This repository is part of the Kafka Contract Enforcement initiative:

• 🔧 Starter: https://github.com/mathias82/spring-kafka-contract-starter
• 🌐 Live Demo: https://mathias82.github.io/spring-kafka-contract-demo/

🧪 Spring Kafka Contract -- Demo Project

A fully runnable demo showcasing fail-fast Kafka Schema Registry contract validation using Spring Boot, Apache Kafka, Avro, and Confluent Schema Registry.

This project demonstrates how the
👉 spring-kafka-contract-starter
can prevent broken Kafka deployments by validating schema contracts at application startup.

---

🎯 What This Demo Shows

This demo answers a very practical question:

What actually happens when schemas are missing, incompatible, or evolve incorrectly in a real Spring Boot Kafka application?

It demonstrates:

• Kafka producer & consumer with Avro\
• Schema Registry subject creation\
• Schema evolution (compatible & incompatible)\
• Fail-fast application startup when contracts are broken\
• How startup validation prevents late production failures

This is not a library, but a reference project you can run, inspect, and experiment with.

---

🧩 Architecture

Spring Boot Producer
↓
Apache Kafka
↓
Spring Boot Consumer
↓
PostgreSQL

Schema Registry (Avro)
↑
Contract validation at startup

---

🚀 Tech Stack

• Java 17\
• Spring Boot 3.x\
• Apache Kafka\
• Confluent Schema Registry\
• Avro\
• PostgreSQL\
• Docker & Docker Compose

---

🔗 Related Project

This demo is built to showcase:

👉 Spring Kafka Contract Starter
https://github.com/mathias82/spring-kafka-contract-starter

The starter enforces Kafka schema contracts before the application starts.

---

📦 How to Run the Demo

1️⃣ Start infrastructure

docker compose up -d

This starts Kafka, Schema Registry and PostgreSQL.

2️⃣ Run the applications

./mvnw spring-boot:run

3️⃣ Observe Startup Validation

• Application starts if schemas are valid\
• Application fails if a subject is missing or incompatible

Broken contracts = no startup.

---

📄 Configuration Example

kafka: contract: enabled: true compatibility: BACKWARD registry: type: confluent url: http://localhost:8081 subjects: - name: order-events-value schema-file: classpath:schemas/order-event.avsc

---

📚 Learn More

Medium article:
https://medium.com/@mstauroy/fail-fast-kafka-schema-contracts-in-spring-boot-before-production-breaks-1b080204b49e

Reddit discussion:
https://www.reddit.com/r/apachekafka/comments/1q43hs6/failfast_kafka_schema_registry_compatibility/

---

👀 Who This Demo Is For

• Kafka + Schema Registry users\
• Spring Boot engineers using Avro\
• Teams dealing with schema evolution\
• Anyone bitten by late Kafka failures

---

⭐ Final Note

This demo exists to make schema contracts visible and testable.

If you find it useful, check out the starter: https://github.com/mathias82/spring-kafka-contract-starter