Distributed Order Processing System

Disclaimer: This project was built for learning purposes to explore event-driven microservices architecture using modern Java technologies. It is not intended for production use.

Overview

An event-driven microservices system where placing an order triggers asynchronous notifications to both a stock service and an email service via Apache Kafka. Orders are persisted to a PostgreSQL database.

Architecture

POST /api/v1/orders
        │
        ├──► PostgreSQL (persist order)
        │
        └──► Kafka "order_topics"
                    │
          ┌─────────┴─────────┐
          ▼                   ▼
    stock-service       email-service
   (updates stock)     (sends email)

Services

Service	Description	Port
order-service	REST API that accepts orders, saves to DB, and publishes to Kafka	8080
stock-service	Kafka consumer that processes stock updates	-
email-service	Kafka consumer that processes email notifications	-
order-base-domains	Shared library containing the OrderEvent class	-

Tech Stack

• Java 17 + Spring Boot 4
• Apache Kafka — event streaming between services
• PostgreSQL — order persistence
• Spring Data JPA — database access
• Docker + Docker Compose — containerized local development
• k6 — load testing
• JUnit 5 + Mockito — unit testing

Running Locally

Prerequisites: Docker Desktop

# Build all services
cd order-base-domains && ./mvnw install -DskipTests
cd ../order-service && ./mvnw package -DskipTests
cd ../stock-service && ./mvnw package -DskipTests
cd ../email-service && ./mvnw package -DskipTests

# Start everything
cd ..
docker compose up --build

API

Place an Order

curl -X POST http://localhost:8080/api/v1/orders \
  -H "Content-Type: application/json" \
  -d '{"name": "iPhone 15", "status": "PENDING"}'

Query Orders in Database

docker exec postgres psql -U postgres -d ordersdb -c "SELECT * FROM orders;"

Running Tests

Unit tests run without any infrastructure (no Docker, no database, no Kafka needed):

cd order-service
./mvnw test

Tests cover OrderController:

• placeOrder returns success message
• placeOrder saves order to database
• placeOrder sends message to Kafka

Load Test Results

Tested with k6 on local Docker environment:

Metric	Result
Concurrent users	200
Throughput	1,867 req/s
p95 latency	20ms
Error rate	0.1%
Total requests	56,198

k6 run load-tests/order-load-test.js