Support for event outbox

[odrotbohm]

[xsreality]

@odrotbohm Excited about this! Curious if ordering of events will also be implemented in the outbox across retries?

[xsreality]

Looked deeper into the implementation branch, looks promising and kudos on the Namastack outbox library @rolandbeisel ! I am currently using Spring Modulith in production to publish events and handle them for at-most once use cases where ordering is not critical. But now would like to extend that to critical use cases with the outbox pattern. I have two questions if you can answer please :)

1. If an existing use of Spring Modulith is switched to outbox (by setting spring.modulith.events.externalization.mode=outbox), how does the behaviour change? What happens to existing events being published and the @ApplicationModuleListeners? Would they stop working?

2. Is the Spring Modulith implementation of outbox with Namastack only meant for externalization of events? I am looking to publish the outbox events and have the handler perform actions on an external system. I don't want to externalize the events to Kafka or other messaging broker. Looking at the Namastack docs it seems to be supported by writing a Handler. But can the same behaviour be implemented via Spring Modulith?

[odrotbohm]

Thanks for the feedback. You'll find a lot of those details in the docs going forward. I'd just like to state that we're going to start with offering Namastack as a drop-in replacement for the externalization of events to provide more advanced guarantees than our current mechanism allows. If the switch is flipped, all events annotated with @Externalized (or otherwise configured as to be externalized) will be handled by Namastack, and registering an OutboxHandler will allow you to do whatever you want with the event. That said, we will provide automatic broker distribution with our existing broker integration artifacts.

Using the outbox approach to the internal event distribution is on our radar already, but we would like to tackle this step by step.

[xsreality]

Thanks @odrotbohm! Glad to see where you intend to take Spring Modulith. I think the modularity features (the main reason we are using Spring Modulith) combined with real Outbox behaviour would make it an invaluable tool.

I would love to test out this feature and help as needed.