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LLD vs HLD

Last Updated: December 16, 2025

Ashish

Ashish Pratap Singh

In software engineering, building a complex system is like constructing a city.

You wouldn't start by laying bricks for a single house without a city plan. You first need to decide where the residential areas, commercial zones, power grids, and roads will go.

This city plan is your High-Level Design (HLD).

Once the city plan is approved, an architect takes a single plot of land in a residential zone and designs the detailed blueprint for a house specifying the number of rooms, the plumbing, the electrical wiring, and the materials to be used.

This detailed house blueprint is your Low-Level Design (LLD).

Both are essential, but they operate at different levels of abstraction and serve different purposes.

In this chapter, we will take a deeper look at their differences.

What is High-Level Design (HLD)?

High-Level Design (HLD) defines the architecture of the system.

It answers the question:

“How should the system be structured, and how will its major components interact?”

The focus here is on the what, not the how.

It answers questions like:

  • What are the major components or microservices? (e.g., User Service, Payment Service, Notification Service, Product Catalog).
  • How will these components communicate? (e.g., via REST APIs, gRPC, or a message queue like RabbitMQ or Kafka).
  • What technology stack will be used? (e.g., Java vs. Python, SQL vs. NoSQL database).
  • How will the system handle scalability, reliability, and availability? (e.g., using load balancers, database replication, CDNs).
  • What are the third-party integrations? (e.g., Stripe for payments, AWS S3 for storage).

The output of HLD is a set of architectural diagrams, data flow diagrams, and technology choices that define the system's skeleton.

What is Low-Level Design (LLD)?

LLD zooms in on a single component or module and translates the abstract architectural concepts into concrete, implementable details that developers can code directly.

It’s where you decide the internal structure of a service — the classes, methods, data models, design patterns, and relationships.

For a single module, it answers questions like:

  • What are the specific classes, and what are their responsibilities?
  • What are the attributes and methods of each class?
  • How do these classes relate to each other (inheritance, composition)?
  • What design patterns are most suitable (e.g., Factory, Singleton, Strategy)?
  • What are the specific method signatures, including parameters, return types, and exceptions?

Key Differences: HLD vs. LLD at a Glance

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Aspect

High-Level Design (HLD)

Low-Level Design (LLD)

Focus

What components exist

How each component is built

Audience

Architects, stakeholders

Engineers, developers

Abstraction

System-level

Module/class-level

Artifacts

System architecture diagrams, tech stack choices

Class diagrams, interaction diagrams, method definitions

Example

"We’ll use a microservices architecture with services for users, orders, and payments"

"The OrderService will use a PaymentProcessor interface with two implementations":

  • StripeProcessor
  • RazorpayProcessor

How HLD and LLD Work Together

HLD and LLD are not alternatives. They're sequential steps in the design process.

The Flow

  1. Requirements define what the system should do
  2. HLD breaks the system into components
  3. LLD designs each component's internals
  4. Code implements the LLD

Now that you understand what Low-Level Design (LLD) is and how it connects to High-Level Design (HLD), lets explore the different types of LLD interviews at companies.