Why additional principles improve quality in LLD - Why This Architecture

Problem Statement

When software grows, code often becomes tangled and hard to change. Without clear guidelines, developers add features in ways that cause bugs, slow down development, and make the system fragile.

Solution

Applying additional design principles guides developers to write cleaner, more organized code. These principles help separate concerns, reduce duplication, and make the system easier to understand and maintain over time.

Architecture

┌───────────────┐       ┌───────────────┐       ┌───────────────┐
│ Untidy Code   │──────▶│ Apply Principles│──────▶│ Clean Code    │
│ (Spaghetti)   │       │ (SOLID, DRY,   │       │ (Modular,     │
│               │       │  KISS, etc.)   │       │  Maintainable)│
└───────────────┘       └───────────────┘       └───────────────┘

This diagram shows how messy code becomes clean and maintainable by applying additional design principles.

Trade-offs

✓ Pros

→

Improves code readability and reduces bugs by enforcing clear structure.

→

Facilitates easier testing and debugging through modular design.

→

Speeds up future development by making code easier to extend and modify.

✗ Cons

→

Requires initial learning and discipline from developers to apply principles correctly.

→

May introduce slight overhead in design time for small or simple projects.

→

Over-applying principles can lead to unnecessary complexity if not balanced.

Use when building medium to large systems where maintainability and scalability are important, typically when the codebase exceeds a few thousand lines or multiple developers are involved.

Avoid strict application in very small or throwaway projects where speed is more important than long-term quality.

Real World Examples

Amazon

Amazon applies SOLID principles to keep their microservices modular and easy to update without breaking other parts.

Netflix

Netflix uses design principles like DRY and KISS to maintain their complex streaming platform with frequent feature updates.

Spotify

Spotify enforces clean code principles to allow multiple teams to work independently on different parts of their music service.

Code Example

The before code mixes pricing logic and printing, and duplicates discount logic. The after code uses inheritance and polymorphism (SOLID principles) to separate item types and pricing rules, removing duplication (DRY) and making it easier to extend.

LLD

### Before applying principles (no clear structure, duplication)
class Order:
    def __init__(self, items):
        self.items = items

    def total_price(self):
        total = 0
        for item in self.items:
            if item['type'] == 'book':
                total += item['price'] * 0.9  # 10% discount
            else:
                total += item['price']
        return total

    def print_order(self):
        for item in self.items:
            print(f"{item['name']}: ${item['price']}")


### After applying SOLID and DRY principles
from abc import ABC, abstractmethod

class Item(ABC):
    def __init__(self, name, price):
        self.name = name
        self.price = price

    @abstractmethod
    def get_price(self):
        pass

class Book(Item):
    def get_price(self):
        return self.price * 0.9  # 10% discount

class OtherItem(Item):
    def get_price(self):
        return self.price

class Order:
    def __init__(self, items):
        self.items = items

    def total_price(self):
        return sum(item.get_price() for item in self.items)

    def print_order(self):
        for item in self.items:
            print(f"{item.name}: ${item.get_price():.2f}")

Alternatives

Code Reviews

Focuses on peer checking of code quality rather than predefined principles.

Use when: Choose when you want human insight and feedback on code quality alongside or instead of strict principles.

Pair Programming

Involves two developers working together to write code, promoting quality through collaboration.

Use when: Choose when real-time knowledge sharing and immediate feedback are priorities.

Summary

Additional design principles guide developers to write cleaner and more maintainable code.

They reduce bugs and speed up future changes by organizing code better and avoiding duplication.

Applying principles thoughtfully improves software quality especially in medium to large projects.