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.