HLD vs LLD: Understanding High-Level and Low-Level Design

You’re preparing for a system design interview. The interviewer asks: “Design a URL shortener.” Do you start drawing boxes labeled “Web Server” and “Database”? Or do you dive into class diagrams and data structures?

The answer depends on whether they want High-Level Design (HLD) or Low-Level Design (LLD). And knowing the difference isn’t just interview trivia—it’s a fundamental skill that separates good engineers from great architects.

Let’s break this down properly.

Where HLD and LLD Fit in Software Development

Before diving into the details, let’s see where these designs fit in the software development lifecycle:

Requirements

Gather and analyze requirements

High-Level Design

Define architecture & components

Low-Level Design

Detail classes, APIs, schemas

Implementation

Write code & unit tests

Deployment

Deploy to production

Requirements

Gather and analyze requirements

High-Level Design

Define architecture & components

Low-Level Design

Detail classes, APIs, schemas

Implementation

Write code & unit tests

Deployment

Deploy to production

Notice how HLD comes first—you need to know the big picture before you can design the details. LLD builds on top of HLD, taking each component and specifying exactly how to implement it.

What is High-Level Design (HLD)?

High-Level Design is like the blueprint of a city. It shows you the major districts, how roads connect them, and where essential services are located. You won’t find the interior layout of individual buildings—that’s not its job.

In software terms, HLD defines:

• System architecture: Microservices vs monolith, cloud vs on-premise
• Major components: What are the key services or modules?
• Data flow: How does information move through the system?
• Technology choices: Which databases, message queues, caching layers?
• External integrations: APIs, third-party services, external systems

A Real HLD Example: URL Shortener

Let’s see what a High-Level Design for a URL shortener looks like:

This diagram tells you:

• We have a load balancer handling traffic distribution
• An API gateway routing requests to appropriate services
• Three main services handling different responsibilities
• Separate databases for different purposes (with caching)

What it doesn’t tell you: How do we generate short URLs? What’s the database schema? How do classes interact? That’s LLD territory.

What HLD Documents Include

Architecture Scalability Technology Stack

• System context diagrams: How the system fits in the broader environment
• Component diagrams: Major modules and their interactions
• Data flow diagrams: How information moves through the system
• Technology decisions: Database choices, frameworks, cloud services
• Non-functional requirements: Scalability, availability, performance targets

What is Low-Level Design (LLD)?

If HLD is the city blueprint, LLD is the architectural plan for each building. It specifies room layouts, electrical wiring, plumbing—every detail needed to actually construct the building.

In software terms, LLD defines:

• Class diagrams: What classes exist and how they relate
• Method signatures: What each function does, its inputs and outputs
• Data structures: How data is organized internally
• Database schemas: Tables, columns, indexes, relationships
• Algorithms: The logic for key operations
• API contracts: Request/response formats, endpoints

The Same URL Shortener in LLD

Now let’s zoom into the URL Shortening Service and design it at a low level:

See the difference? Now we know:

• The exact classes we need to create
• What methods each class has
• How classes depend on each other
• The data model for storing URLs

What LLD Documents Include

Classes Algorithms Schemas

• Class diagrams with methods: Full UML class diagrams
• Sequence diagrams: How objects interact over time
• Database schema: DDL statements, indexes, constraints
• API specifications: OpenAPI/Swagger docs
• Pseudocode or algorithms: For complex business logic
• Error handling: How different errors are handled

HLD vs LLD: The Complete Comparison

Here’s a comprehensive comparison to make the differences crystal clear:

Aspect

HLD

LLD

Abstraction Level

Bird's-eye view of the system

Detailed implementation specifics

Focus

What the system does

How the system does it

Audience

Architects, PMs, stakeholders

Developers and engineers

Artifacts

Architecture diagrams, data flow

Class diagrams, algorithms, schemas

Tools Used

Block diagrams, flowcharts

UML, pseudocode, ERDs

Timing in SDLC

Early planning phase

Before implementation begins

The Analogy That Makes It Click

When to Use HLD vs LLD

Understanding when to apply each type of design is crucial for both interviews and real projects.

