Composition over inheritance in LLD - System Design Exercise

Design: Vehicle Management System

Design focuses on how to model vehicles and their features using composition instead of inheritance. Out of scope are UI design, database persistence, and network communication.

Functional Requirements

FR1: Support different types of vehicles like cars, trucks, and motorcycles

FR2: Allow vehicles to have different features such as engine types, GPS, and entertainment systems

FR3: Enable adding or removing features dynamically without changing vehicle classes

FR4: Provide a way to get vehicle details including all features

FR5: Support future extension with new vehicle types and features easily

Non-Functional Requirements

NFR1: System should be easy to maintain and extend

NFR2: Avoid deep inheritance hierarchies to reduce complexity

NFR3: Ensure code reuse without tight coupling between vehicle types and features

NFR4: Performance should be reasonable for up to 10,000 vehicles managed concurrently

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Vehicle interface or abstract class defining common vehicle behavior

Feature interfaces or classes representing individual capabilities

Composition container inside vehicle to hold features

Factory or builder to assemble vehicles with features

Client code to interact with vehicles and their features

Design Patterns

Composition design pattern

Decorator pattern for adding features dynamically

Strategy pattern for interchangeable behaviors

Interface segregation to keep features focused

Dependency injection for flexible feature assignment

Reference Architecture

  +-------------------+       +-------------------+
  |    Vehicle        |<>-----|   Feature         |
  |  (interface)      |       |  (interface)      |
  +-------------------+       +-------------------+
           ^                            ^
           |                            |
  +-------------------+       +-------------------+
  | Car               |       | EngineFeature     |
  +-------------------+       +-------------------+
  | Truck             |       | GPSFeature        |
  +-------------------+       +-------------------+
  | Motorcycle        |       | EntertainmentFeature|
  +-------------------+       +-------------------+

Vehicles hold a list of Features and delegate feature behavior to them.

Components

Vehicle Interface

Abstract class or interface in any OOP language

Defines common vehicle methods like getDetails()

Concrete Vehicle Classes

Classes like Car, Truck, Motorcycle

Implement Vehicle interface and hold features

Feature Interface

Interface for vehicle features

Defines methods for features like activate(), getDescription()

Concrete Feature Classes

Classes like EngineFeature, GPSFeature

Implement Feature interface with specific behavior

Composition Container

List or collection inside Vehicle classes

Holds features and delegates calls to them

Request Flow

1. Client creates a Vehicle object (e.g., Car)

2. Client creates Feature objects (e.g., EngineFeature, GPSFeature)

3. Client adds Feature objects to Vehicle's feature list

4. When client calls getDetails() on Vehicle, Vehicle calls getDescription() on each Feature

5. Vehicle aggregates feature descriptions and returns full details

6. Client can add or remove features at runtime without changing Vehicle class

Database Schema

Not applicable as this is a design pattern example focusing on code structure rather than persistence.

Scaling Discussion

Bottlenecks

Managing large numbers of features per vehicle can increase memory usage

Frequent dynamic feature changes may cause performance overhead

Complex feature interactions may require careful coordination

Without inheritance, some shared code might be duplicated if not factored well

Solutions

Use efficient data structures for feature storage like hash maps

Cache computed vehicle details if features rarely change

Define clear contracts and interfaces to manage feature interactions

Extract common code into utility classes or base components to avoid duplication

Interview Tips

Time: Spend 10 minutes understanding requirements and clarifying scope, 20 minutes designing the composition-based solution with diagrams, 10 minutes discussing scaling and trade-offs, 5 minutes summarizing.

Explain why inheritance can cause rigid and complex hierarchies

Show how composition allows flexible feature addition and removal

Describe how delegation works from vehicle to features

Discuss patterns like decorator and strategy that support composition

Mention trade-offs such as potential complexity in managing many small components