Design a Library Management System

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What is the Library Management System Problem?

Design a Library Management System that allows members to search, borrow, return, and reserve books. The system should manage multiple copies of books, track due dates, calculate fines for overdue books, and notify members when reserved books become available.

In this problem, you’ll design a system that manages thousands of books and members while ensuring fair access and proper inventory tracking through different user roles.

Why This Problem?

Library Management is an excellent LLD case study because it involves:

• Role Management - Handling different privileges for Members and Librarians.
• Copy Management - Distinguishing between a “Book” (metadata) and a “BookItem” (physical copy).
• Strategy Pattern - Flexible fine calculation for different rules and membership tiers.
• Observer Pattern - Notifying members automatically when reserved items are returned.

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Problem Overview

Design a system to manage library operations, focusing on the catalog, member activities, and the automated handling of loans and fines.

Core Requirements

Functional Requirements:

• Search Capabilities: Search by title, author, ISBN, or genre using in-memory data structures.
• Authentication: Registration and authentication for members to access services.
• Lending Flow: Borrow books (max 5) for a period of 14 days; track due dates accurately.
• Inventory Tracking: Manage multiple physical copies (BookItems) with statuses: Available, Reserved, Loaned.
• Fine System: Automatically calculate overdue fines based on configurable strategies (e.g., $1/day).
• Borrowing Limits: Enforce limits on total books and unpaid fine thresholds (e.g., $50) before borrowing.
• Reservations: Reserve unavailable books; notify members on return; expire reservations after 3 days.
• Librarian Tools: Allow staff to add/remove/update books and manage the physical catalog.

Non-Functional Requirements:

• Object-Oriented Design: Clearly separated roles and responsibilities for each class.
• Thread Safety: Ensure borrow, return, and reservation operations handle concurrent access safely.
• Modularity: Easy to add new media types, membership tiers, or notification channels.
• Extensibility: Use patterns like Factory, Observer, and Strategy to make the system extensible.
• Maintainability: Core logic should be easy to test, understand, and maintain over time.

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What’s Expected?

1. System Architecture

The system is organized around the Library service which manages the Catalog and Account data.

2. Key Classes to Design

classDiagram
    class Book {
        -String isbn
        -String title
        -String author
        -List~BookItem~ copies
    }
    
    class BookItem {
        -String barcode
        -BookStatus status
        -RackLocation location
        +checkout()
        +returnItem()
    }
    
    class Account {
        <<abstract>>
        -String id
        -List~BookItem~ borrowed
        +resetPassword()
    }

    class Member {
        +borrowItem(item)
        +reserveItem(item)
    }

    class Librarian {
        +addBookItem(item)
        +blockMember(member)
    }

    Book *-- BookItem
    Account <|-- Member
    Account <|-- Librarian

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System Flow

Borrowing Flow

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Key Design Challenges

1. Book vs. BookItem

If you treat the “Book” and the “Physical Copy” as the same object, you’ll struggle to manage multiple copies of the same title across different library racks.

Solution: Use Composition. The Book class stores metadata (Title, ISBN), while the BookItem class represents the actual physical copy with its own unique barcode and status.

2. Flexible Fine Calculation

A reference book might have a higher late fee than a children’s book, or a “Gold” member might have lower fines.

Solution: Use the Strategy Pattern. Create a FineStrategy interface. This allows the system to calculate fines dynamically based on the book type and member status without changing the core returnBook logic.

3. The Reservation Queue

When a book is popular, multiple people want the next available copy.

Solution: Use a Waitlist (Queue) for each Book. When a BookItem is returned, the system checks the queue and automatically transitions the status to “Reserved” for the first person in line, rather than making it “Available” to the public.

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What You’ll Learn

By solving this problem, you’ll master:

• ✅ Hierarchical Modeling - Separating concepts from physical instances.
• ✅ Role-Based Access Control - Designing systems with different user privileges.
• ✅ Event Notification - Using Observers to bridge the gap between events and users.
• ✅ Workflow Automation - Managing fines and loan lifecycles.

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View Complete Solution & Practice

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• 🤖 AI-powered review of your design and code

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Related Problems

After mastering Library Management, try these similar problems:

• Hotel Management - Managing room inventory and stays.
• Movie Ticket Booking - Handling time-limited reservations.
• Parking Lot - Tracking available slots and vehicle types.