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Step 01

Pick a build

Browse open builds posted by real companies. Each one is a real engineering problem — not a DSA puzzle or a tutorial project.

Step 02

Push your solution

Work in your own IDE. When you’re done, submit your GitHub repo. That’s the entire submission process.

Step 03

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Your code is evaluated across 7 engineering dimensions. You get a detailed review with scores, feedback, and where you stand — regardless of your rank.

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Every submission is evaluated across 7 engineering dimensions. Not a pass/fail. A detailed architectural breakdown of where you stand.

Stealth Fintech Startup

Payment Reconciliation Service

74/100

Code Quality

Readability
good
Error Handling
good
Testing
fair

Design Quality

Architecture
good
Abstractions
fair
Data & API Design
excellent
Complexity Balance
good
Test SuiteTypeScriptLinting Setup

Feedback

Solid reconciliation service with clean domain separation and a well-thought-out gateway adapter pattern. Testing covers core flows but lacks coverage for partial failure and concurrent processing scenarios that would surface in production.

Strengths

  • Adapter pattern in src/types/GatewayAdapter.ts enables zero-downtime provider rotation across all payment gateways
  • Compound unique indexes in src/models/Transaction.ts prevent duplicate reconciliation entries across restarts
  • Typed error hierarchy in src/errors/ (ReconciliationError, GatewayTimeoutError) enables precise recovery per failure mode

Areas for Improvement

  • Retry logic duplicated across StripeAdapter.ts:61, PayPalAdapter.ts:44, SquareAdapter.ts:52 — extract to shared middleware
  • No integration tests for concurrent batch processing — tests/ covers individual adapter units only
  • Hardcoded timeouts in PayPalAdapter.ts:47 and StripeAdapter.ts:83 — externalize to environment config

Key Issues

major
Retry logic with inconsistent backoff duplicated across 3 gateway adapters

src/adapters/StripeAdapter.ts:61, PayPalAdapter.ts:44, SquareAdapter.ts:52

Extract into shared withRetry() middleware with configurable exponential backoff and jitter.

At production payment companies, retry middleware is shared infrastructure. Duplicated retry logic drifts silently when one handler is updated.

major
No test coverage for concurrent batch processing or partial gateway failure

tests/adapters/

Add integration tests simulating partial gateway failures mid-batch. Verify completed transactions survive.

Payment systems require exhaustive failure-mode testing. Partial batch failures are the production norm.

minor
BatchProcessor imports concrete adapter classes — no dependency injection

src/handlers/BatchProcessor.ts:12–14

Accept adapters via constructor injection for testability and zero-downtime provider rotation.

Dependency injection is how production payment systems add new gateways without modifying handler code.

Open builds

Real engineering problems from real companies. Pick one and start building.

Stealth Fintech Startup

Payment Reconciliation Service

Design a service that reconciles payment records across multiple gateway providers and flags discrepancies in real time.

Node.jsPostgreSQLEvent-Driven

Series A SaaS Company

Multi-Tenant Notification Engine

Build a notification system that supports email, SMS, and in-app channels with per-tenant rate limiting and delivery tracking.

TypeScriptRedisREST API

B2B Logistics Platform

Route Optimization API

Create an API that calculates optimal delivery routes given a set of waypoints, time windows, and vehicle constraints.

PythonFastAPIAlgorithms

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ReadabilityError HandlingTestingArchitectureAbstractionsData & APIDesignComplexityBalance

Based on 7 completed builds

Payment Reconciliation Service

Stealth Fintech Startup

74Good

Top 18%

Multi-Tenant Notification Engine

Series A SaaS Company

81Excellent

Top 9%

Route Optimization API

B2B Logistics Platform

58Fair

Top 32%

Real-Time Chat Backend

EdTech Scale-Up

79Good

Top 12%

Inventory Sync Pipeline

D2C E-Commerce Startup

72Good

Top 22%

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