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Tasks from production.
Not from a textbook.

Implement a rate-limiter with Redis, fix N+1 in an ORM, rewrite a queue for exactly-once. Every task is distilled from a real case. Auto-check plus AI code review — in minutes, not a week of waiting.

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RateLimiter.php submission #14 · review

// Task 03 · Middle

Implement rate-limiter with Redis

100 requests per minute per key. Atomic INCR + EXPIRE. No race condition.

middle PHP Redis concurrency

1class RateLimiter {

2 public function tryHit($key) {

3 // INCR + EXPIRE — две операции

4 Redis::incr($key);

5 Redis::expire($key, 60);

6 }

AI Review 78 / 100 · 78 / 100 · needs fixes

Between INCR and EXPIRE the key can get stuck without a TTL. Wrap both operations in MULTI/EXEC or use a Lua script — that's atomic. Additionally: return bool instead of void so the caller knows the result.

80+

tasks from junior to senior

3 min

avg AI review time

technologies and stacks

×3

attempts without penalty

What's inside

Not "solved — move on"

Every task is a real-world formulation, auto-graded solution, and detailed AI breakdown.

Real cases

Race condition in a queue, deadlock in a transaction, cache invalidation — tasks you encounter at work and in interviews.

Auto-grading

The solution is run through tests automatically. You see what's green, what's red — and where to dig.

AI code review

AI reads your code, explains what's wrong and why. Not "test failed", but "you forgot atomicity — here's how to fix it".

3 difficulty levels

Junior, Middle, Senior. Take what you can handle, progress through tracks. See your level — not by feel, but by completed tasks.

Others' solutions

After submission, other solutions open up. See how a senior from Booking solved the same task — and where yours is shorter.

Progress through tracks

"PHP/Laravel", "Go production", "SQL deeper". Completed tasks accumulate in a track and pull up your level.

How it works

From opening a task to AI review — in one session

No bureaucracy or week-long feedback waiting. Solved — got a breakdown.

// 01

Choose a task

Filter by level, stack, and topic. Requirements, tests, expected result — all on one page.

~2 minutes

// 02

Write your solution

Right in the browser or locally. Submit code, tests run automatically.

~30 minutes

// 03

AI reviews the code

Not just "pass / fail". AI breaks down the architecture, spots race conditions, suggests patterns.

2–3 minutes

// 04

Fix and move forward

Up to 3 attempts without penalty. Done — open others' solutions and the next task in the track.

unlimited

Levels

Clear progression

Not "easy" and "hard" by feel. Each level is about specific skills and types of tasks.

Junior

Basics and CRUD

~30 tasks · 1–2 hours each

Request and form validation
Basic SQL queries
HTTP, REST, status codes
Simple migrations and models

Middle

Production and patterns

~35 tasks · 2–4 hours each

Cache, queues, idempotency
N+1, indexes, EXPLAIN
Background jobs and cron
Authorization, OAuth, JWT

Senior

Distributed systems

~15 tasks · 4–8 hours each

Exactly-once delivery, outbox
Sharding, replication, quorum
Observability and tracing
System Design for interviews

Topics and stack

Only what you encounter in production

No "algorithmic tasks for the sake of tasks". What's really asked in interviews and what breaks in the middle of the night on-call.

PHP / Laravel Go production SQL · PostgreSQL Redis · cache Queues · Kafka / RabbitMQ REST / gRPC API Docker · Kubernetes Observability · logs / traces Concurrency · race conditions Authorization · OAuth / JWT N+1 · indexes · EXPLAIN Idempotency / outbox Rate limiting Testing · mocks / integration CI/CD basics

Tasks from production. Not from a textbook.

Not "solved — move on"

From opening a task to AI review — in one session

Clear progression

Only what you encounter in production

Start with the first task

Tasks from production.
Not from a textbook.