Effectful Edge (IO)              Functional Core (Pure)
┌─────────────────────────────────┐    ┌──────────────────────────┐
│  Router    → request parsing    │    │  Service  → orchestration│
│  Consumer  → event handling     │───▶│  Entity   → domain rules │
│  Client    → external APIs      │    │            → validation  │
│  Producer  → event publishing   │◀───│            → transforms  │
│  Repository→ data persistence   │    │                          │
└─────────────────────────────────┘    └──────────────────────────┘

class OrderService {
  async createOrder(request: CreateOrderRequest): Promise<Result<Order>> {
    // Validate with entity
    const order = Order.fromRequest(request);
    const validation = order.validate();
    if (!validation.ok) return validation;

    // Check business rules
    const inventory = await this.inventoryRepo.checkAvailability(order.items);
    if (!inventory.available) return Err('Items not available');

    // Coordinate persistence
    await this.inventoryRepo.reserve(order.items);
    const saved = await this.orderRepo.save(order.toRecord());

    return Ok(Order.fromRecord(saved));
  }
}

class Order {
  constructor(
    public readonly id: string,
    public readonly customerId: string,
    public readonly items: OrderItem[],
    public readonly status: OrderStatus,
    public readonly total: number
  ) {}

  static fromRequest(req: CreateOrderRequest): Order {
    return new Order(
      generateId(),
      req.customerId,
      req.items.map(i => new OrderItem(i)),
      'pending',
      req.items.reduce((sum, i) => sum + i.price * i.quantity, 0)
    );
  }

  toRecord(): OrderRecord {
    return {
      id: this.id,
      customer_id: this.customerId,
      items: JSON.stringify(this.items),
      status: this.status,
      total: this.total
    };
  }

  validate(): Result<Order> {
    if (this.items.length === 0) {
      return Err('Order must have at least one item');
    }
    if (this.total < 0) {
      return Err('Order total cannot be negative');
    }
    return Ok(this);
  }

  canCancel(): boolean {
    return ['pending', 'confirmed'].includes(this.status);
  }
}

router.post('/orders', async (req, res) => {
  const result = await orderService.createOrder(req.body);

  if (result.ok) {
    res.status(201).json(result.value.toResponse());
  } else {
    res.status(400).json({ error: result.error });
  }
});

class OrderRepository {
  async save(record: OrderRecord): Promise<OrderRecord> {
    return await db.orders.create(record);
  }

  async findById(id: string): Promise<OrderRecord | null> {
    return await db.orders.findOne({ id });
  }
}

1. Entity   → Domain models, validation, transforms
2. Repository → Data access interfaces and implementations
3. Service  → Business logic orchestration
4. Router   → HTTP endpoints

Architect Outputs           →    Testing Inputs
────────────────────────────────────────────────
Component responsibilities  →    What to test
Layer boundaries           →    Where to test
Pure vs effectful          →    Unit vs integration
Entity transformations     →    Property-based tests
Service orchestration      →    Stub-driven tests