Open/Closed Principle in LLD - System Design Guide

Problem Statement

When software modules are changed directly to add new features, existing code can break and cause bugs. This makes the system fragile and hard to maintain as it grows.

Solution

The Open/Closed Principle solves this by designing modules that are open for extension but closed for modification. This means you add new behavior by adding new code, not by changing existing code, keeping the system stable and easier to grow.

Architecture

┌───────────────┐       ┌───────────────┐
│  Existing     │       │  New Feature  │
│  Module       │──────▶│  Module       │
│  (Closed)     │       │  (Extension)  │
└───────────────┘       └───────────────┘
        ▲                      │
        │                      │
        └──────────────────────┘

This diagram shows the existing module that is closed for changes but open to extensions through new modules that add features.

Trade-offs

✓ Pros

→

Reduces bugs by avoiding changes to tested, stable code.

→

Makes adding new features safer and faster.

→

Improves code maintainability and readability.

→

Supports easier testing by isolating new behavior.

✗ Cons

→

Can increase initial design complexity.

→

May lead to more classes or modules, increasing codebase size.

→

Requires developers to plan for extension points upfront.

Use when the system expects frequent feature additions or changes, especially in medium to large codebases with multiple developers.

Avoid when the project is very small or unlikely to change, as the added abstraction may be unnecessary overhead.

Real World Examples

Netflix

Netflix uses the Open/Closed Principle in their streaming service to add new content recommendation algorithms without changing the core playback system.

Amazon

Amazon applies this principle in their payment processing modules to support new payment methods by adding new modules instead of modifying existing ones.

Uber

Uber designs their ride pricing system to be open for extension to add new pricing strategies without modifying the existing calculation logic.

Code Example

The before code mixes all discount rules in one method, so adding new types means changing existing code. The after code uses separate classes for each discount type, so new discounts can be added by creating new classes without modifying existing ones.

LLD

### Before (violates Open/Closed Principle):
class DiscountCalculator:
    def calculate(self, customer_type, amount):
        if customer_type == 'regular':
            return amount * 0.9
        elif customer_type == 'vip':
            return amount * 0.8
        # Adding new customer types requires modifying this method

### After (applies Open/Closed Principle):
from abc import ABC, abstractmethod

class DiscountStrategy(ABC):
    @abstractmethod
    def calculate(self, amount):
        pass

class RegularDiscount(DiscountStrategy):
    def calculate(self, amount):
        return amount * 0.9

class VIPDiscount(DiscountStrategy):
    def calculate(self, amount):
        return amount * 0.8

class DiscountCalculator:
    def __init__(self, strategy: DiscountStrategy):
        self.strategy = strategy

    def calculate(self, amount):
        return self.strategy.calculate(amount)

# To add a new discount, create a new class without changing DiscountCalculator

Alternatives

Modifying Existing Code Directly

Changes are made inside existing modules instead of adding new ones.

Use when: Only when the system is very small and unlikely to change, to keep things simple.

Feature Flags

Uses runtime switches to enable or disable features instead of extending code via new modules.

Use when: When you need to toggle features quickly without deploying new code.

Summary

The Open/Closed Principle helps keep software stable by avoiding changes to existing code.

It encourages adding new features through extensions rather than modifications.

This principle improves maintainability and reduces bugs in growing systems.

Practice

1. What does the Open/Closed Principle (OCP) primarily encourage in software design?

easy

A. Software entities should be open for extension but closed for modification.

B. Software entities should be open for modification but closed for extension.

C. Software entities should be neither open nor closed for changes.

D. Software entities should always be rewritten for new features.

Solution

1. Step 1: Understand the definition of OCP

   The Open/Closed Principle states that software components should allow new functionality to be added without changing existing code.

2. Step 2: Match the principle to the options

   Software entities should be open for extension but closed for modification correctly states that entities should be open for extension (new features) but closed for modification (existing code remains unchanged).

3. Final Answer:

   Software entities should be open for extension but closed for modification. -> Option A

4. Quick Check:

   OCP = Open for extension, closed for modification [OK]

Hint: Remember: extend without changing existing code [OK]

Common Mistakes:

• Confusing 'open for extension' with 'open for modification'
• Thinking OCP means rewriting code for new features
• Ignoring the 'closed for modification' part

2. Which of the following code structures best supports the Open/Closed Principle?

easy

A. Using inheritance or interfaces to add new behavior without changing existing code.

B. Hardcoding all features inside one large class.

C. Copying and pasting code to create new features.

D. Modifying existing classes directly to add new features.

Solution

1. Step 1: Identify structures that allow extension without modification

   Inheritance and interfaces allow new classes to add behavior without changing existing classes.

