Why additional principles improve quality in LLD - Design It to Understand It

Design: Understanding the Impact of Additional Design Principles on Software Quality

In scope: Explanation of design principles impact on quality, examples of principles, benefits. Out of scope: Detailed code implementations or high-level architecture.

Functional Requirements

FR1: Explain how applying more design principles improves software quality

FR2: Show examples of principles that enhance maintainability, scalability, and reliability

FR3: Demonstrate the relationship between principles and reduced bugs or easier changes

Non-Functional Requirements

NFR1: Use simple, clear explanations suitable for beginners

NFR2: Focus on principles relevant to low-level design (LLD)

NFR3: Avoid technical jargon and complex terms

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

Key Components

Single Responsibility Principle (SRP)

Open/Closed Principle (OCP)

Liskov Substitution Principle (LSP)

Interface Segregation Principle (ISP)

Dependency Inversion Principle (DIP)

Design Patterns

Modular design

Encapsulation

Abstraction

Separation of concerns

Reference Architecture

  +-----------------------------+
  |       Design Principles      |
  +-----------------------------+
  | SRP | OCP | LSP | ISP | DIP |
  +-----------------------------+
            |         |         
            v         v         
  +-----------------------------+
  | Improved Code Quality       |
  | - Easier to maintain       |
  | - Easier to extend         |
  | - Fewer bugs               |
  | - Better readability       |
  +-----------------------------+

Components

Single Responsibility Principle (SRP)

Conceptual principle

Ensures each module/class has one reason to change, making code easier to understand and fix.

Open/Closed Principle (OCP)

Conceptual principle

Allows extending behavior without modifying existing code, reducing risk of introducing bugs.

Liskov Substitution Principle (LSP)

Conceptual principle

Ensures subclasses can replace base classes without breaking functionality, improving reliability.

Interface Segregation Principle (ISP)

Conceptual principle

Prevents forcing clients to depend on interfaces they don't use, reducing unnecessary complexity.

Dependency Inversion Principle (DIP)

Conceptual principle

Promotes depending on abstractions, not concrete implementations, enhancing flexibility and testability.

Request Flow

1. 1. Developer applies SRP to split code into focused modules.

2. 2. OCP guides adding new features by extending modules, not changing them.

3. 3. LSP ensures new subclasses behave correctly when replacing base classes.

4. 4. ISP helps create small, client-specific interfaces to avoid bloated dependencies.

5. 5. DIP encourages coding against interfaces, allowing easy swapping of implementations.

6. 6. These principles together improve code clarity, reduce bugs, and simplify changes.

Database Schema

Not applicable as this is a conceptual design principle explanation.

Scaling Discussion

Bottlenecks

Without principles, code becomes tightly coupled and hard to change.

Adding features causes bugs due to unclear responsibilities.

Testing becomes difficult due to dependencies on concrete implementations.

Code readability suffers, increasing onboarding time for new developers.

Solutions

Apply SRP to isolate changes and reduce side effects.

Use OCP to add features without modifying tested code.

Follow LSP to ensure subclass reliability and safe polymorphism.

Implement ISP to keep interfaces lean and relevant.

Adopt DIP to decouple modules and enable easier testing and swapping.

Interview Tips

Time: Spend 10 minutes explaining each key principle with simple examples, 10 minutes discussing how they improve quality, and 5 minutes summarizing benefits.

Explain each principle in simple terms with relatable examples.

Show how principles reduce bugs and improve maintainability.

Discuss how principles enable easier feature addition and testing.

Highlight the importance of clean, modular code for team collaboration.

Mention that applying multiple principles together leads to higher quality software.