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.