📚 Spring-Modulith Library App

Modular Monolith Library Management System
Built using Spring Boot, Spring Modulith & Domain‑Driven Design
Domains: Rental, Inventory, Notifications

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Introduction

This repository contains a sample Library Management System built with Spring Modulith. It demonstrates how to structure a Spring Boot application into modular components with clearly defined context boundaries, adhering to Domain-Driven Design (DDD) principles.

Key characteristics of this architecture:

• Modular Design – Each module encapsulates its own domain logic and data.
• Inter‑module Communication – Modules interact either synchronously via exposed APIs or asynchronously using domain events.
• Loose Coupling – Reduces dependencies between modules, improving maintainability and scalability.
• Microservice‑ready – Clear boundaries simplify transitioning from a monolithic architecture to distributed microservices as the system evolves.

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🔦 Why Spring Modulith?

• Modular Monolith Architecture: Encapsulate functional domains into modules (rental, inventory, notifications), improving maintainability and clarity.
• Structural Verification: Enforce module boundaries using ApplicationModules.of(...).verify() in tests, preventing illegal package dependencies.
• Event‑Driven Communication: Modules interact via domain events instead of direct service injection, leading to loose coupling and high cohesion.
• Self‑Documenting Modules: Automatically generate module documentation diagrams and artifacts via Spring Modulith tooling. module-docs

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⚙️ Workflow Overview

1. Rental API Call

   • POST /api/rentals/rent → RentalService persists rental and publishes RentalEvent.

   • POST /api/rentals/return → RentalService updates rental with return date time and publishes ReturnEvent.

2. Inventory Listener

   • Consumes RentalEvent → decrements available copies and publishes RentalSuccessEvent
   • Consumes ReturnEvent → increments available copies and publishes ReturnSuccessEvent.

3. Notifications Listener

   • Listens to both RentalSuccessEvent and ReturnSuccessEvent events and logs notification metadata (bookId, userId, timestamps).

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📦 Key Features

• Events driven side effects: inventory updates and notifications handled asynchronously.
• Modular isolation: each domain encapsulates its own model and logic.
• Automated architecture enforcement: module verify tests catch coupling violations.

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🧭 Benefits in Real–World Development

1. Clean Domain Separation – Each module owns its model and operations.

2. Evolution‑Friendly Architecture – Easy to add new modules or external integrations (e.g., Kafka, Email).

3. Independent Testing – Modules can be tested individually using @ApplicationModuleTests.

4. Microservice Migration Ready – Event-driven design simplifies extraction into microservices if needed.

🧪 Getting Started

Prerequisites

• Java 17+
• Gradle (wrapper included) or Maven
• Postgres Database

Clone & Run

git clone https://github.com/avidee007/spring-modulith-library-app.git
cd spring-modulith-library-app 

Build application

  ./gradlew build

Run application

  ./gradlew bootRun

Run tests

  ./gradlew test