📚 Library Service API

A production-style REST API for managing a modern digital library system.

The application replaces a fully manual, paper-based process for tracking books, borrowings, users, and payments with a structured, scalable backend system.

The system exposes a fully documented API and integrates external services for payments and notifications.

🚀 Overview

The Library Service API provides functionality for:

• Managing book inventory
• Handling user registration and authentication
• Creating and tracking borrowings
• Processing online payments
• Calculating overdue fines
• Sending asynchronous notifications

The project is designed as a modular backend system with clearly separated responsibilities between services. There is no frontend layer. The system is fully operable via REST API and Swagger UI.

🏗 Architecture

The system follows a modular Django app architecture:

• books/
• users/
• borrowings/
• payments/
• notifications/

Core principles:

• Separation of concerns
• Clear domain boundaries
• Explicit business logic
• Transaction-safe operations
• External integrations abstracted behind service layers
• Asynchronous event handling

🧩 Domain Model

erDiagram
    USER {
        int id PK
        string email
        string first_name
        string last_name
        string password
        bool is_staff
        bool is_active
        datetime date_joined
    }

    BOOK {
        int id PK
        string title
        string author
        string cover
        int inventory
        decimal daily_fee
    }

    BORROWING {
        int id PK
        date borrow_date
        date expected_return_date
        date actual_return_date
        int user_id FK
        int book_id FK
    }

    PAYMENT {
        int id PK
        string status
        string type
        decimal money_to_pay
        string session_id
        string session_url
        datetime created_at
        int borrowing_id FK
    }

    USER ||--o{ BORROWING : "creates"
    BOOK ||--o{ BORROWING : "borrowed in"
    BORROWING ||--o{ PAYMENT : "generates"

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🔐 Authentication & Authorization

• JWT authentication via djangorestframework-simplejwt
• Email-based login
• Access & refresh tokens
• Role-based access control (admin vs regular user)

Security model:

• Users can access only their own borrowings and payments
• Admin users can access all records
• Public endpoints limited to registration and token generation

💳 Stripe Integration

Payments are processed using Stripe Checkout.

Flow:

• Borrowing created
• Stripe Checkout session generated
• Payment record created (PENDING)
• User completes payment
• Stripe webhook updates status to PAID
• Success endpoint confirms payment state
• Fine payments are generated automatically for overdue returns:

 fine = overdue_days * daily_fee * FINE_MULTIPLIER

📖 API Endpoints

Endpoint	Method	Description
/api/user/register/	POST	Register a new user
/api/user/token/	POST	Get access and refresh tokens
/api/user/token/refresh/	POST	Refresh access token
/api/user/me/	GET, PUT, PATCH	Retrieve or update authenticated user's profile
/api/books/	GET	List all books
/api/books/	POST	Create a new book (admin only)
/api/books/{id}/	GET	Retrieve book details
/api/books/{id}/	PUT, PATCH	Update book (admin only)
/api/books/{id}/	DELETE	Delete book (admin only)
/api/borrowings/	GET	List borrowings (filtered by user / active status)
/api/borrowings/	POST	Create a new borrowing
/api/borrowings/{id}/	GET	Retrieve borrowing details
/api/borrowings/{id}/return/	POST	Return a borrowed book
/api/payments/	GET	List payments
/api/payments/{id}/	GET	Retrieve payment details
/api/payments/success/	GET	Check payment status after Stripe redirect
/api/payments/cancel/	GET	Stripe cancel callback
/api/payments/webhooks/stripe/	POST	Stripe webhook for payment confirmation

Testing

• Unit tests for models and business logic
• API endpoint tests
• Stripe session mocked in tests
• Celery tasks mocked

Tests coverage: 96%

• Borrowings: 100%
• Payments (incl. webhook): 100%
• Notifications tasks: 92%
• Overall branch-safe transactional logic covered

⚙️ Technology Stack

• Python 3.11
• Django
• Django REST Framework
• drf-spectacular
• Celery
• Redis
• PostgreSQL
• Stripe SDK

🚀 Running the Project

1. Build containers

docker-compose build

2. Start services

docker-compose up

3. Run migrations

docker-compose exec web python manage.py migrate

4. Create superuser

docker-compose exec web python manage.py createsuperuser

🌐 Access the Application

Service	URL
API	http://localhost:8000
Swagger	http://localhost:8000/api/docs/
ReDoc	http://localhost:8000/api/redoc/