Library Management System

A library management system sounds simple: track books and who has borrowed them. But beneath this simplicity lies a rich domain of interconnected behaviors, business rules, and edge cases that make it an excellent case study for Low-Level Design.

Why Library Management System?

This classic LLD problem appears frequently in technical interviews because it:

• Contains multiple interacting entities with clear responsibilities
• Requires careful state management (book availability, loan status)
• Involves time-based business logic (due dates, fines)
• Demonstrates practical application of design patterns
• Scales naturally from simple to complex requirements

In this module, we'll design a complete Library Management System from first principles, applying everything you've learned about object-oriented design, SOLID principles, and design patterns.

Before writing any code or drawing any diagrams, we must deeply understand the library domain. A library is more than a building with books—it's a carefully orchestrated system of resources, users, policies, and workflows.

The Library Ecosystem:

A modern library operates as a resource sharing system with multiple stakeholders, each with distinct needs and interactions:

• Members: Borrow books, reserve unavailable titles, pay fines, renew loans
• Librarians: Manage inventory, process transactions, handle member issues
• Administrators: Set policies, generate reports, manage the catalog
• The System: Track state, enforce rules, send notifications, calculate penalties

Each stakeholder interaction creates requirements that our system must satisfy.

Functional requirements describe what the system must do—the specific capabilities users expect. We'll organize these by subsystem to maintain clarity and ensure completeness.

Methodology: Use Case Driven Requirements

For each major user action, we identify:

1. Actor: Who performs the action
2. Preconditions: What must be true before the action
3. Trigger: What initiates the action
4. Main Flow: The happy path sequence
5. Alternative Flows: Variations and exceptions
6. Postconditions: What must be true after the action

Non-functional requirements define how the system should behave—the qualities and constraints that shape our design decisions. While functional requirements drive feature implementation, non-functional requirements drive architectural choices.

For a library management system, we focus on requirements that impact real-world usability and maintainability:

Translating NFRs to Design Decisions:

Let's see how non-functional requirements directly influence our class design:

Use cases capture the complete interaction between actors and the system. Let's analyze the most critical use cases in detail, as they directly inform our class design.

Main Success Scenario (Happy Path):

1. Member presents library card and book
2. Librarian scans library card
3. System retrieves member details and validates status
4. Librarian scans book barcode
5. System verifies book is available
6. System checks member hasn't exceeded borrowing limit
7. System creates loan record with:
   • Loan date (today)
   • Due date (based on membership type: 14, 21, or 30 days)
   • Member reference
   • Book copy reference
8. System updates book copy status to 'LOANED'
9. System decrements member's available borrowing slots
10. System prints receipt with book title, due date, renewal info
11. Member takes book

Alternative Flows:

3a. Member account suspended:

• System displays suspension reason
• Transaction cannot proceed
• Librarian advises member to resolve issue

5a. Book is currently on loan to another member:

• System informs this copy is unavailable
• System offers to check other copies or create reservation

5b. Book is reserved by this member:

• System fulfills reservation
• Reservation record marked as fulfilled
• Normal loan created

5c. Book is reserved by another member:

• System prevents checkout
• Librarian explains book is held for another member

6a. Member at borrowing limit:

• System displays current loans
• Transaction cannot proceed
• Librarian advises returning books first

Postconditions:

• Loan record exists in system
• Book copy status is 'LOANED'
• Member's active loan count increased
• Audit trail captures transaction details

Business Rules Identified:

• BR-001: Borrowing limit varies by membership type
• BR-002: Loan period varies by membership type
• BR-003: Suspended members cannot borrow
• BR-004: Reserved books cannot be borrowed by non-reserving members
• BR-005: Each transaction must be traceable

Main Success Scenario:

1. Librarian scans returned book
2. System retrieves active loan record
3. System calculates if book is overdue:
   • If return date > due date → book is overdue
4. System marks loan as returned with return date
5. System checks for reservations on this title
6. System updates book copy status:
   • If reservations exist → status = 'ON_HOLD'
   • If no reservations → status = 'AVAILABLE'
7. System increments member's available borrowing slots
8. If overdue, system calculates and applies fine
9. Receipt printed showing any fine applied

Alternative Flows:

3a. Book is overdue:

• System calculates overdue days
• Fine = overdue_days × daily_fine_rate
• Fine added to member's account
• If fine exceeds threshold, member account suspended

5a. Reservations exist for this book:

• System notifies first member in reservation queue
• Book placed on hold shelf
• Hold expiration timer started (e.g., 7 days)
• If not collected, next reservation is processed

Postconditions:

• Loan record marked as completed
• Book copy status updated appropriately
• Member's borrowing capacity restored
• Fine applied if overdue
• Next reservation holder notified if applicable

Business Rules Identified:

• BR-006: Fine rate varies by membership type
• BR-007: Maximum fine cap may apply per item
• BR-008: Auto-suspension triggered at fine threshold
• BR-009: Reservations processed in FIFO order
• BR-010: Hold period is configurable

In any design exercise—especially interviews—clearly defining what's in scope and out of scope demonstrates senior engineering judgment. It prevents scope creep and shows you can prioritize effectively.

Scope Definition Principles:

1. Start with core functionality that demonstrates design skills
2. Explicitly exclude infrastructure concerns (databases, networking)
3. Note extensions that could be added later
4. Document assumptions to prevent ambiguity

Every design is built on assumptions. Documenting these explicitly prevents misunderstandings and shows thorough thinking. Here are our working assumptions for this Library Management System design:

As we move to design, every class and method should trace back to a requirement. This ensures we build what's needed—nothing more, nothing less.

Here's a preview of how our requirements will map to design elements:

What's Next:

With requirements clearly defined, we'll move to identifying the core entities that will form our domain model. The next page focuses on:

• Extracting entities from requirements (Book, Member, Loan, Librarian, etc.)
• Defining attributes and behaviors for each entity
• Establishing entity boundaries and responsibilities
• Distinguishing entities from value objects