Backend Patterns

Architecture layers

Controller (routes) → Service (business logic) → Repository (data access) → Database

• Controller: HTTP concerns only. Parse request, validate input, call service, format response.
• Service: Business logic. No HTTP concepts (no req/res). Testable in isolation.
• Repository: Data access. Raw queries or ORM calls. No business logic.

API endpoint pattern

// Controller
async function createInvoice(req: Request, res: Response) {
  const dto = CreateInvoiceDto.parse(req.body)  // validate
  const invoice = await invoiceService.create(dto)  // delegate
  res.status(201).json(invoice)  // respond
}

Query optimization

• Select only needed columns: .select('id, name, status') not .select('*')
• Always add .limit() to list queries
• Use database indexes on columns used in WHERE and ORDER BY
• Use pagination (cursor-based preferred over offset for large datasets)

Input validation

• Validate at the boundary (controller) with Zod or class-validator
• Never trust client input inside service or repository layers
• Validate types AND business rules (e.g., amount > 0, date in future)

Error responses

// Consistent error shape
{
  "error": {
    "code": "INVOICE_NOT_FOUND",
    "message": "Invoice with ID abc123 not found",
    "status": 404
  }
}

Nest.js specifics

• One module per domain (InvoicesModule, PaymentsModule, AuthModule)
• Use guards for auth, interceptors for logging/transformation, pipes for validation
• DTOs with class-validator decorators for all input