Spring AMQP is an alternative approach to distributing events, besides Axon Server which is the default.
Axon provides out-of-the-box support to transfer events to and from an AMQP message broker, such as RabbitMQ.
To use the Spring AMQP components from Axon, make sure the axon-amqp module is available on the classpath.
Forwarding events to an AMQP Exchange
The SpringAMQPPublisher forwards events to an AMQP exchange. It is initialized with a SubscribableMessageSource, which is generally the EventBus or EventStore. Theoretically, this could be any source of events that the publisher can subscribe to.
To forward events generated in the application to an AMQP Channel, a single line of application.properties configuration is sufficient:
This will automatically send all published events to the AMQP Exchange with the given name.
By default, no AMQP transactions are used when sending. This can be overridden using the axon.amqp.transaction-mode property, and setting it to transactional or publisher-ack.
Note that exchanges are not automatically created. You must still declare the Queues, Exchanges and Bindings you wish to use. Check the Spring documentation for more information.
Reading events from an AMQP Queue
Spring has extensive support for reading messages from an AMQP Queue. However, this needs to be 'bridged' to Axon, so that these messages can be handled from Axon as if they are regular event messages.
The SpringAMQPMessageSource allows event processors to read messages from a queue instead of the event store or event bus. It acts as an adapter between Spring AMQP and the SubscribableMessageSource needed by these processors.
To receive events from a queue and process them inside an Axon application, you need to configure a SpringAMQPMessageSource: