Design a Library Management System

The library management system is a favorite at consultancies like EPAM because it rewards clean domain modeling over algorithmic flash. The single insight the whole problem turns on is the distinction between a Book (the abstract title, identified by ISBN) and a BookItem (a physical copy you can actually borrow). Candidates who collapse the two into one class produce a model that cannot represent "3 copies, 2 on loan, 1 reserved" — and the interviewer learns everything they need to in the first five minutes.

What the interviewer is testing

• Do you separate identity from instance — the catalog title vs the borrowable copy? This is the load-bearing decision.
• Can you model a lifecycle (a copy moves available → loaned → reserved → lost) as a proper state, not scattered flags?
• Do you give borrowing rules, fine calculation, and notifications their own seams (Strategy, Observer) rather than burying them in a God Library class?
• Can you reason about the hold/reservation queue: who gets the next returned copy, and how is a member notified?

How to frame it

Clarify scope: members and librarians, a max-books-per-member and loan-period limit, reservations when all copies are out, and fines for overdue returns. State your non-functional bar: extensible fine policy, pluggable notification channels, and a clean availability model.

Then lay out the entities. A Library (Singleton aggregate root) owns the catalog of Books; each Book has many BookItem copies. A Member holds an Account with active Loans and Reservations. Checkout finds an available copy, transitions it to loaned, and creates a Loan with a due date. Return frees the copy — and if a reservation queue exists, the next member is notified and the copy is held. A FineCalculator (Strategy) prices overdue days; a NotificationService (Observer) pushes overdue and hold-available alerts.

Drive the loop: search catalog → checkout available copy → loan with due date → on overdue, observer notifies and fine accrues → return → reservation queue head is notified. The worked solution follows the full 9-section template; the Book/BookItem split and the copy state machine are where you score.