Documentation Issue: MessageHandlerInterceptor should return a value when a Message Handler has a return value

[edreyer]

This applies to projects using the Reactor extension only (I think).

If a @CommandHandler method returns a value, any @MessageHandlerInterceptor interceptor methods must also return a value, or that value is lost and null is returned to the message sender.

e.g.

@MessageHandlerInterceptor(messageType = CommandMessage::class)
fun handleTheThing(
  cmdMsg: CommandMessage<*>,
  chain: InterceptorChain
): Any {
  return chain.proceed() // must return or value is lost
}

I discovered this via some trial and error. It might be helpful if the docs also reflected this.

[smcvb]

Thanks for this suggestion @edreyer. Returning values from command handlers is also an option when you're not using Project Reactor.

The addition Axon's Reactor Extension provides is the ReactorResultHandlerInterceptor, which I am guessing you're confusion this requirement with.
No worries though there.

At any note, I agree it would be beneficial to provide some "gotchas" when it comes to constructing interceptors. That set would thus also contain some specifics about return values from message handling functions and that the InterceptorChain's result is paramount to actually get the result back. I'd wager the Message Intercepting page would be ideal for that actually.