HLD vs. LLD: Understanding High-Level and Low-Level Design in Software Engineering

Introduction

When designing a software system, two critical phases guide the development process: High-Level Design (HLD) and Low-Level Design (LLD). These ensure a well-structured, efficient, and scalable application. But what exactly do they mean, and why are they essential?

Think of It Like Planning a Trip

Imagine you and your friends are planning a weekend trip. You decide on the destinations, the trip duration, and the transportation method. This is similar to HLD—it provides an overarching plan.

But is that enough? You also need to arrange accommodation, set departure times, and budget your expenses. This is where LLD comes in—it ensures every small detail is accounted for so the trip runs smoothly.

Similarly, every software development project requires both HLD and LLD—one offers strategic direction, while the other focuses on execution.

What is High-Level Design (HLD)?

The Big Plan

HLD serves as the architectural blueprint of a software system. It provides a broad overview of the system, helping stakeholders and developers understand its core structure.

Key Components of HLD

• System Modules: Identifies major components like databases, microservices, and APIs.
• Component Interactions: Defines how different modules communicate.
• Technology Stack: Specifies programming languages, frameworks, and cloud services (e.g., Python, Java, AWS).
• Design Patterns and Principles: Establishes best practices for maintainability and scalability.

Example of HLD in a Food Delivery App

For a food delivery platform, the HLD includes:

• User System (Handles user authentication and profile management).
• Restaurant System (Stores restaurant information and menus).
• Order System (Manages order placement and payments).
• Delivery System (Assigns delivery agents and tracks orders).

   

HLD answers “What needs to be built?” but not “How will it be implemented?”

What is Low-Level Design (LLD)?

The Small Details

Once HLD is finalized, LLD dives into the specifics, detailing the implementation of each component.

Key Components of LLD

• Class Diagrams and Functions: Defines objects, methods, and their interactions.
• Database Schema: Specifies tables, columns, and relationships.
• API Specifications: Details endpoints, request parameters, and response structures.
• Code Logic: Outlines data flow and algorithmic operations.

Example of LLD in a Food Delivery App

For the Order System, the LLD defines:

• A PaymentService class with methods like processPayment() and refundPayment().
• A database schema with an orders table having columns such as order_id, user_id, and amount.
• API endpoints such as:
  • POST /orders (For order placement).
  • GET /orders/{id} (For order tracking).

     

LLD answers “How will it be built?” with step-by-step implementation guidelines.