What happens when your modular monolith needs to share events with external systems? In this tutorial, I'll show you how Spring Modulith makes it incredibly easy to externalize events to message brokers like Apache Kafka with just a single annotation.

We'll build a Spring Modulith application from scratch that publishes events both internally across modules and externally to Apache Kafka. Starting from Spring Initializr, we'll set up a social media posting app with Docker Compose support for Postgres and Kafka, create internal module communication with application events, and then externalize those same events to a Kafka topic using the @Externalized annotation. By the end, you'll understand the complete flow from event publishing to both internal listeners and external brokers.

- How to set up a Spring Modulith project with Kafka and Postgres using Docker Compose
- Understanding the difference between internal module events and externalized events
- Using the @Externalized annotation to push events to a Kafka topic
- Configuring Kafka consumers to deserialize and read externalized events
- How Spring Modulith and Spring for Apache Kafka work together seamlessly

0:00 - Intro & What We're Building
1:15 - Why Externalize Events?
2:30 - Spring Modulith Documentation Overview
4:15 - Creating the Project on start.spring.io
6:00 - Setting Up Docker Compose (Postgres & Kafka)
7:30 - Building the Post Module (Model, Controller, Service)
12:00 - Publishing Events with ApplicationEventPublisher
14:00 - Internal Event Listener with @ApplicationModuleListener
15:30 - Externalizing Events with @Externalized Annotation
17:00 - Creating a Kafka Listener
18:30 - Running & Testing the Application
21:00 - Fixing the Byte Array Deserialization Issue
22:30 - Recap & Wrap Up


🔗Resources & Links mentioned in this video:
GitHub Repository: https://github.com/danvega/modulith-externalized-events
Spring Modulith Documentation: https://docs.spring.io/spring-modulith/reference/events.html
Spring Initializr: https://start.spring.io

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Dan Vega

A walkthrough tutorial on combining Spring Modulith with Apache Kafka to externalize application events. Starting from scratch using Spring Initializr, it covers setting up a modular monolith with internal event publishing via ApplicationEventPublisher, listening to events internally with @ApplicationModuleListener, and externalizing events to a Kafka topic using the @Externalized annotation. The demo includes Docker Compose setup for Postgres and Kafka, configuring Kafka consumer properties (including value deserializer), and troubleshooting common issues like topic name mismatches and byte array deserialization.

How to Externalize Events in Spring Modulith with Kafka