Open/Closed Principle in LLD - System Design Exercise

Design: Open/Closed Principle Application Design

Focus on designing a module or component demonstrating the Open/Closed Principle. Out of scope are specific UI or database implementations.

Functional Requirements

FR1: Design a software module that can be extended with new features without modifying existing code.

FR2: Ensure the system supports adding new behaviors easily.

FR3: Maintain existing functionality without introducing bugs when extending.

Non-Functional Requirements

NFR1: The system should be maintainable and scalable for future extensions.

NFR2: Changes to existing code should be minimized to reduce risk.

NFR3: Design should promote code reuse and separation of concerns.

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

Key Components

Abstract classes or interfaces to define contracts

Concrete implementations for specific behaviors

Factory or dependency injection to manage object creation

Extension points for adding new features

Design Patterns

Strategy pattern to encapsulate algorithms

Decorator pattern to add responsibilities dynamically

Template method pattern to define skeletons with extension hooks

Dependency inversion principle to depend on abstractions

Reference Architecture

  +---------------------+
  |    Client Code      |
  +----------+----------+
             |
             v
  +----------+----------+
  |   Abstract Feature   |<-- Interface or Abstract Class
  +----------+----------+
             |
    +--------+--------+--------+
    |        |        |        |
+---v---+ +--v---+ +--v---+ +--v---+
|Feature| |Feature| |Feature| |Feature|
| Impl1 | | Impl2 | | Impl3 | | Impl4 |
+-------+ +-------+ +-------+ +-------+

Client interacts with Abstract Feature interface.
New features add new implementations without changing existing code.

Components

Abstract Feature Interface

Interface or Abstract Class

Defines the contract for features that can be extended.

Concrete Feature Implementations

Classes implementing the interface

Provide specific behaviors or features.

Client Code

Any module using the features

Uses the abstract interface to interact with features without knowing concrete details.

Factory or Dependency Injection

Design pattern or framework

Creates and injects concrete feature implementations to client code.

Request Flow

1. Client requests a feature through the abstract interface.

2. Factory or DI provides a concrete implementation of the feature.

3. Client uses the feature implementation without depending on its concrete class.

4. To add new features, create new implementations without modifying existing code.

5. Client can use new features by receiving new implementations via factory or DI.

Database Schema

Not applicable as this design focuses on code structure and extensibility.

Scaling Discussion

Bottlenecks

Too many concrete implementations can make management complex.

If abstractions are not well designed, adding new features may require changes.

Performance overhead if many layers of abstraction or decorators are used.

Solutions

Organize implementations into modules or packages for clarity.

Design clear and stable abstractions anticipating future extensions.

Use composition over inheritance to reduce complexity.

Apply caching or optimization if abstraction layers cause performance issues.

Interview Tips

Time: Spend 10 minutes explaining the principle and its importance, 20 minutes designing the module with diagrams and examples, and 15 minutes discussing scaling and trade-offs.

Explain the Open/Closed Principle simply: software entities should be open for extension but closed for modification.

Show how abstractions help add new features without changing existing code.

Discuss design patterns that support this principle.

Highlight benefits: easier maintenance, fewer bugs, better scalability.

Mention potential challenges and how to address them.

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