Axon save events in event stroe first or publish events in Event Bus first?

Question

I am trying to make a sequence diagram for Axon workflow. However, I am not sure what's the event processing order. Does Axon save events in the Event Store first, or does it publish the events on the Event Bus first?

Answer 1

What might be curious to know is that from Axon's perspective, the EventStore is a more specific implementation of an EventBus. You can see this when looking at the AbstractEventStore implementation, which extends the AbstractEventBus implementation.

The operation to publish will group the events into a single transaction. Or, in Axon terms, the UnitOfWork. Before doing anything with it, it'll invoke the prepareCommit method to prepare the transaction/UnitOfWork for committing. It is here that Axon invokes SubscribingEventProcessors and stores events.

However, storing events happens first, and providing events to the Subscribing Processors comes later. From that perspective, you can state that storing in the EventStore comes first, and publishing on the EventBus comes second.