What is the example of low level design?

An Example of Low-Level Design (LLD) is designing a Library Management System. This example demonstrates how to translate high-level requirements into detailed components, classes, data structures, and their interactions. Below is a comprehensive walkthrough of the LLD for a Library Management System.

Example: Library Management System

1. Understanding the Requirements

Before diving into the design, it's essential to outline the key functionalities and requirements of the Library Management System:

• User Roles:

  • Admin: Manage books, users, and system configurations.
  • Librarian: Handle daily operations like issuing and returning books.
  • Member/User: Search, borrow, and return books.

• Core Functionalities:

  • Book Management: Add, update, delete, and search books.
  • User Management: Register, update, and manage user accounts.
  • Issue/Return Books: Track borrowed books and due dates.
  • Notifications: Alert users about due dates and overdue books.
  • Reporting: Generate reports on book inventory, user activity, etc.

2. High-Level Design (HLD) Overview

At a high level, the system comprises the following modules:

• User Interface (UI): Web or desktop interface for interacting with the system.
• Business Logic Layer (BLL): Handles the core functionalities and processes.
• Data Access Layer (DAL): Manages interactions with the database.
• Database: Stores all persistent data like books, users, transactions.

3. Detailed Low-Level Design (LLD)

A. Classes and Their Responsibilities

1. User

   • Attributes:
     • userId: String
     • name: String
     • email: String
     • password: String
     • role: Role (Enum: ADMIN, LIBRARIAN, MEMBER)
   • Methods:
     • login(email: String, password: String): boolean
     • logout(): void
     • updateProfile(details: UserDetails): void

2. Book

   • Attributes:
     • bookId: String
     • title: String
     • author: String
     • isbn: String
     • publisher: String
     • copiesAvailable: int
   • Methods:
     • addBook(details: BookDetails): void
     • updateBook(details: BookDetails): void
     • searchBook(criteria: SearchCriteria): List<Book>

3. Transaction

   • Attributes:
     • transactionId: String
     • book: Book
     • user: User
     • issueDate: Date
     • dueDate: Date
     • returnDate: Date
   • Methods:
     • issueBook(user: User, book: Book): boolean
     • returnBook(transactionId: String): boolean
     • renewBook(transactionId: String): boolean

4. Notification

   • Attributes:
     • notificationId: String
     • user: User
     • message: String
     • date: Date
     • status: Status (Enum: SENT, PENDING, FAILED)
   • Methods:
     • sendNotification(user: User, message: String): boolean
     • scheduleNotification(user: User, message: String, date: Date): void

5. Report

   • Attributes:
     • reportId: String
     • type: ReportType (Enum: INVENTORY, USER_ACTIVITY, OVERDUE_BOOKS)
     • generatedDate: Date
     • data: String (Could be in PDF, CSV, etc.)
   • Methods:
     • generateReport(type: ReportType): Report
     • exportReport(format: ExportFormat): boolean

6. DatabaseConnection

   • Attributes:
     • connectionString: String
     • connection: Connection
   • Methods:
     • connect(): boolean
     • disconnect(): void
     • executeQuery(query: String): ResultSet
     • executeUpdate(query: String): boolean

B. Data Structures

1. Enum Types

   • Role

     public enum Role {
         ADMIN,
         LIBRARIAN,
         MEMBER
     }

   • Status

     public enum Status {
         SENT,
         PENDING,
         FAILED
     }

   • ReportType

     public enum ReportType {
         INVENTORY,
         USER_ACTIVITY,
         OVERDUE_BOOKS
     }

   • ExportFormat

     public enum ExportFormat {
         PDF,
         CSV,
         EXCEL
     }

2. SearchCriteria

   public class SearchCriteria {
       private String title;
       private String author;
       private String isbn;
       private String publisher;
       // Getters and Setters
   }

3. UserDetails and BookDetails

   public class UserDetails {
       private String name;
       private String email;
       private String password;
       // Getters and Setters
   }
   
   public class BookDetails {
       private String title;
       private String author;
       private String isbn;
       private String publisher;
       private int copiesAvailable;
       // Getters and Setters
   }

C. Module Interactions

1. Issuing a Book

   • Steps:
     1. Member requests to borrow a book via the UI.
     2. Transaction class verifies copiesAvailable in Book.
     3. If available, decrement copiesAvailable and create a new Transaction record.
     4. Schedule a Notification for due date reminder.

2. Returning a Book

   • Steps:
     1. Member initiates return via the UI.
     2. Transaction class updates returnDate and increments copiesAvailable in Book.
     3. Check for any overdue and trigger Notification if necessary.

3. Generating Reports

   • Steps:
     1. Admin/Librarian requests a report via the UI.
     2. Report class gathers relevant data from Book, User, and Transaction classes.
     3. Generate and export the report in the desired format.

