three need a free account, so they follow you everywhere; as a guest the toolbar asks you to sign in.
Ask AI opens Claude or ChatGPT in a new tab with the passage and its context (lesson, stage, the paragraphs around it) already written as a question; Copy prompt gives you the same text for any other assistant. Copy link makes a link that opens the page scrolled to that exact line (on phones, Link also offers your phone's Share sheet). The # next to any heading copies a link to that section.
Keyboard: select text (or Shift + arrows), then H highlight, N note, B bookmark, A Ask AI, L copy link. Shift+F10 opens the toolbar.
The Notebook has three tabs: Highlights, Notes and Bookmarks. Search across all of them, filter by week or colour, edit or delete (with undo). Open at … takes you to the stage you saved it from. + Review card turns a highlight into a flashcard with its key phrase blanked out, which joins your daily standup. Export Markdown downloads everything.
Highlight: a sentence worth remembering, in colour, in the text and the Notebook. Bookmark: a place you want to come back to, with a title and note, in the Notebook (the Bookmark button in a lesson's header saves the whole lesson). Save checkpoint: your reading position for progress, not a Notebook item.
where to find itthe notebook tab; the pencil button in the phone reading toolbar; the Notebook card on the progress page.
A few minutes a day on cards made from your own mistakes. Cards you know come back less and less often; cards you don't come back tomorrow.
Where cards come from: every question you get wrong (or skip) in a trial, right answers you flagged as a guess, and highlights you turned into cards in the Notebook. You never have to make cards by hand.
Boxes (spaced repetition). Every card sits in one of five boxes. A new card starts in Box 1. When it comes up you try to recall the answer, tap Show answer, and grade yourself:
| Grade | What happens | Comes back |
|---|---|---|
| Again | Back to Box 1 | tomorrow |
| Hard | Stays in its box | after that box's interval |
| Good | Up one box | after the next box's interval |
| Easy | Up two boxes | after that box's interval |
Intervals: Box 1 every day · Box 2 every 3 days · Box 3 every 7 days · Box 4 every 16 days · Box 5 about monthly (35 days). A card is due when its date is today or earlier, so the number on the Arena tab and the hub's standup line is simply how many cards are waiting today. Each graded card is worth +5 XP, and a review session (or a finished stage) keeps your day on the streak.
where to find itthe daily standup: from the Arena tab (the orange number is cards due), the Daily standup line on the hub's Continue card, or the progress page. Keys: Space shows the answer, 1–4 grade.
Two ways a world tests you. Trials check what you remember; the boss checks whether you can design under interview pressure.
Trials are a week's multiple-choice questions, split into parts of about 15–30. Each part tests one lesson and opens when you finish that lesson (a “Locked” part still has Start anyway). Questions come one per screen, untimed: choose, then Lock in to see the answer and explanation. Your picks autosave, so a closed tab comes back to the same question.
Pass mark: 80%, rounded up (12 of 15, 16 of 20, 24 of 30). The first pass of each part is worth +100 XP. Fail and the part rests for 10 minutes, then comes back with a new option order; retakes only raise your best score. Everything you missed and every flagged answer goes to your review queue. Practice all puts every question of the week on one untimed page with no XP; its answers are kept apart from your attempts.
Each world ends with a boss: a full system design interview (a URL shortener at scale, a flash sale, an event-driven order system…). Fight it, or just read the model answer like any lesson.
Interview mode: each phase has the capstone's own time budget (the clock runs over budget, it never cuts you off). You write your answer in a scratchpad first; the model answer stays hidden until you reveal it. At the end you grade yourself against the capstone's checklist: 70% defeats the boss for +200 XP, once. Read mode is untimed with the answer open and gives no boss XP. Reading the capstone to the end still completes it as a lesson (+60 XP) and marks the boss node done on the tree.
where to find itthe T boxes and the diamond on each world row of the skill tree; the trial card at the end of each lesson; the “Fight the boss” card at the top of each capstone.
The fun part of the course, built from the same questions and lessons. Two minutes a day keeps a streak alive.
5 questions, 20 seconds each, the same five for everyone that day, drawn from every quick check and trial in the course. It rolls over at 00:00 IST. Run out of time and it counts as a miss.
Your game score is 100 per right answer plus 5 for every second left. Only the first run of the day counts: replays are practice. The first run pays +5 XP per right answer and +10 for 5/5, plus quests (first daily +30, a perfect 5/5 +50, 3-day streak +40, 7-day streak +100). Share your result as a little grid of squares.
