问题
Just want to confirm the intended behavior of Axon, versus what I’m seeing in my application. We have a customized Kafka publisher integrated with the Axon framework, as well as, a custom Cassandra-backed event store.
The issue I’m seeing is as follows: (1) I publish a command (e.g. CreateServiceCommand) which hits the constructor of the ServiceAggregate, and then (2) A ServiceCreatedEvent is applied to the aggregate. (3) We see the domain event persisted in the backend and published over the EventBus (where we have a Kafka consumer).
All well and good, but suppose I publish that same command again with the same aggregate identifier. I do see the ServiceCreatedEvent being applied to the aggregate in the debugger, but since a domain event record already exists with that key, nothing is persisted to the backend. Again, all well and good, however I see the ServiceCreatedEvent being published out to Kafka and consumed by our listener, which is unexpected behavior.
I’m wondering whether this is the expected behavior of Axon, or if our Kafka integrations ought to be ensuring we’re not publishing duplicate events over the EventBus.
Edit:
I swapped in Axon's JPA event store and saw the following log when attempting to issue a command to create the aggregate that already exists. This issue then is indeed due to a defect with our custom event store.
"oracle.jdbc.OracleDatabaseException: ORA-00001: unique constraint (R671659.UK8S1F994P4LA2IPB13ME2XQM1W) violated\n\n\tat oracle.jdbc.driver.T4CTTIoer11.processError
回答1:
The given explanation has a couple of holes which make it odd to be honest, and hard to pinpoint where the problem lies.
In short no, Axon would not publish an event twice as a result of dispatching the exact same command a second time. This depends on your implementation. If the command creates an aggregate, then you should see a constraint violation on the uniqueness requirement of aggregate identifier and (aggregate) sequence number. If it's a command which works on an existing aggregate, it depends on your implementation whether it is idempotent yes/no.
From your transcript I guess you are talking about a command handler which creates an Aggregate. Thus the behavior you are seeing strikes me as odd. Either the event store is custom which inserts this undesired behavior, or it's due to not using Axon's dedicated Kafka Extension.
Also note that using a single solution for event storage and message distribution like Axon Server will omit the problem entirely. You'd no longer need to configure any custom event handling and publication on Kafka at all, saving you personal development work and infrastructure coordination. Added, it provides you the guarantees which I've discussed earlier. From more insights on why/how of Axon Server, you can check this other SO response of mine.
来源：https://stackoverflow.com/questions/64930996/axon-event-published-multiple-times-over-eventbus