Async split/aggregate gateway flows - spring-integration

Question

I'm trying to build a recipe for asynchronous orchestration using spring integration gateways (both inbound and outbound). After seeing an example here, I tried using scatter-gather like this:
#Configuration
public class IntegrationComponents {
#Value("${rest.endpoint.base}")
private String endpointBase;
#Bean
public HttpRequestHandlingMessagingGateway inboundGateway() {
return Http.inboundGateway("/test-inbound-gateway-resource")
.requestMapping(mapping -> mapping.methods(HttpMethod.POST))
.requestTimeout(3000)
.replyTimeout(3000)
.get();
}
#Bean
public HttpRequestExecutingMessageHandler outboundGateway1() {
return Http.outboundGateway(endpointBase + "/test-resource-1")
.httpMethod(HttpMethod.POST)
.expectedResponseType(String.class)
.get();
}
#Bean
public HttpRequestExecutingMessageHandler outboundGateway2() {
return Http.outboundGateway(endpointBase + "/test-resource-2")
.httpMethod(HttpMethod.POST)
.expectedResponseType(String.class)
.get();
}
#Bean
public StandardIntegrationFlow integrationFlow() {
ExecutorService executor = Executors.newCachedThreadPool();
IntegrationFlow flow1 = IntegrationFlows.from(MessageChannels.executor(executor))
.handle(outboundGateway1())
.get();
IntegrationFlow flow2 = IntegrationFlows.from(MessageChannels.executor(executor))
.handle(outboundGateway2())
.get();
return IntegrationFlows
.from(inboundGateway())
.transform(String.class, String::toUpperCase)
.channel(MessageChannels.executor(executor))
.scatterGather(
scatterer -> scatterer
.applySequence(true)
.recipientFlow(flow1)
.recipientFlow(flow2),
gatherer -> gatherer
.outputProcessor(messageGroup -> {
List<Message<?>> list = new ArrayList<>(messageGroup.getMessages());
String payload1 = (String) list.get(0).getPayload();
String payload2 = (String) list.get(1).getPayload();
return MessageBuilder.withPayload(payload1 + "+" + payload2).build();
}))
.get();
}
}
This executes, but my payloads are swapped, because in this case outboundGateway1 takes longer to execute than outboundGateway2. Payload 2 comes first, then payload 1.
Is there a way to tell scatter-gather to define/maintain order when sending to the output processor?
On a similar note, maybe split/aggregate and/or using a router is a better pattern here? But if so, what would that look like?
I tried the following split/route/aggregate, but it failed saying "The 'currentComponent' (org.springframework.integration.router.RecipientListRouter#b016b4e) is a one-way 'MessageHandler' and it isn't appropriate to configure 'outputChannel'. This is the end of the integration flow.":
#Configuration
public class IntegrationComponents {
#Value("${rest.endpoint.base}")
private String endpointBase;
#Bean
public HttpRequestHandlingMessagingGateway inboundGateway() {
return Http.inboundGateway("/test-inbound-gateway-resource")
.requestMapping(mapping -> mapping.methods(HttpMethod.POST))
.requestTimeout(3000)
.replyTimeout(3000)
.get();
}
#Bean
public HttpRequestExecutingMessageHandler outboundGateway1() {
return Http.outboundGateway(endpointBase + "/test-resource-1")
.httpMethod(HttpMethod.POST)
.expectedResponseType(String.class)
.get();
}
#Bean
public HttpRequestExecutingMessageHandler outboundGateway2() {
return Http.outboundGateway(endpointBase + "/test-resource-2")
.httpMethod(HttpMethod.POST)
.expectedResponseType(String.class)
.get();
}
#Bean
public StandardIntegrationFlow integrationFlow() {
ExecutorService executor = Executors.newCachedThreadPool();
IntegrationFlow flow1 = IntegrationFlows.from(MessageChannels.executor(executor))
.handle(outboundGateway1())
.get();
IntegrationFlow flow2 = IntegrationFlows.from(MessageChannels.executor(executor))
.handle(outboundGateway2())
.get();
return IntegrationFlows
.from(inboundGateway())
.transform(String.class, String::toUpperCase)
.split()
.channel(MessageChannels.executor(executor))
.routeToRecipients(r -> r
.recipientFlow(flow1)
.recipientFlow(flow2))
.aggregate()
.get();
}
}

Answer 1

Can you not simply Collections.sort() the list in the output processor? Each message will have a IntegrationMessageHeaderAccessor.SEQUENCE_NUMBER header since you set applySequence.