Decorator pattern in LLD - System Design Exercise

Design: Decorator Pattern Implementation

Design and implement the decorator pattern for a simple component interface and demonstrate how to add responsibilities dynamically. Out of scope: integration with large systems or specific language syntax.

Functional Requirements

FR1: Allow dynamic addition of responsibilities to objects without modifying their code

FR2: Support multiple layers of decoration

FR3: Maintain the original object's interface so decorated objects can be used interchangeably

FR4: Enable flexible and reusable code by composing behaviors at runtime

Non-Functional Requirements

NFR1: Must not alter the original object's source code

NFR2: Should support nesting decorators without performance degradation

NFR3: Design should be simple to understand and maintain

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

Key Components

Component interface or abstract class

Concrete component implementing the interface

Decorator base class implementing the interface and holding a reference to a component

Concrete decorators extending the decorator base to add behaviors

Design Patterns

Decorator pattern structure and roles

Composition over inheritance

Open/Closed principle

Transparent decoration

Reference Architecture

  +-----------------+       +-------------------+       +---------------------+
  |   Component     |<------+ ConcreteComponent |       |   Decorator         |
  |  (Interface)    |       |  (Implements)     |       | (Implements Component) |
  +-----------------+       +-------------------+       +---------------------+
                                      ^                             |
                                      |                             |
                                      |                      +---------------------+
                                      |                      | ConcreteDecorator(s) |
                                      |                      +---------------------+

Components

Component

Interface or Abstract Class

Defines the interface for objects that can have responsibilities added dynamically.

ConcreteComponent

Class implementing Component

The original object to which additional responsibilities can be attached.

Decorator

Abstract Class implementing Component

Holds a reference to a Component object and defines the interface for decorators.

ConcreteDecorator

Class extending Decorator

Adds responsibilities to the component dynamically by overriding methods.

Request Flow

1. Client creates a ConcreteComponent instance.

2. Client wraps the ConcreteComponent with one or more ConcreteDecorator instances.

3. Client calls methods on the decorated object using the Component interface.

4. Each decorator adds its behavior before or after delegating to the wrapped component.

5. The call propagates through all decorators down to the original ConcreteComponent.

Database Schema

Not applicable for this pattern as it focuses on object structure and behavior.

Scaling Discussion

Bottlenecks

Excessive layers of decorators can cause performance overhead due to deep call chains.

Complexity in debugging when many decorators are nested.

Potential difficulty in managing state if decorators add stateful behavior.

Solutions

Limit the number of decorator layers or optimize critical decorators for performance.

Use clear logging and debugging tools to trace decorator calls.

Design decorators to be stateless or carefully manage state to avoid side effects.

Interview Tips

Time: Spend 10 minutes explaining the problem and pattern roles, 15 minutes drawing and describing the design, 10 minutes discussing scaling and trade-offs.

Explain the motivation for the decorator pattern: adding behavior without modifying original code.

Describe the roles of Component, ConcreteComponent, Decorator, and ConcreteDecorator.

Show how decorators maintain the same interface for interchangeability.

Discuss benefits like flexibility and adherence to open/closed principle.

Mention potential drawbacks and how to mitigate them.