How to correctly use the Scatter-Gather pattern?

Question

I am still pretty new when it comes to Spring Integration, I am trying to use IntetrationFlowDefinition.scatterGather() with no avail. The general idea is to:

take a String as an input
use a splitter to tokenize it using the space character
send each token to a set of services, each returning a value
aggregate responses from each service
aggregate previously aggregated responses to a List
transform the list to a single String

I am using a messaging gateway as my input/output:

@MessagingGateway(name = "gateway")
public interface TestGateway {

  @Gateway(requestChannel = "input")
  String process(String input);

}

and invoking it via:

final TestGateway gateway = (TestGateway) ctx.getBean("gateway");
System.out.println(gateway.process("this is a test"));

My IntegrationFlows are:

@Bean
public IntegrationFlow soutFlow() {
  return f -> f.handle(m -> System.out.println(m.toString()));
}

@Bean
public IntegrationFlow flow() {
  return IntegrationFlows.from("input")
    .split(s -> s.delimiters(" "))
    .wireTap(soutFlow())
    .scatterGather(
      sc -> sc.recipientFlow(m -> true, f1 -> f1.handle((p, h) -> p + " - flow 1").wireTap(soutFlow()))
        .recipientFlow(m -> true, f2 -> f2.handle((p, h) -> p + " - flow 2").wireTap(soutFlow()))
        .applySequence(true),
      ga -> ga.outputProcessor(mg -> mg.getMessages()
        .stream()
        .map(m -> m.getPayload().toString())
        .collect(Collectors.joining(", "))),
      sg -> sg.gatherTimeout(1_000))
    .wireTap(soutFlow())
    .aggregate()
    .wireTap(soutFlow())
    .transform((List<String> source) -> source.stream()
      .map(s -> "- " + s)
      .collect(Collectors.joining("\n")))
    .get();
}

I am expecting to see output formatted as something like this:

- this - flow 1, this - flow 2
- is - flow 1, is - flow 2
- a - flow 1, a - flow 2
- test - flow 1, test - flow 2

Unfortunately, this doesn't work and gatherer always times out after 1 second. Wire-tapping debug output shows the following messages:

