// Build
app.get("/api/users", (req, res) => {
res.json({ success: true });
});
Write API
Pass Tests
Handle Edge Cases
Deploy
All tests passed
12/12 test cases
Verified response
Edge cases covered
GET/api/tasks
POST/api/tasks
PUT/api/tasks/:id
DELETE/api/tasks/:id
Active DevelopmentFeedback
Practice Real Backends.
Defend Edge Cases.
Master Production Systems.
Hands-on backend challenges, real-world scenarios, automated verification, and a path to production-ready skills.
Scroll to explore
Experience the Platform
Scroll to explore
The Challenge Catalog
Browse our massive curriculum of real-world backend engineering challenges and architectural scenarios.
Browser-Based IDE
Write, debug, and execute your code in a professional workspace without ever leaving the browser.
Production Edge Cases
Our distributed runner hits your API with high concurrency to catch race conditions and strict RFC compliance issues.
Track Your Mastery
Review your latency, memory usage, and build a verified portfolio of production-ready backend code.
SUPPORTED LANGUAGES & RUNTIMES
Write in whatever you build with.
Submit and test real API endpoints in Python, Go, TypeScript, Rust, Node.js, Java, C#, C++, Ruby, PHP, and more.
Python
FastAPI / Django
Go
Gin / Fiber / Standard
TypeScript
Node / Bun / Deno
Rust
Actix / Axum / Tokio
Node.js
Express / Fastify / NestJS
Java
Spring Boot / Quarkus
Python
FastAPI / Django
Go
Gin / Fiber / Standard
TypeScript
Node / Bun / Deno
Rust
Actix / Axum / Tokio
Node.js
Express / Fastify / NestJS
Java
Spring Boot / Quarkus
Python
FastAPI / Django
Go
Gin / Fiber / Standard
TypeScript
Node / Bun / Deno
Rust
Actix / Axum / Tokio
Node.js
Express / Fastify / NestJS
Java
Spring Boot / Quarkus
C# / .NET
ASP.NET Core / WebAPI
C++
Crow / Boost.Beast
Ruby
Rails / Sinatra / Hanami
PHP
Laravel / Symfony / Swoole
Kotlin
Ktor / Spring Boot
Elixir
Phoenix / Plug / OTP
C# / .NET
ASP.NET Core / WebAPI
C++
Crow / Boost.Beast
Ruby
Rails / Sinatra / Hanami
PHP
Laravel / Symfony / Swoole
Kotlin
Ktor / Spring Boot
Elixir
Phoenix / Plug / OTP
C# / .NET
ASP.NET Core / WebAPI
C++
Crow / Boost.Beast
Ruby
Rails / Sinatra / Hanami
PHP
Laravel / Symfony / Swoole
Kotlin
Ktor / Spring Boot
Elixir
Phoenix / Plug / OTP
0+
Automated Tests Run
0%
Spec Compliance
0+
Backend Scenarios
<0ms
Test Feedback
Competitive programming tests whether you memorized abstract recursion. APIRun tests whether your service survives 50 concurrent transactions, distributed locks, and strict RFC standards.
← Swipe table to compare →
| Engineering Vector | LeetCode & Competitive DSA | APIRun Real Backends |
|---|---|---|
| Core Challenge Type | ✕Inverting binary trees, synthetic array tricks, and abstract recursion problems. | ✓Building real API endpoints, input validation, database updates, and authentication. |
| Edge Case Verification | ✕Array boundary index checks and artificial CPU time limits. | ✓High-concurrency traffic, race condition prevention, and duplicate request handling. |
| API Standards | ✕Single text print output (e.g. System.out.println("true")). | ✓Real HTTP status codes (200, 201, 400, 404, 409, 429) and structured JSON responses. |
| State & Databases | ✕None. Ephemeral memory resets on every run. | ✓Real Redis keys, distributed locks, database transactions, and automatic rollbacks. |
| Developer Workflow | ✕Locked inside a basic browser text box with hidden mock drivers. | ✓Code in the browser editor or test your actual local backend server using our CLI. |
| Real-World Relevance | ✕Rarely encountered in day-to-day backend development or production systems. | ✓Directly mirrors real production backend engineering and system design interviews. |
CURRICULUM & DOMAIN MASTERY
Every track simulates real production architecture patterns required at Staff & Senior levels.
TRACK 01High Concurrency
Concurrency & Distributed Locks
Defend against race conditions and double-spending bugs under heavy traffic using distributed locks and atomic transactions.
Race ConditionsDistributed LocksTransactionsDeadlock Prevention
TRACK 02Traffic Management
Traffic & Rate Limiting
Implement sliding-window counters and token bucket rate limiters to protect your APIs from traffic spikes and abuse.
Rate LimitingRedis Counters429 HandlingToken Bucket
TRACK 03Reliability
Payment Safety & Webhooks
Safely handle payment webhooks with cryptographic signature checks and duplicate request prevention.
Signature ChecksIdempotency KeysDuplicate PreventionWebhook Security
TRACK 04API Standards
API Design & Status Codes
Design clean REST endpoints with proper HTTP status codes, structured error handling, and request validation.
REST EndpointsInput ValidationStatus CodesError Handling
TRACK 05Async Processing
Background Queues & Workers
Build reliable background job workers with automatic retries, failed job queues, and task scheduling.
Task QueuesRetry LogicFailed Job QueuesTask Scheduling
TRACK 06Authentication
Auth, Tokens & Session Security
Implement secure token authentication, refresh token rotation, password hashing, and user role permissions.
JWT TokensToken RefreshSession SecurityRole-Based Access
CHALLENGE REPOSITORY
Popular API Challenges
HTTP FundamentalsBeginner
Ping & Health Check API
Build your first API endpoints: a ping responder and a service health status check.
#HTTP#REST
REST APIBeginner
Echo & Request Body Validation
Learn how to parse JSON request bodies and return 200 OK vs 400 Bad Request.
#HTTP#REST#Validation
HTTP FundamentalsBeginner
Greeting API with Query Parameters
Learn how to read URL query parameters and provide clean fallback defaults.
#HTTP#REST
Data StructuresBeginner
In-Memory Key-Value Store
Build a lightweight in-memory cache API with key-value set, get, and 404 handling.
#HTTP#REST#Cache
OPEN FEEDBACK/EARLY PREVIEW
Help us expand the challenge catalog
Encountered tricky race conditions, idempotency bugs, or token bucket edge cases in production? Share your ideas and help shape upcoming backend challenges.
+ New Challenge Specs+ Framework Requests+ Harness Edge Cases