Applying SOLID to real code in LLD - Architecture Diagram

System Overview - Applying SOLID to real code

This system demonstrates how the SOLID principles guide the design of a simple order processing system. It ensures the code is easy to maintain, extend, and test by separating responsibilities and using interfaces.

Architecture Diagram

User
  |
  v
OrderController
  |
  v
OrderService <--> PaymentService
  |
  v
OrderRepository
  |
  v
Database

Cache <--> OrderService

Components

User

actor

Initiates order requests

OrderController

service

Handles user requests and delegates to services

OrderService

service

Processes orders following business rules

PaymentService

service

Handles payment processing separately

OrderRepository

database_access

Manages data storage and retrieval for orders

Database

database

Stores order data persistently

Cache

cache

Stores frequently accessed order data for quick retrieval

Request Flow - 16 Hops

User → OrderController

OrderController → OrderService

OrderService → Cache

Cache → OrderService

OrderService → OrderRepository

OrderRepository → Database

Database → OrderRepository

OrderRepository → OrderService

OrderService → PaymentService

PaymentService → OrderService

OrderService → OrderRepository

OrderRepository → Database

OrderRepository → OrderService

OrderService → Cache

OrderService → OrderController

OrderController → User

Failure Scenario

Component Fails:Database

Impact:Order data cannot be saved or retrieved, causing order processing failures

Mitigation:Use cache for read requests to serve stale data; queue write requests for retry; alert system admins for database recovery

Architecture Quiz - 3 Questions

Which component is responsible for separating payment processing from order logic?

A PaymentService

B OrderController

C OrderService

D OrderRepository

Design Principle

This architecture applies SOLID principles by separating concerns: single responsibility for each component, open for extension by isolating payment logic, and dependency inversion by using interfaces between services and repositories. This makes the system easier to maintain and extend.

Practice

1. Which SOLID principle encourages a class to have only one reason to change?

easy

A. Dependency Inversion Principle

B. Open/Closed Principle

C. Liskov Substitution Principle

D. Single Responsibility Principle

Solution

1. Step 1: Understand the principle names

   Single Responsibility Principle means one class should do one job only.

2. Step 2: Match principle to description

   Only Single Responsibility Principle talks about one reason to change.

3. Final Answer:

   Single Responsibility Principle -> Option D

4. Quick Check:

   One class, one reason to change = Single Responsibility Principle [OK]

Hint: One class, one job means Single Responsibility Principle [OK]

Common Mistakes:

• Confusing Single Responsibility with Open/Closed
• Thinking Dependency Inversion means single responsibility
• Mixing Liskov Substitution with class responsibilities

2. Which of the following code snippets correctly applies the Dependency Inversion Principle?

easy

A. class Car { Engine engine = new Engine(); }

B. interface Engine { void start(); } class Car { Engine engine; Car(Engine e) { engine = e; } }

C. class Car { void start() { Engine engine = new Engine(); engine.start(); } }

D. class Engine { void start() {} } class Car extends Engine {}

Solution

1. Step 1: Identify Dependency Inversion Principle

   It says depend on abstractions (interfaces), not concrete classes.

2. Step 2: Check which code uses interfaces

   interface Engine { void start(); } class Car { Engine engine; Car(Engine e) { engine = e; } } uses interface Engine and injects it into Car, following DIP.

3. Final Answer:

   Interface with injection to depend on abstraction -> Option B

4. Quick Check:

   Depend on interface, not concrete class = DIP [OK]

Hint: Look for interface usage and constructor injection [OK]

Common Mistakes:

• Creating concrete objects inside classes (violates DIP)
• Extending concrete classes instead of using interfaces
• Not using constructor or setter injection

3. Given this code snippet, what will be the output?

interface Shape { double area(); }
class Rectangle implements Shape {
  double width, height;
  Rectangle(double w, double h) { width = w; height = h; }
  public double area() { return width * height; }
}
class Circle implements Shape {
  double radius;
  Circle(double r) { radius = r; }
  public double area() { return 3.14 * radius * radius; }
}
class AreaCalculator {
  double totalArea(Shape[] shapes) {
    double total = 0;
    for (Shape s : shapes) total += s.area();
    return total;
  }
}
public class Main {
  public static void main(String[] args) {
    Shape[] shapes = { new Rectangle(2,3), new Circle(1) };
    System.out.println(new AreaCalculator().totalArea(shapes));
  }
}

medium

A. 9.14

B. 12.14

C. 15.42

D. Error at runtime

Solution

1. Step 1: Calculate Rectangle area

   Rectangle area = 2 * 3 = 6

2. Step 2: Calculate Circle area

   Circle area = 3.14 * 1 * 1 = 3.14

3. Step 3: Sum areas

   Total area = 6 + 3.14 = 9.14

4. Final Answer:

   9.14 -> Option A

5. Quick Check:

   6 + 3.14 = 9.14 [OK]

Hint: Add areas of shapes using interface method calls [OK]

Common Mistakes:

• Using wrong formula for circle area
• Adding radius instead of area
• Forgetting to sum both shapes

4. Identify the violation of the Open/Closed Principle in this code:

class Report {
  void printReport(String type) {
    if (type.equals("PDF")) { System.out.println("Printing PDF"); }
    else if (type.equals("HTML")) { System.out.println("Printing HTML"); }
  }
}

medium

A. Adding new report types requires changing printReport method

B. Using if-else is always correct for Open/Closed Principle

C. The class is closed for modification

D. The method uses polymorphism correctly

Solution

1. Step 1: Understand Open/Closed Principle

   Classes should be open for extension but closed for modification.

2. Step 2: Analyze printReport method

   Adding new types means editing if-else, so modifying existing code.

3. Final Answer:

   Adding new report types requires changing printReport method -> Option A

4. Quick Check:

   Modifying code to add features breaks Open/Closed Principle [OK]

Hint: If adding features needs code change, Open/Closed is violated [OK]

Common Mistakes:

• Thinking if-else is always fine
• Confusing Open/Closed with Single Responsibility
• Ignoring need for polymorphism

5. You have a payment system with classes for CreditCardPayment and PayPalPayment. To follow the Interface Segregation Principle, how should you design the interfaces?

hard

A. Create one large interface Payment with all methods for all payment types

B. Make CreditCardPayment inherit from PayPalPayment

C. Create separate interfaces like CreditCardPaymentInterface and PayPalPaymentInterface with only relevant methods

D. Use a single abstract class with all payment methods implemented

Solution

1. Step 1: Understand Interface Segregation Principle

   Clients should not depend on methods they do not use; interfaces should be small and specific.

2. Step 2: Apply to payment system

   Separate interfaces for each payment type avoid forcing classes to implement unused methods.

3. Final Answer:

   Create separate interfaces like CreditCardPaymentInterface and PayPalPaymentInterface with only relevant methods -> Option C

4. Quick Check:

   Small, specific interfaces = Interface Segregation Principle [OK]

Hint: Split interfaces by client needs to avoid unused methods [OK]

Common Mistakes:

• Using one big interface for all payment types
• Inheriting unrelated payment classes
• Using abstract class forcing all methods