Why SOLID principles guide maintainable design in LLD - Why This Architecture

Problem Statement

When software grows, code becomes tangled and hard to change. Small changes cause bugs in unrelated parts, and adding new features takes too long because the design is rigid and unclear.

Solution

SOLID principles guide how to write code that is easy to understand, change, and extend. They break down responsibilities, reduce dependencies, and encourage clear interfaces, so developers can safely update or add features without breaking existing code.

Architecture

Single

Responsibility

→Open/Closed

↓

Interface

Segregation

→Dependency

This diagram shows the five SOLID principles as steps leading to maintainable design, illustrating how each principle builds on the previous to improve code structure.

Trade-offs

✓ Pros

→

Improves code readability by separating concerns clearly.

→

Makes adding new features safer and faster by reducing side effects.

→

Facilitates easier testing by isolating components.

→

Encourages reusable and flexible code components.

✗ Cons

→

Initial design takes more time and thought to apply all principles.

→

May introduce more classes and interfaces, increasing codebase size.

→

Over-engineering can occur if principles are applied too rigidly on simple problems.

Use SOLID principles when building medium to large applications expected to evolve over time with multiple developers.

Avoid strict SOLID application in very small or throwaway scripts where simplicity and speed are more important than maintainability.

Real World Examples

Amazon

Amazon applies SOLID principles in their microservices to allow independent teams to develop and deploy features without breaking others.

Netflix

Netflix uses SOLID principles to keep their streaming service code modular, enabling rapid feature rollout and easy bug fixes.

Spotify

Spotify designs their backend services following SOLID to support continuous integration and delivery with minimal downtime.

Code Example

The before code mixes calculation, printing, and saving in one class, violating Single Responsibility Principle (SRP). The after code separates calculation, printing, and saving into different classes, making it easier to extend or modify each part independently, following SOLID principles.

LLD

### Before applying SOLID principles (violating SRP and OCP):

class Report:
    def __init__(self, data):
        self.data = data

    def calculate(self):
        # calculate report data
        return sum(self.data)

    def print_report(self):
        print(f"Report total: {self.calculate()}")

    def save_to_file(self, filename):
        with open(filename, 'w') as f:
            f.write(str(self.calculate()))


### After applying SOLID principles (SRP and OCP applied):

from abc import ABC, abstractmethod

class Calculator(ABC):
    @abstractmethod
    def calculate(self):
        pass

class SumCalculator(Calculator):
    def __init__(self, data):
        self.data = data

    def calculate(self):
        return sum(self.data)

class ReportPrinter:
    def print(self, calculator: Calculator):
        print(f"Report total: {calculator.calculate()}")

class ReportSaver:
    def save(self, calculator: Calculator, filename):
        with open(filename, 'w') as f:
            f.write(str(calculator.calculate()))


# Usage
calculator = SumCalculator([1, 2, 3])
printer = ReportPrinter()
saver = ReportSaver()
printer.print(calculator)
saver.save(calculator, 'report.txt')

Alternatives

Monolithic Design

Combines all functionality in one large codebase without strict separation of concerns.

Use when: Choose when building very small applications or prototypes where speed is more important than long-term maintainability.

Procedural Programming

Focuses on sequences of instructions rather than objects and interfaces.

Use when: Choose for simple scripts or tasks with minimal complexity and no need for extensibility.

Summary

SOLID principles prevent tangled code by promoting clear responsibilities and flexible design.

They help developers add features and fix bugs safely without breaking unrelated parts.

Applying SOLID leads to code that is easier to read, test, and maintain over time.

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