Applying SOLID to real code in LLD - Scalability & System Analysis

Scalability Analysis - Applying SOLID to real code

Growth Table: Applying SOLID Principles

Users / Codebase Size	Code Complexity	Maintainability	Bug Rate	Refactoring Effort
100 users / Small code	Low	High	Low	Minimal
10K users / Medium code	Moderate	Moderate to High	Moderate	Moderate
1M users / Large code	High	Moderate	High	High
100M users / Very large code	Very High	Low without SOLID	Very High	Very High

First Bottleneck

As the codebase grows, the first bottleneck is code maintainability. Without SOLID principles, the code becomes tightly coupled and hard to change. This leads to more bugs and slower feature delivery.

Scaling Solutions with SOLID

• Single Responsibility Principle: Break code into small classes/functions each with one job. Easier to test and change.
• Open/Closed Principle: Design code to add new features by adding code, not changing existing code. Use interfaces and inheritance.
• Liskov Substitution Principle: Subclasses should work wherever base classes are used. Prevents unexpected bugs.
• Interface Segregation Principle: Use many small, specific interfaces instead of one big one. Clients depend only on what they use.
• Dependency Inversion Principle: Depend on abstractions, not concrete classes. Makes swapping implementations easier.

Applying these principles helps the code scale in size and complexity without breaking.

Back-of-Envelope Cost Analysis

For a growing codebase:

• Bug fixes and feature additions take longer without SOLID, increasing developer hours by 2x to 5x at large scale.
• Refactoring legacy code without SOLID can cost weeks or months of effort.
• Applying SOLID early reduces long-term maintenance cost by improving modularity and testability.
• Code review and onboarding new developers is faster with clear, SOLID-based design.

Interview Tip

When discussing scalability of code design, start by explaining the challenges of growing codebases. Then introduce SOLID principles as a way to keep code modular and maintainable. Use simple examples to show how each principle helps avoid common problems. Finally, mention trade-offs like initial design effort versus long-term benefits.

Self Check

Your codebase is growing and becoming hard to maintain. What SOLID principle do you apply first and why?

Answer: Start with the Single Responsibility Principle to break large classes into smaller ones with one job each. This reduces complexity and makes future changes easier.

Key Result

Applying SOLID principles early keeps code maintainable and scalable as user base and code size grow, preventing the first bottleneck of code complexity and bug rate.

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