Design Example: Car Rental System

A Car Rental System (like Hertz or Zipcar) manages vehicle inventory, customer memberships, and the lifecycle of reservations.

Step 1: Requirements Gathering

Core Use Cases:

• Search: Users can search for available cars by location, date, and category (SUV, Sedan, Luxury).
• Reservation: Users can book a car for a specific duration.
• Pick-up/Return: Handle the physical handoff and return of the vehicle.
• Billing: Calculate costs based on duration, fuel level, and damages.
• Inventory Management: Admins can add/remove cars or update their status.

Vehicle States:

• Status: AVAILABLE, RESERVED, LOANED, BEING_SERVICED, LOST.

Step 2: Identify Core Objects

• Vehicle: Abstract base for different car types.
• VehicleLog: Record of a car's history (service, cleanup).
• Account: User (Customer) and Employee (Attendant).
• Reservation: Connects a User to a Vehicle for a specific period.
• Bill: Handles payment calculations and extra charges.
• Location: Specific rental office where cars are stored.

Step 3: Design Class Diagram

Step 4: Implementation in TypeScript

enum VehicleStatus { AVAILABLE, RESERVED, LOANED }
 
abstract class Vehicle {
  constructor(
    public licensePlate: string,
    public dailyRate: number,
    public status: VehicleStatus = VehicleStatus.AVAILABLE
  ) {}
}
 
class SUV extends Vehicle {
  constructor(licensePlate: string) {
    super(licensePlate, 100);
  }
}
 
class Reservation {
  constructor(
    public id: string,
    public user: string,
    public vehicle: Vehicle,
    public start: Date,
    public end: Date
  ) {}
 
  public getDurationInDays(): number {
    return (this.end.getTime() - this.start.getTime()) / (1000 * 3600 * 24);
  }
}
 
class Bill {
  public static calculate(reservation: Reservation): number {
    const days = reservation.getDurationInDays();
    return days * reservation.vehicle.dailyRate;
  }
}

Deep Dive: Handling Dynamic Fleet Management

Using Composition for vehicle features (GPS, Child Seat) instead of inheritance allows the rental system to add/remove add-ons without creating complex class hierarchies.

Wrap Up

Designing a Car Rental System highlights the need for Atomic Reservations. Ensuring that a vehicle is not overbooked across overlapping dates is the primary technical challenge in the LLD of this system.

[!TIP] Use the Strategy Pattern for dynamic insurance pricing based on the user's age or location.