D. Design Patterns Applied

1. Singleton Pattern for DatabaseConnection to ensure only one instance manages the database interactions.

   public class DatabaseConnection {
       private static DatabaseConnection instance;
       private Connection connection;
   
       private DatabaseConnection() {
           // Initialize connection
       }
   
       public static synchronized DatabaseConnection getInstance() {
           if (instance == null) {
               instance = new DatabaseConnection();
           }
           return instance;
       }
   
       // Other methods
   }

2. Factory Pattern for creating different types of reports.

   public class ReportFactory {
       public static Report createReport(ReportType type) {
           switch(type) {
               case INVENTORY:
                   return new InventoryReport();
               case USER_ACTIVITY:
                   return new UserActivityReport();
               case OVERDUE_BOOKS:
                   return new OverdueBooksReport();
               default:
                   throw new IllegalArgumentException("Invalid Report Type");
           }
       }
   }

3. Observer Pattern for the Notification system to alert users about due dates.

   public interface Observer {
       void update(String message);
   }
   
   public class User implements Observer {
       private String email;
   
       @Override
       public void update(String message) {
           // Send email notification
       }
   }
   
   public class NotificationSystem {
       private List<Observer> observers = new ArrayList<>();
   
       public void addObserver(Observer observer) {
           observers.add(observer);
       }
   
       public void notifyAllObservers(String message) {
           for (Observer observer : observers) {
               observer.update(message);
           }
       }
   }

E. Error Handling and Edge Cases

1. Issuing a Book When No Copies Available

   • Solution: Validate copiesAvailable before proceeding. If zero, notify the user that the book is currently unavailable.

2. Returning a Book Not Issued

   • Solution: Check if the transaction exists and is valid before processing the return. Handle cases where the transaction might be missing or already processed.

3. Database Connection Failures

   • Solution: Implement retry mechanisms and fallback procedures. Log errors and alert admins if persistent issues occur.

4. Handling Overdue Books

   • Solution: Automatically flag transactions where dueDate has passed without a returnDate and trigger notifications or fines.

F. Concurrency Considerations

1. Synchronized Methods

   • Ensure thread-safe operations when multiple users attempt to issue or return the same book simultaneously.

   public synchronized boolean issueBook(User user, Book book) {
       if(book.getCopiesAvailable() > 0) {
           book.decrementCopies();
           // Create Transaction
           return true;
       }
       return false;
   }

2. Optimistic Locking

   • Use versioning in the Book records to prevent lost updates when concurrent transactions occur.

G. Database Schema Design

1. Users Table

   Column	Data Type	Constraints
   user_id	VARCHAR	PRIMARY KEY
   name	VARCHAR	NOT NULL
   email	VARCHAR	UNIQUE, NOT NULL
   password	VARCHAR	NOT NULL
   role	ENUM	('ADMIN', 'LIBRARIAN', 'MEMBER')

2. Books Table

   Column	Data Type	Constraints
   book_id	VARCHAR	PRIMARY KEY
   title	VARCHAR	NOT NULL
   author	VARCHAR	NOT NULL
   isbn	VARCHAR	UNIQUE, NOT NULL
   publisher	VARCHAR
   copies_available	INT	NOT NULL

3. Transactions Table

   Column	Data Type	Constraints
   transaction_id	VARCHAR	PRIMARY KEY
   book_id	VARCHAR	FOREIGN KEY REFERENCES Books(book_id)
   user_id	VARCHAR	FOREIGN KEY REFERENCES Users(user_id)
   issue_date	DATE	NOT NULL
   due_date	DATE	NOT NULL
   return_date	DATE

4. Notifications Table

   Column	Data Type	Constraints
   notification_id	VARCHAR	PRIMARY KEY
   user_id	VARCHAR	FOREIGN KEY REFERENCES Users(user_id)
   message	TEXT	NOT NULL
   date	DATE	NOT NULL
   status	ENUM	('SENT', 'PENDING', 'FAILED')

5. Reports Table

   Column	Data Type	Constraints
   report_id	VARCHAR	PRIMARY KEY
   type	ENUM	('INVENTORY', 'USER_ACTIVITY', 'OVERDUE_BOOKS')
   generated_date	DATE	NOT NULL
   data	TEXT

H. Security Considerations

1. Authentication

   • Implement secure login mechanisms using hashed passwords (e.g., bcrypt).
   • Use JWT (JSON Web Tokens) for session management.

2. Authorization

   • Role-based access control (RBAC) to restrict functionalities based on user roles (Admin, Librarian, Member).

3. Data Protection

   • Encrypt sensitive data in transit using TLS.
   • Encrypt sensitive data at rest in the database.

