# Modular Code Rules

CRITICAL: Never write giant monolithic files. Split code into modules, libraries, and microservice-ready components.

## Problem

Agents write enormous single files that are hard to review, test, debug, and maintain. No clear boundaries between features.

## Core Principles

1. **Maximum file size**: 100 lines per file (excluding tests and migrations)
2. **Maximum function/method size**: 30 lines
3. **Maximum class size**: 5 public methods
4. **One responsibility per file**: A file does ONE thing

## Module Structure (Mandatory)

Every feature must be organized as an independent module:

```
{feature}/
├── Controllers/     # HTTP request handling (thin)
├── Services/        # Business logic (fat)
├── Repositories/    # Data access (abstracted)
├── Models/          # Data definitions
├── Routes/           # Route definitions
├── Events/           # Events this module emits
├── Listeners/        # Events this module handles
├── Jobs/             # Async work this module performs
├── Requests/         # Input validation (not in controller)
├── Resources/        # Output transformation (not raw model)
├── Exceptions/       # Module-specific exceptions
├── Tests/            # Module-specific tests
└── ModuleServiceProvider.php  # Module registration
```

## Service Layer Rules

```php
// ❌ BAD: Business logic in controller
class ProductController
{
    public function store(Request $request)
    {
        $product = Product::create($request->all());
        Cache::forget('products');
        event(new ProductCreated($product));
        Mail::to($product->vendor)->send(new NewProduct($product));
        return response()->json($product);
    }
}

// ✅ GOOD: Business logic in service
class ProductController
{
    public function __construct(private ProductService $service) {}
    
    public function store(ProductStoreRequest $request): JsonResponse
    {
        $product = $this->service->create($request->validated());
        return response()->json(new ProductResource($product), 201);
    }
}

class ProductService
{
    public function create(array $data): Product
    {
        $product = $this->repository->create($data);
        $this->clearCache();
        ProductCreated::dispatch($product);
        $this->notifyVendor($product);
        return $product;
    }
}
```

## Repository Pattern (Mandatory for Data Access)

```php
// ❌ BAD: Query in controller or service
$products = Product::where('active', true)->paginate(20);

// ✅ GOOD: Query in repository
interface ProductRepositoryInterface
{
    public function listActive(int $perPage = 20): LengthAwarePaginator;
}

class ProductRepository implements ProductRepositoryInterface
{
    public function __construct(private Product $model) {}
    
    public function listActive(int $perPage = 20): LengthAwarePaginator
    {
        return $this->model->query()
            ->where('is_active', true)
            ->orderBy('created_at', 'desc')
            ->paginate($perPage);
    }
}
```

## Cross-Module Communication

Modules MUST NOT import models or repositories from other modules.

```
❌ Product module imports Order model directly
❌ Order module calls ProductRepository directly

✅ Product module dispatches ProductCreated event
✅ Order module listens to ProductCreated event
✅ Module boundaries enforced via interfaces
```

## Microservice Readiness

Every module must be extractable as an independent service:

1. **Own database migrations**: Module manages its own tables
2. **Own routes**: Module registers its own routes
3. **Own config**: Module has its own configuration
4. **Own tests**: Module tests run independently
5. **Interface contracts**: Module exposes interfaces, not implementations

## File Splitting Rules

When a file exceeds 100 lines:

```
Original: ProductController.php (250 lines)
   ↓ Split into:
ProductController.php        # index, show (thin delegates)
ProductStoreController.php   # store endpoint (thin delegates)
ProductUpdateController.php  # update endpoint (thin delegates)
ProductService.php           # business logic (called by all)
```

When a service exceeds 5 methods:

```
Original: ProductService.php (8 methods)
   ↓ Split into:
ProductCrudService.php      # create, update, delete
ProductSearchService.php    # list, search, filter
ProductPricingService.php   # calculatePrice, applyDiscount
```

## Language-Specific Module Patterns

### Node.js
```
src/modules/product/
├── routes.js
├── controller.js
├── service.js
├── repository.js
├── model.js
├── validators.js
└── __tests__/
```

### Go
```
internal/product/
├── handler.go
├── service.go
├── repository.go
├── model.go
└── handler_test.go
```

### Flutter/Dart
```
lib/features/product/
├── data/
│   ├── repositories/
│   └── models/
├── domain/
│   ├── entities/
│   └── usecases/
└── presentation/
    ├── pages/
    ├── widgets/
    └── providers/
```

## Checklist

- [ ] Every file under 100 lines
- [ ] Every function under 30 lines
- [ ] Every class under 5 public methods
- [ ] Features organized as modules
- [ ] Service layer contains business logic
- [ ] Repository layer abstracts data access
- [ ] Controllers are thin (5-10 lines per method)
- [ ] Cross-module communication via events
- [ ] Each module testable independently
- [ ] Each module extractable to microservice