GenericMessage [payload=this, headers={replyChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceNumber=1, errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=4, correlationId=7c9653a7-55eb-72e4-ff74-62b10782cc3b, id=6c81a1ce-9a7c-83c3-3b69-7fb2f8b3112c, timestamp=1538668173435}]
GenericMessage [payload=this - flow 1, headers={replyChannel=org.springframework.integration.channel.FixedSubscriberChannel@78aea4b9, sequenceNumber=1, gatherResultChannel=ac418949-6ade-4db5-bd4c-3cbbbb7e9a49:1, sequenceDetails=[[7c9653a7-55eb-72e4-ff74-62b10782cc3b, 1, 4]], errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=2, correlationId=8d29aea7-3ec7-2c15-5b92-fbb0af002a3f, id=7ca4de30-f801-ad14-6452-249c85e9ab36, timestamp=1538668173446}]
GenericMessage [payload=this - flow 2, headers={replyChannel=org.springframework.integration.channel.FixedSubscriberChannel@78aea4b9, sequenceNumber=2, gatherResultChannel=ac418949-6ade-4db5-bd4c-3cbbbb7e9a49:1, sequenceDetails=[[7c9653a7-55eb-72e4-ff74-62b10782cc3b, 1, 4]], errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=2, correlationId=8d29aea7-3ec7-2c15-5b92-fbb0af002a3f, id=71b22112-8e7b-c2c5-057f-2a91c49af8c8, timestamp=1538668173446}]
GenericMessage [payload=is, headers={replyChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceNumber=2, errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=4, correlationId=7c9653a7-55eb-72e4-ff74-62b10782cc3b, id=9da2f181-82de-d50e-51bb-229a3e32b998, timestamp=1538668174448}]
GenericMessage [payload=is - flow 1, headers={replyChannel=org.springframework.integration.channel.FixedSubscriberChannel@78aea4b9, sequenceNumber=1, gatherResultChannel=ac418949-6ade-4db5-bd4c-3cbbbb7e9a49:2, sequenceDetails=[[7c9653a7-55eb-72e4-ff74-62b10782cc3b, 2, 4]], errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=2, correlationId=0abfd6a8-713d-e0cb-bc59-a111a85429df, id=d5102417-f6c2-cbac-197a-b9c4f3f51a74, timestamp=1538668174448}]
GenericMessage [payload=is - flow 2, headers={replyChannel=org.springframework.integration.channel.FixedSubscriberChannel@78aea4b9, sequenceNumber=2, gatherResultChannel=ac418949-6ade-4db5-bd4c-3cbbbb7e9a49:2, sequenceDetails=[[7c9653a7-55eb-72e4-ff74-62b10782cc3b, 2, 4]], errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=2, correlationId=0abfd6a8-713d-e0cb-bc59-a111a85429df, id=d9b31390-d3d1-9b61-43fc-45c06ac86931, timestamp=1538668174449}]
GenericMessage [payload=a, headers={replyChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceNumber=3, errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=4, correlationId=7c9653a7-55eb-72e4-ff74-62b10782cc3b, id=f109f368-6859-ac8d-0e0c-3beb563f9594, timestamp=1538668175451}]
GenericMessage [payload=a - flow 1, headers={replyChannel=org.springframework.integration.channel.FixedSubscriberChannel@78aea4b9, sequenceNumber=1, gatherResultChannel=ac418949-6ade-4db5-bd4c-3cbbbb7e9a49:3, sequenceDetails=[[7c9653a7-55eb-72e4-ff74-62b10782cc3b, 3, 4]], errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=2, correlationId=0e347381-a6e2-891a-72ff-c862565d6a1e, id=e0270063-6e19-5827-7674-21fa047ab11d, timestamp=1538668175451}]
GenericMessage [payload=a - flow 2, headers={replyChannel=org.springframework.integration.channel.FixedSubscriberChannel@78aea4b9, sequenceNumber=2, gatherResultChannel=ac418949-6ade-4db5-bd4c-3cbbbb7e9a49:3, sequenceDetails=[[7c9653a7-55eb-72e4-ff74-62b10782cc3b, 3, 4]], errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=2, correlationId=0e347381-a6e2-891a-72ff-c862565d6a1e, id=b08aab99-10ce-4dac-2f3c-b48b4514fd35, timestamp=1538668175451}]
GenericMessage [payload=test, headers={replyChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceNumber=4, errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=4, correlationId=7c9653a7-55eb-72e4-ff74-62b10782cc3b, id=52feeb27-8da4-70c4-889e-f482d2922e40, timestamp=1538668176453}]
GenericMessage [payload=test - flow 1, headers={replyChannel=org.springframework.integration.channel.FixedSubscriberChannel@78aea4b9, sequenceNumber=1, gatherResultChannel=ac418949-6ade-4db5-bd4c-3cbbbb7e9a49:4, sequenceDetails=[[7c9653a7-55eb-72e4-ff74-62b10782cc3b, 4, 4]], errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=2, correlationId=53e7fd2c-d3a4-0c0a-1296-2a14ef5e90ab, id=9152e603-e436-c0a4-abde-1f7e5ae654bf, timestamp=1538668176454}]
GenericMessage [payload=test - flow 2, headers={replyChannel=org.springframework.integration.channel.FixedSubscriberChannel@78aea4b9, sequenceNumber=2, gatherResultChannel=ac418949-6ade-4db5-bd4c-3cbbbb7e9a49:4, sequenceDetails=[[7c9653a7-55eb-72e4-ff74-62b10782cc3b, 4, 4]], errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@649725e3, sequenceSize=2, correlationId=53e7fd2c-d3a4-0c0a-1296-2a14ef5e90ab, id=a7357efd-d34a-6a05-159e-5860e3bc5281, timestamp=1538668176454}]

I must be missing something obvious but Spring documentation is very scarce when it comes to good Java DSL examples.

