Why SOLID principles guide maintainable design in LLD - Design It to Understand It

Design: Maintainable Software Design Using SOLID Principles

Focus on explaining how SOLID principles help create maintainable software design. Out of scope are specific programming language implementations or detailed code examples.

Functional Requirements

FR1: Design software components that are easy to understand and modify

FR2: Ensure components can be extended without changing existing code

FR3: Promote code reuse and reduce duplication

FR4: Allow independent development and testing of components

FR5: Support clear separation of concerns

Non-Functional Requirements

NFR1: Code changes should minimize impact on unrelated parts

NFR2: Design should support scaling team size and codebase

NFR3: Maintain low complexity to reduce bugs and improve readability

NFR4: Design should enable easy debugging and troubleshooting

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

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

Abstraction and interfaces

Loose coupling

Encapsulation

Code reuse

Reference Architecture

 +---------------------+       +---------------------+       +---------------------+
 |  Single Responsibility|       |  Open/Closed        |       |  Liskov Substitution |
 |  Principle (SRP)      |       |  Principle (OCP)    |       |  Principle (LSP)     |
 +----------+----------+       +----------+----------+       +----------+----------+
            |                             |                             |
            v                             v                             v
 +---------------------+       +---------------------+       +---------------------+
 | Interface Segregation|       | Dependency Inversion |       |  Maintainable Design |
 | Principle (ISP)      |       | Principle (DIP)      |       +---------------------+
 +---------------------+       +---------------------+

Components

Single Responsibility Principle (SRP)

Conceptual principle

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

Open/Closed Principle (OCP)

Conceptual principle

Design modules to be open for extension but closed for modification, enabling new features without breaking existing code.

Liskov Substitution Principle (LSP)

Conceptual principle

Guarantee that subclasses can replace base classes without altering correctness, supporting reliable polymorphism.

Interface Segregation Principle (ISP)

Conceptual principle

Create specific interfaces so clients only depend on what they use, reducing unnecessary dependencies.

Dependency Inversion Principle (DIP)

Conceptual principle

Depend on abstractions, not concrete implementations, to reduce coupling and increase flexibility.

Request Flow

1. 1. Identify responsibilities in the system and assign each to a separate module (SRP).

2. 2. Design modules with abstract interfaces that allow adding new features without changing existing code (OCP).

3. 3. Ensure subclasses can be used wherever their base classes are expected without errors (LSP).

4. 4. Split large interfaces into smaller, client-specific ones so modules depend only on needed methods (ISP).

5. 5. Make high-level modules depend on abstractions rather than concrete classes to allow easy swapping of implementations (DIP).

6. 6. Together, these steps create a system where changes affect minimal parts, enabling easier maintenance and extension.

Database Schema

Not applicable as this is a design principle explanation without specific data storage requirements.

Scaling Discussion

Bottlenecks

Tightly coupled code making changes risky and error-prone

Large classes or modules with multiple responsibilities causing confusion

Rigid code that breaks when adding new features

Interfaces forcing clients to depend on unused methods

High-level modules depending on low-level concrete implementations reducing flexibility

Solutions

Apply SRP to split responsibilities into focused modules

Use OCP to extend behavior via new modules rather than modifying existing ones

Follow LSP to ensure safe inheritance and polymorphism

Implement ISP to create fine-grained interfaces tailored to clients

Adopt DIP to invert dependencies and rely on abstractions

Interview Tips

Time: Spend 10 minutes explaining each SOLID principle with simple examples, 10 minutes discussing how they improve maintainability, and 5 minutes summarizing benefits and answering questions.

Explain each SOLID principle clearly and simply

Show how principles reduce complexity and bugs

Describe how principles support team collaboration and scaling

Give relatable examples of maintainability problems solved by SOLID

Highlight the importance of abstraction and loose coupling

Practice

1. Which of the following best describes the main goal of the SOLID principles in software design?

easy

A. To create code that is easy to understand, maintain, and extend

B. To make code run faster by using complex algorithms

C. To reduce the number of lines of code in a program

D. To enforce strict coding styles and formatting rules

Solution

1. Step 1: Understand the purpose of SOLID principles

   SOLID principles focus on writing code that is clean, flexible, and easy to maintain and extend over time.

2. Step 2: Compare options with the purpose

   To create code that is easy to understand, maintain, and extend matches this goal, while others focus on speed, code length, or style, which are not the main SOLID goals.

