is it possible to create a queue listener using web flux spring integration? - spring-boot

Question

#Component
#RequiredArgsConstructor
public class EventListener {
private final EventProcessingService eventProcessingService;
#JmsListener(destination = "inputQueue", constainerFactory = "myContainerFactory)
public void receiveMessage(Message message) {
eventProcessingService.doSome(message).subscribe(); // return Mono<Void>
}
}
#Service
public class EventProcessingService {
public Mono<Void> doSome(Message message) {
//...
}
}
#Configuration
#RequiredArgsConstructor
public class MqIntegration {
private final ConnectionFactory connectionFactory;
#Bean
public Publisher<Message<String>> mqReactiveFlow() {
return IntegrationFlows
.from(Jms.messageDrivenChannelAdapter(this.connectionFactory)
.destination("testQueue"))
.channel(MessageChannels.queue())
.toReactivePublisher();
}
}
I have some webflux application which interacts with ibm mq and a JmsListener which listens for messages from the queue when a message is received EventProcessingService makes requests to other services depending on the messages.
I would like to know how I can create a JmsListener that works with reactive threads using Spring Integration. In other words I want to know if it is possible to create an Integration flow which will receive messages from the queue and call the EvenProcessingService when the messages are received so that it does not have a negative effect on the threads inside webflux application

Answer 1

I think we need to clean up some points in your question.
WebFlux is not a project by itself. It is Spring Framework module about Web on top of reactive server: https://docs.spring.io/spring-framework/docs/current/reference/html/web-reactive.html#spring-webflux
The #JmsListener is a part of another Spring Framework module - spring-jms. And there is nothing relevant to threads used by reactive server for WebFlux layer. https://docs.spring.io/spring-framework/docs/current/reference/html/integration.html#jms
Spring Integration is a separate project which implement EIP on top of Spring Framework dependency injection container. It indeed has its own WebFlux module for channel adapters on top of WebFlux API in Spring Framework: https://docs.spring.io/spring-integration/docs/current/reference/html/webflux.html#webflux. And it also has a JMS module on top of JMS module from Spring Framework: https://docs.spring.io/spring-integration/docs/current/reference/html/jms.html#jms. However there is nothing related to #JmsLisntener since its Jms.messageDrivenChannelAdapter() fully covers that functionality and from a big height it does it the same way - via MessageListenerContainer.
All of this is might not be relevant to the question, but it is better to have a clear context of what you are asking so we will feel that we are on the same page with you.
Now trying to answer to your concern.
As long as you don't deal with JMS from WebFlux layer (#RequestMapping or WebFlux.inboundGateway()), you don't effect those non-blocking thread. The JMS MessageListenerContainer spawns its own threads and perform pulling from the queue and message processing.
What you are explaining with your JMS configuration and service looks more like this:
#Bean
public IntegrationFlow mqReactiveFlow() {
return IntegrationFlows
.from(Jms.messageDrivenChannelAdapter(this.connectionFactory)
.destination("testQueue"))
.handle(this.eventProcessingService)
.nullChannel();
}
There is really no reason to shift messages just after JMS into a QueueChannel since JMS listening is already an async operation.
We need that nullChannel in the end of your flow just because your service method returns Mono and framework knows nothing what to do with that. Starting with version 5.4.3 the NullChannel is able to subscribe to the Publisher payload of the message produced to it.
You could have though a FluxMessageChannel in between to really simulate a back-pressure for JMS listener, but that won't make to much different for your next service.

Answer 2

I think you are going to have to bypass #JmsListener as that is registering an on message, which although asynchronous isn't going to be reactive. JMS is essentially blocking, so patching a reactive layer on top, is going to be just a patch.
You will need to use the Publisher that you have created to generate the back pressure. I think you are going to have to define and instantiate your own listener bean which does something on the lines of :
public Flux<String> mqReactiveListener() {
return Flux.from(mqReactiveFlow())
.map(Message::getPayload);
}