Architecture Overview

• Style: Microservices using Service-Oriented Architecture (SOA).
• Services: 3 isolated services with individual codebases, connected to seperate MongoDB instances.
• Containerization: Docker.
• Communication: Event-driven architecture (message broker).
• Message Broker: RabbitMQ (default), easily swappable with Kafka or NATS.
• Deployment: Docker Compose.

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Microservices

• User Service

  • Framework: Express.js
  • Responsibilities: Manage user data
  • MongoDB Collection: USERS
  • API Endpoints:
    • POST /users – Register a new user
    • GET /users/{id} – Fetch user details
  • Event Handling:
    • Consumes: LoanCreated – update user loanHistory, bookHistory and currentBooks .
    • Consumes: LoanReturned – update currentBooks.

• Book Service

  • Framework: Fastify
  • Responsibilities: Manage books and availability
  • MongoDB Collection: BOOKS
  • API Endpoints:
    • POST /books – Add a new book
    • GET /books/{id} – Fetch book details
  • Event Handling:
    • Consumes: LoanCreated – mark book as unavailable, update currentHolder and currentLoan.
    • Consumes: LoanReturned – mark book as available, set currentHolder and currentLoan to None.

• Loan Service

  • Framework: Koa
  • Responsibilities: Manage loan transactions
  • MongoDB Collection: LOANS
  • API Endpoints:
    • POST /loans – Register a new user
    • GET /loans/{id}/return – Fetch user details
  • Event Handling:
    • Publishes: LoanCreated – (userId, bookId, loanId)
    • Publishes: LoanReturned – (userId, bookId, loanId)

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Data Model

• USERS:

  Field	Type	Description
  id	ObjectId (PK)	Unique user ID
  name	String	Full name of the user
  email	String (Unique)	User's email address
  joinedAt	Date	Timestamp of user registration
  currentBooks	Array of Strings	ID list of all active loaned books associated with a user (if any)
  loanHistory	Array of Strings	ID list of all loans associated with a user (if any)
  bookHistory	Array of Strings	ID list of all books ever loaned to a user (if any)
  status	Enum	User account status (active, suspended)

• BOOKS:

  Field	Type	Description
  id	ObjectId (PK)	Unique book ID
  title	String	Title of the book
  author	String	Author name
  isbn	String (Unique)	International Standard Book Number
  available	Boolean	Indicates if the book is currently available
  currentLoan	String (Nullable)	ID of the current active loan associated with a book (if any)
  currentHolder	String (Nullable)	ID of the user the book was currently loaned out to (if any)
  tags	Array of Strings	Genres or category labels
  addedAt	Date	Date the book was added to the system

• LOANS:

  Field	Type	Description
  _id	ObjectId (PK)	Unique loan ID
  userId	String (FK)	References the ID of a user
  bookId	String (FK)	References the ID of a book
  loanedAt	Date	Date the loan was initiated
  dueDate	Date	Deadline for returning the book
  returnedAt	Date (Nullable)	Date the book was returned
  status	String	Loan status (active, returned, overdue)

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Infrastructure

• Message Broker: RabbitMQ:
  • Services communicate via a central broker.
  • Each service subscribes to a queue/topic.
  • Events are JSON-based messages.
• Deployment:
  • Docker compose setup with user-service, book-service, loan-service, three MongoDB containers, RabbitMQ container.