Parking Lot System Design - Complete Low-Level Design Guide

Design a scalable Parking Lot System using Java and Spring Boot. Learn requirement analysis, UML class diagrams, object-oriented design, SOLID principles, design patterns, parking allocation algorithms, concurrency handling, and real-world enterprise architecture.

Introduction

One of the most popular Low-Level Design interview questions is the Parking Lot System.

Companies like Amazon, Microsoft, Uber, Walmart, PayPal, Goldman Sachs, and many product-based organizations ask this question because it evaluates multiple software engineering skills simultaneously.

Instead of testing syntax, this problem evaluates your ability to design a real-world object-oriented system.

Through this problem you demonstrate knowledge of:

Object-Oriented Programming
SOLID Principles
Design Patterns
UML Modeling
Class Relationships
Extensibility
Concurrency
Clean Code
Real-world Business Modeling

In this article, we will design a production-ready Parking Lot System using Java and Spring Boot.

Problem Statement

Design a parking management system that can:

Park vehicles
Remove vehicles
Issue parking tickets
Calculate parking fees
Track available spaces
Support multiple floors
Support multiple vehicle types
Display parking availability
Scale for large parking facilities

Functional Requirements

The system should support:

Add Vehicle
Remove Vehicle
Allocate Parking Spot
Generate Parking Ticket
Release Parking Spot
Calculate Parking Charges
View Available Spaces
Multiple Parking Floors
Multiple Entry Gates
Multiple Exit Gates
Parking History

Non-Functional Requirements

The system should be:

Highly Available
Scalable
Thread Safe
Extensible
Maintainable
Testable
Easy to Monitor

Real-World Parking Types

The parking lot should support:

Bike
Car
SUV
Electric Vehicle
Bus
Truck

Each vehicle type may require different parking spot sizes.

High-Level Architecture

flowchart TD
    CUSTOMER["Customer"]

    ENTRY["Entry Gate System"]

    PARKING["Parking Allocation Service"]

    SPOT_MGR["Spot Manager Service"]

    DATABASE["Parking Database"]

    EXIT["Exit Gate System"]

    BILLING["Billing Service"]

    PAYMENT["Payment Gateway"]

    CUSTOMER --> ENTRY --> PARKING --> SPOT_MGR --> DATABASE

    EXIT --> BILLING --> PAYMENT

Actors

Actors include:

Driver
Parking Attendant
Administrator
Billing System
Payment Gateway

Main Components

The system consists of:

Parking Lot
Parking Floor
Parking Spot
Vehicle
Ticket
Payment
Entry Gate
Exit Gate
Parking Service
Billing Service

Core Entities

classDiagram

class ParkingLot

class ParkingFloor

class ParkingSpot

class Vehicle

class Ticket

class Payment

ParkingLot --> ParkingFloor

ParkingFloor --> ParkingSpot

ParkingSpot --> Vehicle

Vehicle --> Ticket

Ticket --> Payment

Class Responsibilities

ParkingLot

Responsible for:

Managing Floors
Managing Gates
Overall Capacity

ParkingFloor

Responsible for:

Managing Parking Spots
Displaying Availability

ParkingSpot

Responsible for:

Occupancy
Spot Type
Vehicle Assignment

Vehicle

Stores:

Vehicle Number
Vehicle Type

Ticket

Stores:

Entry Time
Spot Number
Vehicle
Ticket ID

Payment

Stores:

Amount
Payment Method
Status

Parking Spot Types


Small

Compact

Large

Electric

Handicapped

Vehicle Types


Bike

Car

SUV

Truck

Bus

Electric Vehicle

Allocation Strategy

The allocation algorithm should:

Find nearest available spot
Match vehicle type
Reserve immediately
Prevent duplicate allocation

flowchart LR
    VEHICLE["Vehicle Arrival"]

    SEARCH["Spot Search Service"]

    BOOK["Reservation Service"]

    TICKET["Ticket Generation System"]

