 Java, Spring and Web Development tutorials  1. Overview
In this tutorial, we’ll explore the challenges of maintaining data consistency between database operations and messaging. We’ll begin by examining the problem and then implement the transactional outbox pattern to address key issues.
Next, we’ll introduce Eventuate Tram to populate the outbox table with messages ready to be published to a specific topic. Finally, we’ll run the Eventuate CDC Service in its own Docker container to monitor changes in the outbox table and publish the corresponding messages through Kafka.
2. When Do We Need Transactional Messaging?
Just like we often rely on database transactions to ensure atomicity in data operations, we might also need to publish messages to a message broker atomically. For instance, sometimes we need to save data to the database and publish a message to a message broker as a single, atomic operation.
Although this may seem straightforward, it presents some hidden challenges. Let’s explore this problem using a simple use-case where we’ll try to save a Comment entity to the database and publish an event to the baeldung.comment.added Kafka topic.
A naive approach is to publish the message within a transactional block. For instance, if we use Spring Data JPA for database operations and KafkaTemplate to send messages, our domain service might look like this:
@Service
class CommentService {
private final CommentRepository comments;
private final KafkaTemplate<Long, CommentAddedEvent> kafkaTemplate;
// constructor
@Transactional
public Long save(Comment comment) {
Comment saved = this.comments.save(comment);
log.info("Comment created: {}", saved);
CommentAddedEvent commentAdded = new CommentAddedEvent(saved.getId(), saved.getArticleSlug());
kafkaTemplate.send("baeldung.comment.added", saved.getId(), commentAdded);
}
}
However, this approach publishes the Kafka message before the database commit happens. In other words, we risk sending the message even if the transaction fails at commit time, and the operation is rolled back.
On the other hand, if we try removing the @Transactional annotation, Spring won’t roll back the DB inserts when the publishing to Kafka fails. Needless to say, neither approach is ideal, as both can lead to inconsistent data across systems.
3. The Transactional Outbox Pattern
We can implement the Transactional Outbox pattern to ensure eventual consistency in our system. This pattern involves saving messages in a special database table (i.e., the “outbox”) within the same transaction as our data changes.
Afterward, a separate process reads the outbox and publishes the messages to the message broker. It then updates, removes, or marks the records as published to keep track of what has been sent:
A similar issue can happen when publishing the event and updating the outbox table. We want to avoid updating the outbox unless the event is successfully published, to prevent losing events. On the other hand, if the event is sent but the database update fails, the system might retry and send the event again. This can result in duplicate events, but it’s better than losing them.
Overall, this approach ensures “at least once” delivery, prioritizing reliability over avoiding duplicates.
4. Demo Application Overview
In this article, we’ll work with a simple Spring Boot application that manages article comments for a blogging site like Baeldung. Users can add comments to an article by sending POST requests to the /api/articles/{slug}/comments endpoint:
curl --location "http://localhost:8080/api/articles/oop-best-practices/comments" \
--header "Content-Type: application/json" \
--data "{
\"articleAuthor\": \"Andrey the Author\",
\"text\": \"Great article!\",
\"commentAuthor\": \"Richard the Reader\"
}"
For quick testing, we can run this curl command using the post-comment.bat script located in src/rest/resources.
When a Comment entity is saved to the database, the system also publishes a Kafka message. This message includes the newly saved Comment ID and the article slug, and is sent to a topic named baeldung.comment.added.
To set up the local environment, we’ll use Docker to start containers for PostgreSQL, Kafka, and Eventuate’s CDC Service. This can be done easily using the eventuate-docker-compose.yml file located in src/test/resources. We’ll also start the Spring Boot application locally using the eventuate profile:
To see everything in practice, we can also refer to our integration test, EventuateTramLiveTest.
5. Eventuate Tram
Eventuate is a Java platform that supports core microservices patterns like CQRS, Event Sourcing, and Transaction Sagas. One of its components, Eventuate Tram, enables reliable inter-service communication through the transactional outbox pattern and event publishing.
