Learn system design the way the interview tests it.

Concepts built from the ground up, the technologies real designs name, and worked designs that open with the hard part and end with the follow-up questions. Plus a coding track that draws every algorithm as it runs.

Start with Foundations Practise coding

Most of the atlas is free to read, with no account.

DatabaseCacheExternal systemAPI gatewayLoad balancerFocusClick a node for details · ⌘/Ctrl + scroll to zoom

The live architecture from Distributed Rate Limiter. Every design has one: click a box for its role and its trade-off.

Start with the ideas, in order.

Each module builds on the one before: latency and estimation, storage, consistency, caching, messaging, and the patterns every design reuses. The last two cover building with LLMs and the interview itself.

Read Foundations

Every design opens with the hard part.

Not boxes first. Each one states what the interviewer is really testing, then the requirements, the numbers that force the design, an architecture you can click through, and the follow-up questions interviewers ask.

Begin with Twitter / Instagram Feed

Twitter / Instagram FeedThe hard partThe celebrity problem. A design that only works for the median user fails — fan-out cost is bimodal and the code path has to split.Uber / Delivery TrackingThe hard partTwo problems glued together: 1.25M location writes per second that destroy any disk-backed index, and a matching step where two riders must never get the same driver.Payment SystemThe hard partThe one design where consistency beats availability, and where at-least-once plus idempotent stops being a slogan and becomes the mechanism preventing double charges.

The systems real designs name.

One page each: what it is, what it's good at, how it fails, and the moment it becomes the right answer in a design.

Open PostgreSQL

Algorithms you can watch run.

The coding track opens every data structure with a step-through: the structure on one side, the code highlighting line by line on the other. Press play below.

Go to the coding track

lo
hi
answer
dimmed = eliminated

pythonruns with the steps

1def two_sum_sorted(nums, target):
2    lo, hi = 0, len(nums) - 1
3    while lo < hi:
4        total = nums[lo] + nums[hi]
5        if total == target:
6            return [lo, hi]
7        if total > target:
8            hi -= 1
9        else:
10            lo += 1
11    return []

01 / 9Sorted inputpairs 28

Eight sorted values, and a target of 19. Brute force would test all 28 pairs. The order in this array is what makes a cheaper answer possible — unsorted, none of what follows holds.

Then build one real system end to end.

AI Engineering follows a single production application from the first commit, explaining each concept with the project's real code.

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AI Engineering

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