Design Library Management (LLD)

1. Requirements

• Functional

  • Catalog books and copies; members borrow and return with due dates and fine policy.
  • Enforce max concurrent loans per member; place hold when all copies are out.
  • Librarian can add inventory, check in, and waive fines (audited).

• Non-Functional

  • Consistent loan state: a copy cannot be both available and checked out.
  • Extensible fine rules and loan periods by book category or member tier.

• Assumptions / Out of Scope

  • Digital lending and DRM omitted; physical copies only.

2. Core Entities

3. Class Diagram

4. State / Sequence Diagram (where relevant)

5. Design Patterns Applied

• Factory — Create Loan with validated clock and due date. Factory Method pattern.
• Strategy — FinePolicy and LoanPeriodPolicy by category. Strategy pattern.
• Repository (conceptual) — Catalog abstracts persistence; pair with Factory Method pattern for concrete stores when the pattern docs exist.

6. Implementation

Go

package library

type Book struct {
    Identifier, Title, ISBN string
}

type BookCopy struct {
    CopyID  string
    BookID  string
    Status  string
}

type Member struct {
    Identifier string
    Name       string
    ActiveLoanCount int
}

type Circulation struct {
    Catalog   *Catalog
    FineRules FinePolicy
}

func (circ *Circulation) Checkout(memberID, bookID string) (*Loan, error) { /* ... */ }
func (circ *Circulation) Return(copyID string) (fineCents int64, err error) { /* ... */ }

JavaScript

class Catalog {
  constructor() {
    this.booksById = new Map();
    this.copiesById = new Map();
  }
  addBook(book) { /* ... */ }
  addCopy({ copyId, bookId }) { /* ... */ }
}

class CirculationService {
  constructor({ catalog, finePolicy }) {
    this.catalog = catalog;
    this.finePolicy = finePolicy;
  }
  checkout({ memberId, bookId }) { /* ... */ }
  returnCopy({ copyId }) { /* ... */ }
}

7. Concurrency / Thread Safety

• Collisions: Two clerks checking out last available copy; concurrent return and renew.
• Granularity: Transaction wrapping copy status update and loan insert; DB unique constraint on active loan per copy.
• Go: sync.Mutex per copy id in in-memory simulation.

8. Extensibility & Followups

• Inter-library loan as workflow with external partner ids.
• Notification scheduler on due soon and overdue.
• Edge cases: lost book, damaged return, membership suspension.