Let’s integrate Eventuate Tram to ensure at-least-once delivery of Kafka messages in our example. First, we add the necessary dependencies for eventuate-tram-spring-jdbc-kafka and eventuate-tram-spring-events to our pom.xml:
<dependency>
<groupId>io.eventuate.tram.core</groupId>
<artifactId>eventuate-tram-spring-jdbc-kafka</artifactId>
<version>0.36.0-RELEASE</version>
</dependency>
<dependency>
<groupId>io.eventuate.tram.core</groupId>
<artifactId>eventuate-tram-spring-events</artifactId>
<version>0.36.0-RELEASE</version>
</dependency>
Then, we’ll import two configuration classes:
@Configuration
@Import({
TramEventsPublisherConfiguration.class,
TramMessageProducerJdbcConfiguration.class
})
class EventuateConfig {
}
Additionally, we’ll need to change our CommentAddedEvent record and make sure it implements the Eventuate’s DomainEvent interface:
record CommentAddedEvent(Long id, String articleSlug) implements DomainEvent {
}
Finally, we’ll refactor the domain service, which contains all the logic. This time, instead of directly publishing to Kafka, we’ll use the DomainEventPublisher bean to publish a CommentAddedEvent:
@Service
class CommentService {
private final CommentRepository comments;
private final DomainEventPublisher domainEvents;
// constructor
@Transactional
public Long save(Comment comment) {
Comment saved = this.comments.save(comment);
log.info("Comment created: {}", saved);
CommentAddedEvent commentAdded = new CommentAddedEvent(saved.getId(), saved.getArticleSlug());
domainEvents.publish(
"baeldung.comment.added",
saved.getId(),
singletonList(commentAdded)
);
return saved.getId();
}
}
As a result, whenever we persist a Comment entity, we’ll also insert a CommentAddedEvent entry into the eventuate.message table within the same transaction.
Let’s verify this by connecting to the database and querying the comment table:
mydb=# select * from comment;
id | article_slug | comment_author | text
----+-------------------+--------------------+------------------
1 | oop-best-practices | Richard the Reader | Great article!
(1 row)
Let’s also query the message table from the eventuate schema. Assuming the CDC Service is down, we can expect to retrieve only one message, marked as unpublished:
mydb=# select id, destination, published from eventuate.message;
id | destination | published
--------------------------------------+----------------------------+-----------
0000019713d8ffe4-e86a640584cf0000 | baeldung.comment.added | 0
(1 row)
6. Eventuate’s CDC Service
Change data capture (CDC) is a technique used to detect and track changes such as inserts, updates, and deletes in a database so those changes can be captured and sent to other systems. Therefore, the Eventuate CDC Service is the component that captures changes for the outbox table and publishes them as events to our message broker.
The Eventuate CDC service currently supports several message brokers, including Apache Kafka, ActiveMQ, RabbitMQ, and Redis. For databases, it uses efficient transaction log tailing with MySQL through the binlog protocol and with Postgres using WAL. Alternatively, for other JDBC-compatible databases, it falls back on a less efficient polling approach to detect changes.
If we spin up the CDC service and re-run the test, we’ll notice that the entries from eventuate.messages table will be marked as published:
mydb=# select id, destination, published from eventuate.message;
id | destination | published
--------------------------------------+----------------------------+-----------
0000019713d8ffe4-e86a640584cf0000 | baeldung.comment.added | 1
(1 row)
Lastly, we can use kafka-console-consumer.sh to verify that the message was successfully published to our topic:
{
"payload": "{ \"id\": 1, \"articleSlug\": \"oop-best-practices\" }",
"headers": {
"PARTITION_ID": "1",
"event-aggregate-type": "baeldung.comment.added",
"DATE": "Tue, 27 May 2025 22:24:37 GMT",
"event-aggregate-id": "1",
"event-type": "com.baeldung.eventuate.tram.domain.CommentAddedEvent",
"DESTINATION": "baeldung.comment.added",
"ID": "0000019713d8ffe4-e86a640584cf0000"
}
}
As expected, the message was delivered and the outbox table was updated accordingly.
7. Conclusion
In this article, we explored the complexities of transactional messaging, beginning with the challenge of atomically performing a database operation and publishing a domain event. We uncovered hidden difficulties and saw how the transactional outbox pattern helps address them.
Then, we used the Eventuate Tram framework, which implements this pattern for us. Together with the Eventuate CDC Service, which leverages change data capture to monitor the outbox table and send messages to Kafka, we achieved eventual consistency and guaranteed at-least-once delivery in our system.
As always, the code presented in this article is available over on GitHub. The post Transactional Messaging for Microservices Using Eventuate Tram first appeared on Baeldung.
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