Throw exception in saga

Axon Framework
, , ,
1

I have the below api to create a biller

      @PostMapping
      fun createBiller(@RequestBody @Valid action: CreateBillerRequest, principal: Principal): Mono<ExternalId> {
        return commandGateway.send(
          RequestCreateBiller(
            externalId = ExternalId(),
            code = action.code,
            description = action.description,
            type = action.type,
            enabled = action.enabled
          )
        )
      }

a simple aggregate

      @Aggregate
      internal class BillerAggregate() {

        @AggregateIdentifier
        private lateinit var externalId: ExternalId

        @CommandHandler
        @CreationPolicy(AggregateCreationPolicy.CREATE_IF_MISSING)
        fun handle(command: RequestCreateBiller) {
          AggregateLifecycle.apply(
            CreateBillerRequested(
              command.externalId, command.code, command.description, command.type, command.enabled, command.actionBy
            )
          )
        }

        @EventSourcingHandler
        fun on(event: CreateBillerRequested) {
          externalId = event.externalId
        }
      }

In the below saga i invoke an external system to create the biller. In the error case i throw the exception WebClientFunctionalException

	  @StartSaga
      @SagaEventHandler(associationProperty = "externalId")
      fun handle(event: CreateBillerRequested) {
        SagaLifecycle.associateWith("externalId", event.externalId.asReference())

        webClient.post().uri(applicationProperties.edg.urls.biller)
          .body(Mono.just(event), CreateBillerRequested::class.java).retrieve()
          .onStatus({ it.isError }) {
            val statusCode = it.statusCode()
            it.bodyToMono(String::class.java)
              .switchIfEmpty(Mono.just(statusCode.value().toString() + " " + statusCode.reasonPhrase))
              .flatMap { response ->
                val message = WebClientErrorHandler.getOriginalApiErrorMessage(it, response).orElse(response)
                return@flatMap Mono.error(WebClientFunctionalException(message))
              }
          }.bodyToMono(ConfirmBillerResponse::class.java)
          .block()
      }

The problem is when i test the api in the error case, the api returns always status 200.
So my questions are:
Is the invokation of the external system in this case in the right place?
In case of error, how can i make the api returns the exception WebClientFunctionalException?

2

The main issue here is that the event handler that starts the saga runs asynchronously, so the command is already completed before the event handler is executed. If the call to the external system is critical for the command to succeed, you can execute this in the command handler.