06 Exercise: Architectural Layer Classification

January 2026 (858 Words, 5 Minutes)

Introduction

In a layered architecture, code is organized into horizontal layers where each layer has a specific responsibility:

• Presentation Layer: User interface, HTTP routes, UI callbacks, HTML/JSON rendering
• Business Layer: Domain logic, validation rules, calculations, business entities
• Data Access Layer: Database queries, repositories, object-relational mapping
• Database: Persistent storage (tables, schemas)

Learning Objectives:

• Identify which architectural layer a code snippet belongs to
• Understand the responsibilities of each layer
• Recognize patterns that indicate layer membership
• Justify layer classification decisions

Instructions:

1. Study the Layer Responsibilities reference
2. Read each code snippet carefully
3. Classify each snippet by layer and justify your answer
4. Check your answers against the solution

Total Points: 16 Time: ~15 minutes

Reference: Layer Responsibilities

Key principle: Dependencies should only point downward (Presentation → Business → Data Access → Database).

Code Snippets

Study the following Python code snippets carefully.

# Snippet A
def validate_email(email: str) -> bool:
    return "@" in email and "." in email.split("@")[1]

# Snippet B
@app.route('/users/<id>')
def get_user(id):
    user = user_service.find(id)
    return render_template('user.html', user=user)

# Snippet C
def save_user(user: User) -> None:
    cursor.execute("INSERT INTO users VALUES (?, ?)", (user.id, user.name))

# Snippet D
def calculate_discount(order: Order) -> float:
    if order.total > 100:
        return order.total * 0.1
    return 0.0

# Snippet E
def render_order_summary(order: Order) -> str:
    return f"""
    <div class="order">
        <h2>Order #{order.id}</h2>
        <p>Total: €{order.total:.2f}</p>
    </div>
    """

# Snippet F
class OrderRepository:
    def find_by_customer(self, customer_id: str) -> list[Order]:
        rows = self.db.execute(
            "SELECT * FROM orders WHERE customer_id = ?",
            (customer_id,)
        )
        return [Order(**row) for row in rows]

# Snippet G
@dataclass
class Order:
    id: str
    customer_id: str
    items: list[OrderItem]

    @property
    def total(self) -> float:
        return sum(item.price * item.quantity for item in self.items)

# Snippet H
@callback(Output('chart', 'figure'), Input('dropdown', 'value'))
def update_chart(selected_id):
    data = service.get_analysis(selected_id)
    return create_figure(data)

Task: Classify Code Snippets (16 points)

For each code snippet (A-H), identify which architectural layer it belongs to and provide a brief justification (1-2 sentences).

Your Answer:

(1 point per correct layer, 1 point per valid justification = 16 points total)

Solution