Publish events to Spring Cloud Stream channels

[mehdichitforoosh]

Hi.
Can Axon framework provide publish events directly to Spring cloud stream?
Or we should use spring-amqp in event-driven microservices based on Spring cloud?
Thanx.

[smcvb]

This extension is currently mainly focused on using Spring Cloud discovery to route commands from one Axon instance to another.

I'd assume Spring Cloud Stream would be a workable fit to route events from one instance to another, sure, but that's not what this extension is currently focused on.

A pull request to provide this functionality would definitely be valuable of course!
If however you're hard pressed to provide that, then I'd like to recommend you to use Axon Server to route any type of message in your system (note that Axon Server has a free edition). Biggest benefit of picking Axon Server is that you have to provide very little configuration at all.

If this is not a workable fit for you, you can indeed use the Axon AMQP or Axon Kafka (still in beta) extensions.

Ow and lastly, @mehdichitforoosh, would you mind sharing whether you'll provide a PR to implement Spring Cloud Stream support for message?

[mehdichitforoosh]

@smcvb
Hi.
Yes I'm interested in doing this but need help in reading the codes and integrate Axon API with Spring Cloud Stream.

[mehdichitforoosh]

@smcvb
It seems we should start with implementing EventBus interface in org.axonframework.eventhandling package with Spring Cloud Stream API in publish methods and auto configure the SpringCloudStreamEventBus for avoiding creation of SimpleEventBus in AxonAutoConfiguration.
Do you agree?

[smcvb]

An EventBus would very likely be a step to far in this scenario.

Firstly, you need to deduce whether the Spring Cloud Stream API is a push or pull mechanism.
Based on this, you can figure out whether this approach could be an event source you subscribe on (pushing message) or an event source you can stream from (pulling messages).

Both solution have a dedicated interface in Axon, respectively the SubscribableMessageSource and the StreamableMessageSource. An EventBus for example only implements theSubscribableMessageSource, whilst an EventStore also is a StreamableMessageSource.

To see actual worthwhile code samples for both usages, I'd suggest you have a look at the Axon AMQP Extension for the subscribing solution and Axon Kafka Extension for the streaming solution.

[mehdichitforoosh]

Spring Cloud Stream API is push and also a pull mechanism(Source,Sink and Process interfaces).It has SubscribableChannel for input messages and a MessageChannelfor output messages from Spring Messaging package.It seems we should use both of EventStore and EventBus.I am currently reading source codes from Axon Framework and Spring Cloud Stream to understand relationships between objects.
I think solutions from Axon AMQP Extension can help.
Soon, I'll share with you what I've find out for implementing classes.
Thanx.

[smcvb]

Good stuff @mehdichitforoosh!
However, I'd like to suggest to focus on the SubscribableMessageSource and StreamableMessageSource interfaces in the framework than. Both message sources are the required inputs for respectivelky providing events to a SubscribingEventProcessor and a TrackingEventProcessor, so staying closer to that API is beneficial I think.

[mehdichitforoosh]

Hi. @smcvb @abuijze
There are classes with names InboundEventMessageChannelAdapter and OutboundEventMessageChannelAdapter in Axon Spring module based on Spring Messaging MessageChannel and MessageHandler interfaces.
Spring Cloud Stream is based on Spring Messaging MessageChannel interface,too.
If we configure Spring Cloud Stream and inject Output channel (MessageChannel) object created by Stream to OutboundEventMessageChannelAdapter object for sending messages and subscribe InboundEventMessageChannelAdapter object as a MessageHandler to SubscribableChannel object from Stream.It seems to work without developing more stuff.But I have not tested yet.
But for PollableMessageSource interface that have been added in Spring Cloud Stream 2.0,I think some classes are needed.
Do you agree?

[mehdichitforoosh]

Hi.
I don't have permission to push the code changes to axon-springcloud-4.1.x branch.
I mistakenly created a new pull request.This is the first time i want to contribute in a github project.:-)
Please check these examples for integrating Axon Framework with Spring Cloud Stream and Spring Integration.
Integrating Axon with Spring Cloud Stream
It works for me.Of course this is an initial version.
@smcvb @abuijze