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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:
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.
By the end of this page, you will be able to: • Systematically gather and categorize requirements for a library system • Distinguish between functional and non-functional requirements • Identify scope boundaries and make intentional design decisions • Document assumptions clearly to prevent scope creep • Map high-level use cases to concrete system behaviors
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:
Each stakeholder interaction creates requirements that our system must satisfy.
| Concept | Definition | Design Relevance |
|---|---|---|
| Catalog | Complete inventory of all library materials | Central data store, search capabilities, metadata management |
| Circulation | The lending and returning workflow | State transitions, rule enforcement, transaction history |
| Reservation | Holding a book for future borrowing | Queue management, notification triggers, priority handling |
| Hold | Temporary claim on a specific copy | Expiration logic, conflict resolution |
| Fine | Monetary penalty for overdue items | Calculation logic, payment processing, accumulation rules |
| Renewal | Extending a loan period | Rule validation, limit enforcement, availability checking |
Notice how each domain concept implies specific behaviors and constraints. 'Circulation' isn't just data movement—it enforces that a book can't be borrowed if already checked out. 'Fine' isn't just a number—it involves calculation rules, accumulation over time, and payment workflows. Understanding these implications early prevents design gaps later.
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:
FR-CAT-01: Add new books to the catalog • System stores book metadata (title, author, ISBN, publisher, publication date) • Multiple copies of the same book are tracked individually • Each copy has a unique barcode/ID
FR-CAT-02: Update book information • Librarians can modify book details • Changes apply to the title record, not individual copies
FR-CAT-03: Remove books from catalog • Books can only be removed if no active loans exist • Removal preserves historical loan records
FR-CAT-04: Search the catalog • Members can search by title, author, ISBN, subject • Search returns books with availability status • Results support pagination and filtering
FR-MEM-01: Register new members • Capture member details (name, address, phone, email) • Assign unique member ID and library card number • Set membership type (Student, Faculty, General Public)
FR-MEM-02: Update member profiles • Members can update contact information • Librarians can modify membership type and status
FR-MEM-03: Deactivate members • Members with overdue books cannot be deactivated • Outstanding fines must be settled first • Historical records retained for audit
FR-MEM-04: View member activity • Show current loans, reservation history, fine history • Display borrowing limits and current utilization
FR-CIR-01: Borrow a book • Member presents library card and book • System validates member status (active, not suspended) • System checks member hasn't exceeded borrowing limit • System checks book is available (not loaned, reserved by others) • Loan record created with due date based on membership type
FR-CIR-02: Return a book • System records return date • If overdue, calculate and apply fine • If book was reserved, notify next member in queue • Update book status to available
FR-CIR-03: Renew a loan • Member can renew if no reservations exist • Maximum renewal count enforced (typically 2-3) • Due date extended from current date, not original due date
FR-CIR-04: Reserve a book • Members can reserve books currently on loan • Reservation queue maintains FIFO order • When book returned, first reserving member is notified • Reservation expires if not collected within hold period
FR-FIN-01: Calculate overdue fines • Fine accrues daily based on membership type rates • Calculation: (days overdue) × (daily rate) • Maximum fine cap per item may apply
FR-FIN-02: Process fine payments • Accept partial or full payment • Update member fine balance • Generate payment receipt
FR-FIN-03: Waive fines • Librarians can waive fines with authorization • Reason for waiver must be recorded • Waiver history maintained for audit
FR-FIN-04: Suspend members for unpaid fines • Auto-suspend when fine exceeds threshold • Suspended members cannot borrow until fine cleared
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:
| Category | Requirement | Design Impact |
|---|---|---|
| Performance | Search returns results within 2 seconds | Index design, query optimization, caching strategy |
| Performance | Checkout/return completes in < 500ms | Transaction efficiency, minimal blocking |
| Scalability | Support 10,000+ members | Data partitioning, efficient queries |
| Scalability | Support 100,000+ books in catalog | Pagination, lazy loading, search indexing |
| Availability | System available during library hours (99%) | Error handling, graceful degradation |
| Reliability | No data loss for transactions | Transaction integrity, audit logging |
| Maintainability | Easy to add new membership types | Open/Closed Principle, strategy patterns |
| Maintainability | Configurable fine rates without code changes | External configuration, separation of concerns |
| Security | Member data protected | Access control, data encryption considerations |
| Auditability | All transactions traceable | Comprehensive logging, immutable history |
In Low-Level Design interviews, you won't implement distributed databases or load balancers. However, demonstrating awareness of these requirements shows senior-level thinking. Design your classes to enable these qualities even if you don't implement the infrastructure. For example: design the Loan class with an audit trail in mind, even if logging infrastructure isn't in scope.
Translating NFRs to Design Decisions:
Let's see how non-functional requirements directly influence our class design:
| NFR | Design Decision |
|---|---|
| Configurable fine rates | Extract FinePolicy as a separate configurable component |
| Easy to add membership types | Use Strategy pattern for membership-specific behaviors |
| All transactions traceable | Add timestamp and actor tracking to all state-changing operations |
| Support large catalog | Design BookRepository to support pagination and filtering |
| Checkout completes quickly | Avoid heavy computations in the checkout path |
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.
Primary Actor: Member Stakeholders: Member (wants book), Librarian (processes transaction), Library (tracks inventory)
Preconditions: • Member is registered and active • Member has not exceeded borrowing limit • Member has no overdue fines above threshold
Trigger: Member approaches counter with library card and book
Main Success Scenario (Happy Path):
Alternative Flows:
3a. Member account suspended:
5a. Book is currently on loan to another member:
5b. Book is reserved by this member:
5c. Book is reserved by another member:
6a. Member at borrowing limit:
Postconditions:
Business Rules Identified:
BR-001: Borrowing limit varies by membership typeBR-002: Loan period varies by membership typeBR-003: Suspended members cannot borrowBR-004: Reserved books cannot be borrowed by non-reserving membersBR-005: Each transaction must be traceablePrimary Actor: Member (or Librarian on behalf) Stakeholders: Member (completes transaction), Library (regains inventory), Waiting members (may get notified)
Preconditions: • Book was previously borrowed • Active loan record exists
Trigger: Member returns book to circulation desk or drop box
Main Success Scenario:
Alternative Flows:
3a. Book is overdue:
5a. Reservations exist for this book:
Postconditions:
Business Rules Identified:
BR-006: Fine rate varies by membership typeBR-007: Maximum fine cap may apply per itemBR-008: Auto-suspension triggered at fine thresholdBR-009: Reservations processed in FIFO orderBR-010: Hold period is configurableIn 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:
In an interview, explicitly state your scope assumptions: 'I'll focus on the core domain model and circulation logic. I'll assume we have a persistence layer but won't detail the database schema. I'll design notification triggers but not the email delivery infrastructure. Does this scope alignment work for you?' This shows maturity and prevents wasted effort on tangential features.
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:
In real interviews, state assumptions aloud and ask if they're reasonable: 'I'm assuming borrowing limits vary by membership type—Student: 5, Faculty: 10, General: 3. Does that match what you have in mind?' This invites feedback and shows collaborative design thinking.
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:
| Requirement | Design Element | Pattern Applied |
|---|---|---|
| FR-CIR-01: Borrow a book | Loan class, LibraryService.borrowBook() | — |
| FR-CIR-02: Return a book | Loan class state transition, LibraryService.returnBook() | State Pattern |
| FR-CIR-04: Reserve a book | Reservation class, ReservationQueue | Observer Pattern |
| BR-001: Varying borrowing limits | MemberType.getBorrowingLimit() | Strategy Pattern |
| BR-006: Varying fine rates | FinePolicy interface with implementations | Strategy Pattern |
| NFR: Configurable fine rates | ExternalConfiguration, FinePolicy injection | Dependency Injection |
| NFR: Transactions traceable | Audit logging in domain events | Observer Pattern |
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:
You now have a comprehensive understanding of the Library Management System requirements. We've covered functional requirements across catalog, member, circulation, and fine management subsystems; non-functional requirements that shape architecture; detailed use case analysis for critical workflows; clear scope boundaries and documented assumptions. This foundation ensures our design will be purposeful and complete.