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

Problem Statement

When using plain strings or integers to represent categories like vehicle types or parking spot types, the code becomes error-prone and hard to maintain. Typos or inconsistent values can cause bugs that are difficult to trace, and adding new types requires changes scattered across the codebase.

Solution

Enums provide a fixed set of named constants that represent categories such as vehicle types or spot types. By using enums, the code gains clarity, type safety, and easier maintenance because only predefined values are allowed, preventing invalid inputs and simplifying updates.

Architecture

VehicleType

┌───────────┐

↓

ParkingSpot Class

- spot_type: SpotType

↓

Vehicle Class

- vehicle_type: VehicleType

This diagram shows enums VehicleType and SpotType defining fixed categories. These enums are used as types in Vehicle and ParkingSpot classes to ensure only valid types are assigned.

Trade-offs

✓ Pros

→

Prevents invalid or inconsistent category values by restricting to predefined constants.

→

Improves code readability by giving meaningful names to category values.

→

Simplifies maintenance when adding or changing categories in one place.

→

Enables type checking and better IDE support for autocompletion and error detection.

✗ Cons

→

Adds slight verbosity compared to using plain strings or integers.

→

Requires understanding of enum syntax and usage, which may be new to beginners.

→

May require conversion when interacting with external systems expecting strings or numbers.

Use enums when your system has fixed sets of categories like vehicle types or spot types that rarely change and must be validated consistently across the codebase.

Avoid enums if the category values are highly dynamic, user-defined, or frequently changing at runtime, as enums are static and require code changes to update.

Real World Examples

Uber

Uber uses enums to represent vehicle types such as 'Sedan', 'SUV', and 'Bike' to ensure consistent categorization across their ride matching and pricing systems.

Airbnb

Airbnb uses enums for property types like 'Apartment', 'House', and 'Bed & Breakfast' to validate listing categories and simplify filtering logic.

LinkedIn

LinkedIn uses enums for job types such as 'Full-time', 'Part-time', and 'Contract' to standardize job posting categories and improve search accuracy.

Code Example

Before using enums, vehicle types are plain strings prone to typos and inconsistent values. After applying enums, only predefined VehicleType values are allowed, preventing errors and improving code clarity.

LLD

### Before: Using plain strings (error-prone)
class Vehicle:
    def __init__(self, vehicle_type):
        self.vehicle_type = vehicle_type  # No validation

car = Vehicle('car')
motorcycle = Vehicle('motorcycle')  # Typo fixed here


### After: Using enums (safe and clear)
from enum import Enum

class VehicleType(Enum):
    CAR = 'car'
    MOTORCYCLE = 'motorcycle'
    TRUCK = 'truck'

class Vehicle:
    def __init__(self, vehicle_type: VehicleType):
        self.vehicle_type = vehicle_type

car = Vehicle(VehicleType.CAR)
motorcycle = Vehicle(VehicleType.MOTORCYCLE)  # No typo possible

# Trying to create with invalid type raises error
# invalid_vehicle = Vehicle('motocycle')  # ValueError: 'motocycle' is not a valid VehicleType

Alternatives

String constants

Uses plain strings instead of fixed enum types, allowing any string value.

Use when: Choose when categories are very dynamic or user-generated and cannot be fixed in code.

Database lookup tables

Stores category types in a database table rather than code enums, allowing runtime updates.

Use when: Choose when categories need to be managed by non-developers or updated frequently without code deployments.

Summary

Using enums prevents bugs caused by invalid or inconsistent category values.

Enums improve code clarity and maintainability by defining fixed sets of named constants.

They are best used when categories are stable and rarely change at runtime.