axon-cqrs-sample

This sample is a simple demonstration of how to use the Java based AxonFramework for CQRS/ES. This demo aims to illustrates the following features of the platform...

• Command and Query Responsibility Separation (CQRS).
• Event Sourcing using an Aggregate Root based Repository
• Using MongoDb as your EventStore
• Publishing and Subscribing to distributed events using RabbitMQ

Before you start:

1. Install Java and check it's working for you.
2. Install Maven and check it's working for you.
3. Install RabbitMQ then go to the Management Console and add a user called 'test' with a password of 'password'.
4. Install MongoDb (running locally against localhost requires no username or password)

Run the demo:

To run the sample, either execute the ToDoItemDemoRunner.java class in your IDE or use Maven from the command line...

mvn clean compile exec:java

If everything is hanging together as intended, you should see a whole bunch console entries describing what has happened.

After running the demo:

The ToDoItemDemoRunner.java demonstrates CQRS and EventSourcing using the AxonFramework. The ToDoItem.java class models the "Aggregate Root" for a To-Do Item (Aggregate Root is a concept taken from the Domain Driven Design discipline by Eric Evans, popular in the CQRS/ES community).

The code in the commands & events packages model the commands an events from the business domain for this use case. As you'll discover the CreateToDoItemCommand causes a ToDoItemCreatedEvent and the MarkCompletedCommand causes a ToDoItemCompletedEvent. The handling of commands and the firing of events is handled by the ToDoItem class (the 'aggregate root').

In Axon, commands are sent to a CommandGateway and delivered to the 'aggregate root' by Axon. Axon takes care of storing and publishing any events that follow the command being handled using it's built-in 'EventRepository'. This EventRepository brings together MongoDb as the database persistence mechanism and RabbitMQ as the Pub/Sub mechanism for events.The spring configuration in sampleContext.xml describes how all the integrated components are wired together. Annotations wire much of this functionality together.

Once the demo has run, look in the mongodb 'cqrs' database's 'events' collection for your stored events. It should contain your event documents. Likewise, look in the RabbitMQ Console to see your message queue's metrics. It should show that messages did indeed flow through the Axon.EventBus exchange & it's associated queue when the demo was run.

Finally, the ToDoMaterialViewManager.java is an event listener that manages the content of the simple materialised view held in the MaterialisedView.java singleton. In CQRS, materialised views are used as the basis of queries and offer a simplified means of presenting information to users and systems without the need to refer to the more complicated 'aggregate root' model.

For more information on CQRS and Event Sourcing, check out Exploring CQRS and Event Sourcing by Microsoft Press.