CQRS + Event sourcing using Spring Cloud Stream

CQRS stands for Command Query Responsibility segregation, which means that you can use a different model to update information and a different model to read information. Typically by different model we mean actual object model and probably on different machines.

I this example I am using

• Kafka as event store (for event sourcing)
• MongoDB for Write store
• Could use Elasticsearch for Read store (full text capabilities)
• Spring Cloud Stream to abstract the event store and the publish/subscribe mechanism

This is actually a precursor project to applying Spring Data Flow which will handle instance count, partitioning between consumer/producers and ad-hoc creation of data micro-services using familiar Java API and known semantics from Spring Integration like Sink, Source, Transformer

Producer

On the producer side, a small app backed by mongodb is created. Spring Data provides special hooks like afterSave, afterDelete that are overridden in the application to apply event sourcing.

Consumer

Consumer handles the event and from the producer and just logs it but could eventually save it to elasticsearch for full text querying

class UserListener extends AbstractMongoEventListener<User> {

	......

	@Override
	public void onAfterSave(AfterSaveEvent<User> event) {
		...
	}

	@Override
	public void onAfterDelete(AfterDeleteEvent<User> event) {
		...
	}

}

How do I set this up?

Run Kafka, Zookeeper, Kafka manager, MongoDB

docker-compose up -d

Build Producer/Consumer

mvn -f producer/pom.xml clean package mvn -f consumer/pom.xml clean package

Run Producer

java -jar producer/target/producer.jar

Run Consumer

java -jar consumer/target/consumer.jar

Do a GET request on producer

curl http://localhost:8080/