Spring Integration - Messaging Gateway with a returning value - spring-integration

Question

I'm new with Spring Integration. I need to implement a Messaging Gateway with a returning value. In order to continue some processing asynchronously after executing some synchronous steps. So I made 2 activators
#Slf4j
#MessageEndpoint
public class Activator1 {
#ServiceActivator(inputChannel = "asyncChannel")
public void async(){
log.info("Just async message");
try {
Thread.sleep(500);
} catch (InterruptedException e) {
log.error("I don't want to sleep now");
}
}
}
and
#Slf4j
#MessageEndpoint
public class Activator2 {
#ServiceActivator(inputChannel = "syncChannel")
public ResponseEntity sync(){
try {
Thread.sleep(500);
return ResponseEntity.ok("Return Http Message");
} catch (InterruptedException e) {
log.error("I don't want to sleep");
}
return ResponseEntity.badRequest().build();
}
}
The pipeline
#Configuration
public class Pipeline {
#Bean
MessageChannel asyncChannel() {
return new DirectChannel();
}
#Bean
public MessageChannel syncChannel() {
return MessageChannels.direct().get();
}
}
the gateway
#MessagingGateway
public interface ReturningGateway {
#Gateway(requestChannel = "asyncChannel", replyChannel = "syncChannel")
public ResponseEntity getSyncHttpResponse();
}
And Controller
#Slf4j
#RestController
#RequestMapping("/sync")
public class ResponseController {
#Autowired
ReturningGateway returningGateway;
#PostMapping("/http-response")
public ResponseEntity post() {
return returningGateway.getSyncHttpResponse();
}
}
So I'm not sure if thats the correct way to do what I want to do
Can you give me hand?

Answer 1

Let me try to explain some things first of all!
#Gateway(requestChannel = "asyncChannel", replyChannel = "syncChannel")
The requestChannel is where a gateway sends a message. But since you don't have any arguments in the gateway method and there is no a payloadExpression, the behavior is to "receive" from that channel. See docs for more info: https://docs.spring.io/spring-integration/docs/current/reference/html/messaging-endpoints.html#gateway-calling-no-argument-methods.
The replyChannel is where to wait for a reply, not send. In most cases a gateway relies on the replyChannel header for correlation. The request-reply pattern in messaging. We need an explicit replyChannel if it is a PublishSubscribeChannel to track a reply somehow or when we deal with the flow which we can't modify to rely on the replyChannel header. See the same gateway chapter in the docs.
Your use-case is not clear for me: you say async continuation, but at the same time the return from your gateway contract looks like a result of that sync() method. From here, please make yourself familiar with the gateway contract and then come back to us with refreshed vision for your solution.