Availability: 99.9% uptime, graceful degradation if notification service fails

Security: Secure member authentication, role-based access (member vs librarian), audit logging

Usability: Intuitive search interface, clear error messages, accessibility compliance

Use Case Diagram

graph TB
    subgraph Actors
        Member[👤 Member]
        Librarian[👔 Librarian]
        System[🖥️ System]
    end

    subgraph "Member Use Cases"
        UC1[🔍 Search Catalog]
        UC2[📖 Checkout Book]
        UC3[📚 Return Book]
        UC4[🔄 Renew Book]
        UC5[📅 Reserve Book]
        UC6[❌ Cancel Reservation]
        UC7[💳 Pay Fine]
        UC8[📜 View Borrowing History]
        UC9[📊 View Account Status]
    end

    subgraph "Librarian Use Cases"
        UC10[➕ Add Book to Catalog]
        UC11[✏️ Update Book Information]
        UC12[🗑️ Remove Book]
        UC13[👥 Register Member]
        UC14[🔧 Manage Member Account]
        UC15[💰 Waive Fine]
        UC16[📊 Generate Reports]
        UC17[🏷️ Mark Book Lost/Damaged]
        UC18[✅ Issue Replacement Card]
    end

    subgraph "System Use Cases"
        UC19[🔔 Send Notifications]
        UC20[⚠️ Calculate Fines]
        UC21[📢 Process Reservation Queue]
        UC22[🚫 Auto-Cancel Expired Holds]
    end

    Member --> UC1
    Member --> UC2
    Member --> UC3
    Member --> UC4
    Member --> UC5
    Member --> UC6
    Member --> UC7
    Member --> UC8
    Member --> UC9

    Librarian --> UC10
    Librarian --> UC11
    Librarian --> UC12
    Librarian --> UC13
    Librarian --> UC14
    Librarian --> UC15
    Librarian --> UC16
    Librarian --> UC17
    Librarian --> UC18
    Librarian -.-> UC1

    System --> UC19
    System --> UC20
    System --> UC21
    System --> UC22

    style UC1 fill:#e1f5ff,stroke:#01579b,stroke-width:2px
    style UC2 fill:#c8e6c9,stroke:#2e7d32,stroke-width:2px
    style UC3 fill:#fff9c4,stroke:#f57f17,stroke-width:2px
    style UC4 fill:#f3e5f5,stroke:#6a1b9a,stroke-width:2px
    style UC5 fill:#ffe0b2,stroke:#e65100,stroke-width:2px
    style UC6 fill:#ffcdd2,stroke:#c62828,stroke-width:2px
    style UC7 fill:#b2dfdb,stroke:#00695c,stroke-width:2px
    style UC8 fill:#d1c4e9,stroke:#4527a0,stroke-width:2px
    style UC9 fill:#b2ebf2,stroke:#006064,stroke-width:2px
    style UC10 fill:#c5e1a5,stroke:#558b2f,stroke-width:2px
    style UC11 fill:#ffccbc,stroke:#bf360c,stroke-width:2px
    style UC12 fill:#ffab91,stroke:#d84315,stroke-width:2px
    style UC13 fill:#ce93d8,stroke:#6a1b9a,stroke-width:2px
    style UC14 fill:#ffe082,stroke:#f9a825,stroke-width:2px
    style UC15 fill:#a5d6a7,stroke:#388e3c,stroke-width:2px
    style UC16 fill:#90caf9,stroke:#1565c0,stroke-width:2px
    style UC17 fill:#ef9a9a,stroke:#c62828,stroke-width:2px
    style UC18 fill:#b39ddb,stroke:#5e35b1,stroke-width:2px
    style UC19 fill:#80deea,stroke:#00695c,stroke-width:2px
    style UC20 fill:#ffcc80,stroke:#ef6c00,stroke-width:2px
    style UC21 fill:#bcaaa4,stroke:#5d4037,stroke-width:2px
    style UC22 fill:#f48fb1,stroke:#ad1457,stroke-width:2px

Class Diagram

The design demonstrates multiple key patterns:

Strategy Pattern: FineCalculationStrategy for flexible fine policies
Observer Pattern: NotificationService observers for multi-channel notifications
State Pattern: BookItemState for book lifecycle management
Command Pattern: Library operations (CheckoutCommand, ReturnCommand, etc.)
Composite Pattern: SearchCriteria for complex catalog queries

classDiagram
    class Library {
        -String name
        -Address address
        -Catalog catalog
        -List~Member~ members
        -List~Librarian~ librarians
        -NotificationService notificationService

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