HLD and LLD differences with explanations, real life product examples using JavaScript

 

High-Level Design (HLD) vs Low-Level Design (LLD) with Examples and JavaScript Concepts

In software engineering, High-Level Design (HLD) and Low-Level Design (LLD) are two distinct phases of the design process, each serving a different purpose in creating software systems. Let’s break down their differences and provide real-life product examples with explanations.

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1. High-Level Design (HLD)

Definition:

• HLD focuses on the system architecture and design of the entire system or software product. It is abstract and deals with components, their relationships, and the flow of data between them.
• HLD is typically done at the beginning of the design process, providing an overview of the solution, including key system components, modules, and technologies involved.

Purpose:

• To identify the system's components (such as servers, databases, third-party services) and define how they interact with each other.
• To provide a clear roadmap of how different subsystems will integrate and operate together.

Key Characteristics:

• Architecture: Involves defining the system's structure.
• Components and Modules: Identifying major system parts.
• Database Design: High-level database schema (tables, relationships, etc.)
• Technology Stack: Choosing technologies, platforms, and tools.
• Communication Protocols: Deciding how components will interact (e.g., REST APIs, WebSockets).
• Non-Functional Requirements: Security, scalability, performance considerations.

Real-life Product Example: Let’s say you're building an online e-commerce platform.

• System Components:
  • Frontend: React or Angular for the user interface.
  • Backend: Node.js with Express for the RESTful API.
  • Database: MongoDB for product catalog and customer data storage.
  • Authentication Service: OAuth or JWT for user login.
  • Payment Service: Integration with third-party services like Stripe or PayPal.
  • Notification System: Email and SMS notifications using third-party services like SendGrid.

In HLD, you'd describe these components and how they interact. For example:

• Frontend (React) communicates with Backend (Node.js/Express) using HTTP REST APIs.
• The Backend interacts with the Database (MongoDB) to fetch product details.
• When a user makes a purchase, the Backend communicates with a Payment Gateway (e.g., Stripe) and sends a confirmation via Notification Service.

plaintext
User <--> Frontend (React) <--> Backend (Node.js) <--> Database (MongoDB)
                                  |                          |
                                  V                          V
                                Payment (Stripe)          Notification Service (SendGrid)

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2. Low-Level Design (LLD)

Definition:

• LLD dives deeper into the specific details of each module/component defined in HLD. It is a more concrete, detailed design focused on how each component will be implemented, including specific algorithms, class definitions, functions, data structures, and error handling.

Purpose:

• To define how each individual component or module will function in terms of actual code.
• It provides a detailed blueprint for developers to implement the system, ensuring that all components interact correctly.

Key Characteristics:

• Classes and Methods: Specific classes, objects, and methods needed for implementation.
• Algorithms: Detailed algorithms for solving specific subproblems.
• Data Structures: Specific data structures used within components.
• Flow of Data: Detailed flow of data within individual components or between functions.
• Error Handling and Logging: How errors are caught and logged.

Real-life Product Example (continuation from HLD): Let’s continue with the e-commerce platform and break it down into LLD components.

LLD for Authentication Service (JWT-based)

• Authentication Module: This is responsible for verifying the user's credentials (username/password).
• You would define:
  • Login function: A method that checks if the provided credentials match those in the database.
  • JWT Token Generation: After successful login, the system generates a JWT token to authenticate subsequent requests.

javascript
// Example of LLD for Authentication Service

const jwt = require('jsonwebtoken');
const bcrypt = require('bcrypt');

// User login function
function login(username, password) {
  // Fetch user from the database (simulate database call)
  const user = getUserFromDatabase(username);
  if (!user) {
    throw new Error('User not found');
  }

  // Compare passwords
  const isPasswordValid = bcrypt.compareSync(password, user.passwordHash);
  if (!isPasswordValid) {
    throw new Error('Invalid password');
  }

  // Generate JWT token
  const token = jwt.sign({ userId: user.id }, 'secretKey', { expiresIn: '1h' });
  return token;
}

// Simulated database call (for example purposes)
function getUserFromDatabase(username) {
  // In a real-world scenario, this would query the database
  const users = [
    { id: 1, username: 'alice', passwordHash: bcrypt.hashSync('password123', 10) },
    { id: 2, username: 'bob', passwordHash: bcrypt.hashSync('password456', 10) },
  ];

  return users.find(user => user.username === username);
}

• JWT Verification: Define how the backend will verify JWT tokens sent by the frontend for subsequent requests.

javascript
// JWT verification middleware
function verifyToken(req, res, next) {
  const token = req.headers['authorization'];
  if (!token) {
    return res.status(403).send('Token is required');
  }

  jwt.verify(token, 'secretKey', (err, decoded) => {
    if (err) {
      return res.status(403).send('Invalid or expired token');
    }
    req.userId = decoded.userId;
    next(); // Proceed to the next middleware/handler
  });
}

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Key Differences Between HLD and LLD

Aspect	High-Level Design (HLD)	Low-Level Design (LLD)
Focus	System architecture, components, and module interactions.	Detailed implementation of components, methods, and algorithms.
Level of Detail	Abstract, focuses on overall structure.	Detailed, specific to the inner workings of each component.
Audience	Stakeholders, architects, and senior developers.	Developers who will implement the code.
Time of Creation	Created in the early stages of development.	Created after HLD, during or just before implementation.
Example (E-commerce)	Identifying Frontend, Backend, Payment Gateway components.	Writing code for the Authentication service or Payment module.
Technologies Involved	High-level technologies (e.g., React, Node.js, MongoDB).	Specific libraries, functions, or algorithms (e.g., JWT, bcrypt).

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Real-World Example with JavaScript

Example: Build a Chat Application

HLD:

• Components:
  • Frontend: React.js for UI components (chat window, message list).
  • Backend: Node.js and Express.js for the chat API.
  • Database: MongoDB for storing user messages.
  • WebSocket Service: To manage real-time communication between users.
  • Authentication: JWT for login sessions.
  • Message Storage: Store and retrieve messages from MongoDB.

plaintext
User <--> Frontend (React) <--> Backend (Node.js + Express) <--> Database (MongoDB)
                             <--> WebSocket Service (Socket.io)

LLD (Message Sending Module):

• Message Class (Define message schema)

javascript
const mongoose = require('mongoose');

const messageSchema = new mongoose.Schema({
  sender: String,
  recipient: String,
  content: String,
  timestamp: { type: Date, default: Date.now },
});

const Message = mongoose.model('Message', messageSchema);

module.exports = Message;

• Socket.io Integration (Real-time messaging)

javascript
const socketIo = require('socket.io');

const io = socketIo(server);

io.on('connection', socket => {
  console.log('User connected');
  
  socket.on('sendMessage', (message) => {
    // Save message to the database
    const newMessage = new Message(message);
    newMessage.save().then(() => {
      io.emit('newMessage', message);  // Broadcast message to all connected clients
    });
  });

  socket.on('disconnect', () => {
    console.log('User disconnected');
  });
});

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Conclusion:

• HLD gives you the big picture: how components interact, what technologies to use, and how modules fit together.
• LLD focuses on the details: how you write code, implement features, handle errors, and design algorithms.

In real-life products, these designs ensure that your system is scalable, maintainable, and well-structured. For example, in the chat application, the HLD describes how components work together, while LLD dives into specific implementations like message handling and WebSocket integration.