Signed-in learners are ranked every week by leaderboard points. Everyone starts in the L1 Intern league. When the week ends (Monday 00:00 IST), the top 20% of each league's active players (at least one, and only with 60+ points) move up a league; a week with zero points drops you one league. Reaching the top 10 of the whole board in a week earns the Top 10 quest (+100 XP).
| Action | Points | Rule |
|---|---|---|
| Lesson completed | +50 | once per lesson |
| Stage cleared | +5 | once per stage, up to 12 a lesson |
| Quick check, first try right | +3 | once per question, up to 10 a lesson; answer re-checked on the server |
| Daily challenge | +8 each | per correct answer (re-checked), +10 for 5/5, up to +10 for speed; first run of the day only |
| Trial part passed | +40 | once per week and part |
| Review session | +10 | once a day |
| Active day | +5 | a day with course activity on your streak |
| Daily cap | 400 | most points one day can add, whatever you do |
One daily challenge is worth at most 60 points (5 × 8 + 10 + 10). Points are re-checked on the server against the course, duplicates are dropped, and guests see an estimate of “local points” instead of a rank.
Arena games are 60–90 second rounds: Latency Ladder, Pick the Store, Fix the Outage (+40 XP each on the first clear). The Playground boards (Cache or Miss, Hash Ring, Trip the Breaker, Back of the Envelope, Race Condition, Incident Commander) are longer simulations (+60 XP each). Every card lists the lessons that teach it, and best scores stay in your browser.
where to find itthe arena tab; the leaderboard (“full board”); the daily challenge.
XP is your running total. Every amount below comes straight from the code; each one-time reward is paid once, however often you repeat the thing.
| Action | XP | How often |
|---|---|---|
| Read a stage | +10 | once per stage |
| Finish a lesson | +60 | once per lesson |
| Quick check right on the first try | +5 | once per question |
| Every quick check in a stage right first try | +10 | once per stage |
| Pass a trial part | +100 | first pass of each part |
| Defeat a boss (interview mode, 70%+) | +200 | once per boss |
| Clear a world (lessons + boss + trial parts) | +300 | once per world, with its badge |
| Finish a side quest | +120 | once (60 for the lesson + 60 bonus) |
| Grade a review card | +5 | every card, every day |
| Daily challenge | +5 / right | first run of the day, +10 for 5/5 |
| Arena game / Playground board | +40 / +60 | first clear of each |
| Case file: solve its activity | +25 | once per case |
| Case file: right call on its debrief question | +50 | once per case that has one |
| Level | From | Unlocks |
|---|---|---|
| L1 · Intern | 0 XP | Everything is open. Nothing is gated. |
| L2 · Junior | 100 XP | The terminal learns more commands. |
| L3 · SDE | 300 XP | Blueprint skin and hidden post-mortem notes. |
| L4 · Senior | 600 XP | Director's cut notes on every case file. |
| L5 · Staff | 1,000 XP | Your handle on the Hall of Fame. |
| L6 · Principal | 1,500 XP | You beat the portfolio. |
Quests are one-time goals with an XP reward: the site's main quests (open the terminal, survive Tatkal Rush, read a case file to the end…) live in the quest log; course quests (First blood, daily streaks, Top 10, each game) are awarded as you play. Badges mark bigger milestones: one per cleared world plus site badges like First blood, Daily streak 7, Top 10, Arena regular and Case cracker. Hover a badge to see how to earn it.
where to find itclick the level chip (e.g. “L4 · Senior”) in the header for the quest log and badges; the progress page lists every badge and an XP log of recent rewards.
The Progress tab answers “where am I, how am I doing, what next”. Guests see this browser's data; signed in, your account merged with it.
Overview: level and XP with what the next level unlocks, course % (lessons, bosses and trial parts done out of all of them), your current streak (days with a cleared stage, a lesson, a trial part or a review), reading time, lessons done and a bar per week. On the right: resume where you left off, the next trial part to take and today's standup.
Course map: every lesson, boss, trial part and side quest of every week as a grid. Black is completed, an orange corner is in progress, orange trial cells are passed (with your best %), outlined ones were tried but below 80%. Click any cell to open it.
Your rhythm: a 12-week heatmap (darker = more done that day; tap a day for details), minutes per week against your goal, current and longest streak, best day and time, and the XP log.
Weekly goal: pick minutes or lessons per week and, if you like, a finish date. The page projects when you'll finish at your recent pace and how much a week you need to make your date. What stuck: best trial score per part, review cards by box, Notebook counts and every badge.
Share my progress: turn on a public page for friends and choose what it shows. Off until you turn it on; all of it is in the next section.
Your data. Download my data saves one JSON file with everything your account holds (signed in) plus this browser's copy. Signed in, an Import card offers to move progress you made as a guest in this browser into your account. Reset clears exactly what you choose, in three steps: what to reset, which data, confirm.
| Scope | What it covers | Data you can clear |
|---|---|---|
| One lesson | a single lesson, boss or trial | Lesson progress, Quiz answers & trials, Notes & highlights, Bookmarks |
| One week | every lesson, boss and trial of a week | Lesson progress, Quiz answers & trials, Notes & highlights, Bookmarks |
| Whole course | all of System Design Mastery | Lesson progress, Quiz answers & trials, Notes & highlights, Bookmarks, Streaks & activity |
| Everything | the course plus XP, quests and streaks | Lesson progress, Quiz answers & trials, Notes & highlights, Bookmarks, Streaks & activity, XP & quests |
where to find itthe progress tab. Sections: overview · map · activity · goals · mastery · share · data (the sticky row under the header jumps between them).
