order-api

Order API — Live Coding Practice

Build a small food-ordering REST API in Spring Boot, one story at a time: create and read an order, then list, update and cancel, then validation with proper error bodies, then idempotent retries and per-customer rate limiting. Delivery Hero candidates have reported being asked to "implement 2 REST APIs" and "implement a CRUD API" in their live rounds, followed by production questions about retries and abusive clients. This exercise covers that whole path: HTTP status codes, a server-side computed total, Bean Validation, RFC 9457 ProblemDetail, an Idempotency-Key that survives concurrent retries, and a Retry-After header. Time box: 45 minutes.

Tech Stack

• Java 25, Spring Boot 4.0.2 (Web MVC, Bean Validation), Maven wrapper
• JUnit 5, AssertJ, MockMvc (@SpringBootTest + @AutoConfigureMockMvc) plus plain unit tests
• No database: an in-memory ConcurrentHashMap store is enough

Getting Started

• JDK 25+
• ./mvnw spring-boot:run starts the app on http://localhost:8080. Every endpoint answers 501 Not Implemented until you build it.
• ./mvnw test. The first real test is @Disabled("story 1: remove this line to begin").

curl -i -X POST http://localhost:8080/orders \
  -H 'Content-Type: application/json' \
  -H 'Idempotency-Key: 5f1c2a' \
  -d '{"customerId":"c-1","restaurantId":"r-42",
       "items":[{"name":"Margherita","quantity":2,"priceCents":950}]}'

The skeleton gives you the application class, the controller with every endpoint mapped, an empty OrderService, the request/response records, a Clock bean and an empty RateLimiter. Change any of it.

How the Interview Runs

One user story at a time. Open INTERVIEW_TASKS.md, cover everything below the current task, start a timer. Talk through the design first, then write the tests.

Solution

git checkout solutions/order-api then ./mvnw test.

Reset to the skeleton: git checkout main -- java/order-api.