LLD Introduction

LLD vs HLD

Understanding the difference between Low-Level Design and High-Level Design, and when to apply each in system design.

8 min readEssential

1Overview

Real-World Analogy

Building a House

HLD is the architect's blueprint: how many floors, room layout, where plumbing goes. LLD is the detailed construction plan: exact measurements, which screws to use, wiring diagrams.

Writing a Book

HLD is the outline: chapters, main plot points, character arcs.LLD is writing each sentence, choosing words, formatting paragraphs.

High-Level Design (HLD) focuses on system architecture - components, how they communicate, technology choices, and scaling strategies.

Low-Level Design (LLD) focuses on implementation details - classes, methods, data structures, algorithms, and design patterns.

2Key Differences

Aspect	HLD (High-Level)	LLD (Low-Level)
Focus	System architecture	Code structure
Abstraction	High (boxes and arrows)	Low (classes and methods)
Diagrams	System diagrams, flowcharts	UML class/sequence diagrams
Concerns	Scalability, availability, latency	Maintainability, extensibility, testability
Technology	Databases, message queues, CDNs	Classes, interfaces, design patterns
Who creates	System architects	Software engineers
Interview examples	Design Twitter, Design URL Shortener	Design Parking Lot, Design Chess

3High-Level Design (HLD)

HLD Covers:

System components and their interactions
Database choices (SQL vs NoSQL)
Caching strategies (Redis, Memcached)
Message queues (Kafka, RabbitMQ)
Load balancing approaches

API design (REST, GraphQL, gRPC)
Scaling strategies (horizontal/vertical)
Data partitioning and sharding
CDN and content delivery
Failure handling and redundancy

Example: URL Shortener HLD

1. API Gateway receives requests

2. Application servers handle URL creation/redirection

3. Distributed key-value store for URL mapping

4. Cache layer for hot URLs

5. Analytics service for click tracking

6. CDN for faster global access

4Low-Level Design (LLD)

LLD Covers:

Class design and relationships
Interface definitions
Design patterns (Singleton, Factory, etc.)
SOLID principles application
Data structures for entities

Method signatures and logic
Error handling strategies
Concurrency considerations
UML diagrams (Class, Sequence)
Code organization and modules

Example: Parking Lot LLD

1. Classes: ParkingLot, Floor, ParkingSpot, Vehicle, Ticket

2. Interfaces: ParkingStrategy, PaymentProcessor

3. Patterns: Singleton for ParkingLot, Strategy for spot allocation

4. Enums: VehicleType, SpotType, TicketStatus

5. Relationships: ParkingLot has many Floors (Composition)

5Interview Context

HLD Interview Problems

Design Twitter/Instagram
Design URL Shortener
Design Netflix/YouTube
Design Uber/Lyft
Design WhatsApp
Design E-commerce Platform

LLD Interview Problems

Design Parking Lot
Design Chess Game
Design Elevator System
Design Library Management
Design ATM Machine
Design Tic-Tac-Toe

Interview Strategy

HLD: Start with requirements, estimate scale, design components, discuss tradeoffs
LLD: Clarify requirements, identify entities, design classes, apply patterns, discuss extensibility
Both: Ask clarifying questions, think out loud, consider edge cases

6Key Takeaways

1HLD = System architecture (what components, how they connect)

2LLD = Code design (classes, methods, patterns)

3HLD concerns: scalability, availability, performance

4LLD concerns: maintainability, extensibility, testability

5HLD uses system diagrams; LLD uses UML class/sequence diagrams.

6Both are essential skills for senior engineering roles.