PrepKitBeta
LLD›LLD Case Studies›Design a Library Management System

Design a Library Management System

LLD Case Studies

This question tests your ability to model a system with different types of users and complex rules. The goal is to design a system to manage the catalog of a library and the borrowing of books by its members.

1. Clarify Requirements

  • What are the primary functions? The system should be able to add new books, search for books, and manage user accounts.
  • Who are the users? There are two types of users: Librarian (an admin) and Member.
  • What can a Librarian do? Add/remove books, add/remove members, and view all borrowed books.
  • What can a Member do? Search for books, borrow books, and return books.
  • What are the rules for borrowing? A member can borrow up to a certain limit (e.g., 5 books). A book cannot be borrowed if it's already checked out.
  • How is the book catalog organized? Books can be searched by title, author, or subject.
  • Is there a system for fines? Yes, if a book is returned after its due date, a fine is calculated.

2. Identify the Core Classes and Objects

  • Library: The main class that manages the catalog and user accounts.
  • Book: Represents a single book.
  • BookItem: Represents a specific copy of a book (since a library can have multiple copies). This is a key distinction.
  • User: An abstract class for users.
  • Librarian, Member: Subclasses of User.
  • Catalog: A class dedicated to searching the book collection.
  • BookLending: A record that tracks which member has borrowed which BookItem and when it's due.

3. Design the System - Class Diagram

LibraryCatalogBookBookItemBookLendingFine<<abstract>>UserLibrarianMember11*11*1*11..*10..*110..1

4. Key Design Decisions & Implementation Details

Book vs. BookItem:

This is a critical design choice. A Book is a conceptual entity (e.g., "The Lord of the Rings"), while a BookItem is a physical copy with a unique barcode.

public class Book {
    private String title;
    private String author;
    private String ISBN;
    // ...
}

public class BookItem extends Book {
    private String barcode;
    private Date issueDate;
    private BookStatus status; // e.g., AVAILABLE, RESERVED, LOANED
}

The Catalog Class:

This class encapsulates the searching logic, adhering to the Single Responsibility Principle.

public class Catalog implements Searchable {
    private Map<String, List<Book>> bookTitles;
    private Map<String, List<Book>> bookAuthors;

    @Override
    public List<Book> searchByTitle(String title) {
        return bookTitles.get(title);
    }
    // ... other search methods
}

Interview Focus: Key Discussion Points

  • Single Responsibility Principle: Point out how you've separated concerns. The Catalog is only for searching. The Library class is for high-level management. The BookLending class is only for tracking loans.
  • User Roles and Permissions: How would you handle the different permissions for Librarian and Member? You could use a simple role check, or for a more complex system, you could introduce a Permission class or even use the Decorator Pattern to add responsibilities to User objects dynamically.
  • Scalability: If the library has millions of books, a simple Map for the catalog might consume too much memory. You could discuss using a database with proper indexing on the title and author columns to make searching efficient.
  • Concurrency: What if two members try to borrow the last copy of a book at the same time? The borrowBook method in the Library class should be synchronized or use a locking mechanism on the specific BookItem to ensure that only one user can successfully borrow it.
Back to Course