UPDATE:

Using wireTap() does, in fact, affect the flow. I have removed it and set up some breakpoints. It appears that the gatherer is correctly processing the scattered and processed messages, the problem seems to be in sending them further down the stream. The exception is:

org.springframework.messaging.MessagingException: Failed to handle Message; nested exception is org.springframework.messaging.core.DestinationResolutionException:
 failed to look up MessageChannel with name '27cf1bec-7e06-424e-80f8-9f2f67ebaf87:2' in the BeanFactory.;
 nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No bean named '27cf1bec-7e06-424e-80f8-9f2f67ebaf87:2' available,
 failedMessage=GenericMessage [payload=is - flow 1, is - flow 2, headers={replyChannel=org.springframework.integration.channel.FixedSubscriberChannel@4bafe935, sequenceNumber=2, gatherResultChannel=27cf1bec-7e06-424e-80f8-9f2f67ebaf87:2, sequenceDetails=[[2baa7135-5c02-932f-6b5d-cfd4052a965b, 2, 4]], errorChannel=org.springframework.messaging.core.GenericMessagingTemplate$TemporaryReplyChannel@87b5b49, sequenceSize=2, correlationId=c04f280d-83b7-74e9-a0a6-0f89efdf795c, id=ffec13cf-531e-0721-1484-e8b26b705330, timestamp=1538735548924}]

I would imagine that these intermediate channels would be created automatically by Spring when using IntegrationFlow. Is that not the case?

How does it work if you remove `wireTap()` in all your `recipientFlow()` ? — Artem Bilan, Oct 04 '18 at 16:04
This is not so typical use-case and not so short code to be present in the Reference Manual as a sample. So, far your code is good. I only have doubt about that `wireTap()` in the end which just simple may make your sub-flows as *one-way*... — Artem Bilan, Oct 04 '18 at 16:06

score 3 · Accepted Answer · answered Oct 05 '18 at 13:11

The problem is an incorrect/incorrectly set-up gatherer. My assumption was that if the input of the scatter-gather is a single String (a tokenized word), the output should also be a single String. Unfortunately, the aggregator created this way was not restoring sequence/correlation headers correctly causing the aggregator which is next in line to wait indefinitely to complete the group. Providing a null gatherer restores the default behavior. The downside is that the scatter-gather output is now a List. This can be easily mitigated by putting a transformer that flattens the list before the next aggregator. The working code below:

@Bean
public IntegrationFlow flow() {
  return IntegrationFlows.from("input")
    .split(s -> s.delimiters(" "))
    .scatterGather(
      sc -> sc
        .applySequence(true)
        .recipientFlow(m -> true, f1 -> f1.handle((p, h) -> p + " - flow 1"))
        .recipientFlow(m -> true, f2 -> f2.handle((p, h) -> p + " - flow 2")),
      null
    )
    .transform((List<String> l) -> l.stream().collect(Collectors.joining(", ")))
    .aggregate()
    .transform((List<String> source) -> source.stream()
      .map(s -> "- " + s)
      .collect(Collectors.joining("\n")))
    .get();
}

And the output is now exactly how it was expected to be:

- this - flow 1, this - flow 2
- is - flow 1, is - flow 2
- a - flow 1, a - flow 2
- test - flow 1, test - flow 2

That's great! You can accept your own answer. Meanwhile I'll take a look off-line what's going on there with the `scatter-gather`as a nested process of the outer `split-aggregate`... — Artem Bilan, Oct 05 '18 at 15:28
I confirm that we have a problem: https://jira.spring.io/browse/INT-4542. Thank you for finding this and providing workaround! — Artem Bilan, Oct 08 '18 at 19:10

How to correctly use the Scatter-Gather pattern?

1 Answers1