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Train in unfamiliar codebases. Identify your blind spots. Expand your engineering range.

  • Real codebases
  • Constraint-driven challenges
  • Fast validation
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THE PROBLEM

System familiarity creates blind spots.

Most engineers grow inside the same codebase for years. You become comfortable within it and since you know where everything lives, it all becomes intuitive

Inside that environment, you feel capable.

But when you face an interview, join a new team or tackle a different stack that familiarity doesn’t help. You slow down because you haven’t trained for unfamiliar systems.

Knowing Isn't the Same as Doing

KNOWING
  • ✓ Understand what an N+1 query is
  • ✓ Know that transactions prevent partial failure
  • ✓ Know caching improves performance
  • ✓ Understand race conditions conceptually
≠
DOING
  • ? Refactor a controller to eliminate N+1 without breaking pagination
  • ? Wrap a multi-step purchase flow in a safe transaction
  • ? Implement cache invalidation without serving stale data
  • ? Fix a race condition inside an existing service layer
Why This Matters

1. Interviews. 2. New teams. 3. New problems.

You slow down in unfamiliar systems because you didn't build them or haven't lived in them yet

How Widebase Works

01

Step into a real codebase

You receive a full project - controllers, models, services, migrations, tests. This isn't a blank canvas but a system with history. Choose the stack you want to test yourself in

project structure - Laravel / PHP 
app/
├── Http/Controllers/
│   ├── OrderController.php
│   ├── ProductController.php
│   └── Api/InventoryController.php
├── Models/
│   ├── Order.php
│   └── Product.php
├── Services/
│   └── PurchaseService.php
├── database/migrations/
└── tests/Feature/
    └── StockManagementTest.php
src/
├── routes/
│   ├── orders.js
│   ├── products.js
│   └── inventory.js
├── models/
│   ├── Order.js
│   └── Product.js
├── services/
│   └── purchaseService.js
├── middleware/
│   └── auth.js
└── __tests__/
    └── stockManagement.test.js
inventory/
├── views/
│   ├── orders.py
│   ├── products.py
│   └── purchase.py
├── models/
│   ├── order.py
│   └── product.py
├── serializers/
│   └── inventory.py
├── migrations/
└── tests/
    └── test_stock_management.py
src/main/java/com/app/
├── controller/
│   ├── OrderController.java
│   └── InventoryController.java
├── model/
│   ├── Order.java
│   └── Product.java
├── service/
│   └── PurchaseService.java
├── repository/
│   └── ProductRepository.java
└── src/test/java/
    └── StockManagementTest.java
internal/
├── handlers/
│   ├── orders.go
│   ├── products.go
│   └── inventory.go
├── models/
│   ├── order.go
│   └── product.go
├── services/
│   └── purchase.go
├── middleware/
│   └── auth.go
└── tests/
    └── stock_management_test.go
src/
├── app/
│   ├── api/orders/route.ts
│   ├── api/products/route.ts
│   └── api/inventory/route.ts
├── lib/
│   ├── models/order.ts
│   ├── models/product.ts
│   └── services/purchase.ts
├── prisma/
│   └── schema.prisma
└── __tests__/
    └── stockManagement.test.ts
02

Read the challenge

The purchase endpoint checks stock and decrements it in separate steps - no locking. Two users buy the last item simultaneously, and stock goes negative.

PROBLEM.md 
## Fix Stock Race Condition
Difficulty: Hard · 60-90 min
Skills:    DB Transactions, Pessimistic
           Locking, Concurrency

The purchase endpoint has a classic race
condition. Two concurrent buyers can both
pass the stock check and decrement -
leaving stock at -1.

Use transactions + locking to prevent
overselling. Don't break existing tests.
03

Submit and validate

Your solution runs against real tests. No subjective grading, just automated verification that your fix works.

test results 
> Validating solution...

✓ stock_never_goes_negative ........ PASS
✓ concurrent_purchases_handled ..... PASS
✓ sold_out_returns_409 ............. PASS
✓ successful_purchase_decrements ... PASS
✓ order_created_on_success ......... PASS

5 passed - Challenge complete.
A REAL CHALLENGE

Here's what working inside the platform looks like

Widebase already includes multiple challenges, with more continuously being added. Each one drops you into a full project with failing tests and a clear objective.

Hard

Stock Race Condition

Fix a concurrency bug where two buyers oversell inventory

Medium

Fix N+1 Query

Eliminate N+1 queries causing slow order listings

Medium

React Data Table with Export

Implement a dynamic table component that supports sorting, filtering, and CSV export for large datasets

Hard

Cache Invalidation

Add caching that never serves stale data

Hard

Idempotent Orders

Prevent duplicate orders on client retries

+ More challenges

Fix Stock Race Condition

Hard 60-90 min
Context

The inventory purchase endpoint has a critical bug: it checks and decrements stock without any database transaction or locking. This means:

  • Race condition - Two users buying the last item simultaneously can both succeed, driving stock negative
  • Partial failure - If the second item in a multi-item purchase fails the stock check, the first item's stock is already decremented and never rolled back
Your Task
  • Wrap the entire purchase in a database transaction
  • Use row-level locking when reading product stock to prevent concurrent reads
  • If any item fails the stock check, the entire transaction should roll back
  • Return a 409 response for insufficient stock
Modify
app/Http/Controllers/Api/InventoryController.php
Hints
  • Transactions auto-rollback on exceptions
  • Row-level locks prevent concurrent reads on the same row
  • Throwing a runtime exception inside the transaction triggers the rollback
DB Transactions Pessimistic Locking Concurrency 5 test assertions
What This Is Not

Not algorithm puzzles. Not abstract problems. Not memorizing patterns.

This is Hands-on practice with real code, real constraints, and problems or features you've only read about.

If you feel strong in your own codebase
but slower in unfamiliar ones,
Widebase was built for you.

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