Enum usage (VehicleType, SpotType) in LLD - System Design Exercise

Design: Parking Lot System with Enum Usage

Design focuses on enum usage for vehicle and spot types, spot assignment logic, and basic parking lot management. Payment, user management, and multi-level parking are out of scope.

Functional Requirements

FR1: Support different types of vehicles: Motorcycle, Car, Truck

FR2: Support different types of parking spots: MotorcycleSpot, CompactSpot, LargeSpot

FR3: Assign vehicles to appropriate spot types based on vehicle type

FR4: Prevent parking a vehicle in an incompatible spot

FR5: Allow querying available spots by spot type

FR6: Track occupied and free spots

Non-Functional Requirements

NFR1: System should handle up to 500 vehicles parked simultaneously

NFR2: Spot assignment and release operations should respond within 100ms

NFR3: System availability target: 99.9% uptime

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Enum definitions for VehicleType and SpotType

ParkingSpot manager to track spot availability

Vehicle class with type attribute

Assignment logic mapping VehicleType to allowed SpotTypes

Storage for spot occupancy status

Design Patterns

Enum pattern for fixed set of constants

Strategy pattern for spot assignment rules

Observer pattern for spot availability updates

Reference Architecture

  +----------------+       +------------------+       +----------------+
  |   Vehicle      |       |  ParkingSpot     |       |  ParkingLot     |
  | - vehicleType  |<----->| - spotType       |<----->| - spots[]       |
  +----------------+       | - isOccupied     |       | - assignSpot()  |
                           +------------------+       | - releaseSpot() |
                                                      +----------------+

Enums:
VehicleType = {Motorcycle, Car, Truck}
SpotType = {MotorcycleSpot, CompactSpot, LargeSpot}

Mapping:
Motorcycle -> MotorcycleSpot, CompactSpot, LargeSpot
Car -> CompactSpot, LargeSpot
Truck -> LargeSpot

Components

VehicleType Enum

Enum (language built-in)

Defines fixed vehicle categories: Motorcycle, Car, Truck

SpotType Enum

Enum (language built-in)

Defines fixed parking spot categories: MotorcycleSpot, CompactSpot, LargeSpot

Vehicle Class

Object-oriented class

Represents a vehicle with a VehicleType attribute

ParkingSpot Class

Object-oriented class

Represents a parking spot with SpotType and occupancy status

ParkingLot Manager

Service or class

Manages spot availability, assignment, and release

Request Flow

1. 1. Vehicle arrives with a VehicleType.

2. 2. ParkingLot queries available spots compatible with VehicleType.

3. 3. Assign the first available compatible spot to the vehicle.

4. 4. Mark the spot as occupied.

5. 5. When vehicle leaves, release the spot and mark it free.

Database Schema

Entities: - Vehicle (id, vehicle_type) - ParkingSpot (id, spot_type, is_occupied) Relationships: - Vehicle assigned to one ParkingSpot (1:1 when parked) VehicleType and SpotType are enums stored as strings or integers in the database.

Scaling Discussion

Bottlenecks

Single ParkingLot manager becomes a bottleneck with many concurrent spot assignments

Database contention when updating spot occupancy status

Latency increases when searching for available spots in large parking lots

Solutions

Partition parking lot into zones with separate managers to reduce contention

Use in-memory caching (e.g., Redis) for spot availability with eventual consistency to database

Index spots by type and availability for fast lookup

Implement optimistic locking or transactions to handle concurrent updates safely

Interview Tips

Time: Spend 10 minutes clarifying requirements and constraints, 20 minutes designing enums and assignment logic, 10 minutes discussing scaling and trade-offs, 5 minutes summarizing.

Explain why enums are useful for fixed categories and type safety

Describe how vehicle types map to spot types and why this matters

Discuss how to handle spot assignment and release efficiently

Mention how to scale the system for larger parking lots

Highlight trade-offs between strict type matching and flexibility