2. Step 2: Evaluate options against OCP

   Using inheritance or interfaces to add new behavior without changing existing code uses inheritance/interfaces, which aligns with OCP. Other options involve modifying or duplicating code, violating OCP.

3. Final Answer:

   Using inheritance or interfaces to add new behavior without changing existing code. -> Option A

4. Quick Check:

   Inheritance/interfaces = OCP support [OK]

Hint: Choose inheritance or interfaces for extension [OK]

Common Mistakes:

• Thinking direct modification supports OCP
• Believing code duplication is acceptable
• Ignoring design patterns that enable extension

3. Consider this code snippet:

class Shape {
  area() { return 0; }
}

class Rectangle extends Shape {
  constructor(width, height) {
    super();
    this.width = width;
    this.height = height;
  }
  area() { return this.width * this.height; }
}

class Circle extends Shape {
  constructor(radius) {
    super();
    this.radius = radius;
  }
  area() { return Math.PI * this.radius * this.radius; }
}

function totalArea(shapes) {
  return shapes.reduce((sum, shape) => sum + shape.area(), 0);
}

What principle does this design demonstrate?

medium

A. Liskov Substitution Principle by replacing shapes with unrelated classes.

B. Single Responsibility Principle by separating area calculation.

C. Dependency Inversion Principle by depending on concrete classes.

D. Open/Closed Principle by allowing new shapes without modifying totalArea.

Solution

1. Step 1: Analyze how new shapes are added

   New shape classes (Rectangle, Circle) extend Shape and implement area(). totalArea works without changes.

2. Step 2: Match design to OCP

   Since totalArea does not change when new shapes are added, the design follows Open/Closed Principle.

3. Final Answer:

   Open/Closed Principle by allowing new shapes without modifying totalArea. -> Option D

4. Quick Check:

   Adding shapes without changing totalArea = OCP [OK]

Hint: Look for unchanged code when adding new features [OK]

Common Mistakes:

• Confusing OCP with other SOLID principles
• Thinking totalArea must change for new shapes
• Ignoring inheritance usage

4. Given this code violates the Open/Closed Principle:

class Report {
  generate(type) {
    if (type === 'pdf') {
      // generate PDF
    } else if (type === 'html') {
      // generate HTML
    }
  }
}

What is the best fix to comply with OCP?

medium

A. Add more if-else cases inside generate() for new report types.

B. Create separate classes for each report type implementing a common interface.

C. Remove the generate() method and write report code inline.

D. Use global variables to control report type.

Solution

1. Step 1: Identify why current code violates OCP

   Adding new report types requires modifying generate() method, breaking OCP.

2. Step 2: Choose design that allows extension without modification

   Separate classes for each report type implementing a common interface allow adding new types without changing existing code.

3. Final Answer:

   Create separate classes for each report type implementing a common interface. -> Option B

4. Quick Check:

   Separate classes = extend without modify [OK]

Hint: Use separate classes, not if-else chains [OK]

Common Mistakes:

• Adding more if-else blocks inside methods
• Ignoring interface or inheritance use
• Using global variables for behavior control

5. You are designing a payment system that must support new payment methods without changing existing code. Which approach best follows the Open/Closed Principle?

hard

A. Rewrite the payment processor each time a new method is added.

B. Add new payment method code inside a large switch statement in the payment processor.

C. Implement a Payment interface and create new classes for each payment method.

D. Use global flags to select payment methods inside one class.

Solution

1. Step 1: Understand the requirement for extensibility without modification

   The system must allow adding new payment methods without changing existing code.

2. Step 2: Evaluate design options for OCP compliance

   Implementing a Payment interface and creating new classes for each method allows extension by adding classes, not modifying existing ones.

3. Final Answer:

   Implement a Payment interface and create new classes for each payment method. -> Option C

4. Quick Check:

   Interface + new classes = OCP compliance [OK]

Hint: Use interfaces and new classes for new features [OK]

Common Mistakes:

• Using switch statements that require modification
• Rewriting existing code repeatedly
• Using global flags for behavior control