Class design (Book, Member, Librarian, Loan) in LLD - System Design Exercise

Design: Library Management System

Design the core classes and their relationships for managing books, members, librarians, and loans. Exclude UI and database implementation details.

Functional Requirements

FR1: Allow members to borrow and return books

FR2: Track which books are loaned and their due dates

FR3: Librarians can add, update, and remove books

FR4: Members can view their loan history and current loans

FR5: Support multiple members and librarians

Non-Functional Requirements

NFR1: Handle up to 10,000 members and 100,000 books

NFR2: Operations should be efficient for quick lookups

NFR3: Ensure data consistency for loans and returns

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Book class with details like title, author, ISBN

Member class with personal info and loan records

Librarian class with permissions to manage books

Loan class to represent borrowing details

Design Patterns

Association between Member and Loan

Inheritance if Librarian and Member share common user attributes

Encapsulation to protect class data

Use of methods to perform borrow and return actions

Reference Architecture

Library Management System Classes

+----------------+       +----------------+       +----------------+
|     Book       |       |    Member      |       |   Librarian    |
+----------------+       +----------------+       +----------------+
| - bookId       |       | - memberId     |       | - librarianId  |
| - title        |       | - name         |       | - name         |
| - author       |       | - email        |       | - email        |
| - ISBN         |       | - loans: List  |       |                |
| - isAvailable  |       +----------------+       +----------------+
+----------------+               |                          |
         ^                       |                          |
         |                       |                          |
         |                       |                          |
         |                       |                          |
         |                       |                          |
         |                       |                          |
         |                       v                          v
         |               +----------------+
         |               |      Loan      |
         |               +----------------+
         |               | - loanId       |
         |               | - book: Book   |
         |               | - member: Member|
         |               | - loanDate     |
         |               | - dueDate      |
         |               | - returnDate   |
         |               +----------------+

Components

Book

Class

Represents a book with details and availability status

Member

Class

Represents a library member with personal info and loan records

Librarian

Class

Represents a librarian who manages books

Loan

Class

Represents a loan record linking a book and a member with dates

Request Flow

1. Member requests to borrow a book

2. System checks if the book is available

3. If available, create a Loan object with loanDate and dueDate

4. Mark the book as not available

5. Add the loan to the member's loan list

6. Member returns the book

7. System updates the Loan with returnDate

8. Mark the book as available again

9. Librarian adds or updates book details in the system

Database Schema

Entities: - Book(bookId PK, title, author, ISBN, isAvailable) - Member(memberId PK, name, email) - Librarian(librarianId PK, name, email) - Loan(loanId PK, bookId FK, memberId FK, loanDate, dueDate, returnDate) Relationships: - One Member can have many Loans (1:N) - One Book can have many Loans over time (1:N) - Loan links one Book and one Member

Scaling Discussion

Bottlenecks

Searching books by title or author becomes slow with many books

Tracking loans for many members can cause performance issues

Concurrent borrow requests for the same book may cause conflicts

Solutions

Use indexing on book attributes for fast search

Paginate loan records and cache frequent queries

Implement locking or transactions to prevent double borrowing

Interview Tips

Time: Spend 10 minutes understanding requirements and clarifying scope, 20 minutes designing classes and relationships, 10 minutes discussing scaling and improvements, 5 minutes summarizing.

Explain class responsibilities clearly

Show relationships and how they support requirements

Discuss how to handle book availability and loan tracking

Mention data consistency and concurrency handling

Highlight how the design can scale with more users and books