Backend Engineer Homework – Spring Boot Microservice

This project is a lightweight Spring Boot microservice built as a Backend Engineer homework assignment.
It demonstrates core backend engineering skills including REST API design, relational data modeling, event logging, data visualization, and containerization.

---

✨ Tech Stack

• Java 11
• Spring Boot 2.7.18
• Spring Web, Spring Data JPA
• MySQL 8
• Hibernate / JPA
• XChart (data visualization)
• Springdoc OpenAPI (Swagger)
• Docker & Docker Compose

---

📌 Features Overview

• CRUD REST API for Employee
• One-to-Many relationship: Employee → Events
• Automatic event logging on each CRUD operation
• Graph endpoint that visualizes events over time
• Global error handling & validation
• Health check endpoint
• Fully dockerized (App + MySQL)

---

📁 Domain Model

Employee

Field	Type
id	Long
name	String
role	String
createdAt	Timestamp

Event

Field	Type
id	Long
employee	Many-to-One
type	GET / POST / PUT / DELETE
timestamp	Timestamp

Relationship Employee 1 ──── * Event

Each CRUD operation on Employee automatically creates a corresponding Event.

---

🚀 REST API Endpoints

Employee CRUD

Method	Endpoint	Description
POST	/employee	Create employee
GET	/employee/{id}	Get employee by id
GET	/employees	List all employees
PUT	/employee/{id}	Update employee
DELETE	/employee/{id}	Delete employee

---

Graph / Visualization

Method	Endpoint	Description
POST	/events/graph	Returns graph metadata (JSON)
POST	/events/graph/image	Streams PNG graph

Request example

{ "empIds": [1,2,3], "from": "01-12-2025 00:00", "to": "31-12-2025 23:59", "bucket": "DAY" } The PNG graph visualizes event counts per employee within the given time range.

🧪 Validation & Error Handling @Valid request validation

Consistent error responses

Proper HTTP status codes

Example Error Response { "status": 400, "message": "name must not be blank" } Error Mapping Scenario Status Validation error 400 Malformed JSON / enum 400 Resource not found 404 Unexpected error 500

🩺 Health Check GET /actuator/health Response:

{ "status": "UP" }

📊 Swagger / OpenAPI Swagger UI is available at:

http://localhost:8080/swagger-ui/index.html All endpoints, request/response models, and validations are documented.

🐳 Running with Docker Compose 1️⃣ Build & Start docker compose up --build

This starts:

Spring Boot application MySQL 8 database

2️⃣ Stop & Clean docker compose down -v

🔍 Sample cURL Commands Create Employee

curl -u user:user
-H "Content-Type: application/json"
-d '{"name":"Merve","role":"Backend Engineer"}'
http://localhost:8080/employee Get Employee

curl -u user:user http://localhost:8080/employee/1 Graph Image

curl -u user:user
-H "Content-Type: application/json"
-d '{"empIds":[1],"from":"01-12-2025 00:00","to":"31-12-2025 23:59","bucket":"DAY"}'
http://localhost:8080/events/graph/image
--output graph.png

Update Employee curl -i -u user:user
-H "Content-Type: application/json"
-d '{"name":"Merve D.","role":"Senior Backend Engineer"}'
-X PUT http://localhost:8080/employee/1

🗄 Database Schema (MySQL) Tables:

employees events

Foreign key: events.employee_id → employees.id (ON DELETE CASCADE) Indexes ensure efficient graph queries by employee and time.

📦 Project Structure css Copy code src/main/java ├── employee │ ├── EmployeeController │ ├── EmployeeService │ ├── EmployeeRepository │ └── dto ├── event │ ├── Event │ ├── EventService │ └── graph └── common ├── GlobalExceptionHandler ├── ApiError └── NotFoundException

🎯 Design Decisions & Assumptions Event logging is part of the core business logic GET operations also produce events (explicit requirement) Graph endpoint prioritizes clarity over heavy analytics JPA lifecycle callbacks are used for timestamps Docker Compose preferred over Kubernetes for simplicity

✅ Evaluation Criteria Coverage ✔ Clean REST API design ✔ Proper JPA & MySQL usage ✔ One-to-Many mapping ✔ Event logging correctness ✔ Graph filtering by employee & time ✔ Validation & error handling ✔ Swagger documentation ✔ Containerized setup

👤 Author Merve Döker Senior Java Backend Engineer

This project was created as a focused technical exercise to demonstrate backend engineering fundamentals without unnecessary complexity.