    VEHICLE --> SEARCH --> BOOK --> TICKET

Parking Flow

sequenceDiagram

participant Driver

participant EntryGate

participant ParkingService

participant SpotManager

Driver->>EntryGate: Enter

EntryGate->>ParkingService: Request Spot

ParkingService->>SpotManager: Find Available Spot

SpotManager-->>ParkingService: Spot

ParkingService-->>Driver: Parking Ticket

Exit Flow

sequenceDiagram

participant Driver

participant ExitGate

participant Billing

participant Payment

Driver->>ExitGate: Exit

ExitGate->>Billing: Calculate Fee

Billing->>Payment: Collect Payment

Payment-->>Driver: Receipt

Parking Fee Calculation

Possible pricing model:

First Hour
Per Hour
Daily Maximum
Lost Ticket Charge

Example:

Duration	Charge
1 Hour	$5
2 Hours	$8
Additional Hour	$3

Spot Status

Each parking spot can be:


Available

Occupied

Reserved

Maintenance

Design Patterns Used

Singleton

ParkingLot

Only one parking lot manager.

Factory Pattern

Vehicle Factory

Creates:

Car
Bike
Truck
Bus

Strategy Pattern

Parking Allocation Strategy

Examples:

Nearest Spot
Lowest Floor
Random

Observer Pattern

Display Boards

Automatically update available parking spaces.

State Pattern

Parking Spot States

Available
Occupied
Reserved

SOLID Principles

SRP

Each class has one responsibility.

OCP

Add new vehicle types without modifying existing code.

LSP

Every vehicle behaves like a Vehicle.

ISP

Separate billing from parking operations.

DIP

ParkingService depends on interfaces instead of concrete implementations.

Concurrency

Multiple cars may enter simultaneously.

Challenges:

Double allocation
Race conditions
Duplicate tickets

Solutions:

Locks
Optimistic Locking
Atomic Operations
Transactions

Database Design

Tables:


ParkingLot

ParkingFloor

ParkingSpot

Vehicle

Ticket

Payment

Spring Boot Layers

flowchart LR

Controller

-->

Service

-->

Repository

-->

Database

REST APIs

Park Vehicle

POST /vehicles/park

Exit Vehicle

POST /vehicles/exit

Available Spots

GET /parking/spots

Parking History

GET /tickets

Enterprise Architecture

flowchart TD
    APP["Mobile App"]

    API["API Gateway"]

    PARKING["Parking Service"]

    SPOT["Spot Service"]

    BILL["Billing Service"]

    PAY["Payment Gateway"]

    PG["PostgreSQL"]

    REDIS["Redis Cache"]

    APP --> API --> PARKING

    PARKING --> SPOT
    PARKING --> BILL

Scaling Considerations

Large parking systems may include:

Multiple Buildings
Hundreds of Floors
Thousands of Spots
Multiple Cities

Scaling techniques:

Redis Cache
Distributed Locking
Event-Driven Architecture
Kafka Notifications

Possible Enhancements

Future features:

EV Charging Stations
Online Reservation
QR Code Entry
License Plate Recognition
Mobile Payments
Dynamic Pricing
Membership Plans
Monthly Parking
AI-based Spot Recommendation

Common Mistakes

❌ Large God Classes

❌ Hardcoded Pricing

❌ No Concurrency Handling

❌ Ignoring SOLID Principles

❌ Tight Coupling

❌ No Parking Strategy

Interview Questions

How would you support multiple parking lots?
How would you allocate the nearest parking spot?
How would you handle concurrent parking requests?
Which design patterns would you use?
How would you support EV charging spots?
How would you calculate dynamic pricing?
How would you make the system scalable?
How would you design the database?
How would you support reservations?
How would you prevent duplicate parking allocations?

Summary

The Parking Lot System is an excellent Low-Level Design problem because it combines object-oriented modeling, SOLID principles, design patterns, concurrency, and scalable architecture.

A production-ready solution typically includes:

Well-defined domain models
Layered architecture
Factory, Strategy, Singleton, and Observer patterns
Thread-safe parking allocation
REST APIs
Spring Boot services
Database persistence
Extensible pricing and allocation strategies

Mastering this design builds a strong foundation for tackling more advanced LLD interview problems such as Library Management System, Hotel Booking System, Movie Ticket Booking, Food Delivery Platform, Cab Booking, and E-Commerce Order Management.