🚀 Spring Boot CQRS Example

Project Architecture

• ✅ CQRS (Command Query Responsibility Segregation) pattern implementation for modern e-commerce systems.

Used Technologies in The Project:

📚 What is CQRS?

CQRS = Separate Write and Read operations!

Traditional Approach:

User → API → Single Database (PostgreSQL)
        ↓
   Write & Read (Slow!)

CQRS Approach:

User → API → Command (Write) → PostgreSQL
           → Query (Read)    → Elasticsearch (Fast!)

🎯 Why CQRS?

Real World Scenario:

In an e-commerce site:

• 🛒 Product purchase: 10% (LOW)
• 👀 Product viewing: 90% (HIGH)

Problem:

• You're both writing and reading from PostgreSQL
• Millions of users searching products simultaneously → SLOWNESS!

Solution: CQRS

• Write (Command) → PostgreSQL (Strong, ACID)
• Read (Query) → Elasticsearch (Fast, Search)

---

🎬 Scenario: Product System

📝 Command Side (Write - 10%)

User → "Add new product" 
     → Command API
     → Save to PostgreSQL
     → Send event to RabbitMQ

Endpoint:

POST /api/commands/products
Content-Type: application/json

{
  "name": "iPhone 15 Pro",
  "price": 45000.0
}

📖 Query Side (Read - 90%)

User → "Search iPhone"
     → Query API  
     → Fetch from Elasticsearch (⚡ Fast!)

Endpoint:

GET /api/products?name=iPhone

🔄 Event Flow

1. Admin adds new product

   • Written to PostgreSQL

2. RabbitMQ publishes event

   • ProductCreatedEvent

3. Listener catches event

   • Written to Elasticsearch

4. Users search quickly

   • Read from Elasticsearch

---

🛠️ Technologies

Technology	Purpose	Why?
PostgreSQL	Write Database	ACID, reliable writes
Elasticsearch	Read Database	Fast search, full-text search
RabbitMQ	Event Bus	Asynchronous communication
Spring Boot	Framework	Rapid development

---

🚀 Setup

1️⃣ Start Services with Docker Compose

docker-compose up -d

Database Configuration

• PostgreSQL
  • URL: http://localhost:5432
  • Database: cqrs_example
  • Username: postgres
  • Password: postgres

• Elasticsearch
  • URL: http://localhost:9200
  • Username: elastic
  • Password: elasticsearch

• RabbitMQ Management
  • URL: http://localhost:15672
  • Username: rabbitmq
  • Password: rabbitmq

2️⃣ Run the Application

./gradlew bootRun

3️⃣ Test It

Create product:

curl -X POST http://localhost:8080/api/commands/products \
  -H "Content-Type: application/json" \
  -d '{"name":"Laptop","price":15000.0}'

List products:

curl http://localhost:8080/api/products

---

📂 Project Structure

src/main/java/com/furkankayam/
├── command/                            # 📝 WRITE side
│   ├── Product.java                    # JPA Entity
│   ├── ProductCommandRepository.java
│   ├── ProductCommandService.java
│   └── ProductCommandController.java
│
├── query/                              # 📖 READ side  
│   ├── ProductDocument.java            # Elasticsearch Document
│   ├── ProductQueryRepository.java
│   ├── ProductQueryService.java
│   └── ProductQueryController.java
│
├── event/                              # 🔄 Events
│   ├── ProductEvent.java
│   ├── ProductEventPublisher.java
│   └── ProductEventListener.java
│
├── dto/                                # 📦 Data Transfer Objects
│   ├── request/
│   │   ├── CreateProductRequest.java
│   │   └── UpdateProductRequest.java
│   └── response/
│       └── ProductResponse.java
│
└── config/                             # ⚙️ Configuration
    └── RabbitMQConfig.java

---

🎯 API Endpoints

Command API (Write)

POST   /api/commands/products      # Create product
PUT    /api/commands/products/{id} # Update product
DELETE /api/commands/products/{id} # Delete product

Query API (Read)

GET /api/products                              # All products
GET /api/products/{id}                         # Product by ID
GET /api/products/search?name=...              # Search by name
GET /api/products/search/price?min=...&max=... # Search by price

---

🔥 Real World Example

❌ Without CQRS:

1000 users searching "iPhone" simultaneously
  └─> PostgreSQL JOIN queries
  └─> Slow! (500ms - 2s)
  └─> Database locked!

✅ With CQRS:

1000 users searching "iPhone" simultaneously
  └─> Elasticsearch full-text search
  └─> Fast! (10ms - 50ms)
  └─> PostgreSQL unaffected!

---

📊 Performance Comparison

Operation	Traditional	CQRS
Create product	50ms	50ms
Search product	800ms	20ms
1000 users searching	DB locked	Working smoothly

---

🤔 When to Use CQRS?

✅ Use:

• Unbalanced read/write ratio (90% reads, 10% writes)
• High traffic
• Complex search requirements
• E-commerce, blogs, news sites

❌ Don't Use:

• Simple CRUD applications
• Low traffic
• Balanced read/write operations

License

This project is licensed under the MIT License. See the LICENSE file for details

Created by Mehmet Furkan KAYA