1. Java 17: The latest long-term support version with enhancements in performance and usability.
2. Spring Boot: Simplifies the development of new Spring applications through convention over configuration.
3. Maven: Handles project dependencies and builds.
4. H2 Database: An in-memory database that is easy to set up and suitable for development and testing phases. For production, you might consider a more robust option like PostgreSQL(since I didn't have time to set up PostgreSQL db I have used Mysql ) for its reliability and rich feature set.
5. JUnit and Mockito for Testing: Standard libraries for unit and integration testing in Java projects.

The project would have the following main components:

• Borrower: Entity and repository to manage borrower data.
• Book: Entity and repository to manage book data including handling multiple copies with the same ISBN.
• LibraryService: Business logic for borrowing and returning books.
• LibraryController: RESTful endpoints for interacting with the library system.

• id: Unique identifier (auto-generated)
• name: Name of the borrower
• email: Email address of the borrower

• id: Unique identifier (auto-generated)
• isbn: ISBN number of the book
• title: Title of the book
• author: Author of the book
• borrowedBy: Borrower id

URL FOR swagger-ui: http://localhost:2222/swagger-ui/index.html#/

• POST /api/borrowers - Register a new borrower.

curl --location 'localhost:2222/api/borrowers' \
--header 'Content-Type: application/json' \
--data-raw '{
    "id":"borrowers1",
    "name":"supun",
    "email":"[email protected]"
}'

• POST /api/books - Register a new book.

curl --location 'localhost:2222/api/books' \
--header 'Content-Type: application/json' \
--data '{
    "id":"bookid1",
    "isbn":"isbn1",
    "title":"AAAAAAAAAAAAAAAAAAAAA",
    "author":"SADADADADAD"
}'

• GET /api/books - Retrieve a list of all books.

curl --location 'localhost:2222/api/books' \
--data ''

• PUT /api/borrowers/{borrowerId}/books/{bookId}/borrow - Borrow a book by id.

curl --location --request PUT 'localhost:2222/api/borrowers/borrowers1/books/bookid1/borrow' \
--data ''

• PUT /api/borrowers/{borrowerId}/books/{bookId}/return - Return a borrowed book.

curl --location --request PUT 'localhost:2222/api/borrowers/borrowers1/books/bookid1/return' \
--data ''

• Concurrent Borrowing: Synchronization at the application or database level to ensure a book can only be borrowed by one person at a time.
• Validation: Using Spring's @Valid annotation to enforce input validations.
• Error Handling: Global error handler using @ControllerAdvice to manage exceptions and return proper HTTP status codes.

H2 is chosen for development due to its simplicity and ease of configuration. For production, a transition to PostgreSQL would be recommended for its robustness, support for concurrent transactions, and extensive industry adoption.

Use Spring profiles to configure different settings for development, testing, and production environments. Configuration files like application-dev.properties, and application-prod.properties would help manage these environments.

• Swagger or Springdoc OpenAPI for interactive API documentation.
• README.md to include setup instructions, environment configuration details, and basic usage.

• Borrower's email is unique and can be used as a secondary identifier.
• Each book borrowal is tracked as a unique event, allowing for historical borrowing data without affecting the current state of book availability.
• The system does not manage overdue books or fines; it only tracks the state of a book (borrowed/available).

• Dockerizing the application for easier deployment.
• Using Jenkins or GitHub Actions for CI/CD.
• Writing comprehensive unit and integration tests with Mockito and JUnit.
• Adhering to the 12 Factor App methodology, especially regarding config, logging, and disposability.

• Configure DB username and password in application.properties (Location: application-dev.properties or application-prod.properties)
• Go to project folder librarySystem Open the CMD interface in the project folder and run "mvn clean install"
• After building successfully, run the command. “mvn spring-boot:run”