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README.md

Delivery Food System — Interview Practice (Code Review & Refactoring)

A Spring Boot order processing system designed as a live coding interview practice repository. Unlike the Bookstore and Todo API projects (which have CRUD gaps to fill), this project is a code review and refactoring challenge — the candidate reviews existing code, identifies code smells, proposes improvements, and implements the most important ones.

The codebase is intentionally messy with architectural issues, tight coupling, missing patterns, and unused infrastructure — giving interviewers realistic scenarios to test a candidate's ability to read, critique, and improve existing code.

Originally sourced from ensiehrahbar/delivery-food-system.

Tech Stack

  • Java 21
  • Spring Boot 3.4.1
  • Spring Data MongoDB
  • Spring Kafka (Redpanda)
  • Docker Compose (MongoDB + Redpanda)
  • Maven
  • Lombok

Getting Started

Prerequisites

  • Java 21+
  • Docker & Docker Compose
  • Maven 3.9+

Start Infrastructure

docker-compose up -d

This starts:

  • MongoDB on port 27017
  • Redpanda (Kafka-compatible) on port 9092
  • Redpanda Console on port 8090

Run the Application

./mvnw spring-boot:run

The app starts on http://localhost:8080.

Run Tests

./mvnw clean test

Domain

A developer created an order processing system where users can place orders, and the system processes them by applying discounts, generating invoices, and dispatching events. When the order is processed, it is marked as processed. If the order value is above 100 EUR, the application applies a 10% discount.

Order API

  • GET /order/hello — Health check
  • POST /order — Place an order

Example Endpoints (reference implementations)

  • GET /test/mongo — Test MongoDB integration
  • GET /test/kafka — Test Kafka integration

Project Structure

src/main/java/
├── com/foodtakeway/          ← Main code (focus here)
│   ├── DeliveryFoodSystemApplication.java
│   ├── Order.java                ← Entity (many issues)
│   ├── OrderApi.java             ← Controller + nested DTO
│   └── OrderService.java         ← Service (in-memory, blocking, tightly coupled)
└── examples/                     ← Reference implementations (MongoDB, Kafka)
    ├── ExampleApi.java
    ├── ExampleKafka.java
    ├── ExampleMongoDB.java
    └── event/
        └── Event.java

Key Code Smells (for interviewer reference)

Area Issue
Persistence Orders stored in ArrayList — MongoDB dependency exists but isn't used
Coupling placeOrder() synchronously calls processOrder() which blocks for 10 seconds
Encapsulation Fields are package-private, no getters/setters, no @Id on Order
Architecture No repository pattern, no layered architecture, business logic in service
Validation No input validation on API or domain level
Error handling No exception handling, no global error handler
Logging Uses System.out.println instead of SLF4J
Event system Kafka available but not integrated — events only logged to console
ID generation Random.nextInt(1000) — collision risk
HTTP responses Returns 200 for creation (should be 201), generic string responses
Code organization OrderRequest DTO nested inside controller file
Testing Only contextLoads() — no business logic tests

Sample Requests

Place an Order

curl -X POST http://localhost:8080/order \
  -H "Content-Type: application/json" \
  -d '{"userEmail": "john@example.com", "amount": 150.0}'

Note: This request will block for ~10 seconds due to the intentional Thread.sleep(10000) in processOrder().

Interview Tasks

See INTERVIEW_TASKS.md for the full task list organized by level (Junior / Mid / Senior).

Tips for Interviewers

  • Let candidates explore the codebase for 10 minutes before starting
  • The examples/ package shows how MongoDB and Kafka work — candidates can reference it
  • Encourage candidates to talk through what they see before coding
  • If a change would take too long, a TODO comment is sufficient
  • Focus on thought process and prioritization as much as implementation