Low-Level Design (LLD): What It Covers and a Practical Example in C#

When it comes to software development, Low-Level Design (LLD) is the step where high-level architectural ideas are converted into detailed, implementable designs. It’s where abstract concepts meet the reality of coding, helping developers build systems that are maintainable, efficient, and scalable.

In this blog, we’ll explore what LLD encompasses, its importance, and walk through a practical example in C# to bring these concepts to life.

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What Does LLD Cover?

Low-Level Design focuses on:

1. Class Design:
   • Defining attributes, methods, and relationships between classes.
2. Object Interactions:
   • Explaining how objects collaborate to fulfill functionality.
3. Algorithm Design:
   • Writing detailed logic for processing data.
4. Database Schema:
   • Mapping data needs into relational tables or NoSQL structures.
5. Validation and Error Handling:
• Planning for edge cases, exceptions, and input validation.

How Does LLD Differ from HLD?

Practical Example: Online Library System

Scenario

You’re tasked with designing an Online Library System where users can:

1. Borrow books.
2. Return books.
3. View available books.

We’ll use LLD principles to design this system.

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Step 1: Requirements Breakdown

1. Functional Requirements:

   • Users can view the list of books.
   • Users can borrow books if available.
   • Users can return borrowed books.

2. Non-Functional Requirements:

   • Ensure accurate availability status.
   • Handle edge cases (e.g., returning a book not borrowed).

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Step 2: Class Design

Class Diagram

The main classes include:

• Book: Represents a book.
• User: Represents a library user.
• Library: Handles book inventory and user operations.

Step 3: LLD Implementation in C#

1. The Book Class

public class Book
{
    public int Id { get; set; }
    public string Title { get; set; }
    public string Author { get; set; }
    public bool IsAvailable { get; set; } = true;

    public Book(int id, string title, string author)
    {
        Id = id;
        Title = title;
        Author = author;
    }
}

2. The User Class

public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<Book> BorrowedBooks { get; set; } = new List<Book>();

    public User(int id, string name)
    {
        Id = id;
        Name = name;
    }
}

3. The Library Class

public class Library
{
    private List<Book> _books = new List<Book>();

    public Library()
    {
        // Preload books into the library
        _books.Add(new Book(1, "The Great Gatsby", "F. Scott Fitzgerald"));
        _books.Add(new Book(2, "1984", "George Orwell"));
        _books.Add(new Book(3, "To Kill a Mockingbird", "Harper Lee"));
    }

    public List<Book> GetAvailableBooks()
    {
        return _books.Where(book => book.IsAvailable).ToList();
    }

    public string BorrowBook(User user, int bookId)
    {
        var book = _books.FirstOrDefault(b => b.Id == bookId && b.IsAvailable);
        if (book == null)
        {
            return "Book is not available.";
        }

        book.IsAvailable = false;
        user.BorrowedBooks.Add(book);
        return $"{user.Name} borrowed \"{book.Title}\".";
    }

    public string ReturnBook(User user, int bookId)
    {
        var book = user.BorrowedBooks.FirstOrDefault(b => b.Id == bookId);
        if (book == null)
        {
            return "This book was not borrowed by the user.";
        }

        book.IsAvailable = true;
        user.BorrowedBooks.Remove(book);
        return $"{user.Name} returned \"{book.Title}\".";
    }
}

4. Client Code

public class Program
{
    public static void Main()
    {
        var library = new Library();
        var user = new User(1, "Alice");

        Console.WriteLine("Available Books:");
        foreach (var book in library.GetAvailableBooks())
        {
            Console.WriteLine($"{book.Id}. {book.Title} by {book.Author}");
        }

        Console.WriteLine("\nBorrowing a Book:");
        Console.WriteLine(library.BorrowBook(user, 1));

        Console.WriteLine("\nAvailable Books After Borrowing:");
        foreach (var book in library.GetAvailableBooks())
        {
            Console.WriteLine($"{book.Id}. {book.Title} by {book.Author}");
        }

        Console.WriteLine("\nReturning a Book:");
        Console.WriteLine(library.ReturnBook(user, 1));

        Console.WriteLine("\nAvailable Books After Returning:");
        foreach (var book in library.GetAvailableBooks())
        {
            Console.WriteLine($"{book.Id}. {book.Title} by {book.Author}");
        }
    }
}

Output

Available Books:
1. The Great Gatsby by F. Scott Fitzgerald
2. 1984 by George Orwell
3. To Kill a Mockingbird by Harper Lee

Borrowing a Book:
Alice borrowed "The Great Gatsby".

Available Books After Borrowing:
2. 1984 by George Orwell
3. To Kill a Mockingbird by Harper Lee

Returning a Book:
Alice returned "The Great Gatsby".

Available Books After Returning:
1. The Great Gatsby by F. Scott Fitzgerald
2. 1984 by George Orwell
3. To Kill a Mockingbird by Harper Lee

Key Elements Highlighted in LLD

1. Attributes and Methods:
   • Clear definitions for Book, User, and Library.
2. Object Interactions:
   • Library manages operations involving User and Book.
3. Edge Case Handling:
   • Borrowing an unavailable book.
   • Returning a book not borrowed.
4. Scalability:
   • The design can be extended with additional features like late fees or reservations.

Conclusion

Low-Level Design (LLD) bridges the gap between high-level ideas and actual implementation. By focusing on classes, relationships, and workflows, LLD ensures your system is ready for real-world use. The Online Library System example demonstrates how to apply LLD principles in C# to create a functional, maintainable application.

What’s your take on LLD? Let us know how you approach detailed designs in your projects!