📚 Book Loan Service

A lightweight, domain‑driven service that manages book borrowing operations through a clean and extensible API.

Managing the lifecycle of borrowed books may seem simple at first glance, yet it hides a surprisingly rich set of rules:
books can be unavailable, customers may reach borrowing limits, loans can expire, and overdue items must be handled gracefully.
Book Loan Service was created to explore this domain in depth — not as a full library system, but as a clean, expressive, and testable foundation for book‑borrowing logic.

This project demonstrates how a well‑designed domain model, a thin application layer, and lightweight infrastructure can work together to form a robust, maintainable service.
It is intentionally minimalistic, focusing on clarity, correctness, and architectural quality rather than feature breadth.

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🎯 Project Goals

This project serves as a focused exploration of domain‑driven design applied to a simple but meaningful problem: managing the lifecycle of borrowed books.

The goals are:

• To model a clear and expressive domain with well‑defined business rules
• To demonstrate how a thin application layer orchestrates domain behavior
• To keep infrastructure lightweight, replaceable, and free of business logic
• To provide a fully testable architecture (unit + integration)
• To offer a clean, readable codebase suitable for learning, teaching, or portfolio use

Rather than building a full library management system, this project isolates the core borrowing logic to highlight architectural clarity and maintainability.

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✨ What this project offers

• A domain‑driven model for book loans
• Borrowing, returning, and validating book availability
• Overdue detection and loan limit enforcement
• A clean, extensible application service
• In‑memory repositories for fast development
• A fully tested core (unit + integration tests)
• A structure designed for long‑term maintainability

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🏛️ Architecture Overview

The project follows a domain‑centric, layered architecture:

Domain

• Entities (Book, Customer, Loan)
• Business rules (availability, overdue logic, loan limits)

Application

• Commands (BorrowBooksCommand)
• Services (LibraryService)
• Error messages and result handling

Infrastructure

• In‑memory repositories
• Dependency injection setup

Test

• Unit tests (mocked repositories)
• Integration tests (real in‑memory repos + scoped DI fixture)
• Test helpers (RandomHelper, LoanTestHelper)

The Domain layer is pure and independent.
The Application layer orchestrates use cases.
The Infrastructure layer provides replaceable technical details.
The Test layer ensures correctness at every level.

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🧱 Requirements

• .NET 8 SDK (or later)
• Git (for cloning the repository)
• A terminal or command prompt (Windows, macOS, or Linux)

No external database or message broker is required — everything runs in memory.

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🚀 Getting Started

Clone the repository:

git clone https://github.com/Argos78/book-loan-service.git
cd book-loan-service

Run the test suite:

dotnet test

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▶️ Running the API

To start the API locally:

dotnet run --project src/BookLoanService.Api

Once running, the service will be available at:

http://localhost:5160

(Or another port depending on your environment.)

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📘 Swagger UI

Once the API is running, you can explore the available endpoints through the built‑in Swagger UI:

http://localhost:5160/swagger

This interface provides a complete, interactive description of the API, including request/response schemas and example payloads.

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📡 API Overview (high‑level)

Even though a Swagger UI is available, here is a brief overview of the main capabilities:

Borrow books

Submit a list of book IDs for a given customer.
The service returns:

• BorrowedBooks → successfully borrowed items
• RejectedBooks → items rejected with reason codes
  (NOT_FOUND, NOT_AVAILABLE, LOAN_OVERDUE, LIMIT_REACHED)

List all books

Retrieve the current catalog, including books added at runtime.

Add a book

Add a new book with an auto‑assigned ID.

This section intentionally stays concise to avoid duplicating the Swagger contract.

🧪 Testing Strategy

Testing is central to this project.

Unit Tests

• Mocked repositories
• Validate business rules and service behavior
• Fast and deterministic

Integration Tests

• Real in‑memory repositories
• Scoped DI container via AppTestFixture
• End‑to‑end scenarios (borrowing, rejecting, overdue logic, etc.)

Test Helpers

• RandomHelper for stable random IDs
• LoanTestHelper for generating active, overdue, or returned loans

🗺️ Roadmap

• Add persistence layer (SQL or NoSQL)
• Add authentication / authorization
• Add book return endpoint
• Add overdue notifications
• Add CI/CD pipeline and build badge

🤝 Contributing

Contributions, suggestions, and improvements are welcome.
Feel free to open an issue or submit a pull request.

📄 License

This project is licensed under the MIT License.
You are free to use, modify, and distribute it with attribution.