3. Final Answer:

   To create code that is easy to understand, maintain, and extend -> Option A

4. Quick Check:

   SOLID = Maintainable code [OK]

Hint: SOLID = clean, flexible, maintainable code [OK]

Common Mistakes:

• Confusing SOLID with performance optimization
• Thinking SOLID is about code formatting
• Believing SOLID reduces code length only

2. Which of the following is the correct description of the Single Responsibility Principle (SRP)?

easy

A. A class should be responsible for error logging only

B. A class should have only one reason to change

C. A class should handle all user interface and database logic

D. A class should inherit from multiple classes

Solution

1. Step 1: Recall the Single Responsibility Principle

   SRP states that a class should have only one reason to change, meaning it should have a single responsibility.

2. Step 2: Evaluate each option

   A class should have only one reason to change correctly states SRP. A class should inherit from multiple classes describes multiple inheritance, not SRP. A class should handle all user interface and database logic mixes responsibilities, violating SRP. A class should be responsible for error logging only is too narrow and not a general principle.

3. Final Answer:

   A class should have only one reason to change -> Option B

4. Quick Check:

   SRP = One reason to change [OK]

Hint: SRP = one responsibility per class [OK]

Common Mistakes:

• Confusing SRP with inheritance rules
• Thinking SRP means one class per feature
• Believing SRP limits class functionality too much

3. Consider this code snippet following the Open/Closed Principle (OCP):

class Shape:
    def area(self):
        pass

class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius
    def area(self):
        return 3.14 * self.radius * self.radius

class Square(Shape):
    def __init__(self, side):
        self.side = side
    def area(self):
        return self.side * self.side

shapes = [Circle(2), Square(3)]
areas = [shape.area() for shape in shapes]
print(areas)

What will be the output?

medium

A. [6.28, 9]

B. [12.56, 6]

C. Error: area method not implemented

D. [12.56, 9]

Solution

1. Step 1: Calculate area of Circle with radius 2

   Area = 3.14 * 2 * 2 = 12.56

2. Step 2: Calculate area of Square with side 3

   Area = 3 * 3 = 9

3. Step 3: Collect areas in list and print

   Output list is [12.56, 9]

4. Final Answer:

   [12.56, 9] -> Option D

5. Quick Check:

   Circle area + Square area = [12.56, 9] [OK]

Hint: Calculate each shape's area separately [OK]

Common Mistakes:

• Using wrong formula for circle area
• Mixing side length with radius
• Expecting error due to base class method

4. A developer wrote this code violating the Dependency Inversion Principle (DIP):

class Database:
    def connect(self):
        print('Connecting to MySQL')

class UserService:
    def __init__(self):
        self.db = Database()
    def get_user(self):
        self.db.connect()
        return 'User data'

What is the main problem here?

medium

A. UserService depends directly on a concrete Database class, making it hard to change database types

B. Database class should not have a connect method

C. UserService should not have a get_user method

D. Database connection should be established outside UserService

Solution

1. Step 1: Understand Dependency Inversion Principle

   DIP says high-level modules should depend on abstractions, not concrete classes.

2. Step 2: Analyze the code dependency

   UserService creates a concrete Database instance directly, tightly coupling them and making changes hard.

3. Final Answer:

   UserService depends directly on a concrete Database class, making it hard to change database types -> Option A

4. Quick Check:

   DIP = Depend on abstractions, not concretes [OK]

Hint: Check if high-level class depends on concrete class [OK]

Common Mistakes:

• Thinking connect method is wrong
• Confusing method placement with DIP violation
• Ignoring tight coupling issues

5. You are designing a payment system that must support multiple payment methods (credit card, PayPal, crypto). To follow the Interface Segregation Principle (ISP) and keep the system maintainable, which design approach is best?

hard

A. Use one class with many conditional statements to handle all payment types

B. Create a single large interface with all payment methods combined, and implement it in one class

C. Create separate interfaces for each payment method and implement only the needed ones in each class

D. Avoid interfaces and use global functions for each payment method

Solution

1. Step 1: Recall Interface Segregation Principle

   ISP states clients should not be forced to depend on interfaces they do not use; smaller, specific interfaces are better.

2. Step 2: Evaluate design options

   Create separate interfaces for each payment method and implement only the needed ones in each class creates separate interfaces per payment method, so classes implement only what they need, improving maintainability.

3. Final Answer:

   Create separate interfaces for each payment method and implement only the needed ones in each class -> Option C

4. Quick Check:

   ISP = Small, specific interfaces [OK]

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

Common Mistakes:

• Using one big interface for all methods
• Mixing all logic in one class with conditionals
• Ignoring interface design principles