I. Performance and Scalability Enhancements

1. Caching

   • Implement caching for frequently accessed data like book details using in-memory caches (e.g., Redis).

2. Load Balancing

   • Distribute incoming requests across multiple servers to ensure no single server becomes a bottleneck.

3. Database Optimization

   • Index frequently queried fields (e.g., isbn, email) to speed up search operations.
   • Use database replication for read-heavy operations.

4. Asynchronous Processing

   • Handle notifications and report generation asynchronously to improve responsiveness.

4. Implementation Example: Issuing a Book

Below is a simplified Java implementation of the issueBook method within the Transaction class, demonstrating how different components interact.

public class Transaction {
    private String transactionId;
    private Book book;
    private User user;
    private Date issueDate;
    private Date dueDate;
    private Date returnDate;

    private NotificationSystem notificationSystem;
    private DatabaseConnection dbConnection;

    public Transaction() {
        this.notificationSystem = NotificationSystem.getInstance();
        this.dbConnection = DatabaseConnection.getInstance();
    }

    public synchronized boolean issueBook(User user, Book book) {
        if (book.getCopiesAvailable() > 0) {
            book.decrementCopies();
            this.transactionId = UUID.randomUUID().toString();
            this.book = book;
            this.user = user;
            this.issueDate = new Date();
            Calendar cal = Calendar.getInstance();
            cal.setTime(this.issueDate);
            cal.add(Calendar.DAY_OF_MONTH, 14); // 2 weeks due date
            this.dueDate = cal.getTime();

            // Save transaction to database
            String query = "INSERT INTO Transactions (transaction_id, book_id, user_id, issue_date, due_date) " +
                           "VALUES ('" + this.transactionId + "', '" + book.getBookId() + "', '" + user.getUserId() + "', '" +
                           this.issueDate + "', '" + this.dueDate + "')";
            boolean isSaved = dbConnection.executeUpdate(query);

            if (isSaved) {
                // Send due date notification
                String message = "Dear " + user.getName() + ", your borrowed book '" + book.getTitle() +
                                 "' is due on " + this.dueDate + ". Please return it by the due date to avoid fines.";
                notificationSystem.sendNotification(user, message);
                return true;
            } else {
                // Rollback if transaction save fails
                book.incrementCopies();
            }
        }
        return false;
    }

    // Other methods...
}

Explanation:

1. Synchronization: The issueBook method is synchronized to prevent race conditions when multiple users attempt to issue the same book simultaneously.

2. Transaction Creation:

   • Checks if the book has available copies.
   • Decrements the copiesAvailable count.
   • Generates a unique transactionId and sets the issueDate and dueDate.

3. Database Interaction:

   • Inserts the new transaction into the Transactions table.
   • If the database operation fails, it rolls back by incrementing the copiesAvailable.

4. Notification:

   • Sends a due date notification to the user using the NotificationSystem.

5. Best Practices Demonstrated in the Example

1. Object-Oriented Principles:

   • Encapsulation: Each class manages its own data and behavior.
   • Single Responsibility: Each class has a clear responsibility (e.g., User handles user-related operations).

2. Design Patterns:

   • Singleton: DatabaseConnection and NotificationSystem use the Singleton pattern to ensure only one instance manages these resources.
   • Factory: ReportFactory creates different types of reports based on the requested ReportType.

3. Error Handling:

   • Implements rollback mechanisms if the transaction cannot be saved to the database.
   • Validates resource availability before proceeding with operations.

4. Scalability and Performance:

   • Uses caching and load balancing strategies to optimize performance (not fully shown in the example but considered in the design).
   • Employs asynchronous notifications to prevent blocking the main transaction flow.

5. Security:

   • Ensures secure user authentication and role-based access control (details abstracted for brevity).

6. Conclusion

This Library Management System example showcases how Low-Level Design involves detailing classes, their attributes, methods, interactions, and applying design patterns to create a robust and maintainable system. By breaking down the system into manageable components and addressing aspects like error handling, concurrency, and security, you demonstrate a thorough understanding of designing complex applications.

Preparation Tips for LLD Interviews:

• Practice Common Scenarios: Familiarize yourself with typical LLD interview questions like designing a parking lot system, a URL shortener, or a cache system.
• Diagramming Skills: Be comfortable sketching class diagrams, sequence diagrams, and other UML diagrams to visualize your design.
• Explain Your Thought Process: Clearly articulate your reasoning behind design choices, data structure selections, and pattern applications.
• Optimize and Justify: Always consider performance, scalability, and maintainability, and be prepared to discuss trade-offs.

For further preparation, explore resources like Grokking the System Design Interview and System Design Primer The Ultimate Guide, which offer in-depth insights and practice problems to enhance your design skills.