Architectural · App Structure · DWG NO. 045

Layered Architecture

Organize the codebase into layers — presentation, business, data — where each layer only depends on the one directly below it.

TypeApplication Structure
ScopeWhole application
ComplexityLow–Medium
Common inTraditional web apps, enterprise systems, most frameworks by default
01 · The problem

Without a rule, every layer starts calling every other layer

Without an agreed structure, a controller might query the database directly, a data-access class might contain business rules, and a service might format HTML — the codebase becomes a web of dependencies where changing one thing risks breaking something in a completely different concern.

02 · Structure

Strict one-directional dependency

Presentation depends on business logic; business logic depends on data access; data access depends on nothing above it. Each layer only calls downward, never upward or sideways past its neighbor — the presentation layer never touches the database directly.

Presentationcontrollers, viewsBusiness Logicservices, rulesData Accessrepositories, queries
03 · Reference implementation

Three layers, each calling only downward

The controller never imports the repository directly — it only ever calls through the service layer sitting between them.

layered-app.js 
// Data access layer — knows about storage, nothing else.
const orderRepository = {
  save: (order) => console.log(`db: saved order ${order.id}`),
};

// Business logic layer — knows about rules, calls data access.
const orderService = {
  placeOrder(items) {
    if (items.length === 0) throw new Error("order must have items");
    const order = { id: "order-1", items };
    orderRepository.save(order);
    return order;
  },
};

// Presentation layer — knows about requests/responses, calls business logic.
function handleCreateOrderRequest(body) {
  const order = orderService.placeOrder(body.items);
  return { status: 201, body: order }; // never touches orderRepository directly
}

console.log(handleCreateOrderRequest({ items: ["book", "pen"] }));
04 · When to use it

Trade-offs