Your progress is private. Turn on a public page and you get one link, showing only the parts you pick, to send on WhatsApp, LinkedIn or X.
Turn it on in the share section of the Progress page (it needs an account). The Public page switch is off until you flip it or press Publish my page. While it's off nobody can see your progress, and switching it off again makes your link show a 404 straight away.
Handle. Your page lives at himanshukukreja.in/u/<handle>. A handle is 3–24 lowercase letters, numbers or single hyphens. One is suggested from your name, and whatever you type is checked as you go: taken, reserved and offensive handles are refused. Changing it later breaks links you already sent.
Name on the page: your full name, first name + initial or anonymous (just the @handle); new pages start with first name + initial. Your profile photo appears only if you tick it, your account has one and you aren't anonymous. Then pick what visitors see (all six start on). The preview on the right follows every change, and Save changes updates a live page at once.
| Switch | What it shows |
|---|---|
| Level & course progress | level, XP, course %, lessons, world map |
| Skill tree | every world, boss and trial node, read-only |
| Streak | current and best streak |
| Arena scores | league, rank, weekly points, best daily, daily streak |
| Badges | badges you've earned |
| Milestones | worlds cleared, bosses beaten, recent wins |
Your email, account id, notes, highlights, bookmarks and exact study times are never shown, whatever you switch on. Public pages are kept out of search engines, and a handle that doesn't exist looks exactly like one that's switched off (both are a 404), so nobody can check who has a page.
Every Share button opens the same sheet: your link with Copy link, WhatsApp, LinkedIn and Post (X) with a message already written, and copy message + link. On a phone, Share… opens your phone's own share menu.
Page still off? The sheet offers Turn on & get my link (it publishes with your saved choices, or the defaults) or Choose what's visible, which brings you to the settings. As a guest it asks you to sign in to get your share link, and the progress in this browser comes with you; you can also share the course itself instead.
| Where | Button | The link opens |
|---|---|---|
| Progress page, under your level | share my progress | your page |
| Progress page, share section | Share my page | your page |
| Course hub, next to Skill tree | Share | your page |
| Daily challenge result | Challenge a friend | today's daily challenge (works without a page or an account) |
| World cleared, level up | Share it | that world's or level's achievement page |
| Boss debrief, after a win | Share the win | that boss's achievement page |
| End of a lesson, once complete | Share | your page |
| Progress page, badges | share | your page |
| Leaderboard, your own row | share icon | your league rank's achievement page |
What a friend sees when they open your link, no account needed: your name or @handle, your level on the L1 Intern → L6 Principal ladder with XP, course %, streak, lessons and worlds done, a world map, a read-only copy of your skill tree, your Arena scores, badges and recent milestones. Each part is there only if its switch is on.
The page ends with an invitation: Start the free course or Take today's daily challenge, plus “Can you beat …?” next to your scores. When you open your own page signed in, a “your page” bar adds Edit what's visible and Share; visitors never see it.
Achievement pages (/u/<handle>/a/…) show one win, big: a cleared world, a beaten boss, a level, your streak, your best daily, your league rank this week or a badge. They exist only for what your page shows; otherwise the link opens your page.
Link previews. Paste your link into WhatsApp, Slack, LinkedIn or X and it unfolds into a preview card of your page or the achievement.
When someone arrives through your link and creates an account within 30 days, the sign-up is counted for your link (the first link a browser arrived from is the one that counts). Only the site owner sees these numbers, and nothing about the visitor appears on your page.
where to find itthe share section of the progress page (signed in); the Share buttons above, at the moments worth sharing; your page itself at /u/<handle>.
Production incidents, rush hours and build logs told as stories with blueprints. They live outside the course, under Case files.
A case file is closed when you have read it: the same reading rules as lessons apply to its checkpoint sections, or use “Close the case” once most of the reading time is in. Reading your first case file to the end is a quest (+50 XP) and earns the On-call badge. A case can also end with a debrief question (the Tatkal case does): make the right call for +50 XP, one answer per case.
11 case files end with an interactive activity: a simulation or spot-the-bug drill in a few steps. Score 70% to solve it for +25 XP; rerun it as often as you like to raise your best. Solve all of them for the Case cracker quest (+150 XP) and badge.
where to find itCase files in the site header (outside the course). Your record (closed · solved) is at the top of the index.
No. Every lesson, quick check, trial, boss, game and the daily challenge work without one. An account adds a few things.
Sign in with Google or an email code from the arrow button in the header. The first account that signs in on a browser picks up that browser's guest progress; a second account on the same browser starts from its own.
What's stored: page views and reading time, learning events (lessons opened and completed, answers, games), and if you sign in your email, name and progress. No ads, nothing sold, and nothing about you is public unless you turn on a share page. The privacy page lists every tool and has a switch to turn analytics off in your browser (also “opt out” in the site footer).
where to find itsign in from the header; progress → your data to export or reset; privacy & analytics to opt out.