documentation.txt

# E-Commerce Spring Boot Backend – Complete Technical Documentation

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## 1. Project Overview

The **E-Commerce Spring Boot Backend** is a **microservices-based distributed system** designed to power a scalable, secure, and production-ready e-commerce platform. The system follows **modern cloud-native architecture principles**, leveraging Spring Boot, Spring Cloud, service discovery, API gateway, centralized configuration, security, and multiple databases.

This backend supports core e-commerce functionalities such as:

* User authentication and authorization
* Product catalog management
* Shopping cart handling
* Order processing
* Payment processing
* Email notifications

The application is designed to be **highly scalable**, **fault-tolerant**, and **easily deployable** using containers and orchestration tools.

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## 2. Key Architectural Principles

* **Microservices Architecture**
* **Single Responsibility per Service**
* **Loose Coupling & High Cohesion**
* **Centralized Configuration Management**
* **Service Discovery & Load Balancing**
* **API Gateway Pattern**
* **Secure-by-Design (Spring Security)**

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## 3. Technology Stack

### Core Backend

* **Java 17+**
* **Spring Boot**
* **Spring Cloud**

### Microservices & Cloud

* Spring Cloud **Eureka Server** (Service Discovery)
* Spring Cloud **Gateway** (API Gateway)
* Spring Cloud **Config Server** (Centralized Configuration)

### Security

* **Spring Security**
* In-Memory Authentication
* Database-backed Authentication
* Role-Based Access Control (RBAC)

### Databases

* **PostgreSQL** – Relational Data (Users, Orders, Payments)
* **MongoDB** – NoSQL Data (Products, Carts)

### Messaging & Communication

* RESTful APIs
* WebClient / OpenFeign (inter-service communication)

### Notifications

* **Email Service** (SMTP / JavaMailSender)

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## 4. High-Level System Architecture

```
                ┌──────────────────┐
                │  Config Server   │
                └────────▲─────────┘
                         │
        ┌────────────────┴────────────────┐
        │          Eureka Server            │
        └────────▲───────────▲─────────────┘
                 │           │
        ┌────────┴───┐   ┌───┴────────┐
        │ API Gateway│   │ Auth Service│
        └────▲───────┘   └────▲────────┘
             │                │
 ┌───────────┼────────────────┼────────────────────┐
 │           │                │                    │
 │   Product Service   Cart Service   Order Service  │
 │           │                │                    │
 │      Payment Service     Email Service           │
 └──────────────────────────────────────────────────┘
```

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## 5. Microservices Overview

| Service Name     | Responsibility                 | Database          |
| ---------------- | ------------------------------ | ----------------- |
| Config Server    | Centralized configuration      | None              |
| Eureka Server    | Service discovery              | None              |
| API Gateway      | Routing, security              | None              |
| UserAuth Service | Authentication & authorization | PostgreSQL        |
| Product Service  | Product catalog                | MongoDB           |
| Cart Service     | Shopping cart                  | PostgreSQL        |
| Order Service    | Order lifecycle                | MongoDB           |
| Payment Service  | Payment handling               | PostgreSQL        |
| Email Service    | Notifications                  | SMTP              |

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## 6. Config Server

### Purpose

* Centralized management of configuration files
* Environment-specific configs (dev, test, prod)

### Benefits

* No hard-coded configs in services
* Dynamic refresh using Spring Cloud Bus (optional)

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## 7. Eureka Server (Service Discovery)

### Purpose

* Registers all microservices
* Enables dynamic service lookup

### Benefits

* No hard-coded service URLs
* Load balancing and fault tolerance

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## 8. API Gateway

### Responsibilities

* Single entry point for all client requests
* Request routing
* Authentication & authorization
* Rate limiting and logging (optional)

### Security Implementation

#### Authentication Methods

1. **In-Memory Authentication**

   * Used for admin / testing
2. **Database Authentication**

   * Validates users from PostgreSQL

#### Authorization

* Role-based access control (ADMIN, USER)
* Endpoint-level security

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## 9. UserAuth Service

### Responsibilities

* User registration
* Login & credential validation
* Role management

### Database (PostgreSQL)

* Users table
* Roles table

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## 10. Product Service

### Responsibilities

* Product creation & updates
* Inventory management
* Product search

### Database (MongoDB)

* Flexible schema for product attributes
* High read performance

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## 11. Cart Service

### Responsibilities

* Add/remove items from cart
* Maintain cart per user
* Calculate cart totals

### Database (MongoDB)

* Fast updates
* Document-based cart structure

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## 12. Order Service

### Responsibilities

* Order creation
* Order status tracking
* Interaction with payment service

### Database (PostgreSQL)

* ACID transactions
* Strong consistency

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## 13. Payment Service

### Responsibilities

* Payment initiation
* Payment verification
* Transaction logging

### Database (PostgreSQL)

* Secure transaction storage

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## 14. Email Service

### Responsibilities

* Order confirmation emails
* Payment success/failure notifications
* Account alerts

### Technology

* JavaMailSender
* SMTP (Gmail / SES / Outlook)

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## 15. Inter-Service Communication

* REST APIs
* Service name resolution via Eureka
* Gateway-based routing

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## 16. Security Design

### Gateway-Level Security

* Centralized authentication
* Token/session validation

### Service-Level Security

* Internal service-to-service trust



## 17. Database Strategy (Polyglot Persistence)

### Why MongoDB?

* Product & cart data is schema-flexible
* High read/write throughput

### Why PostgreSQL?

* Orders & payments need strong consistency
* Relational constraints

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This **E-Commerce Spring Boot Backend** is a **robust, scalable, and secure microservices system** following industry best practices. It demonstrates real-world backend engineering skills, making it suitable for **production deployment, interviews, and enterprise use cases**.

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