Choose your track

Reqflow

Go from confused
to confident.

Interactive system design, FDE, and DSA: run the artifact, don't read the slide.

Choose your track →New here? 5-min tour

Free to start · No account needed for the first lesson

binary-search.pyLive

20

51

82

123

164

235

386

567

lohi

Search for 23 in sorted array

PythonJavaScript

1def binary_search(arr, target):

2 lo, hi = 0, len(arr) - 1

3 while lo <= hi:

4 mid = (lo + hi) // 2

5 if arr[mid] == target: return mid

6 elif arr[mid] < target: lo = mid+1

7 else: hi = mid - 1

step 1 / 5

Pick a track

Choose the role or skill you're preparing for.

Open something live

A running system, lesson, or algorithm, not a slide deck.

Step through it

Play, pause, and inspect every decision point.

Check yourself

Quizzes and spaced repetition show what stuck.

What are you preparing for?

Pick one track. Each is its own product, same idea: learn by running something real.

Not sure where to begin? Open URL Shortener (~12 min): the smallest live system.

System Design

Watch how Instagram, Uber, and other real systems handle a request, including when they fail.

FAANG & senior architecture interviews29 live systemsJump in anytime

Forward Deployed Engineer

Learn the customer-facing AI role: fast demos, messy environments, and shipping without losing the room.

Applied AI & FDE interviews11 lessons~122 min total

Data Structures & Algorithms

Step through sorting, searching, and trees with live visuals, synced code, and spaced-repetition quizzes.

Interview prep & fundamentals15 algorithmsQuiz + compare tools

29Live systems

11FDE lessons

15DSA algorithms

Tools you can open in one click

Every track has something live to try, systems, algorithms, lessons, and sandboxes.

> POST /feed/home

→ API gateway · 12ms

→ feed service · cache miss

✓ rendered in 180ms

Trace a live request

Follow traffic through Instagram, Uber, and more, including failure modes, not just happy paths.

> binary_search([2,5,8,12,23], 23)

step 4/7 · mid=12 · 12 < 23

→ discard left half

✓ found at index 4

Step through every line

Play, pause, or scrub. Comparisons, swaps, and pointers render as the algorithm runs.

Try binary search

> 48-hour customer demo

· scope the win

· deploy into their env

✓ keep the room

Lessons from the field

Customer demos, messy environments, and shipping under pressure, cited, practical, interview-ready.

[6,3,8,1,9,2,7,4]

Bubble sort 47 steps

Merge sort 19 steps

↓ 28 fewer steps

Two algorithms, same input

Race any two sorts side by side and see which burns fewer steps on your array.

O(n²) 40,000 ops

O(n log n) 1,529 ops

Feel O(n²) vs O(n log n)

Drag n and watch quadratic blow up while log-linear barely moves.

visited: [4, 2, 6, 1, 3]

BFS, DFS, BST, live

Animated SVG trees with queue/stack strips and path tracing as you step.

Also in DSA:Spaced-repetition quizTree visualizersCheat sheet

Ready when you are

Open a live lesson in under a minute

No account required to start. Pick a track and try the first artifact free.

Choose your track →5-min tour

Reviewer program

The byline on every system is a real engineer.

Every Reqflow explanation should be signed off by someone who has actually shipped that kind of system. Read a system you know, flag what's wrong, and your name goes on the page you reviewed.

Founding reviewers wanted. Be the first byline.