HLD in System Design Interviews

When an interviewer asks you to “design Twitter” or “design a rate limiter,” they typically expect HLD first. Here’s the approach:

1

Clarify Requirements (2-3 minutes)

Ask about scale (users, requests/second), features (core vs nice-to-have), and constraints (latency, consistency).

Define High-Level Components (5-7 minutes)

Draw the major services, databases, caching layers. Explain why you chose each component.

Deep Dive on Critical Paths (10-15 minutes)

Pick 1-2 important flows (like posting a tweet) and explain the data flow in detail.

Address Non-Functional Requirements (5 minutes)

Discuss scalability, availability, fault tolerance. How does your design handle 10x traffic?

LLD in Object-Oriented Design Interviews

LLD interviews are different. Here you’re expected to write actual class structures, often with code. The approach:

1

Understand the Problem (2-3 minutes)

What are the core entities? What operations do we need? What are the edge cases?

Identify Classes and Relationships (5-7 minutes)

List the main classes. Define is-a (inheritance) and has-a (composition) relationships.

Define Interfaces First (3-5 minutes)

Start with interfaces/abstract classes. This enables flexibility and testability (Dependency Inversion Principle).

Implement Core Methods (10-15 minutes)

Write the key methods. Apply SOLID principles and relevant design patterns.

Consider Edge Cases (3-5 minutes)

Error handling, concurrency, validation. Show you think about production code.

SOLID Principles in LLD

LLD interviews often test your knowledge of SOLID principles:

Single Responsibility Principle (SRP)

Each class should have one reason to change. In our URL shortener, we have separate classes for:

• URLShortenerService - Business logic for URL operations
• URLRepository - Data persistence
• IDGenerator - Generating unique short codes

If we need to change how IDs are generated, we only touch IDGenerator.

Open/Closed Principle (OCP)

Liskov Substitution Principle (LSP)

Interface Segregation Principle (ISP)

Dependency Inversion Principle (DIP)

Design Patterns You’ll Use in LLD

Here are the most common patterns that appear in LLD interviews:

Pattern	Use Case	Example
Factory	Creating objects without specifying exact class	IDGeneratorFactory.create("base62")
Strategy	Swapping algorithms at runtime	Different URL validation strategies
Observer	Notifying multiple objects of changes	Analytics listeners on URL clicks
Singleton	Ensuring single instance	Configuration manager
Builder	Constructing complex objects step by step	Building URL objects with many optional params
Decorator	Adding behavior dynamically	Adding caching to repository

Common Interview Questions

HLD Questions

• Design a URL shortener (like bit.ly)
• Design a social media feed (like Twitter)
• Design a ride-sharing service (like Uber)
• Design a video streaming platform (like YouTube)
• Design a messaging system (like WhatsApp)
• Design an e-commerce platform (like Amazon)

LLD Questions

How HLD and LLD Work Together

They’re not isolated—they’re two parts of the same design process:

Notice the bidirectional arrows—it’s not a waterfall. LLD might reveal that an HLD decision doesn’t work, requiring adjustments. Good architects iterate between these levels.

Key Takeaways

Practice Resources

Ready to practice? Here are some paths forward:

For HLD:

• Start with classic problems: URL shortener, Twitter, Uber
• Focus on scalability, availability, and trade-offs
• Practice drawing clear architecture diagrams

For LLD:

• Master SOLID principles and common design patterns
• Practice with parking lots, elevators, and chess games
• Write actual code, not just diagrams
• Use our LLD Playground to practice with AI-powered feedback

The best engineers are comfortable at both levels. They can zoom out to see the big picture and zoom in to handle implementation details. That versatility is what separates good engineers from great architects.

Now go practice. Pick a system, design it at both levels, and see how they connect. That’s how you build real design skills.