Library Management System

Design a library management system where members search the catalogue, borrow and return physical copies, place holds, and pay fines. It's mostly an entity-modelling exercise: the key insight is separating a book (the title) from its copies (the physical items).

Requirements

Functional

• Librarians add books (title, authors, ISBN) and copies (barcode, rack location).
• Members search by title, author, or ISBN.
• Members borrow available copies; limits depend on membership tier (e.g. 3 basic, 6 premium); loan period 14 days.
• Members return copies; overdue returns incur a per-day fine with a cap.
• Members place a hold on a book when no copy is available; when a copy is returned, the next member in the hold queue is notified and the copy is reserved for them for 48 hours.
• Members with unpaid fines above a threshold cannot borrow.

Non-functional

• Correct under concurrent checkouts at multiple counters.
• Search should be fast for a catalogue of ~1M books (indexes, not scans).
• Notifications are asynchronous and shouldn't block returns.