Service Layer

What is a Service Layer?

The Service Layer is an architectural pattern that defines an application's boundary with a layer of services that establishes a set of available operations and coordinates the application's response in each operation.

Responsibilities

The Service Layer:

• Orchestrates business logic: Coordinates multiple operations
• Enforces business rules: Validates domain constraints
• Manages transactions: Ensures data consistency
• Handles errors: Translates technical errors to business errors
• Provides API boundary: Clean interface for controllers/UI

Structure

Controller/API → Service Layer → Repository Layer → Database

Services aggregate repository calls and add business logic:

class OrderService {
  constructor(
    private orderRepo: OrderRepository,
    private inventoryRepo: InventoryRepository,
    private paymentService: PaymentService
  ) {}

  async placeOrder(order: Order): Promise<OrderResult> {
    // Business logic: check inventory
    const available = await this.inventoryRepo.checkStock(order.items)
    if (!available) throw new OutOfStockError()

    // Business logic: process payment
    const payment = await this.paymentService.charge(order.total)
    if (!payment.success) throw new PaymentFailedError()

    // Coordinate: save order
    await this.orderRepo.save(order)
    await this.inventoryRepo.decrementStock(order.items)

    return { success: true, orderId: order.id }
  }
}

When to Use

Use Service Layer when:

• Business operations involve multiple steps
• Logic needs to be reused across different controllers
• Transactions span multiple repositories
• Business rules are complex

Skip when:

• Application is simple CRUD
• No complex business logic exists
• Controllers can directly call repositories

Anti-Patterns

Anemic Services: Services that just pass through to repositories (no value added)

God Services: Massive services that do everything (violates single responsibility)

Transaction Script: Services become procedural scripts instead of domain-focused

Best Practices

• Keep services focused on a single domain area
• Services should be stateless
• Use dependency injection for testability
• Return domain objects, not database entities
• Handle all exceptions and translate to business errors

The Service Layer is where your business logic lives—keep it clean and focused.