Transaction Service - Presentation for BrowserStack meetup in Lisbon

A Spring Boot application that processes P2P transactions asynchronously. It integrates with a Risk API to evaluate each transaction and publishes approved/rejected events to Apache Kafka.

Tech Stack

• Java 21
• Spring Boot 4.0.6
• Apache Kafka
• MockServer (for local Risk API simulation)

How It Works

1. A transaction is submitted via POST /transactions/p2p.
2. The service calls the Risk API to check if the transaction is allowed.
3. The result is published to a Kafka topic (transactions).
4. If the Risk API is unavailable, the transaction is sent to a dead-letter topic (transaction-dead-letter).
5. The status of a transaction can be queried via GET /transactions/{transactionId}/status.

API Endpoints

Method	Endpoint	Description
POST	/transactions/p2p	Submit a new P2P transaction
GET	/transactions/{transactionId}/status	Get the status of a transaction

Request Body (POST /transactions/p2p)

{
  "transactionId": "abc123",
  "amount": 100.0,
  "sender": "user1",
  "receiver": "user2"
}

Status Response

{
  "transactionId": "abc123",
  "status": "APPROVED",
  "reason": null
}

Running Locally

Prerequisites

• Java 21
• Docker & Docker Compose

1. Start infrastructure with Docker Compose

docker-compose up -d

This starts:

Service	Description	Port
mockServer	Simulates the Risk API	1080
zookeeper	Kafka coordination service	2181
kafka	Kafka broker	9092

2. Build and run the application

./gradlew bootRun

The application will be available at http://localhost:8080.

3. Stop infrastructure

docker-compose down

Configuration

Key properties in src/main/resources/application.properties:

Property	Default	Description
risk.api.url	http://localhost:1080/risk	Risk API endpoint
kafka.bootstrap-servers	localhost:9092	Kafka broker address
kafka.topic.transactions	transactions	Main Kafka topic
kafka.topic.dead-letter	transaction-dead-letter	Dead-letter Kafka topic
kafka.enabled	true	Enable/disable Kafka

Running Tests

./gradlew test

Risk API

GET /risk?transactionId=1

Response:

{
    "result": "allowed"
}

or 

{
    "result": "notAllowed"
}

Accessing MockServer Dashboard

http://localhost:1080/mockserver/dashboard