🧠 Kafka Order Processing System (Spring Boot + Kafka + MongoDB + Docker)

🚀 Overview

This project is a Spring Boot based system demonstrating event driven order processing using Apache Kafka (KRaft mode) and MongoDB.

The system simulates an e-commerce order pipeline:

• The Order Service produces order events to a Kafka topic (orders).
• The Warehouse Service consumes those events to process inventory and update stock.
• Each order goes through status changes: PENDING → PROCESSED.

The entire system runs in Docker containers - no ZooKeeper required (Kafka KRaft mode).

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🧩 Features

✅ Event-driven architecture using Apache Kafka
✅ Kafka KRaft mode (no ZooKeeper dependency)
✅ MongoDB for persistence of orders and stock
✅ Real-time warehouse stock updates
✅ Order status tracking (PENDING, PROCESSED, FAILED)
✅ Clean DTO-based REST API design
✅ Fully Dockerized setup (Kafka, MongoDB, Spring Boot)

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🧰 Tech Stack

Component	Technology
Language	Java 21
Framework	Spring Boot 3.x
Messaging	Apache Kafka 3.8.0 (KRaft mode)
Database	MongoDB 7.0
Build Tool	Maven
Containerization	Docker & Docker Compose

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🧩 Project Structure Overview

Project structure (directory tree)

Kafka_Order_Processing_System/
├─ docker-compose.yml
├─ Dockerfile
├─ HELP.md
├─ mvnw
├─ mvnw.cmd
├─ pom.xml
├─ README.md
├─ src/
│  ├─ main/
│  │  ├─ java/
│  │  │  └─ com/KafkaOrderProcessingSystem/OrderProcessingSystem/
│  │  │     ├─ GlobalExceptionHandler.java
│  │  │     ├─ OrderProcessingSystemApplication.java
│  │  │     ├─ config/
│  │  │     │  └─ SwaggerConfig.java
│  │  │     ├─ controller/
│  │  │     │  ├─ InventoryController.java
│  │  │     │  └─ OrderController.java
│  │  │     ├─ dto/
│  │  │     │  ├─ OrderRequestDTO.java
│  │  │     │  ├─ OrderResponseDTO.java
│  │  │     │  └─ WarehouseStockDTO.java
│  │  │     ├─ entity/
│  │  │     │  ├─ Order.java
│  │  │     │  └─ WarehouseStock.java
│  │  │     ├─ repository/
│  │  │     │  ├─ OrderRepository.java
│  │  │     │  └─ WarehouseRepository.java
│  │  │     ├─ service/
│  │  │     │  ├─ InventoryService.java
│  │  │     │  ├─ OrderProducerService.java
│  │  │     │  ├─ WarehouseConsumerService.java
│  │  │     │  └─ Impl/
│  │  │     │     ├─ InventoryServiceImpl.java
│  │  │     │     ├─ OrderProducerServiceImpl.java
│  │  │     │     └─ WarehouseConsumerServiceImpl.java
│  │  │     └─ utils/
│  │  │        └─ WarehouseStockUpdate.java
│  │  └─ resources/
│  │     ├─ application.yaml
│  │     └─ diagrams/
│  │        ├─ ActivityDiagram_InventoryManagemant.png
│  │        ├─ ActivityDiagram_OrderProducer.png
│  │        ├─ ActivityDiagram_WarehouseConsumer.png
│  │        ├─ ClassDiagram.png
│  │        └─ SequenceDiagram.png
│  └─ test/
│     └─ java/
│        └─ com/KafkaOrderProcessingSystem/OrderProcessingSystem/
│           ├─ OrderProcessingSystemApplicationTests.java
│           └─ (controller, service tests...)
└─ target/ (build output)

The project follows a modular layered architecture, organized into the following components:

1. Controller Layer (controller/)

Handles incoming REST API requests and delegates business logic to the service layer.

• OrderController - Exposes endpoints for placing orders and checking order status.
• InventoryController - Exposes endpoints for adding stock to the warehouse.

2. DTO Layer (dto/)

Contains Data Transfer Objects used for communication between layers.

• OrderRequestDTO – Request payload for placing an order.
• OrderResponseDTO – Response structure for order details.
• WarehouseStockDTO – Represents stock update information.

3. Entity Layer (entity/)

Defines domain entities and enums mapped to the database.

• Order, WarehouseStock.

4. Repository Layer (repository/)

Manages database access using Spring Data JPA.

• OrderRepository, WarehouseRepository.

5. Service Layer (service/Impl/)

Contains the core business logic and Kafka message handling.

• InventoryServiceImpl – Interacts to perform crud operations in warehouse.
• OrderProducerServiceImpl – Sends order events to Kafka topics.
• WarehouseConsumerServiceImpl – Listens to stock updates from Kafka.

6. Configuration & Bootstrapping

• KafkaOrderProcessingSystemApplication.java – Main entry point for Spring Boot.
• application.yml – Contains Kafka, database, and application configurations.

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🧮 System Design Diagrams

To better visualize the architecture and flow of the Kafka Order Processing System, several UML diagrams have been included.

All diagrams are located in: 📁 src/main/resources/diagrams/

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🐳 Running the Application with Docker

1️⃣ Build and Start Containers

docker-compose build
docker-compose up -d

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2️⃣ Verify Services

Spring Boot App → http://localhost:8080

Kafka Broker → http://localhost:9092

Kafka UI → http://localhost:8081

MongoDB → http://localhost:27017